RARE DISEASE
Osteopetrosis and related disorders
Osteopetrosis and related disorders
Osteopetrosis and related disorders
Drug discovery
1
drug
With orphan designation
Overview
Osteopetrosis is a group of rare genetic disorders characterized by defective osteoclast-mediated bone resorption, leading to abnormally dense, brittle bones and complications such as fractures, bone marrow failure, and cranial nerve compression. Major types include autosomal dominant (ADO, Albers-Schönberg disease), autosomal recessive (ARO), and intermediate forms, with severity ranging from asymptomatic to life-threatening manifestations in infancy [1][3][7][12].
Burden
Mortality: Untreated ARO is fatal in infancy/early childhood due to marrow failure or infections [3][7].
Morbidity: Chronic pain, recurrent fractures, vision/hearing loss, and growth retardation [1][12].
Healthcare costs: High due to complex management, HSCT, and lifelong multidisciplinary care [5][15].
Therapies
Severe ARO: Hematopoietic stem cell transplantation (HSCT) to address bone marrow failure and metabolic defects; limited efficacy in neurodegenerative subtypes [5][9].
Mild/ADO: Supportive care (fracture management, physical therapy), calcium/vitamin D supplementation, interferon gamma-1b to enhance osteoclast activity [2][9][15].
Complications: Surgery for fractures/deformities, antibiotics for osteomyelitis, and multidisciplinary care for neurological, dental, and hematological issues [2][12].
Categories: rare bone diseases, rare developmental anomalies during embryogenesis, rare genetic diseases, rare transplant-related disorders
Research Papers
1,713 drug discovery papers about Osteopetrosis and related disorders, with 2 first-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
1,713 drug discovery papers about Osteopetrosis and related disorders, with 2 first-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
categories:
Small molecules
small molecules
2026-08-15 | PIDDosome deficiency impairs bone remodeling by increasing osteoclast ploidy and function.
Osteoporosis is a chronic disease driven by an imbalance between bone-building osteoblasts and bone-resorbing osteoclasts, whose hyperactivation can promote this condition. Osteoclasts are multinucleated cells, and their activity directly correlates with their ploidy level. Multinucleation associates with the accumulation of extra centrosomes that can activate the PIDDosome pathway. Depending on cell type, this can lead to a p53/p21-mediated cell cycle arrest, or BCL2-regulated apoptosis. Here, we report that the PIDDosome controls polyploidization in osteoclasts and its absence triggers bone erosion in mice. PIDDosome activation in osteoclasts depends on the presence of extra centrosomes bearing the distal appendage protein ANKRD26. Consistently, loss of Ankrd26 phenocopies PIDDosome-deficiency in osteoclasts. Surprisingly, p53 and p21, which restrict cell cycle progression in the presence of extra centrosomes, are not involved in limiting osteoclast polyploidization and function. In support of this notion, bones from p53-/- mice display a higher trabecular bone mass phenotype, and osteoclasts generated from p21-/- mice show normal OC ploidy and function. Altogether, we document that the PIDDosome regulates bone homeostasis by regulating osteoclast polyploidization and their bone-resorbing function independently of the canonical p53/p21 axis, hinting towards unknown downstream effectors. We propose that modulation of PIDDosome activation might be therapeutically exploited for osteoporosis treatment, while its inhibition may ameliorate osteopetrosis symptoms.
2026-07-27 | OSTM1 SUPPRESSES B-CELL MALIGNANCY THROUGH GLYCOSYLATION REGULATED UBIQUITIN E3 LIGASE ACTIVITY
Osteopetrosis-associated transmembrane protein 1 (OSTM1) is a glycosylated membrane protein essential for lysosomal homeostasis, with loss-of-function mutations causing autosomal recessive osteopetrosis. Using a whole-genome CRISPR/Cas9 screen, we identified OSTM1 as a previously unrecognized tumor suppressor in B-cell malignancies. Consistently, OSTM1 is frequently deleted or downregulated across diverse human B-cell lymphomas. In mice, B-cell specific monoallelic or biallelic ablation of Ostm1 cooperates with Cdkn2a deletion to drive lymphomagenesis with near-complete penetrance. Mechanistically, we identify a cytosolic, non-glycosylated pool of OSTM1 that functions as a ubiquitin E3 ligase, promoting proteasomal degradation of phosphodiesterase 3B (PDE3B). As PDE3B hydrolyzes cAMP, loss of OSTM1 stabilizes PDE3B, attenuates the tumor suppressive cAMP/PKA/CREB/CREBBP signaling, and enhances oncogenic transformation. Notably, genetic or pharmacological inhibition of PDE3B limits tumor cell growth and sensitizes cells to PI3K inhibition. Together, these findings establish OSTM1 as a tumor suppressor and reveal PDE3B signaling as a therapeutic vulnerability in B-cell lymphoma.
2026-06-25 | Expression of Phosphomimetic OSTM1-T328E/S329D Variant Partially Restores Bone Resorption Defect in LRRK1-Deficient Mice.
LRRK1 is essential for osteoclast-mediated bone resorption, and loss of LRRK1 function causes osteopetrosis in mice and humans. However, the mechanisms by which LRRK1 regulates osteoclast activity remain incompletely defined. We previously identified that phosphorylation of OSTM1 at threonine 328 and serine 329 was compromised in LRRK1-deficient osteoclasts. To test the role for OSTM1 phosphorylation in LRRK1 regulation of osteoclast functions, we expressed a phosphomimetic OSTM1 variant in LRRK1-null osteoclasts. Overexpression of phosphomimetic, but not a dephosphomimetic variant, partially restored resorptive activity in LRRK1-deficient osteoclasts in vitro. To test OSTM1's role in rescuing defective bone resorption in Lrrk1-null mice, we generated Ostm1-T328E/S329D knock-in (KI) mice and crossed them onto the Lrrk1-deficient background. Ostm1-T328E/S329D KI mice displayed normal skeletal development and bone remodeling. When crossed to the Lrrk1-deficient background, OSTM1-T328E/S329D expression increased osteoclast resorptive activity and bone formation and partially improved trabecular architecture, although bone volume remained unchanged. These findings demonstrate that OSTM1 phosphorylation contributes to LRRK1-dependent regulation of osteoclast function and identify the LRRK1-OSTM1 pathway as a mechanistic node controlling bone resorption. Our work provides new insight into the molecular basis of LRRK1-mediated osteoclast function and highlights OSTM1 phosphorylation as a potential therapeutic target for metabolic bone diseases.
2026-04-28 | HDAC6 inhibition for lysosomal trafficking restoration in osteopetrosis
Selective HDAC6 inhibitors enhance α-tubulin acetylation, improving microtubule stability and lysosomal trafficking to the ruffled border, potentially compensating for defective ClCN7 or TCIRG1 function in osteoclasts
2025-09-07 | Probing the proteome-wide impact of inhibitors of Leucine-rich Repeat Kinases 1 and 2 on protein-protein interactions and phosphorylation
Abstract The Leucine-rich repeat kinases 1 and 2 (LRRK1 and 2) are large, multidomain proteins and closely related members of the Roco protein family. They share a high similarity in domain structure and are both phosphorylate members of the Rab GTPase family. However, despite these similarities, there are substantial differences between the two kinases. While mutations to LRRK1 are only implicated in rare cases of osteopetrosis, LRRK2 is associated with multiple diseases, most prominently with familial and sporadic forms of Parkinson’s disease, where pathogenic LRRK2 is associated with an increased kinase activity. While LRRK2 has received major attention from the research community, LRRK1 has been largely understudied. In this work, we employ proximity labelling mass spectrometry in combination with quantitative phosphoproteomics in a model cell line to obtain the cellular interactomes of LRRK1 and LRRK2 and corresponding phosphorylation sites. We then use this dataset to characterize the impact of small molecules targeting both LRRK1 and 2. We identify phosphorylation sites across the proteome that are impacted by these inhibitors and identify novel candidate substrates for LRRK2, including MICALL2. Taken together our data provide a powerful resource for future studies on the cellular role and function of LRRK proteins and their potential use as therapeutic targets.
proteins
2026-04-28 | Combination therapy: Interferon-γ with traditional Shanzhuyuwan formula
Interferon-γ enhances osteoclast formation through STAT1 activation while Cornus officinalis and Rehmannia glutinosa components provide bone matrix support via IGF-1 upregulation and collagen synthesis enhancement, addressing both cellular and structural aspects of osteopetrosis
2026-04-24 | Heparan sulfate buffers the bone resorptive activity of Cathepsin K in bone homeostasis.
To understand the physiological significance of the interaction between Cathepsin K (CtsK) and heparan sulfate (HS) in bone resorption, we manipulated HS-CtsK interaction genetically by mutating three basic residues of CtsK responsible for binding HS. This knockin strain (CtskAAA) expresses an HS-binding deficient CtsK variant. In contrast to CtsK-KO mice, which display profound osteopetrosis, under C57BL/6 background CtskAAA/AAA mice display an osteoporotic phenotype due to enhanced bone resorption by the mutant osteoclasts. This phenotype is consistent with our finding that HS inhibits the collagenase activity of CtsK, suggesting that HS functions as a restraining mechanism to dampen CtsK activity. Surprisingly, under 129S1 background, CtskAAA/AAA mice display increased bone mass due to reduced bone resorptive activity of the mutant osteoclasts, opposite to the phenotype found under C57BL/6 background. This phenotype appears to reflect another biochemical property of HS-CtsK interaction, where HS can stabilize CtsK and extend its half-life. Combined, our data provide strong genetic evidence that CtsK-HS interaction is required for normal osteoclast activity in bone homeostasis through two mechanisms. Endogenous HS likely functions as a buffering agent to prevent excessive resorption and promote sustained resorption, and the balance point of the buffering can be greatly affected by genetic backgrounds.
2025-05-02 | Pycnodysostosis: a case series of eight Saudi patients with cathepsin K gene mutation and a literature review.
Pycnodysostosis, a rare osteopetrosis subtype, is mainly caused by homozygous or compound heterozygous biallelic pathogenic mutation of the cathepsin K (CTSK) gene. The cohort included eight patients (four males and four females) with a mean current age of 13 years (SD ± 3.6) and a mean age at diagnosis of 5 years (SD ± 2). All patients had a positive family history of pycnodysostosis and were born to consanguineous parents. Genetic analysis revealed that all individuals carried the same mutation: NM_000396.3(CTSK):c.244-29A>G. Clinically, they exhibited characteristic craniofacial features and skeletal deformities consistent with the diagnosis. Bone fractures were reported in 7 out of 8 patients, highlighting a significant clinical burden. All affected individuals received growth hormone therapy(GHT), though response to treatment varied among the group. These findings emphasize the importance of early genetic screening, particularly in families with a known history of pycnodysostosis, to enable timely diagnosis and intervention. Although pycnodysostosis is typically described as a nonprogressive skeletal dysplasia, the presence of complications such as osteomyelitis and recurrent fractures may contribute to a more complex and progressive clinical course in some patients.
2025-03-27 | Sorting nexin 10 regulates lysosomal ionic homeostasis via ClC-7 by controlling PI(3,5)P2.
Mutations or ablation of Snx10 are associated with neurodegeneration, blindness, and osteopetrosis. The similarities between osteoclasts and macrophages prompted us to analyze the role of Snx10 in phagocytosis. Deletion of Snx10 impaired phagosome resolution. Defective resolution was caused by reduced Cl- accumulation within (phago)lysosomes, replicating the phenotype reported in macrophages lacking ClC-7, a lysosomal 2Cl-/H+ antiporter. Delivery of ClC-7 to (phago)lysosomes was unaffected by ablation of Snx10, but its activity was markedly depressed. Snx10 was found to regulate ClC-7 activity indirectly by controlling the availability of phosphatidylinositol 3,5-bisphosphate (PI[3,5]P2), which inhibits ClC-7. By limiting the formation of PI(3,5)P2, Snx10 enables the accumulation of luminal Cl- in phagosomes and lysosomes, which is required for their optimal degradative function. Our data suggest that Snx10 regulates the delivery of PI 3-phosphate (PI[3]P), the precursor of PI(3,5)P2, from earlier endocytic compartments to (phago)lysosomes. By controlling the traffic of phosphoinositides, Snx10 regulates phagosomal resolution and possibly accounts for the impaired bone resorption in Snx10-deficient osteoclasts.
2024-02-14 | Kisspeptin-10 binding to Gpr54 in osteoclasts prevents bone loss by activating Dusp18-mediated dephosphorylation of Src.
Osteoclasts are over-activated as we age, which results in bone loss. Src deficiency in mice leads to severe osteopetrosis due to a functional defect in osteoclasts, indicating that Src function is essential in osteoclasts. G-protein-coupled receptors (GPCRs) are the targets for ∼35% of approved drugs but it is still unclear how GPCRs regulate Src kinase activity. Here, we reveal that GPR54 activation by its natural ligand Kisspeptin-10 (Kp-10) causes Dusp18 to dephosphorylate Src at Tyr 416. Mechanistically, Gpr54 recruits both active Src and the Dusp18 phosphatase at its proline/arginine-rich motif in its C terminus. We show that Kp-10 binding to Gpr54 leads to the up-regulation of Dusp18. Kiss1, Gpr54 and Dusp18 knockout mice all exhibit osteoclast hyperactivation and bone loss, and Kp-10 abrogated bone loss by suppressing osteoclast activity in vivo. Therefore, Kp-10/Gpr54 is a promising therapeutic target to abrogate bone resorption by Dusp18-mediated Src dephosphorylation.
cell therapies
2026-08-14 | Neonatal hypocalcemia and hydrocephalus as early manifestations of intermediate osteopetrosis: successful hematopoietic stem cell transplantation despite negative genetic testing: a case report.
Osteopetrosis is a rare genetic skeletal disorder caused by defective osteoclast-mediated bone resorption, leading to increased bone density and complications including hypocalcemia, pancytopenia, cranial nerve compression, and, more rarely, hydrocephalus. We report a rare case of intermediate osteopetrosis presenting with neonatal hypocalcemia and progressive macrocephaly. Brain magnetic resonance imaging confirmed obstructive hydrocephalus, which was treated with ventriculoperitoneal shunting. Clinical and radiological findings supported the diagnosis of osteopetrosis, although targeted genetic testing and whole exome sequencing did not identify a causative variant. The patient was treated with calcium gluconate and vitamin D supplementation, followed by hematopoietic stem cell transplantation from an HLA-matched unrelated donor using peripheral blood stem cells. This case highlights the importance of early recognition and timely hematopoietic stem cell transplantation to promote successful engraftment and improve long-term outcomes, even in the absence of molecular confirmation.
2026-08-13 | Osteopetrorickets and calcium homeostasis in children with osteopetrosis: an endocrinological single-center study
Background Osteopetrosis is a rare inherited disorder caused by impaired osteoclast number or function, leading to increased bone density, fractures, neurologic complications, and disturbances in calcium-phosphate homeostasis. This study aimed to describe the endocrine manifestations of childhood osteopetrosis, particularly osteopetrorickets, and to evaluate treatment responses and post-transplant calcium disorders. Methods We retrospectively reviewed 17 children diagnosed with osteopetrosis at a single tertiary center between 2015 and 2025. Clinical, biochemical, radiologic, genetic, and treatment data were analyzed. Results The median age at diagnosis was 14 months (range, 15 days–130 months), and short stature was observed in 13 of 17 patients (76.4%). Ophthalmologic abnormalities were present in 10 patients (58.8%), hearing loss in 7 patients (41.1%), and hepatosplenomegaly in 7 patients (41.1%). TCIRG1 was the most frequent mutation, followed by CLCN7, TNFSF11, TNFRSF11A , and CA2 . Osteopetrorickets was identified in 13 of 17 patients (76.4%); among these patients, hypocalcemia occurred in 11 of 13, hypophosphatemia in 8 of 13, and vitamin D deficiency in 4 of 13. Generalized osteosclerosis was observed in all patients, whereas classic osteopetrosis-associated radiographic findings, including bone-in-bone appearance, sandwich vertebrae, and Erlenmeyer flask deformity, were identified only in a subset of patients. Hematopoietic stem cell transplantation was performed in 11 of 13 patients with osteopetrorickets. Four of these 11 patients died during the early post-transplant period. Among patients with osteopetrorickets who underwent hematopoietic stem cell transplantation (HSCT) (n=11), seven surviving patients achieved complete resolution of rickets (7/11, 63.6%), allowing discontinuation of replacement therapy within 15 days to 8 months. Post-transplant hypercalcemia developed in 4 of 13 transplant recipients (30.8%). Conclusions Osteopetrorickets is a frequent and clinically significant complication of pediatric osteopetrosis. Early recognition of mineral disturbances, genotype-based treatment planning, and close surveillance for rebound hypercalcemia after transplantation are essential to improve outcomes.
2026-06-29 | A Novel Homozygous T-Cell Immune Regulator 1 (TCIRG1) Stop-Gain Variant Causing Malignant Infantile Osteopetrosis
Malignant infantile osteopetrosis (MIOP) is a rare life-threatening autosomal recessive disorder characterized by defective osteoclast-mediated bone resorption, most commonly caused by pathogenic variants in the TCIRG1 gene. Without hematopoietic stem cell transplantation (HSCT), the disease is associated with severe morbidity and early mortality. We report a 15-month-old Moroccan girl born to first-cousin consanguineous parents who initially presented during infancy with failure to thrive, persistent bicytopenia, developmental delay, axial hypotonia, divergent strabismus with nystagmus, and facial dysmorphism. Skeletal radiographs demonstrated diffuse osteosclerosis, obliteration of medullary cavities, and characteristic Erlenmeyer flask deformities, strongly suggestive of MIOP. Whole-exome sequencing identified a novel homozygous pathogenic variant in T-cell immune regulator 1 (TCIRG1) (NM_006019.4:c.1897C>T; p.Gln633Ter). The variant is absent from the gnomAD database and was classified as pathogenic according to the American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) criteria. To the best of our knowledge, this variant has not previously been reported in affected individuals. The patient is currently undergoing evaluation for HSCT. This case expands the mutational spectrum of TCIRG1-associated MIOP and highlights the importance of early genomic testing in infants from consanguineous families presenting with unexplained cytopenias, growth failure, and characteristic skeletal abnormalities.
2026-06-25 | Subtype-specific bone mineralization defects and early treatment amelioration in murine models of autosomal recessive osteopetrosis revealed by Raman spectroscopy.
Autosomal recessive osteopetrosis is a rare genetic disorder caused by impaired osteoclast function, leading to excessive bone mass, defective remodeling, and fragility fractures. While bone density alterations are well recognized, compositional parameters remain poorly characterized. Here, we applied Raman spectroscopy to characterize bone defects in two murine models, the RANKL-deficient Rankl-/- (mild form) and the TCIRG1-deficient oc/oc (severe form), compared with healthy controls. Raman analysis revealed a gradient in mineral-to-matrix ratio, progressively showing lower values from healthy to Rankl-/- and to oc/oc (skull: 2.93 ± 0.02 vs. 2.19 ± 0.31 vs. 1.72 ± 0.15, p < 0.05; long bone: 3.59 ± 0.56 vs. 2.60 ± 0.53 vs. 1.86 ± 0.17, p < 0.05 for all pairs except Rankl-/- vs oc/oc). Crystallinity showed overall lower values in osteopetrotic bones compared to WT, with partially overlapping distributions between Rankl-/- and oc/oc mice (skull: 0.050 ± 0.001 vs. 0.046 ± 0.001 vs. 0.048 ± 0.001; long bone: 0.050 ± 0.001 vs. 0.047 ± 0.001 vs. 0.048 ± 0.001 for WT, Rankl-/- and oc/oc, respectively). Multivariate analysis achieved 95% classification accuracy under exploratory animal-level cross-validation. In oc/oc mice treated with bone marrow transplantation, Raman analysis detected a significantly higher mineral-to-matrix ratio at day 18 compared to untreated oc/oc mice (2.668 ± 0.124 vs. 2.207 ± 0.311; p = 0.040, respectively). These findings identify genotype-specific compositional alterations in ARO models and provide proof-of-concept that Raman spectroscopy can characterize mineralization defects in intact bones ex vivo, suggesting that further development of non-invasive or minimally invasive Raman-based approaches may enable future studies of ARO progression and therapeutic response in clinically relevant settings.
2026-05-21 | Hematopoietic Stem Cell Transplantation in Infantile Osteopetrosis: Lessons from a Resource-Limited Setting.
Infantile Osteopetrosis is a rare inherited bone disorder characterized by a reduced or complete lack of osteoclast function, leading to defective bone resorption and increased bone density. The defective osteoclast differentiation or function is of hemopoietic origin, thus making Hematopoietic stem cell transplantation (HSCT) the only curative treatment option for this condition. This retrospective study analysed the data of 10 cases of osteopetrosis, undergoing fully HLA-matched HSCT, at AFBMTC from April 2016 to December 2025. Among the cohort, 6(60%) were male with a mean age of 16.09 ± 14.39 months at the time of HSCT. Genetic mutation analysis was available in only four cases (3 had TCIRG1, and 1 case had RANK mutation). All patients received myeloablative conditioning with Fludarabine and Busulfan. Bone marrow harvest was the source of stem cells in all cases except one. Cyclosporine-induced hypertension was the most common complication documented in 100% of cases, followed by Neutropenic fever in 9(90%) cases, mucositis in 6(60%) cases, and VOD in 3 cases (30%). Acute GVHD was documented in 3 (30%) cases, while chronic GVHD was documented in 2(20%). Two (20%) cases had post-HSCT hypercalcemia. Mixed Chimerism was observed in 8 cases (80%) in the first 100 days but did not affect transplant outcomes. OS and DFS for the study were 80%. The cause of death was refractory seizures in one patient, while another patient died due to gut GVHD and septicemia. HSCT at an early age in IO can lead to a good quality of life in this potentially fatal disease.
antibodies
2026-05-21 | The pathogenesis and intersecting mechanisms of monogenic bone mass disorders.
Monogenic disorders of bone mass arise from variants in single genes disrupting the intricate processes of bone formation and resorption. Genes underlying bone mass disorders are involved in various intersecting molecular mechanisms in cells of the mesenchymal and haematopoietic lineages. Signalling pathways, including WNT-β-catenin, TGFβ-BMP and NF-κB, have central roles in bone metabolism, and variants in their regulators or effectors can have opposing effects on bone mass. Defects in osteoblast differentiation, collagen biosynthesis and extracellular matrix mineralization primarily drive the pathogenesis of low bone mass (LBM) conditions, exemplified by various forms of osteogenesis imperfecta. Conversely, impaired osteoclast differentiation and function, or enhanced osteoblastic activity underlie high bone mass (HBM) disorders, such as osteopetrosis and osteosclerosis. A subset of genes have pleiotropic effects, contributing to both LBM and HBM disorders depending on the genetic background. Advances in genetics have not only elucidated pathogenic mechanisms, but also enabled the development of targeted therapies, including anti-sclerostin antibodies and enzyme replacement therapies. However, most LBM and HBM conditions still have no cure. Understanding the shared and divergent mechanisms of bone mass regulation will offer insights into future therapeutic strategies.
2025-09-18 | Elevated surface La promotes hyperfusion and contributes to impaired resorption in osteopetrosis.
The skeleton is a living, biological tissue responding to the biomechanical demands placed upon it throughout life. The individual bones creating this physiological system are each shaped by a multinucleated cell type - the osteoclast - that sculpts each bone in collaboration with local cellular partners, which offer chemical and even tactile feedback of many sorts. Unfortunately, the perturbation of osteoclast formation and function underpins a broad range of human skeletal pathologies, including osteopetrosis - a systemic pathology characterized by impaired osteoclast resorption leading to skeletal thickening, brittle bones, frailty, and lethality. Here, we describe a molecular dysfunction observed in murine and human models of two forms of osteoclast-rich, autosomal recessive osteopetrosis, and our approach for exploiting this molecular dysfunction to correct pathologic osteoclast hyperfusion and resorptive impairment. We find that La - a manager of osteoclast fusion and subsequent resorptive activity - is greatly elevated at the surface of osteoclasts upon loss of SNX10 or OSTM1. Using inhibitory antibodies, we suppress excessive La surface function in these mutant osteoclasts, impede osteopetrotic hyperfusion and restore osteoclast resorptive function. We share these observations as proofs-of-principle that osteoclast fusion represents a viable therapeutic target for addressing osteoclast dysfunction in diseases underpinned by excessive osteoclast multinucleation and perturbed resorptive function.
2024-11-06 | Osteoprotegerin secretion and its inhibition by RANKL in osteoblastic cells visualized using bioluminescence imaging.
Bone remodeling is regulated by the interaction between receptor activator of nuclear factor kappa-B ligand (RANKL) and its receptor RANK on osteoblasts and osteoclasts, respectively. Osteoprotegerin (OPG) is secreted from osteoblasts and inhibits osteoclast differentiation by acting as a decoy receptor for RANKL. Despite its importance, the mechanism underlying the secretion of OPG remains poorly understood. Here, we applied a method of video-rate bioluminescence imaging using a fusion protein with Gaussia luciferase (GLase) and visualized the secretion of OPG from living mouse osteoblastic MC3T3-E1 cells. The bioluminescence imaging revealed that the secretion of OPG fused to GLase (OPG-GLase) occurred frequently and widely across the cell surface. Notably, co-expression of RANKL significantly reduced the secretion of OPG-GLase, indicating an inhibitory role of RANKL on OPG secretion within cells. Further imaging and biochemical analyses using deletion mutants of OPG and RANKL, as well as RANKL mutants that cause autosomal recessive osteopetrosis, demonstrated the essential role of protein-protein interaction between OPG and RANKL in the inhibition of OPG secretion. Treatment with proteasome inhibitors resulted in increased levels of OPG in both culture medium and cell lysates. However, the fold-increase of OPG was similar regardless of the presence or absence of RANKL, suggesting that the regulation of OPG secretion by RANKL is independent of proteasome activity. This report visualized the secretion of OPG from living cells and provided evidence for a novel intracellular inhibitory effect of RANKL on OPG secretion.
2024-09-02 | Integrin α2 is an early marker for osteoclast differentiation that contributes to key steps in osteoclastogenesis.
Osteoclasts determine bone tissue turnover. Their increased activity causes osteoporosis, their dysfunction osteopetrosis. Murine monocytic ER-Hoxb8 cells differentiate into OCs upon treatment with M-CSF and RANKL and upregulate the collagen-binding integrin α2β1 distinctly earlier than other OC markers, such as the OC-associated receptor, OSCAR. Integrin α2β1 promotes OC differentiation at multiple levels by stimulating differentiation-relevant genes, by regulating cell matrix adhesion and the formation of adhesion-promoting protrusions, and by the upregulation of proteins involved in precursor cell fusion. The two key factors in osteoclastogenesis, RANK and NFATc1, were essentially unaffected after knocking out the ITGA2 gene encoding integrin α2 subunit. However, compared to integrin α2β1 expressing ER-Hoxb8 cells, ITGA2-deficient cells adhered differently with more branched filopodia and significantly longer tunneling nanotubes. Despite the higher number of fusion-relevant TNTs, they form fewer syncytia. They also resorb less hydroxyapatite, because integrin α2β1 regulates expression of lacuna proteins necessary for bone matrix resorption. The impaired syncytia formation of ITGA2-deficient OC precursor cells also correlated with reduced gene activation of fusion-supporting DC-STAMP and with an almost abolished transcription of tetraspanin CD9. CD9 only partially colocalized with integrin α2β1 in TNTs and filopodia of integrin α2β1-expressing OC precursors. Our findings define integrin α2β1 as an early marker of OC differentiation.
2024-07-06 | Rankl genetic deficiency and functional blockade undermine skeletal stem and progenitor cell differentiation.
Skeletal Stem Cells (SSCs) are required for skeletal development, homeostasis, and repair. The perspective of their wide application in regenerative medicine approaches has supported research in this field, even though so far results in the clinic have not reached expectations, possibly due also to partial knowledge of intrinsic, potentially actionable SSC regulatory factors. Among them, the pleiotropic cytokine RANKL, with essential roles also in bone biology, is a candidate deserving deep investigation. To dissect the role of the RANKL cytokine in SSC biology, we performed ex vivo characterization of SSCs and downstream progenitors (SSPCs) in mice lacking Rankl (Rankl-/-) by means of cytofluorimetric sorting and analysis of SSC populations from different skeletal compartments, gene expression analysis, and in vitro osteogenic differentiation. In addition, we assessed the effect of the pharmacological treatment with the anti-RANKL blocking antibody Denosumab (approved for therapy in patients with pathological bone loss) on the osteogenic potential of bone marrow-derived stromal cells from human healthy subjects (hBMSCs). We found that, regardless of the ossification type of bone, osteochondral SSCs had a higher frequency and impaired differentiation along the osteochondrogenic lineage in Rankl-/- mice as compared to wild-type. Rankl-/- mice also had increased frequency of committed osteochondrogenic and adipogenic progenitor cells deriving from perivascular SSCs. These changes were not due to the peculiar bone phenotype of increased density caused by lack of osteoclast resorption (defined osteopetrosis); indeed, they were not found in another osteopetrotic mouse model, i.e., the oc/oc mouse, and were therefore not due to osteopetrosis per se. In addition, Rankl-/- SSCs and primary osteoblasts showed reduced mineralization capacity. Of note, hBMSCs treated in vitro with Denosumab had reduced osteogenic capacity compared to control cultures. We provide for the first time the characterization of SSPCs from mouse models of severe recessive osteopetrosis. We demonstrate that Rankl genetic deficiency in murine SSCs and functional blockade in hBMSCs reduce their osteogenic potential. Therefore, we propose that RANKL is an important regulatory factor of SSC features with translational relevance.
other
2026-06-01 | Genetic Bone Diseases: A Scoping Review of Pathology, Symptoms, Diagnosis, Treatment, and New Horizons
ABSTRACT Genetic bone diseases are a rare group of afflictions suffered by the general population. However, their rarity should not diminish research efforts to help patients understand and treat their diseases. This review summarizes the pathology, symptoms, diagnosis, and treatment insight into six well‐known genetic bone diseases. Only six bone diseases are included due to the relatively low prevalence of them as whole limiting our scope to ensure accurate information and attention is provided for each disease individually. A literature search of PubMed is conducted, including studies published within the past five years (January 2020–December 2025). Thirty‐six studies met inclusion criteria, and no significant risk of bias is identified among the selected articles. Study findings are synthesized into disease overview, clinical and radiographic features, and diagnostic and treatment approaches. Actively developing or novel therapies relevant to each disease are also included. These treatments include: fresolimumab for osteogenesis imperfecta, small interfering ribonucleic acid (RNA) therapy for Osteopetrosis, denosumab for Paget's disease of bone, vosoritide/recifercept/infigratinib for achondroplasia, mesenchymal stem cell therapy for craniosynostosis, and combination losartan and atenolol therapy for Marfan syndrome. These treatments are generally more recently acknowledged in literature and are either actively undergoing research or require further research to determine their efficacy.
2026-05-27 | Clinical and Molecular Characterization of TCIRG1-Related Autosomal Recessive Osteopetrosis with Current Therapeutic Approaches.
Background/Objectives:TCIRG1-associated infantile osteopetrosis is a severe hereditary disorder caused by impaired osteoclast function, leading to osteosclerosis, hematological abnormalities, neurological complications, and early mortality. Early diagnosis and intervention are critical. Methods: A literature-based analysis was performed on clinical manifestations, outcomes of allogeneic hematopoietic stem cell transplantation (HSCT), immunomodulatory therapy, and experimental gene therapy and cell-based approaches, including lentiviral vectors and patient-derived induced pluripotent stem cells (iPSCs). Results: Allogeneic HSCT is the only established curative therapy, restoring osteoclast function and preventing severe complications. Early transplantation with HLA-matched donors and myeloablative conditioning provides optimal outcomes. Interferon γ1b can transiently enhance osteoclast activity but is not curative and shows variable efficacy. Preclinical studies demonstrate that lentiviral TCIRG1 delivery and transgenic correction in patient-derived iPSCs restore osteoclast function and bone resorption, with stable gene expression and minimal toxicity. Base and prime editing approaches offer potential for precise correction of single-nucleotide TCIRG1 variants, minimizing risks associated with double-strand DNA breaks. Conclusions: Allogeneic HSCT remains the standard therapy for TCIRG1-associated infantile osteopetrosis. Gene therapy and cell-based strategies represent promising adjuncts or alternatives, potentially avoiding immune-related complications and expanding therapeutic options. Further studies are needed to ensure safety, stable engraftment, and long-term efficacy, supporting translation of gene therapy into clinical practice.
2026-04-10 | Ex Vivo Expansion of Hematopoietic Stem and Progenitor Cells from Human Mobilized Peripheral Blood for Gene Therapy Applications
Abstract Ex vivo expansion of mobilized peripheral blood (mPB) hematopoietic stem cells (HSCs) represents a promising approach to advance cell and gene therapy strategies yet is hampered by loss of stem cell function when applying commonly used culture protocols. We performed in-depth characterization of mPB expansion cultures by single cell RNA sequencing, which highlighted differentiation trajectories with preservation of lineage fidelity in committed progenitors. Defining a putative HSC cluster allowed an estimation of transduction efficiency in ex vivo cultures, which correlated with long-term gene marking in xenografts and patients enrolled in a gene therapy study. We then developed a clinically translatable, GMP-compliant process to expand lentivirus (LV)-transduced HSCs from mPB of pediatric patients and adult donors, by biologically informed protocol improvements of cytokine supplementation, media choice, timing of LV transduction and combinations of small molecules preventing the activation of differentiation programs. Our optimized process outperforms validated state-of-the-art cord blood expansion protocols when applied to mPB. LV integration site analysis and genomic barcode-based clonal tracking provided definitive proof for symmetric HSC self-renewal divisions occurring during ex vivo culture. These results warrant clinical testing of this HSC transduction/expansion process in an upcoming clinical gene therapy trial for autosomal recessive osteopetrosis (EU CT 2024-518972-30). One Sentence Summary A mobilized peripheral blood HSC expansion protocol optimized for gene therapy allows robust polyclonal long-term engraftment of LV-transduced cells.
2026-01-02 | Modern approaches to autosomal-recessive osteopetrosis treatment
The review is devoted to modern developments in the treatment of osteopetrosis, the autosomal-recessive form of which has a high endemicity for the indigenous population of Chuvash and Mari El Republicы. Autosomal-recessive osteopetrosis (ARO) is a major medical and social problem, primarily due to the severity of clinical manifestations, low efficiency of pathogenetic therapy and the limitations of etiotropic therapy methods. Until now, the generally accepted standard for treating the autosomal-recessive form is transplantation of pluripotent hematopoietic stem cells. However, information about the development of alternative ARO treatment methods based on innovative technologies appears in medical publications. Therefore, the study of alternative methods of treating this disease is relevant.
2026-01-01 | Human bone-on-a-chip system for preclinical investigations of new therapeutic approaches for autosomal recessive osteopetrosis
This project introduces a human Bone-on-a-Chip system to connect fundamental research and translational applications. Utilizing the HUMIMIC Chip2 platform from TissUse, our approach is based on a decellularized human bone scaffold and sequential colonization allowing for long-term cultivation of 6 weeks. Within this cultivation period, we observe the maintenance and self-organization of primary bone forming cells, bone resorbing cells and immune cells, which enable us to assess the dynamics of the bone matrix and immune responses within the bone marrow microenvironment. In parallel, a system was developed utilizing induced pluripotent stem cells (iPSCs). These cells offer the advantage that genetic diseases can be mimicked and the phenotype and (patho)physiology of gene-corrected cells can be studied. In this thesis, the rare genetic disease autosomal recessive osteopetrosis (ARO) was examined as an example using the developed system. ARO is characterized by impaired bone resorption, for which the only treatment available is allogeneic stem cell transplantation. This therapy bears the risks of infections during patient conditioning and a graft-versus-host reaction. Autologous therapies offer a promising treatment strategy that can be developed and tested with our animal-free system. During long-term cultivation, the system is monitored by analyzing soluble factors and validating immune cell populations by fluorescence-activated cell sorting (FACS). By co-registering the scaffold before and after cultivation, it is possible to visualize very pronounced bone resorption and formation and to reflect a potential imbalance, as it is the case for ARO. To validate the physiological relevance of our in vitro bone model, we performed a single nuclei ribonucleic acid (snRNA) sequencing analysis to offer insights into the transcriptional landscape of individual cells within the bone construct. Additionally, histological stainings provide a structural comparison between the in vitro and ex vivo tissues. Currently, our Bone-on-a-Chip system is being used to preclinically test two gene therapy strategies for targeting ARO. In conclusion, our Bone-on-a-Chip system is a versatile translational research tool. Long-term culture allows us to observe orchestrated cell behavior, enabling detailed investigation of bone dynamics and immune responses. The integration of iPSCs improves the modeling of genetic diseases and expands applications in both fundamental and translational research.
small molecules
2026-08-15 | PIDDosome deficiency impairs bone remodeling by increasing osteoclast ploidy and function.
Osteoporosis is a chronic disease driven by an imbalance between bone-building osteoblasts and bone-resorbing osteoclasts, whose hyperactivation can promote this condition. Osteoclasts are multinucleated cells, and their activity directly correlates with their ploidy level. Multinucleation associates with the accumulation of extra centrosomes that can activate the PIDDosome pathway. Depending on cell type, this can lead to a p53/p21-mediated cell cycle arrest, or BCL2-regulated apoptosis. Here, we report that the PIDDosome controls polyploidization in osteoclasts and its absence triggers bone erosion in mice. PIDDosome activation in osteoclasts depends on the presence of extra centrosomes bearing the distal appendage protein ANKRD26. Consistently, loss of Ankrd26 phenocopies PIDDosome-deficiency in osteoclasts. Surprisingly, p53 and p21, which restrict cell cycle progression in the presence of extra centrosomes, are not involved in limiting osteoclast polyploidization and function. In support of this notion, bones from p53-/- mice display a higher trabecular bone mass phenotype, and osteoclasts generated from p21-/- mice show normal OC ploidy and function. Altogether, we document that the PIDDosome regulates bone homeostasis by regulating osteoclast polyploidization and their bone-resorbing function independently of the canonical p53/p21 axis, hinting towards unknown downstream effectors. We propose that modulation of PIDDosome activation might be therapeutically exploited for osteoporosis treatment, while its inhibition may ameliorate osteopetrosis symptoms.
2026-07-27 | OSTM1 SUPPRESSES B-CELL MALIGNANCY THROUGH GLYCOSYLATION REGULATED UBIQUITIN E3 LIGASE ACTIVITY
Osteopetrosis-associated transmembrane protein 1 (OSTM1) is a glycosylated membrane protein essential for lysosomal homeostasis, with loss-of-function mutations causing autosomal recessive osteopetrosis. Using a whole-genome CRISPR/Cas9 screen, we identified OSTM1 as a previously unrecognized tumor suppressor in B-cell malignancies. Consistently, OSTM1 is frequently deleted or downregulated across diverse human B-cell lymphomas. In mice, B-cell specific monoallelic or biallelic ablation of Ostm1 cooperates with Cdkn2a deletion to drive lymphomagenesis with near-complete penetrance. Mechanistically, we identify a cytosolic, non-glycosylated pool of OSTM1 that functions as a ubiquitin E3 ligase, promoting proteasomal degradation of phosphodiesterase 3B (PDE3B). As PDE3B hydrolyzes cAMP, loss of OSTM1 stabilizes PDE3B, attenuates the tumor suppressive cAMP/PKA/CREB/CREBBP signaling, and enhances oncogenic transformation. Notably, genetic or pharmacological inhibition of PDE3B limits tumor cell growth and sensitizes cells to PI3K inhibition. Together, these findings establish OSTM1 as a tumor suppressor and reveal PDE3B signaling as a therapeutic vulnerability in B-cell lymphoma.
2026-06-25 | Expression of Phosphomimetic OSTM1-T328E/S329D Variant Partially Restores Bone Resorption Defect in LRRK1-Deficient Mice.
LRRK1 is essential for osteoclast-mediated bone resorption, and loss of LRRK1 function causes osteopetrosis in mice and humans. However, the mechanisms by which LRRK1 regulates osteoclast activity remain incompletely defined. We previously identified that phosphorylation of OSTM1 at threonine 328 and serine 329 was compromised in LRRK1-deficient osteoclasts. To test the role for OSTM1 phosphorylation in LRRK1 regulation of osteoclast functions, we expressed a phosphomimetic OSTM1 variant in LRRK1-null osteoclasts. Overexpression of phosphomimetic, but not a dephosphomimetic variant, partially restored resorptive activity in LRRK1-deficient osteoclasts in vitro. To test OSTM1's role in rescuing defective bone resorption in Lrrk1-null mice, we generated Ostm1-T328E/S329D knock-in (KI) mice and crossed them onto the Lrrk1-deficient background. Ostm1-T328E/S329D KI mice displayed normal skeletal development and bone remodeling. When crossed to the Lrrk1-deficient background, OSTM1-T328E/S329D expression increased osteoclast resorptive activity and bone formation and partially improved trabecular architecture, although bone volume remained unchanged. These findings demonstrate that OSTM1 phosphorylation contributes to LRRK1-dependent regulation of osteoclast function and identify the LRRK1-OSTM1 pathway as a mechanistic node controlling bone resorption. Our work provides new insight into the molecular basis of LRRK1-mediated osteoclast function and highlights OSTM1 phosphorylation as a potential therapeutic target for metabolic bone diseases.
2026-04-28 | HDAC6 inhibition for lysosomal trafficking restoration in osteopetrosis
Selective HDAC6 inhibitors enhance α-tubulin acetylation, improving microtubule stability and lysosomal trafficking to the ruffled border, potentially compensating for defective ClCN7 or TCIRG1 function in osteoclasts
2025-09-07 | Probing the proteome-wide impact of inhibitors of Leucine-rich Repeat Kinases 1 and 2 on protein-protein interactions and phosphorylation
Abstract The Leucine-rich repeat kinases 1 and 2 (LRRK1 and 2) are large, multidomain proteins and closely related members of the Roco protein family. They share a high similarity in domain structure and are both phosphorylate members of the Rab GTPase family. However, despite these similarities, there are substantial differences between the two kinases. While mutations to LRRK1 are only implicated in rare cases of osteopetrosis, LRRK2 is associated with multiple diseases, most prominently with familial and sporadic forms of Parkinson’s disease, where pathogenic LRRK2 is associated with an increased kinase activity. While LRRK2 has received major attention from the research community, LRRK1 has been largely understudied. In this work, we employ proximity labelling mass spectrometry in combination with quantitative phosphoproteomics in a model cell line to obtain the cellular interactomes of LRRK1 and LRRK2 and corresponding phosphorylation sites. We then use this dataset to characterize the impact of small molecules targeting both LRRK1 and 2. We identify phosphorylation sites across the proteome that are impacted by these inhibitors and identify novel candidate substrates for LRRK2, including MICALL2. Taken together our data provide a powerful resource for future studies on the cellular role and function of LRRK proteins and their potential use as therapeutic targets.
proteins
2026-04-28 | Combination therapy: Interferon-γ with traditional Shanzhuyuwan formula
Interferon-γ enhances osteoclast formation through STAT1 activation while Cornus officinalis and Rehmannia glutinosa components provide bone matrix support via IGF-1 upregulation and collagen synthesis enhancement, addressing both cellular and structural aspects of osteopetrosis
2026-04-24 | Heparan sulfate buffers the bone resorptive activity of Cathepsin K in bone homeostasis.
To understand the physiological significance of the interaction between Cathepsin K (CtsK) and heparan sulfate (HS) in bone resorption, we manipulated HS-CtsK interaction genetically by mutating three basic residues of CtsK responsible for binding HS. This knockin strain (CtskAAA) expresses an HS-binding deficient CtsK variant. In contrast to CtsK-KO mice, which display profound osteopetrosis, under C57BL/6 background CtskAAA/AAA mice display an osteoporotic phenotype due to enhanced bone resorption by the mutant osteoclasts. This phenotype is consistent with our finding that HS inhibits the collagenase activity of CtsK, suggesting that HS functions as a restraining mechanism to dampen CtsK activity. Surprisingly, under 129S1 background, CtskAAA/AAA mice display increased bone mass due to reduced bone resorptive activity of the mutant osteoclasts, opposite to the phenotype found under C57BL/6 background. This phenotype appears to reflect another biochemical property of HS-CtsK interaction, where HS can stabilize CtsK and extend its half-life. Combined, our data provide strong genetic evidence that CtsK-HS interaction is required for normal osteoclast activity in bone homeostasis through two mechanisms. Endogenous HS likely functions as a buffering agent to prevent excessive resorption and promote sustained resorption, and the balance point of the buffering can be greatly affected by genetic backgrounds.
2025-05-02 | Pycnodysostosis: a case series of eight Saudi patients with cathepsin K gene mutation and a literature review.
Pycnodysostosis, a rare osteopetrosis subtype, is mainly caused by homozygous or compound heterozygous biallelic pathogenic mutation of the cathepsin K (CTSK) gene. The cohort included eight patients (four males and four females) with a mean current age of 13 years (SD ± 3.6) and a mean age at diagnosis of 5 years (SD ± 2). All patients had a positive family history of pycnodysostosis and were born to consanguineous parents. Genetic analysis revealed that all individuals carried the same mutation: NM_000396.3(CTSK):c.244-29A>G. Clinically, they exhibited characteristic craniofacial features and skeletal deformities consistent with the diagnosis. Bone fractures were reported in 7 out of 8 patients, highlighting a significant clinical burden. All affected individuals received growth hormone therapy(GHT), though response to treatment varied among the group. These findings emphasize the importance of early genetic screening, particularly in families with a known history of pycnodysostosis, to enable timely diagnosis and intervention. Although pycnodysostosis is typically described as a nonprogressive skeletal dysplasia, the presence of complications such as osteomyelitis and recurrent fractures may contribute to a more complex and progressive clinical course in some patients.
2025-03-27 | Sorting nexin 10 regulates lysosomal ionic homeostasis via ClC-7 by controlling PI(3,5)P2.
Mutations or ablation of Snx10 are associated with neurodegeneration, blindness, and osteopetrosis. The similarities between osteoclasts and macrophages prompted us to analyze the role of Snx10 in phagocytosis. Deletion of Snx10 impaired phagosome resolution. Defective resolution was caused by reduced Cl- accumulation within (phago)lysosomes, replicating the phenotype reported in macrophages lacking ClC-7, a lysosomal 2Cl-/H+ antiporter. Delivery of ClC-7 to (phago)lysosomes was unaffected by ablation of Snx10, but its activity was markedly depressed. Snx10 was found to regulate ClC-7 activity indirectly by controlling the availability of phosphatidylinositol 3,5-bisphosphate (PI[3,5]P2), which inhibits ClC-7. By limiting the formation of PI(3,5)P2, Snx10 enables the accumulation of luminal Cl- in phagosomes and lysosomes, which is required for their optimal degradative function. Our data suggest that Snx10 regulates the delivery of PI 3-phosphate (PI[3]P), the precursor of PI(3,5)P2, from earlier endocytic compartments to (phago)lysosomes. By controlling the traffic of phosphoinositides, Snx10 regulates phagosomal resolution and possibly accounts for the impaired bone resorption in Snx10-deficient osteoclasts.
2024-02-14 | Kisspeptin-10 binding to Gpr54 in osteoclasts prevents bone loss by activating Dusp18-mediated dephosphorylation of Src.
Osteoclasts are over-activated as we age, which results in bone loss. Src deficiency in mice leads to severe osteopetrosis due to a functional defect in osteoclasts, indicating that Src function is essential in osteoclasts. G-protein-coupled receptors (GPCRs) are the targets for ∼35% of approved drugs but it is still unclear how GPCRs regulate Src kinase activity. Here, we reveal that GPR54 activation by its natural ligand Kisspeptin-10 (Kp-10) causes Dusp18 to dephosphorylate Src at Tyr 416. Mechanistically, Gpr54 recruits both active Src and the Dusp18 phosphatase at its proline/arginine-rich motif in its C terminus. We show that Kp-10 binding to Gpr54 leads to the up-regulation of Dusp18. Kiss1, Gpr54 and Dusp18 knockout mice all exhibit osteoclast hyperactivation and bone loss, and Kp-10 abrogated bone loss by suppressing osteoclast activity in vivo. Therefore, Kp-10/Gpr54 is a promising therapeutic target to abrogate bone resorption by Dusp18-mediated Src dephosphorylation.
cell therapies
2026-08-14 | Neonatal hypocalcemia and hydrocephalus as early manifestations of intermediate osteopetrosis: successful hematopoietic stem cell transplantation despite negative genetic testing: a case report.
Osteopetrosis is a rare genetic skeletal disorder caused by defective osteoclast-mediated bone resorption, leading to increased bone density and complications including hypocalcemia, pancytopenia, cranial nerve compression, and, more rarely, hydrocephalus. We report a rare case of intermediate osteopetrosis presenting with neonatal hypocalcemia and progressive macrocephaly. Brain magnetic resonance imaging confirmed obstructive hydrocephalus, which was treated with ventriculoperitoneal shunting. Clinical and radiological findings supported the diagnosis of osteopetrosis, although targeted genetic testing and whole exome sequencing did not identify a causative variant. The patient was treated with calcium gluconate and vitamin D supplementation, followed by hematopoietic stem cell transplantation from an HLA-matched unrelated donor using peripheral blood stem cells. This case highlights the importance of early recognition and timely hematopoietic stem cell transplantation to promote successful engraftment and improve long-term outcomes, even in the absence of molecular confirmation.
2026-08-13 | Osteopetrorickets and calcium homeostasis in children with osteopetrosis: an endocrinological single-center study
Background Osteopetrosis is a rare inherited disorder caused by impaired osteoclast number or function, leading to increased bone density, fractures, neurologic complications, and disturbances in calcium-phosphate homeostasis. This study aimed to describe the endocrine manifestations of childhood osteopetrosis, particularly osteopetrorickets, and to evaluate treatment responses and post-transplant calcium disorders. Methods We retrospectively reviewed 17 children diagnosed with osteopetrosis at a single tertiary center between 2015 and 2025. Clinical, biochemical, radiologic, genetic, and treatment data were analyzed. Results The median age at diagnosis was 14 months (range, 15 days–130 months), and short stature was observed in 13 of 17 patients (76.4%). Ophthalmologic abnormalities were present in 10 patients (58.8%), hearing loss in 7 patients (41.1%), and hepatosplenomegaly in 7 patients (41.1%). TCIRG1 was the most frequent mutation, followed by CLCN7, TNFSF11, TNFRSF11A , and CA2 . Osteopetrorickets was identified in 13 of 17 patients (76.4%); among these patients, hypocalcemia occurred in 11 of 13, hypophosphatemia in 8 of 13, and vitamin D deficiency in 4 of 13. Generalized osteosclerosis was observed in all patients, whereas classic osteopetrosis-associated radiographic findings, including bone-in-bone appearance, sandwich vertebrae, and Erlenmeyer flask deformity, were identified only in a subset of patients. Hematopoietic stem cell transplantation was performed in 11 of 13 patients with osteopetrorickets. Four of these 11 patients died during the early post-transplant period. Among patients with osteopetrorickets who underwent hematopoietic stem cell transplantation (HSCT) (n=11), seven surviving patients achieved complete resolution of rickets (7/11, 63.6%), allowing discontinuation of replacement therapy within 15 days to 8 months. Post-transplant hypercalcemia developed in 4 of 13 transplant recipients (30.8%). Conclusions Osteopetrorickets is a frequent and clinically significant complication of pediatric osteopetrosis. Early recognition of mineral disturbances, genotype-based treatment planning, and close surveillance for rebound hypercalcemia after transplantation are essential to improve outcomes.
2026-06-29 | A Novel Homozygous T-Cell Immune Regulator 1 (TCIRG1) Stop-Gain Variant Causing Malignant Infantile Osteopetrosis
Malignant infantile osteopetrosis (MIOP) is a rare life-threatening autosomal recessive disorder characterized by defective osteoclast-mediated bone resorption, most commonly caused by pathogenic variants in the TCIRG1 gene. Without hematopoietic stem cell transplantation (HSCT), the disease is associated with severe morbidity and early mortality. We report a 15-month-old Moroccan girl born to first-cousin consanguineous parents who initially presented during infancy with failure to thrive, persistent bicytopenia, developmental delay, axial hypotonia, divergent strabismus with nystagmus, and facial dysmorphism. Skeletal radiographs demonstrated diffuse osteosclerosis, obliteration of medullary cavities, and characteristic Erlenmeyer flask deformities, strongly suggestive of MIOP. Whole-exome sequencing identified a novel homozygous pathogenic variant in T-cell immune regulator 1 (TCIRG1) (NM_006019.4:c.1897C>T; p.Gln633Ter). The variant is absent from the gnomAD database and was classified as pathogenic according to the American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) criteria. To the best of our knowledge, this variant has not previously been reported in affected individuals. The patient is currently undergoing evaluation for HSCT. This case expands the mutational spectrum of TCIRG1-associated MIOP and highlights the importance of early genomic testing in infants from consanguineous families presenting with unexplained cytopenias, growth failure, and characteristic skeletal abnormalities.
2026-06-25 | Subtype-specific bone mineralization defects and early treatment amelioration in murine models of autosomal recessive osteopetrosis revealed by Raman spectroscopy.
Autosomal recessive osteopetrosis is a rare genetic disorder caused by impaired osteoclast function, leading to excessive bone mass, defective remodeling, and fragility fractures. While bone density alterations are well recognized, compositional parameters remain poorly characterized. Here, we applied Raman spectroscopy to characterize bone defects in two murine models, the RANKL-deficient Rankl-/- (mild form) and the TCIRG1-deficient oc/oc (severe form), compared with healthy controls. Raman analysis revealed a gradient in mineral-to-matrix ratio, progressively showing lower values from healthy to Rankl-/- and to oc/oc (skull: 2.93 ± 0.02 vs. 2.19 ± 0.31 vs. 1.72 ± 0.15, p < 0.05; long bone: 3.59 ± 0.56 vs. 2.60 ± 0.53 vs. 1.86 ± 0.17, p < 0.05 for all pairs except Rankl-/- vs oc/oc). Crystallinity showed overall lower values in osteopetrotic bones compared to WT, with partially overlapping distributions between Rankl-/- and oc/oc mice (skull: 0.050 ± 0.001 vs. 0.046 ± 0.001 vs. 0.048 ± 0.001; long bone: 0.050 ± 0.001 vs. 0.047 ± 0.001 vs. 0.048 ± 0.001 for WT, Rankl-/- and oc/oc, respectively). Multivariate analysis achieved 95% classification accuracy under exploratory animal-level cross-validation. In oc/oc mice treated with bone marrow transplantation, Raman analysis detected a significantly higher mineral-to-matrix ratio at day 18 compared to untreated oc/oc mice (2.668 ± 0.124 vs. 2.207 ± 0.311; p = 0.040, respectively). These findings identify genotype-specific compositional alterations in ARO models and provide proof-of-concept that Raman spectroscopy can characterize mineralization defects in intact bones ex vivo, suggesting that further development of non-invasive or minimally invasive Raman-based approaches may enable future studies of ARO progression and therapeutic response in clinically relevant settings.
2026-05-21 | Hematopoietic Stem Cell Transplantation in Infantile Osteopetrosis: Lessons from a Resource-Limited Setting.
Infantile Osteopetrosis is a rare inherited bone disorder characterized by a reduced or complete lack of osteoclast function, leading to defective bone resorption and increased bone density. The defective osteoclast differentiation or function is of hemopoietic origin, thus making Hematopoietic stem cell transplantation (HSCT) the only curative treatment option for this condition. This retrospective study analysed the data of 10 cases of osteopetrosis, undergoing fully HLA-matched HSCT, at AFBMTC from April 2016 to December 2025. Among the cohort, 6(60%) were male with a mean age of 16.09 ± 14.39 months at the time of HSCT. Genetic mutation analysis was available in only four cases (3 had TCIRG1, and 1 case had RANK mutation). All patients received myeloablative conditioning with Fludarabine and Busulfan. Bone marrow harvest was the source of stem cells in all cases except one. Cyclosporine-induced hypertension was the most common complication documented in 100% of cases, followed by Neutropenic fever in 9(90%) cases, mucositis in 6(60%) cases, and VOD in 3 cases (30%). Acute GVHD was documented in 3 (30%) cases, while chronic GVHD was documented in 2(20%). Two (20%) cases had post-HSCT hypercalcemia. Mixed Chimerism was observed in 8 cases (80%) in the first 100 days but did not affect transplant outcomes. OS and DFS for the study were 80%. The cause of death was refractory seizures in one patient, while another patient died due to gut GVHD and septicemia. HSCT at an early age in IO can lead to a good quality of life in this potentially fatal disease.
antibodies
2026-05-21 | The pathogenesis and intersecting mechanisms of monogenic bone mass disorders.
Monogenic disorders of bone mass arise from variants in single genes disrupting the intricate processes of bone formation and resorption. Genes underlying bone mass disorders are involved in various intersecting molecular mechanisms in cells of the mesenchymal and haematopoietic lineages. Signalling pathways, including WNT-β-catenin, TGFβ-BMP and NF-κB, have central roles in bone metabolism, and variants in their regulators or effectors can have opposing effects on bone mass. Defects in osteoblast differentiation, collagen biosynthesis and extracellular matrix mineralization primarily drive the pathogenesis of low bone mass (LBM) conditions, exemplified by various forms of osteogenesis imperfecta. Conversely, impaired osteoclast differentiation and function, or enhanced osteoblastic activity underlie high bone mass (HBM) disorders, such as osteopetrosis and osteosclerosis. A subset of genes have pleiotropic effects, contributing to both LBM and HBM disorders depending on the genetic background. Advances in genetics have not only elucidated pathogenic mechanisms, but also enabled the development of targeted therapies, including anti-sclerostin antibodies and enzyme replacement therapies. However, most LBM and HBM conditions still have no cure. Understanding the shared and divergent mechanisms of bone mass regulation will offer insights into future therapeutic strategies.
2025-09-18 | Elevated surface La promotes hyperfusion and contributes to impaired resorption in osteopetrosis.
The skeleton is a living, biological tissue responding to the biomechanical demands placed upon it throughout life. The individual bones creating this physiological system are each shaped by a multinucleated cell type - the osteoclast - that sculpts each bone in collaboration with local cellular partners, which offer chemical and even tactile feedback of many sorts. Unfortunately, the perturbation of osteoclast formation and function underpins a broad range of human skeletal pathologies, including osteopetrosis - a systemic pathology characterized by impaired osteoclast resorption leading to skeletal thickening, brittle bones, frailty, and lethality. Here, we describe a molecular dysfunction observed in murine and human models of two forms of osteoclast-rich, autosomal recessive osteopetrosis, and our approach for exploiting this molecular dysfunction to correct pathologic osteoclast hyperfusion and resorptive impairment. We find that La - a manager of osteoclast fusion and subsequent resorptive activity - is greatly elevated at the surface of osteoclasts upon loss of SNX10 or OSTM1. Using inhibitory antibodies, we suppress excessive La surface function in these mutant osteoclasts, impede osteopetrotic hyperfusion and restore osteoclast resorptive function. We share these observations as proofs-of-principle that osteoclast fusion represents a viable therapeutic target for addressing osteoclast dysfunction in diseases underpinned by excessive osteoclast multinucleation and perturbed resorptive function.
2024-11-06 | Osteoprotegerin secretion and its inhibition by RANKL in osteoblastic cells visualized using bioluminescence imaging.
Bone remodeling is regulated by the interaction between receptor activator of nuclear factor kappa-B ligand (RANKL) and its receptor RANK on osteoblasts and osteoclasts, respectively. Osteoprotegerin (OPG) is secreted from osteoblasts and inhibits osteoclast differentiation by acting as a decoy receptor for RANKL. Despite its importance, the mechanism underlying the secretion of OPG remains poorly understood. Here, we applied a method of video-rate bioluminescence imaging using a fusion protein with Gaussia luciferase (GLase) and visualized the secretion of OPG from living mouse osteoblastic MC3T3-E1 cells. The bioluminescence imaging revealed that the secretion of OPG fused to GLase (OPG-GLase) occurred frequently and widely across the cell surface. Notably, co-expression of RANKL significantly reduced the secretion of OPG-GLase, indicating an inhibitory role of RANKL on OPG secretion within cells. Further imaging and biochemical analyses using deletion mutants of OPG and RANKL, as well as RANKL mutants that cause autosomal recessive osteopetrosis, demonstrated the essential role of protein-protein interaction between OPG and RANKL in the inhibition of OPG secretion. Treatment with proteasome inhibitors resulted in increased levels of OPG in both culture medium and cell lysates. However, the fold-increase of OPG was similar regardless of the presence or absence of RANKL, suggesting that the regulation of OPG secretion by RANKL is independent of proteasome activity. This report visualized the secretion of OPG from living cells and provided evidence for a novel intracellular inhibitory effect of RANKL on OPG secretion.
2024-09-02 | Integrin α2 is an early marker for osteoclast differentiation that contributes to key steps in osteoclastogenesis.
Osteoclasts determine bone tissue turnover. Their increased activity causes osteoporosis, their dysfunction osteopetrosis. Murine monocytic ER-Hoxb8 cells differentiate into OCs upon treatment with M-CSF and RANKL and upregulate the collagen-binding integrin α2β1 distinctly earlier than other OC markers, such as the OC-associated receptor, OSCAR. Integrin α2β1 promotes OC differentiation at multiple levels by stimulating differentiation-relevant genes, by regulating cell matrix adhesion and the formation of adhesion-promoting protrusions, and by the upregulation of proteins involved in precursor cell fusion. The two key factors in osteoclastogenesis, RANK and NFATc1, were essentially unaffected after knocking out the ITGA2 gene encoding integrin α2 subunit. However, compared to integrin α2β1 expressing ER-Hoxb8 cells, ITGA2-deficient cells adhered differently with more branched filopodia and significantly longer tunneling nanotubes. Despite the higher number of fusion-relevant TNTs, they form fewer syncytia. They also resorb less hydroxyapatite, because integrin α2β1 regulates expression of lacuna proteins necessary for bone matrix resorption. The impaired syncytia formation of ITGA2-deficient OC precursor cells also correlated with reduced gene activation of fusion-supporting DC-STAMP and with an almost abolished transcription of tetraspanin CD9. CD9 only partially colocalized with integrin α2β1 in TNTs and filopodia of integrin α2β1-expressing OC precursors. Our findings define integrin α2β1 as an early marker of OC differentiation.
2024-07-06 | Rankl genetic deficiency and functional blockade undermine skeletal stem and progenitor cell differentiation.
Skeletal Stem Cells (SSCs) are required for skeletal development, homeostasis, and repair. The perspective of their wide application in regenerative medicine approaches has supported research in this field, even though so far results in the clinic have not reached expectations, possibly due also to partial knowledge of intrinsic, potentially actionable SSC regulatory factors. Among them, the pleiotropic cytokine RANKL, with essential roles also in bone biology, is a candidate deserving deep investigation. To dissect the role of the RANKL cytokine in SSC biology, we performed ex vivo characterization of SSCs and downstream progenitors (SSPCs) in mice lacking Rankl (Rankl-/-) by means of cytofluorimetric sorting and analysis of SSC populations from different skeletal compartments, gene expression analysis, and in vitro osteogenic differentiation. In addition, we assessed the effect of the pharmacological treatment with the anti-RANKL blocking antibody Denosumab (approved for therapy in patients with pathological bone loss) on the osteogenic potential of bone marrow-derived stromal cells from human healthy subjects (hBMSCs). We found that, regardless of the ossification type of bone, osteochondral SSCs had a higher frequency and impaired differentiation along the osteochondrogenic lineage in Rankl-/- mice as compared to wild-type. Rankl-/- mice also had increased frequency of committed osteochondrogenic and adipogenic progenitor cells deriving from perivascular SSCs. These changes were not due to the peculiar bone phenotype of increased density caused by lack of osteoclast resorption (defined osteopetrosis); indeed, they were not found in another osteopetrotic mouse model, i.e., the oc/oc mouse, and were therefore not due to osteopetrosis per se. In addition, Rankl-/- SSCs and primary osteoblasts showed reduced mineralization capacity. Of note, hBMSCs treated in vitro with Denosumab had reduced osteogenic capacity compared to control cultures. We provide for the first time the characterization of SSPCs from mouse models of severe recessive osteopetrosis. We demonstrate that Rankl genetic deficiency in murine SSCs and functional blockade in hBMSCs reduce their osteogenic potential. Therefore, we propose that RANKL is an important regulatory factor of SSC features with translational relevance.
other
2026-06-01 | Genetic Bone Diseases: A Scoping Review of Pathology, Symptoms, Diagnosis, Treatment, and New Horizons
ABSTRACT Genetic bone diseases are a rare group of afflictions suffered by the general population. However, their rarity should not diminish research efforts to help patients understand and treat their diseases. This review summarizes the pathology, symptoms, diagnosis, and treatment insight into six well‐known genetic bone diseases. Only six bone diseases are included due to the relatively low prevalence of them as whole limiting our scope to ensure accurate information and attention is provided for each disease individually. A literature search of PubMed is conducted, including studies published within the past five years (January 2020–December 2025). Thirty‐six studies met inclusion criteria, and no significant risk of bias is identified among the selected articles. Study findings are synthesized into disease overview, clinical and radiographic features, and diagnostic and treatment approaches. Actively developing or novel therapies relevant to each disease are also included. These treatments include: fresolimumab for osteogenesis imperfecta, small interfering ribonucleic acid (RNA) therapy for Osteopetrosis, denosumab for Paget's disease of bone, vosoritide/recifercept/infigratinib for achondroplasia, mesenchymal stem cell therapy for craniosynostosis, and combination losartan and atenolol therapy for Marfan syndrome. These treatments are generally more recently acknowledged in literature and are either actively undergoing research or require further research to determine their efficacy.
2026-05-27 | Clinical and Molecular Characterization of TCIRG1-Related Autosomal Recessive Osteopetrosis with Current Therapeutic Approaches.
Background/Objectives:TCIRG1-associated infantile osteopetrosis is a severe hereditary disorder caused by impaired osteoclast function, leading to osteosclerosis, hematological abnormalities, neurological complications, and early mortality. Early diagnosis and intervention are critical. Methods: A literature-based analysis was performed on clinical manifestations, outcomes of allogeneic hematopoietic stem cell transplantation (HSCT), immunomodulatory therapy, and experimental gene therapy and cell-based approaches, including lentiviral vectors and patient-derived induced pluripotent stem cells (iPSCs). Results: Allogeneic HSCT is the only established curative therapy, restoring osteoclast function and preventing severe complications. Early transplantation with HLA-matched donors and myeloablative conditioning provides optimal outcomes. Interferon γ1b can transiently enhance osteoclast activity but is not curative and shows variable efficacy. Preclinical studies demonstrate that lentiviral TCIRG1 delivery and transgenic correction in patient-derived iPSCs restore osteoclast function and bone resorption, with stable gene expression and minimal toxicity. Base and prime editing approaches offer potential for precise correction of single-nucleotide TCIRG1 variants, minimizing risks associated with double-strand DNA breaks. Conclusions: Allogeneic HSCT remains the standard therapy for TCIRG1-associated infantile osteopetrosis. Gene therapy and cell-based strategies represent promising adjuncts or alternatives, potentially avoiding immune-related complications and expanding therapeutic options. Further studies are needed to ensure safety, stable engraftment, and long-term efficacy, supporting translation of gene therapy into clinical practice.
2026-04-10 | Ex Vivo Expansion of Hematopoietic Stem and Progenitor Cells from Human Mobilized Peripheral Blood for Gene Therapy Applications
Abstract Ex vivo expansion of mobilized peripheral blood (mPB) hematopoietic stem cells (HSCs) represents a promising approach to advance cell and gene therapy strategies yet is hampered by loss of stem cell function when applying commonly used culture protocols. We performed in-depth characterization of mPB expansion cultures by single cell RNA sequencing, which highlighted differentiation trajectories with preservation of lineage fidelity in committed progenitors. Defining a putative HSC cluster allowed an estimation of transduction efficiency in ex vivo cultures, which correlated with long-term gene marking in xenografts and patients enrolled in a gene therapy study. We then developed a clinically translatable, GMP-compliant process to expand lentivirus (LV)-transduced HSCs from mPB of pediatric patients and adult donors, by biologically informed protocol improvements of cytokine supplementation, media choice, timing of LV transduction and combinations of small molecules preventing the activation of differentiation programs. Our optimized process outperforms validated state-of-the-art cord blood expansion protocols when applied to mPB. LV integration site analysis and genomic barcode-based clonal tracking provided definitive proof for symmetric HSC self-renewal divisions occurring during ex vivo culture. These results warrant clinical testing of this HSC transduction/expansion process in an upcoming clinical gene therapy trial for autosomal recessive osteopetrosis (EU CT 2024-518972-30). One Sentence Summary A mobilized peripheral blood HSC expansion protocol optimized for gene therapy allows robust polyclonal long-term engraftment of LV-transduced cells.
2026-01-02 | Modern approaches to autosomal-recessive osteopetrosis treatment
The review is devoted to modern developments in the treatment of osteopetrosis, the autosomal-recessive form of which has a high endemicity for the indigenous population of Chuvash and Mari El Republicы. Autosomal-recessive osteopetrosis (ARO) is a major medical and social problem, primarily due to the severity of clinical manifestations, low efficiency of pathogenetic therapy and the limitations of etiotropic therapy methods. Until now, the generally accepted standard for treating the autosomal-recessive form is transplantation of pluripotent hematopoietic stem cells. However, information about the development of alternative ARO treatment methods based on innovative technologies appears in medical publications. Therefore, the study of alternative methods of treating this disease is relevant.
2026-01-01 | Human bone-on-a-chip system for preclinical investigations of new therapeutic approaches for autosomal recessive osteopetrosis
This project introduces a human Bone-on-a-Chip system to connect fundamental research and translational applications. Utilizing the HUMIMIC Chip2 platform from TissUse, our approach is based on a decellularized human bone scaffold and sequential colonization allowing for long-term cultivation of 6 weeks. Within this cultivation period, we observe the maintenance and self-organization of primary bone forming cells, bone resorbing cells and immune cells, which enable us to assess the dynamics of the bone matrix and immune responses within the bone marrow microenvironment. In parallel, a system was developed utilizing induced pluripotent stem cells (iPSCs). These cells offer the advantage that genetic diseases can be mimicked and the phenotype and (patho)physiology of gene-corrected cells can be studied. In this thesis, the rare genetic disease autosomal recessive osteopetrosis (ARO) was examined as an example using the developed system. ARO is characterized by impaired bone resorption, for which the only treatment available is allogeneic stem cell transplantation. This therapy bears the risks of infections during patient conditioning and a graft-versus-host reaction. Autologous therapies offer a promising treatment strategy that can be developed and tested with our animal-free system. During long-term cultivation, the system is monitored by analyzing soluble factors and validating immune cell populations by fluorescence-activated cell sorting (FACS). By co-registering the scaffold before and after cultivation, it is possible to visualize very pronounced bone resorption and formation and to reflect a potential imbalance, as it is the case for ARO. To validate the physiological relevance of our in vitro bone model, we performed a single nuclei ribonucleic acid (snRNA) sequencing analysis to offer insights into the transcriptional landscape of individual cells within the bone construct. Additionally, histological stainings provide a structural comparison between the in vitro and ex vivo tissues. Currently, our Bone-on-a-Chip system is being used to preclinically test two gene therapy strategies for targeting ARO. In conclusion, our Bone-on-a-Chip system is a versatile translational research tool. Long-term culture allows us to observe orchestrated cell behavior, enabling detailed investigation of bone dynamics and immune responses. The integration of iPSCs improves the modeling of genetic diseases and expands applications in both fundamental and translational research.
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Drug Discovery Landscape
1 orphan drug designation for Osteopetrosis and related disorders.
1 orphan drug designation for Osteopetrosis and related disorders.
Drug | Therapy type | Regulator | Orphan designation | Approval | Sponsor |
|---|---|---|---|---|---|
Autologous haematopoietic stem and progenitor cell population containing CD34+ cells transduced with a lentiviral vector encoding the TCIRG1 cDNA ex vivo expanded | gene therapies | EMA | 2021-10-15 | — | Fondazione Telethon Ets |
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