RARE DISEASE
X-linked sideroblastic anemia
X-linked sideroblastic anemia
X-linked sideroblastic anemia
Synonyms: XLSA
Synonyms: XLSA
Synonyms: XLSA
Drug discovery
1
drug
With orphan designation
Overview
X-linked sideroblastic anemia (XLSA) is an X-linked recessive disorder caused by ALAS2 gene mutations, impairing heme synthesis. It manifests as microcytic hypochromic anemia with ring sideroblasts on bone marrow biopsy and iron overload. Clinical features range from asymptomatic cases to fatigue, pallor, and organ damage from iron accumulation. Diagnosis combines hematological analysis, iron studies, genetic testing, and exclusion of thalassemia. Approximately 60-70% of patients respond partially to pyridoxine (B6), with iron reduction strategies critical for managing complications [1][2][10].
Burden
Chronic anemia requiring lifelong monitoring, with iron overload contributing to mortality via cirrhosis/cardiomyopathy [1][2][10].
Transfusion-dependent cases face iron overload acceleration and reduced quality of life [4][10].
~30% of patients develop pyridoxine resistance, necessitating costly interventions (e.g., stem cell transplant) [8][11][14].
Therapies
Pyridoxine: Lifelong supplementation in responders (partial hematological improvement in 60-70%) [1][10][13].
Iron management: Phlebotomy/chelation for overload; transfusions in severe anemia [1][13].
Emerging therapies: Gene therapy (lentiviral transfer) and azacitidine (for refractory female XLSA) show preclinical promise [3][11][14].
Categories: rare genetic diseases, rare hematological diseases, rare inborn errors of metabolism
Research Papers
61 drug discovery papers about X-linked sideroblastic anemia, with 1 first-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
61 drug discovery papers about X-linked sideroblastic anemia, with 1 first-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
categories:
Small molecules
small molecules
2025-02-25 | Case report: A novel 11-bp deletion in exon 11 causing a frameshift in the C-terminal of the ALAS2 gene leading to X-linked sideroblastic anemia-a family study.
X-linked sideroblastic anemia (XLSA) (MIM 300752) is the most common genetic form of sideroblastic anemia, a heterogeneous group of disorders characterized by iron deposits in the mitochondria of erythroid precursors. It is due to mutations of the erythroid-specific enzyme ALAS2, the first enzyme of the heme biosynthetic pathway. Herein, we report a novel 11-bp deletion in exon 11 leading to a frameshift in the C-terminal region of the ALAS2 gene with a non-functional longer polypeptide of 614 amino acids leading to a loss-of-function mutation manifested as an X-linked sideroblastic anemia phenotype. The proband was a 29-year-old man with moderately severe microcytic hypochromic anemia with splenomegaly and increased ring sideroblasts in the bone marrow with considerable iron overload. Sanger sequencing documented a missense mutation leading to a frameshift with an elongated polypeptide of 614 AA instead of the normal 587 AA protein c.1743_1753 del (p.Gln581Hisfs*35). This mutation affected the interaction with cofactor pyridoxal 5'-phosphate since the patient's hemoglobin improved with oral administration of pyridoxine tablets. His iron overload also responded to sustained oral iron chelation therapy with deferasirox. The screening of the entire family's kindred revealed that two other male siblings were also hemizygous for the same mutation with hypochromic microcytic anemia and tissue iron overload, whereas, three female siblings and their mother were heterozygous for the mutant allele. They did not have anemia or iron overload.
2024-11-05 | Uncoupling Alas2 from Iron-Regulatory Proteins Demonstrates the Protective Role of Alas2 Iron-Responsive Element in Protoporphyrias
Introduction. Many genes relevant to iron metabolism include a transcribed cis-regulatory element called iron-responsive element (IRE) that couples their expression to the availability of intracellular iron. Canonical IREs consist of a conserved stem-loop structure in the 5‘ or 3‘ untranslated region (UTR) of these mRNAs. IREs are bound by two iron-regulatory proteins (IRP1 & IRP2) to either limit the translation of the targeted transcript or to stabilize it. Among the 5'UTR IRE- genes, ALAS2, the erythroid-specific aminolevulinate synthase, catalyzes the first step of heme biosynthesis. Loss-of-function and gain-of-function mutations cause X-linked Sideroblastic Anemia (XLSA) and X-linked Protoporphyria (XLPP). A deficiency in the last enzyme of the pathway, ferrochelatase (FECH), causes the most common Erythropoietic Protoporphyria (EPP). A single-family with protoporphyria has been described with a mutation in the mitochondrial unfoldase CLPX. In the mouse, targeted deletion of IRP2 also results in protoporphyria. Several lines of evidence suggest a role for Alas2 in modifying the severity of erythroid porphyrias. In 4 patients with congenital erythropoietic porphyria due to the same UROS genotype, an XLPP mutation was also found in the most severely affected individual. A mutation in the Alas2-IRE has been described as a modifier in the CLPX-family. Furthermore, increased levels of Alas2 mRNA have been reported in EPP patients. Lastly, other reports suggest that iron deficiency may be protective in protoporphyrias. Methods. We generated a mouse line lacking the loop of Alas2-IRE (Alas2ΔLOOP), which should uncouple ALAS2 expression from IRP-dependent regulation. We phenotyped hemizygous male Alas2ΔLOOP/Y mice from 1 to 8 weeks of age. We compared their phenotype to established mouse models of protoporphyria (EPP FECHm1Pas/m1Pas, XLPP Alas2Q548X/Y, and IRP2-/-) at baseline. We also challenged animals with iron deficiency and iron overload. Lastly, we investigated the severity of EPP animals in the presence or absence of Alas2-IRE. Results. (a) Phenotype. Alas2ΔLOOP/Y mice have a transitory protoporphyria that resolves nearly completely by 8 weeks of age. The RBC protoporphyric phenotype is similar to XLPP mice, but substantially less severe than the EPP, XLPP, and IRP2 models. (b) Iron status. EPP, XLPP, and Alas2ΔLOOP male animals were challenged with either iron deficiency or iron overload at weaning. In iron-poor conditions, all three models have an increase in erythroid PPIX, with the Alas2ΔLOOPanimals having by far the most dramatic change after 5 weeks (mean MFI PPIX [SD] for Alas2ΔLOOPiron-replete: 12.8[6.0] v. iron poor 752[374]). However, only the Alas2 mutants (XLPP, Alas2ΔLOOP) see a modest but prolonged increase in erythroid PPIX when injected once with iron dextran. (c) Disease modifier. The double mutant Alas2ΔLOOP/Y FECHm1Pas/m1Pas (EPP-ΔLOOP) accumulate more erythroid PPIX than the EPP animals at 8 weeks of age (MFI PPIX mean [SD] for Alas2ΔLOOP 18.8[8.9], EPP 128.6 [19.1], EPP-ΔLOOP 659.6[384.7]). The fraction of fluorocytes (erythroid cells in peripheral blood abnormally containing PPIX) is also increased (%mean [SD] for EPP 57.8[8.9], EPP-ΔLOOP 97.6[2.6]). XLPP animals lacking one allele of IRP2 (Alas2Q548X/Y IRP2wt/-) present a phenotype similar to the EPP-ΔLOOP animals while Alas2Q548X/Y IRP2-/- are not viable. Conclusion. Altogether, our results confirm that iron status strongly modifies the erythroid PPIX accumulation, particularly when Alas2 expression is uncoupled from the IRPs control. We also demonstrated that the IRE-IRP system plays a critical role in mitigating the severity of protoporphyria in the rodents, providing evidence that targeting the Alas2-IRE may have pharmacological benefits.
2024-10-25 | A novel pathogenic variant in ALAS2 gene in young Indian male with X-linked sideroblastic anemia: a case report
Congenital sideroblastic anemias are heterogenous disorders with variable phenotypic expression. The most common form is X-linked and is caused by pathogenic variants of the 5-aminolevulinate synthase 2 (ALAS2) enzyme of heme biosynthesis. A 19 year old Indian young male presented with severe microcytic anemia. Bone marrow examination revealed ring sideroblasts. Using second generation sequencing, a novel pathogenic variant (Arg204Leu) was found in exon 5 of the ALAS2 gene, establishing the diagnosis of X-linked sideroblastic anemia. On treatment with oral pyridoxine supplement the patient’s hemoglobin level returned to normal. More than 100 pathogenic variants in the ALAS2 gene have been reported to date. The Arg204Leu variant in our case adds to the XLSA pathogenic variant data base. The associated anemia is fully responsive to pyridoxine supplementation.
2024-07-02 | Elucidating the Role of Human ALAS2 C-terminal Mutations Resulting in Loss of Function and Disease.
The conserved enzyme aminolevulinic acid synthase (ALAS) initiates heme biosynthesis in certain bacteria and eukaryotes by catalyzing the condensation of glycine and succinyl-CoA to yield aminolevulinic acid. In humans, the ALAS isoform responsible for heme production during red blood cell development is the erythroid-specific ALAS2 isoform. Owing to its essential role in erythropoiesis, changes in human ALAS2 (hALAS2) function can lead to two different blood disorders. X-linked sideroblastic anemia results from loss of ALAS2 function, while X-linked protoporphyria results from gain of ALAS2 function. Interestingly, mutations in the ALAS2 C-terminal extension can be implicated in both diseases. Here, we investigate the molecular basis for enzyme dysfunction mediated by two previously reported C-terminal loss-of-function variants, hALAS2 V562A and M567I. We show that the mutations do not result in gross structural perturbations, but the enzyme stability for V562A is decreased. Additionally, we show that enzyme stability moderately increases with the addition of the pyridoxal 5'-phosphate (PLP) cofactor for both variants. The variants display differential binding to PLP and the individual substrates compared to wild-type hALAS2. Although hALAS2 V562A is a more active enzyme in vitro, it is less efficient concerning succinyl-CoA binding. In contrast, the M567I mutation significantly alters the cooperativity of substrate binding. In combination with previously reported cell-based studies, our work reveals the molecular basis by which hALAS2 C-terminal mutations negatively affect ALA production necessary for proper heme biosynthesis.
2024-06-20 | Murine models of erythroid 5ALA synthesis disorders and their conditional synthetic lethal dependency on pyridoxine.
X-linked sideroblastic anemia (XLSA) and X-linked protoporphyria (XLPP) are uncommon diseases caused by loss-of-function and gain-of-function mutations, respectively, in the erythroid form of 5-aminolevulinic acid synthetase (ALAS), ALAS2, which encodes the first enzyme in heme biosynthesis. A related congenital sideroblastic anemia (CSA) is due to mutations in SLC25A38 (solute carrier family 25 member A38), which supplies mitochondrial glycine for ALAS2 (SLC25A38-CSA). The lack of viable animal models has limited the studies on pathophysiology and development of therapies for these conditions. Here, using CRISPR-CAS9 gene editing technology, we have generated knockin mouse models that recapitulate the main features of XLSA and XLPP; and using conventional conditional gene targeting in embryonic stem cells, we also developed a faithful model of the SLC25A38-CSA. In addition to examining the phenotypes and natural history of each disease, we determine the effect of restriction or supplementation of dietary pyridoxine (vitamin B6), the essential cofactor of ALAS2, on the anemia and porphyria. In addition to the well-documented response of XLSA mutations to pyridoxine supplementation, we also demonstrate the relative insensitivity of the XLPP/EPP protoporphyrias, severe sensitivity of the XLSA models, and an extreme hypersensitivity of the SLC25A38-CSA model to pyridoxine deficiency, a phenotype that is not shared with another mouse hereditary anemia model, Hbbth3/+ β-thalassemia intermedia. Thus, in addition to generating animal models useful for examining the pathophysiology and treatment of these diseases, we have uncovered an unsuspected conditional synthetic lethality between the heme synthesis-related CSAs and pyridoxine deficiency. These findings have the potential to inform novel therapeutic paradigms for the treatment of these diseases.
gene therapies
2026-07-15 | Red lines and green lights: Gene therapy for inherited erythroid disorders beyond the haemoglobinopathies.
Gene therapy is revolutionizing treatment paradigms for inherited haematological and immunological conditions. Recent successes, including the United States Food and Drug Administration (FDA) approval of gene therapy products for sickle cell disease and beta-thalassaemia, highlight the translational path of gene therapies for erythroid-specific disorders. In contrast, gene therapy development for other inherited erythroid disorders remains largely preclinical. Here, we examine the emerging landscape of gene therapies for inherited non-haemoglobinopathy erythroid disorders, focusing on the status of gene therapies for Diamond-Blackfan anaemia (DBA), pyruvate kinase deficiency (PKD), X-linked sideroblastic anaemia (XLSA), congenital erythropoietic porphyria (CEP) and congenital dyserythropoietic anaemia (CDA). We discuss the latest cellular engineering approaches being applied to developing therapies for these erythroid disorders and evolving strategies for conditioning and engraftment of modified cells. Despite the rarity of these disorders individually, several convergent biological and translational themes have emerged. Leveraging shared insights across diseases may accelerate clinical translation and broaden the curative potential of gene therapy for inherited erythroid disorders beyond the haemoglobinopathies.
2025-08-07 | [Emerging perspectives on sideroblastic anemia].
Sideroblastic anemias (SAs) represent a diverse group of congenital and acquired disorders, characterized by anemia and the presence of ring sideroblasts in the bone marrow. Congenital sideroblastic anemia (CSA) arises from genetic mutations that disrupt heme and iron metabolism within mitochondria. The most common form of CSA is X-linked sideroblastic anemia (XLSA), caused by mutations in the erythroid-specific aminolevulinate synthase 2 (ALAS2) gene, a key enzyme in the heme biosynthesis pathway in erythroid cells. On the other hand, the most common form of acquired SA is myelodysplastic syndrome with ring sideroblasts (MDS-RS). The review explores the current understanding and emerging perspectives on the pathophysiology of SAs, with a particular focus on XLSA and MDS-RS.
2024-12-10 | An erythroid-specific lentiviral vector improves anemia and iron metabolism in a new model of XLSA.
X-linked sideroblastic anemia (XLSA) is a congenital anemia caused by mutations in ALAS2, a gene responsible for heme synthesis. Treatments are limited to pyridoxine supplements and blood transfusions, offering no definitive cure except for allogeneic hematopoietic stem cell transplantation, only accessible to a subset of patients. The absence of a suitable animal model has hindered the development of gene therapy research for this disease. We engineered a conditional Alas2-knockout (KO) mouse model using tamoxifen administration or treatment with lipid nanoparticles carrying Cre-mRNA and conjugated to an anti-CD117 antibody. Alas2-KOBM animals displayed a severe anemic phenotype characterized by ineffective erythropoiesis (IE), leading to low numbers of red blood cells, hemoglobin, and hematocrit. In particular, erythropoiesis in these animals showed expansion of polychromatic erythroid cells, characterized by reduced oxidative phosphorylation, mitochondria's function, and activity of key tricarboxylic acid cycle enzymes. In contrast, glycolysis was increased in the unsuccessful attempt to extend cell survival despite mitochondrial dysfunction. The IE was associated with marked splenomegaly and low hepcidin levels, leading to iron accumulation in the liver, spleen, and bone marrow and the formation of ring sideroblasts. To investigate the potential of a gene therapy approach for XLSA, we developed a lentiviral vector (X-ALAS2-LV) to direct ALAS2 expression in erythroid cells. Infusion of bone marrow (BM) cells with 0.6 to 1.4 copies of the X-ALAS2-LV in Alas2-KOBM mice improved complete blood cell levels, tissue iron accumulation, and survival rates. These findings suggest our vector could be curative in patients with XLSA.
2010-04-05 | Glutaredoxin 5 deficiency causes sideroblastic anemia by specifically impairing heme biosynthesis and depleting cytosolic iron in human erythroblasts
Glutaredoxin 5 (GLRX5) deficiency has previously been identified as a cause of anemia in a zebrafish model and of sideroblastic anemia in a human patient. Here we report that GLRX5 is essential for iron-sulfur cluster biosynthesis and the maintenance of normal mitochondrial and cytosolic iron homeostasis in human cells. GLRX5, a mitochondrial protein that is highly expressed in erythroid cells, can homodimerize and assemble [2Fe-2S] in vitro. In GLRX5-deficient cells, [Fe-S] cluster biosynthesis was impaired, the iron-responsive element–binding (IRE-binding) activity of iron regulatory protein 1 (IRP1) was activated, and increased IRP2 levels, indicative of relative cytosolic iron depletion, were observed together with mitochondrial iron overload. Rescue of patient fibroblasts with the WT GLRX5 gene by transfection or viral transduction reversed a slow growth phenotype, reversed the mitochondrial iron overload, and increased aconitase activity. Decreased aminolevulinate δ, synthase 2 (ALAS2) levels attributable to IRP-mediated translational repression were observed in erythroid cells in which GLRX5 expression had been downregulated using siRNA along with marked reduction in ferrochelatase levels and increased ferroportin expression. Erythroblasts express both IRP-repressible ALAS2 and non-IRP–repressible ferroportin 1b. The unique combination of IRP targets likely accounts for the tissue-specific phenotype of human GLRX5 deficiency.
2003-04-02 | Involvement of ABC7 in the biosynthesis of heme in erythroid cells: interaction of ABC7 with ferrochelatase
Abstract A mitochondrial half-type ATP-binding cassette (ABC) protein, ABC7, plays a role in iron homeostasis in mitochondria, and defects in human ABC7 were shown to be responsible for the inherited disease X-linked sideroblastic anemia/ataxia. We examined the role of ABC7 in the biosynthesis of heme in erythroid cells where hemoglobin is a major product of iron-containing compounds. RNA blots showed that the amount of ABC7 mRNA in dimethylsulfoxide (Me2SO)-treated mouse erythroleukemia (MEL) cells increased markedly in parallel with the induction of the mRNA expression of ferrochelatase, the last enzyme in the pathway to synthesize heme. The transfection of the antisense oligonucleotide to mouse ABC7 mRNA into Me2SO-treated MEL cells led to a decrease of heme production, as compared with sense oligonucleotide–transfected cells. ABC7 protein was shown to be colocalized with ferrochelatase in mitochondria, as assessed by immunostaining. Furthermore, in vitro and in vivo pull-down assays revealed that ABC7 protein is interacted with the carboxy-terminal region containing the iron-sulfur cluster of ferrochelatase. The transient expression of ABC7 in mouse embryo liver BNL-CL2 cells resulted in an increase in the activity and level of ferrochelatase and thioredoxin, a cytosolic protein containing iron-sulfur. These increases were also observed in MEL cells stably expressing ABC7. When ABC7 transfectants were treated with Me2SO, an increase in cellular heme concomitant with a marked induction of the expression of ferrochelatase was observed. The extent of these increases was 3-fold greater than in control cells. The results indicated that ABC7 positively regulates not only the expression of extramitochondrial thioredoxin but also that of an intramitochondrial iron-sulfur–containing protein, ferrochelatase. Then, the expression of ABC7 contributes to the production of heme during the differentiation of erythroid cells.
cell therapies
2026-01-12 | X‐Linked Sideroblastic Anemia Caused by ALAS2 Intron 1 Mutation Successfully Treated by Allogenic Hematopoietic Stem Cell Transplant
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Data sharing is not applicable to this article as no new data were created or analyzed in this study.
2023-02-13 | Allogenic Hematopoietic Stem Cell Transplant in Iranian Patients With Congenital Sideroblastic Anemia: A Single-Center Experience.
Congenital sideroblastic anemia is characterized by anemia and intramitochondrial iron accumulation in erythroid precursors that form ring sideroblasts. The most common recessive forms are caused by sequence variations in the ALAS2 and SLC25A38 genes. In patients with transfusion-dependent and pyridoxine- resistant severe congenital sideroblastic anemia, hematopoietic stem celltransplantis the only curative option. Herein, we described successful implementations of allogeneic hematopoietic stem cell transplant in 4 Iranian children with congenital sideroblastic anemia. The patients had presented with clinical manifestations of anemia early in life, and the diagnoses of congenital sideroblastic anemia were established through blood tests and bone marrow aspiration. Congenital sideroblastic anemia was further confirmed by the identification of pathogenic variants in SLC25A38 in 2 patients. All 4 patients received allogeneic hematopoietic stem cell transplant with myeloablative conditioning regimen that included busulfan, cyclophosphamide, andrabbit antithymocyte globulin. A combination of cyclosporine A and methotrexate or mycophenolate mofetil was used for graft-versus-host disease prophylaxis. Bone marrow and peripheral blood from sibling or related donors with fully matched human leukocyte antigen profiles were applied. The outcomes of hematopoietic stem celltransplantin patients with congenital sideroblastic anemia were favorable. Three patients achieved full donor chimerism (>95%, 98%, and 100%), and the other patient showed mixed chimerism (75%). All patients remained transfusion independent. Hemato- poietic stem celltransplantis a curative treatmentthat can provide long-term survival for patients with congenital sideroblastic anemia, particularly when used in a timely manner. There remain ongoing challenges in various aspects of hematopoietic stem celltransplantin patients with congenital sideroblastic anemia, which remain to be elucidated.
2018-08-01 | Reduced‐toxicity allogeneic hematopoietic stem cell transplantation in congenital sideroblastic anemia
Key Clinical Message The case of an infant girl with severe congenital sideroblastic anemia associated with a novel molecular defect in mitochondrial transporter SLC25A38 is presented. Her transfusion dependence was fully reversed following allogeneic hematopoietic stem cell transplantation using a modified reduced‐intensity conditioning regimen, and she remains healthy 5 years posttransplant.
small molecules
2025-02-25 | Case report: A novel 11-bp deletion in exon 11 causing a frameshift in the C-terminal of the ALAS2 gene leading to X-linked sideroblastic anemia-a family study.
X-linked sideroblastic anemia (XLSA) (MIM 300752) is the most common genetic form of sideroblastic anemia, a heterogeneous group of disorders characterized by iron deposits in the mitochondria of erythroid precursors. It is due to mutations of the erythroid-specific enzyme ALAS2, the first enzyme of the heme biosynthetic pathway. Herein, we report a novel 11-bp deletion in exon 11 leading to a frameshift in the C-terminal region of the ALAS2 gene with a non-functional longer polypeptide of 614 amino acids leading to a loss-of-function mutation manifested as an X-linked sideroblastic anemia phenotype. The proband was a 29-year-old man with moderately severe microcytic hypochromic anemia with splenomegaly and increased ring sideroblasts in the bone marrow with considerable iron overload. Sanger sequencing documented a missense mutation leading to a frameshift with an elongated polypeptide of 614 AA instead of the normal 587 AA protein c.1743_1753 del (p.Gln581Hisfs*35). This mutation affected the interaction with cofactor pyridoxal 5'-phosphate since the patient's hemoglobin improved with oral administration of pyridoxine tablets. His iron overload also responded to sustained oral iron chelation therapy with deferasirox. The screening of the entire family's kindred revealed that two other male siblings were also hemizygous for the same mutation with hypochromic microcytic anemia and tissue iron overload, whereas, three female siblings and their mother were heterozygous for the mutant allele. They did not have anemia or iron overload.
2024-11-05 | Uncoupling Alas2 from Iron-Regulatory Proteins Demonstrates the Protective Role of Alas2 Iron-Responsive Element in Protoporphyrias
Introduction. Many genes relevant to iron metabolism include a transcribed cis-regulatory element called iron-responsive element (IRE) that couples their expression to the availability of intracellular iron. Canonical IREs consist of a conserved stem-loop structure in the 5‘ or 3‘ untranslated region (UTR) of these mRNAs. IREs are bound by two iron-regulatory proteins (IRP1 & IRP2) to either limit the translation of the targeted transcript or to stabilize it. Among the 5'UTR IRE- genes, ALAS2, the erythroid-specific aminolevulinate synthase, catalyzes the first step of heme biosynthesis. Loss-of-function and gain-of-function mutations cause X-linked Sideroblastic Anemia (XLSA) and X-linked Protoporphyria (XLPP). A deficiency in the last enzyme of the pathway, ferrochelatase (FECH), causes the most common Erythropoietic Protoporphyria (EPP). A single-family with protoporphyria has been described with a mutation in the mitochondrial unfoldase CLPX. In the mouse, targeted deletion of IRP2 also results in protoporphyria. Several lines of evidence suggest a role for Alas2 in modifying the severity of erythroid porphyrias. In 4 patients with congenital erythropoietic porphyria due to the same UROS genotype, an XLPP mutation was also found in the most severely affected individual. A mutation in the Alas2-IRE has been described as a modifier in the CLPX-family. Furthermore, increased levels of Alas2 mRNA have been reported in EPP patients. Lastly, other reports suggest that iron deficiency may be protective in protoporphyrias. Methods. We generated a mouse line lacking the loop of Alas2-IRE (Alas2ΔLOOP), which should uncouple ALAS2 expression from IRP-dependent regulation. We phenotyped hemizygous male Alas2ΔLOOP/Y mice from 1 to 8 weeks of age. We compared their phenotype to established mouse models of protoporphyria (EPP FECHm1Pas/m1Pas, XLPP Alas2Q548X/Y, and IRP2-/-) at baseline. We also challenged animals with iron deficiency and iron overload. Lastly, we investigated the severity of EPP animals in the presence or absence of Alas2-IRE. Results. (a) Phenotype. Alas2ΔLOOP/Y mice have a transitory protoporphyria that resolves nearly completely by 8 weeks of age. The RBC protoporphyric phenotype is similar to XLPP mice, but substantially less severe than the EPP, XLPP, and IRP2 models. (b) Iron status. EPP, XLPP, and Alas2ΔLOOP male animals were challenged with either iron deficiency or iron overload at weaning. In iron-poor conditions, all three models have an increase in erythroid PPIX, with the Alas2ΔLOOPanimals having by far the most dramatic change after 5 weeks (mean MFI PPIX [SD] for Alas2ΔLOOPiron-replete: 12.8[6.0] v. iron poor 752[374]). However, only the Alas2 mutants (XLPP, Alas2ΔLOOP) see a modest but prolonged increase in erythroid PPIX when injected once with iron dextran. (c) Disease modifier. The double mutant Alas2ΔLOOP/Y FECHm1Pas/m1Pas (EPP-ΔLOOP) accumulate more erythroid PPIX than the EPP animals at 8 weeks of age (MFI PPIX mean [SD] for Alas2ΔLOOP 18.8[8.9], EPP 128.6 [19.1], EPP-ΔLOOP 659.6[384.7]). The fraction of fluorocytes (erythroid cells in peripheral blood abnormally containing PPIX) is also increased (%mean [SD] for EPP 57.8[8.9], EPP-ΔLOOP 97.6[2.6]). XLPP animals lacking one allele of IRP2 (Alas2Q548X/Y IRP2wt/-) present a phenotype similar to the EPP-ΔLOOP animals while Alas2Q548X/Y IRP2-/- are not viable. Conclusion. Altogether, our results confirm that iron status strongly modifies the erythroid PPIX accumulation, particularly when Alas2 expression is uncoupled from the IRPs control. We also demonstrated that the IRE-IRP system plays a critical role in mitigating the severity of protoporphyria in the rodents, providing evidence that targeting the Alas2-IRE may have pharmacological benefits.
2024-10-25 | A novel pathogenic variant in ALAS2 gene in young Indian male with X-linked sideroblastic anemia: a case report
Congenital sideroblastic anemias are heterogenous disorders with variable phenotypic expression. The most common form is X-linked and is caused by pathogenic variants of the 5-aminolevulinate synthase 2 (ALAS2) enzyme of heme biosynthesis. A 19 year old Indian young male presented with severe microcytic anemia. Bone marrow examination revealed ring sideroblasts. Using second generation sequencing, a novel pathogenic variant (Arg204Leu) was found in exon 5 of the ALAS2 gene, establishing the diagnosis of X-linked sideroblastic anemia. On treatment with oral pyridoxine supplement the patient’s hemoglobin level returned to normal. More than 100 pathogenic variants in the ALAS2 gene have been reported to date. The Arg204Leu variant in our case adds to the XLSA pathogenic variant data base. The associated anemia is fully responsive to pyridoxine supplementation.
2024-07-02 | Elucidating the Role of Human ALAS2 C-terminal Mutations Resulting in Loss of Function and Disease.
The conserved enzyme aminolevulinic acid synthase (ALAS) initiates heme biosynthesis in certain bacteria and eukaryotes by catalyzing the condensation of glycine and succinyl-CoA to yield aminolevulinic acid. In humans, the ALAS isoform responsible for heme production during red blood cell development is the erythroid-specific ALAS2 isoform. Owing to its essential role in erythropoiesis, changes in human ALAS2 (hALAS2) function can lead to two different blood disorders. X-linked sideroblastic anemia results from loss of ALAS2 function, while X-linked protoporphyria results from gain of ALAS2 function. Interestingly, mutations in the ALAS2 C-terminal extension can be implicated in both diseases. Here, we investigate the molecular basis for enzyme dysfunction mediated by two previously reported C-terminal loss-of-function variants, hALAS2 V562A and M567I. We show that the mutations do not result in gross structural perturbations, but the enzyme stability for V562A is decreased. Additionally, we show that enzyme stability moderately increases with the addition of the pyridoxal 5'-phosphate (PLP) cofactor for both variants. The variants display differential binding to PLP and the individual substrates compared to wild-type hALAS2. Although hALAS2 V562A is a more active enzyme in vitro, it is less efficient concerning succinyl-CoA binding. In contrast, the M567I mutation significantly alters the cooperativity of substrate binding. In combination with previously reported cell-based studies, our work reveals the molecular basis by which hALAS2 C-terminal mutations negatively affect ALA production necessary for proper heme biosynthesis.
2024-06-20 | Murine models of erythroid 5ALA synthesis disorders and their conditional synthetic lethal dependency on pyridoxine.
X-linked sideroblastic anemia (XLSA) and X-linked protoporphyria (XLPP) are uncommon diseases caused by loss-of-function and gain-of-function mutations, respectively, in the erythroid form of 5-aminolevulinic acid synthetase (ALAS), ALAS2, which encodes the first enzyme in heme biosynthesis. A related congenital sideroblastic anemia (CSA) is due to mutations in SLC25A38 (solute carrier family 25 member A38), which supplies mitochondrial glycine for ALAS2 (SLC25A38-CSA). The lack of viable animal models has limited the studies on pathophysiology and development of therapies for these conditions. Here, using CRISPR-CAS9 gene editing technology, we have generated knockin mouse models that recapitulate the main features of XLSA and XLPP; and using conventional conditional gene targeting in embryonic stem cells, we also developed a faithful model of the SLC25A38-CSA. In addition to examining the phenotypes and natural history of each disease, we determine the effect of restriction or supplementation of dietary pyridoxine (vitamin B6), the essential cofactor of ALAS2, on the anemia and porphyria. In addition to the well-documented response of XLSA mutations to pyridoxine supplementation, we also demonstrate the relative insensitivity of the XLPP/EPP protoporphyrias, severe sensitivity of the XLSA models, and an extreme hypersensitivity of the SLC25A38-CSA model to pyridoxine deficiency, a phenotype that is not shared with another mouse hereditary anemia model, Hbbth3/+ β-thalassemia intermedia. Thus, in addition to generating animal models useful for examining the pathophysiology and treatment of these diseases, we have uncovered an unsuspected conditional synthetic lethality between the heme synthesis-related CSAs and pyridoxine deficiency. These findings have the potential to inform novel therapeutic paradigms for the treatment of these diseases.
gene therapies
2026-07-15 | Red lines and green lights: Gene therapy for inherited erythroid disorders beyond the haemoglobinopathies.
Gene therapy is revolutionizing treatment paradigms for inherited haematological and immunological conditions. Recent successes, including the United States Food and Drug Administration (FDA) approval of gene therapy products for sickle cell disease and beta-thalassaemia, highlight the translational path of gene therapies for erythroid-specific disorders. In contrast, gene therapy development for other inherited erythroid disorders remains largely preclinical. Here, we examine the emerging landscape of gene therapies for inherited non-haemoglobinopathy erythroid disorders, focusing on the status of gene therapies for Diamond-Blackfan anaemia (DBA), pyruvate kinase deficiency (PKD), X-linked sideroblastic anaemia (XLSA), congenital erythropoietic porphyria (CEP) and congenital dyserythropoietic anaemia (CDA). We discuss the latest cellular engineering approaches being applied to developing therapies for these erythroid disorders and evolving strategies for conditioning and engraftment of modified cells. Despite the rarity of these disorders individually, several convergent biological and translational themes have emerged. Leveraging shared insights across diseases may accelerate clinical translation and broaden the curative potential of gene therapy for inherited erythroid disorders beyond the haemoglobinopathies.
2025-08-07 | [Emerging perspectives on sideroblastic anemia].
Sideroblastic anemias (SAs) represent a diverse group of congenital and acquired disorders, characterized by anemia and the presence of ring sideroblasts in the bone marrow. Congenital sideroblastic anemia (CSA) arises from genetic mutations that disrupt heme and iron metabolism within mitochondria. The most common form of CSA is X-linked sideroblastic anemia (XLSA), caused by mutations in the erythroid-specific aminolevulinate synthase 2 (ALAS2) gene, a key enzyme in the heme biosynthesis pathway in erythroid cells. On the other hand, the most common form of acquired SA is myelodysplastic syndrome with ring sideroblasts (MDS-RS). The review explores the current understanding and emerging perspectives on the pathophysiology of SAs, with a particular focus on XLSA and MDS-RS.
2024-12-10 | An erythroid-specific lentiviral vector improves anemia and iron metabolism in a new model of XLSA.
X-linked sideroblastic anemia (XLSA) is a congenital anemia caused by mutations in ALAS2, a gene responsible for heme synthesis. Treatments are limited to pyridoxine supplements and blood transfusions, offering no definitive cure except for allogeneic hematopoietic stem cell transplantation, only accessible to a subset of patients. The absence of a suitable animal model has hindered the development of gene therapy research for this disease. We engineered a conditional Alas2-knockout (KO) mouse model using tamoxifen administration or treatment with lipid nanoparticles carrying Cre-mRNA and conjugated to an anti-CD117 antibody. Alas2-KOBM animals displayed a severe anemic phenotype characterized by ineffective erythropoiesis (IE), leading to low numbers of red blood cells, hemoglobin, and hematocrit. In particular, erythropoiesis in these animals showed expansion of polychromatic erythroid cells, characterized by reduced oxidative phosphorylation, mitochondria's function, and activity of key tricarboxylic acid cycle enzymes. In contrast, glycolysis was increased in the unsuccessful attempt to extend cell survival despite mitochondrial dysfunction. The IE was associated with marked splenomegaly and low hepcidin levels, leading to iron accumulation in the liver, spleen, and bone marrow and the formation of ring sideroblasts. To investigate the potential of a gene therapy approach for XLSA, we developed a lentiviral vector (X-ALAS2-LV) to direct ALAS2 expression in erythroid cells. Infusion of bone marrow (BM) cells with 0.6 to 1.4 copies of the X-ALAS2-LV in Alas2-KOBM mice improved complete blood cell levels, tissue iron accumulation, and survival rates. These findings suggest our vector could be curative in patients with XLSA.
2010-04-05 | Glutaredoxin 5 deficiency causes sideroblastic anemia by specifically impairing heme biosynthesis and depleting cytosolic iron in human erythroblasts
Glutaredoxin 5 (GLRX5) deficiency has previously been identified as a cause of anemia in a zebrafish model and of sideroblastic anemia in a human patient. Here we report that GLRX5 is essential for iron-sulfur cluster biosynthesis and the maintenance of normal mitochondrial and cytosolic iron homeostasis in human cells. GLRX5, a mitochondrial protein that is highly expressed in erythroid cells, can homodimerize and assemble [2Fe-2S] in vitro. In GLRX5-deficient cells, [Fe-S] cluster biosynthesis was impaired, the iron-responsive element–binding (IRE-binding) activity of iron regulatory protein 1 (IRP1) was activated, and increased IRP2 levels, indicative of relative cytosolic iron depletion, were observed together with mitochondrial iron overload. Rescue of patient fibroblasts with the WT GLRX5 gene by transfection or viral transduction reversed a slow growth phenotype, reversed the mitochondrial iron overload, and increased aconitase activity. Decreased aminolevulinate δ, synthase 2 (ALAS2) levels attributable to IRP-mediated translational repression were observed in erythroid cells in which GLRX5 expression had been downregulated using siRNA along with marked reduction in ferrochelatase levels and increased ferroportin expression. Erythroblasts express both IRP-repressible ALAS2 and non-IRP–repressible ferroportin 1b. The unique combination of IRP targets likely accounts for the tissue-specific phenotype of human GLRX5 deficiency.
2003-04-02 | Involvement of ABC7 in the biosynthesis of heme in erythroid cells: interaction of ABC7 with ferrochelatase
Abstract A mitochondrial half-type ATP-binding cassette (ABC) protein, ABC7, plays a role in iron homeostasis in mitochondria, and defects in human ABC7 were shown to be responsible for the inherited disease X-linked sideroblastic anemia/ataxia. We examined the role of ABC7 in the biosynthesis of heme in erythroid cells where hemoglobin is a major product of iron-containing compounds. RNA blots showed that the amount of ABC7 mRNA in dimethylsulfoxide (Me2SO)-treated mouse erythroleukemia (MEL) cells increased markedly in parallel with the induction of the mRNA expression of ferrochelatase, the last enzyme in the pathway to synthesize heme. The transfection of the antisense oligonucleotide to mouse ABC7 mRNA into Me2SO-treated MEL cells led to a decrease of heme production, as compared with sense oligonucleotide–transfected cells. ABC7 protein was shown to be colocalized with ferrochelatase in mitochondria, as assessed by immunostaining. Furthermore, in vitro and in vivo pull-down assays revealed that ABC7 protein is interacted with the carboxy-terminal region containing the iron-sulfur cluster of ferrochelatase. The transient expression of ABC7 in mouse embryo liver BNL-CL2 cells resulted in an increase in the activity and level of ferrochelatase and thioredoxin, a cytosolic protein containing iron-sulfur. These increases were also observed in MEL cells stably expressing ABC7. When ABC7 transfectants were treated with Me2SO, an increase in cellular heme concomitant with a marked induction of the expression of ferrochelatase was observed. The extent of these increases was 3-fold greater than in control cells. The results indicated that ABC7 positively regulates not only the expression of extramitochondrial thioredoxin but also that of an intramitochondrial iron-sulfur–containing protein, ferrochelatase. Then, the expression of ABC7 contributes to the production of heme during the differentiation of erythroid cells.
cell therapies
2026-01-12 | X‐Linked Sideroblastic Anemia Caused by ALAS2 Intron 1 Mutation Successfully Treated by Allogenic Hematopoietic Stem Cell Transplant
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Data sharing is not applicable to this article as no new data were created or analyzed in this study.
2023-02-13 | Allogenic Hematopoietic Stem Cell Transplant in Iranian Patients With Congenital Sideroblastic Anemia: A Single-Center Experience.
Congenital sideroblastic anemia is characterized by anemia and intramitochondrial iron accumulation in erythroid precursors that form ring sideroblasts. The most common recessive forms are caused by sequence variations in the ALAS2 and SLC25A38 genes. In patients with transfusion-dependent and pyridoxine- resistant severe congenital sideroblastic anemia, hematopoietic stem celltransplantis the only curative option. Herein, we described successful implementations of allogeneic hematopoietic stem cell transplant in 4 Iranian children with congenital sideroblastic anemia. The patients had presented with clinical manifestations of anemia early in life, and the diagnoses of congenital sideroblastic anemia were established through blood tests and bone marrow aspiration. Congenital sideroblastic anemia was further confirmed by the identification of pathogenic variants in SLC25A38 in 2 patients. All 4 patients received allogeneic hematopoietic stem cell transplant with myeloablative conditioning regimen that included busulfan, cyclophosphamide, andrabbit antithymocyte globulin. A combination of cyclosporine A and methotrexate or mycophenolate mofetil was used for graft-versus-host disease prophylaxis. Bone marrow and peripheral blood from sibling or related donors with fully matched human leukocyte antigen profiles were applied. The outcomes of hematopoietic stem celltransplantin patients with congenital sideroblastic anemia were favorable. Three patients achieved full donor chimerism (>95%, 98%, and 100%), and the other patient showed mixed chimerism (75%). All patients remained transfusion independent. Hemato- poietic stem celltransplantis a curative treatmentthat can provide long-term survival for patients with congenital sideroblastic anemia, particularly when used in a timely manner. There remain ongoing challenges in various aspects of hematopoietic stem celltransplantin patients with congenital sideroblastic anemia, which remain to be elucidated.
2018-08-01 | Reduced‐toxicity allogeneic hematopoietic stem cell transplantation in congenital sideroblastic anemia
Key Clinical Message The case of an infant girl with severe congenital sideroblastic anemia associated with a novel molecular defect in mitochondrial transporter SLC25A38 is presented. Her transfusion dependence was fully reversed following allogeneic hematopoietic stem cell transplantation using a modified reduced‐intensity conditioning regimen, and she remains healthy 5 years posttransplant.
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Drug Discovery Landscape
1 orphan drug designation for X-linked sideroblastic anemia.
1 orphan drug designation for X-linked sideroblastic anemia.
Drug | Therapy type | Regulator | Orphan designation | Approval | Sponsor |
|---|---|---|---|---|---|
Dextran and deferoxamine | — | FDA | 1991-03-08 | — | Biomedical Frontiers, Inc. |
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