RARE DISEASE
Isolated complex I deficiency
Isolated complex I deficiency
Isolated complex I deficiency
Synonyms: Isolated NADH-CoQ reductase deficiency, Isolated NADH-coenzyme Q reductase deficiency, Isolated NADH-ubiquinone reductase deficiency, Isolated mitochondrial respiratory chain complex I deficiency
Synonyms: Isolated NADH-CoQ reductase deficiency, Isolated NADH-coenzyme Q reductase deficiency, Isolated NADH-ubiquinone reductase deficiency, Isolated mitochondrial respiratory chain complex I deficiency
Synonyms: Isolated NADH-CoQ reductase deficiency, Isolated NADH-coenzyme Q reductase deficiency, Isolated NADH-ubiquinone reductase deficiency, Isolated mitochondrial respiratory chain complex I deficiency
Drug discovery
4
drugs
With orphan designations
Overview
Isolated complex I deficiency is a mitochondrial disorder caused by mutations in nuclear or mitochondrial genes encoding subunits or assembly factors of complex I (NADH:ubiquinone oxidoreductase). It is the most common enzymatic defect in mitochondrial diseases, presenting with heterogeneous manifestations such as lactic acidosis, encephalopathy, cardiomyopathy, and progressive neurological decline. Clinical phenotypes include Leigh syndrome, Leber hereditary optic neuropathy, and MELAS syndrome, often with early-onset, severe progression, and high mortality [1][2][4][6].
Burden
High morbidity/mortality: Neonatal cases often fatal; Leigh syndrome causes death within 2–3 years of symptom onset [4][6][18].
Multisystem impact: Neurological, cardiac, hepatic, and renal dysfunction necessitate lifelong multidisciplinary care [1][4][6].
Healthcare costs: Frequent hospitalizations, intensive monitoring, and limited curative options strain families and healthcare systems [4][18].
Therapies
Supportive care: Management of seizures, acidosis, and organ-specific complications [3][8][18].
Cofactors/supplements: Coenzyme Q10, riboflavin, thiamine, and antioxidants (e.g., idebenone), though efficacy is often anecdotal [3][8][18].
Clinical trials: Emerging therapies targeting mitochondrial biogenesis, ROS reduction, and gene-specific approaches (e.g., deoxynucleoside supplementation) [3][12].
Categories: rare genetic diseases, rare inborn errors of metabolism, rare neurological diseases
Research Papers
142 drug discovery papers about Isolated complex I deficiency, with 1 first-in-class and 7 next-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
142 drug discovery papers about Isolated complex I deficiency, with 1 first-in-class and 7 next-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
categories:
Small molecules
gene therapies
2024-12-01 | Proteomics sample preparation v1
Protocol for proteomic analysis and data acquisition used in: "Selective Deficiency of MitochondrialRespiratory Complex I Subunits Ndufs4/6 Causes Tumor Immunogenicity". The paper's abstract is as follows: Cancer cells frequently rewire their metabolism to support proliferation and evade immune surveillance, but little is known about metabolic targets that could increase immune surveillance. Here we show a specific means of mitochondrial respiratory Complex I (CI) inhibition that improves tumor immunogenicity and sensitivity to immune checkpoint blockade (ICB). Targeted genetic deletion of either Ndufs4 or Ndufs6, but not other CI subunits, induces an immune-dependent growth attenuation in melanoma and breast cancer models. We show that deletion of Ndufs4 induces expression of the MHC class-I co-activator Nlrc5 and antigen presentation machinery components, most notably H2-K1. This induction of MHC-related genes is driven by a pyruvate dehydrogenase-dependent accumulation of mitochondrial acetyl-CoA, which leads to an increase in histone H3K27-acetylation within the Nlrc5 and H2-K1 promoters. Taken together, this work shows that selective CI inhibition restricts tumor growth, and that specific targeting of Ndufs4 or Ndufs6, increases T-cell surveillance and ICB responsiveness.
2024-05-09 | Identification of a novel MT-ND3 variant and restoring mitochondrial function by allotopic expression of MT-ND3 gene.
Mitochondrial diseases are caused by nuclear, or mitochondrial DNA (mtDNA) variants and related co-factors. Here, we report a novel m.10197G > C variant in MT-ND3 in a patient, and two other patients with m.10191 T > C. MT-ND3 variants are known to cause Leigh syndrome or mitochondrial complex I deficiency. We performed the functional analyses of the novel m.10197G > C variant that significantly lowered MT-ND3 protein levels, causing complex I assembly and activity deficiency, and reduction of ATP synthesis. We adapted a previously described re-engineering technique of delivering mitochondrial genes into mitochondria through codon optimization for nuclear expression and translation by cytoplasmic ribosomes to rescue defects arising from the MT-ND3 variants. We constructed mitochondrial targeting sequences along with the codon-optimized MT-ND3 and imported them into the mitochondria. To achieve the goal, we imported codon-optimized MT-ND3 into mitochondria in three patients with m.10197G > C and m.10191 T > C missense variants in the MT-ND3. Nuclear expression of the MT-ND3 gene partially restored protein levels, complex I deficiency, and significant improvement of ATP production indicating a functional rescue of the mutant phenotype. The codon-optimized nuclear expression of mitochondrial protein and import inside the mitochondria can supplement the requirements for ATP in energy-deficient mitochondrial disease patients.
2023-06-09 | IQGAP1 mediates the communication between the nucleus and the mitochondria via NDUFS4 alternative splicing
Abstract Constant communication between mitochondria and nucleus ensures cellular homeostasis and adaptation to mitochondrial stress. Anterograde regulatory pathways involving a large number of nuclear-encoded proteins control mitochondrial biogenesis and functions. Such functions are deregulated in cancer cells, resulting in proliferative advantages, aggressive disease and therapeutic resistance. Transcriptional networks controlling the nuclear-encoded mitochondrial genes are known, however alternative splicing (AS) regulation has not been implicated in this communication. Here, we show that IQGAP1, a scaffold protein regulating AS of distinct gene subsets in gastric cancer cells, participates in AS regulation that strongly affects mitochondrial respiration. Combined proteomic and RNA-seq analyses of IQGAP1KO and parental cells show that IQGAP1KO alters an AS event of the mitochondrial respiratory chain complex I (CI) subunit NDUFS4 and downregulates a subset of CI subunits. In IQGAP1KO cells, CI intermediates accumulate, resembling assembly deficiencies observed in patients with Leigh syndrome bearing NDUFS4 mutations. Mitochondrial CI activity is significantly lower in KO compared to parental cells, while exogenous expression of IQGAP1 reverses mitochondrial defects of IQGAP1KO cells. Our work sheds light to a novel facet of IQGAP1 in mitochondrial quality control that involves fine-tuning of CI activity through AS regulation in gastric cancer cells relying highly on mitochondrial respiration.
2021-07-08 | Biallelic mutations in NDUFA8 cause complex I deficiency in two siblings with favorable clinical evolution.
Isolated complex I (CI) deficiency is the most common cause of oxidative phosphorylation (OXPHOS) dysfunction. Whole-exome sequencing identified biallelic mutations in NDUFA8 (c.[293G > T]; [293G > T], encoding for an accessory subunit of CI, in two siblings with a favorable clinical evolution. The individuals reported here are practically asymptomatic, with the exception of slight failure to thrive and some language difficulties at the age of 6 and 9 years, respectively. These observations are remarkable since the vast majority of patients with CI deficiency, including the only NDUFA8 patient reported so far, showed an extremely poor clinical outcome. Western blot studies demonstrated that NDUFA8 protein was strongly reduced in the patients' fibroblasts and muscle extracts. In addition, there was a marked and specific decrease in the steady-state levels of CI subunits. BN-PAGE demonstrated an isolated defect in the assembly and the activity of CI with impaired supercomplexes formation and abnormal accumulation of CI subassemblies. Confocal microscopy analysis in fibroblasts showed rounder mitochondria and diminished branching degree of the mitochondrial network. Functional complementation studies demonstrated disease-causality for the identified mutation as lentiviral transduction with wild-type NDUFA8 cDNA restored the steady-state levels of CI subunits and completely recovered the deficient enzymatic activity in immortalized mutant fibroblasts. In summary, we provide additional evidence of the involvement of NDUFA8 as a mitochondrial disease-causing gene associated with altered mitochondrial morphology, CI deficiency, impaired supercomplexes formation, and very mild progression of the disease.
2020-05-09 | A homozygous variant in NDUFA8 is associated with developmental delay, microcephaly, and epilepsy due to mitochondrial complex I deficiency.
Abstract Mitochondrial complex I deficiency is caused by pathogenic variants in mitochondrial and nuclear genes associated with complex I structure and assembly. We report the case of a patient with NDUFA8‐related mitochondrial disease. The patient presented with developmental delay, microcephaly, and epilepsy. His fibroblasts showed apparent biochemical defects in mitochondrial complex I. Whole‐exome sequencing revealed that the patient carried a homozygous variant in NDUFA8 . His fibroblasts showed a reduction in the protein expression level of not only NDUFA8, but also the other complex I subunits, consistent with assembly defects. The enzyme activity of complex I and oxygen consumption rate were restored by reintroducing wild‐type NDUFA8 cDNA into patient fibroblasts. The functional properties of the variant in NDUFA8 were also investigated using NDUFA8 knockout cells expressing wild‐type or mutated NDUFA8 cDNA. These experiments further supported the pathogenicity of the variant in complex I assembly. This is the first report describing that the loss of NDUFA8, which has not previously been associated with mitochondrial disease, causes severe defect in the assembly of mitochondrial complex I, leading to progressive neurological and developmental abnormalities.
cell therapies
2022-06-21 | Mesenchymal stem cells improve redox homeostasis and mitochondrial respiration in fibroblast cell lines with pathogenic MT-ND3 and MT-ND6 variants.
The most frequent biochemical defect of inherited mitochondrial disease is isolated complex I deficiency. There is no cure for this disorder, and treatment is mainly supportive. In this study, we investigated the effects of human mesenchymal stem cells (MSCs) on skin fibroblast derived from three individuals with complex I deficiency carrying different pathogenic variants in mitochondrial DNA-encoded subunits (MT-ND3, MT-ND6). Complex I-deficient fibroblasts were transiently co-cultured with bone marrow-derived MSCs. Mitochondrial transfer was analysed by fluorescence labelling and validated by Sanger sequencing. Levels of reactive oxygen species (ROS) were measured using MitoSOX Red. Moreover, mitochondrial respiration was analysed by Seahorse XFe96 Extracellular Flux Analyzer. Levels of antioxidant proteins were investigated via immunoblotting. Co-culturing of complex I-deficient fibroblast with MSCs lowered cellular ROS levels. The effect on ROS production was more sustained compared to treatment of patient fibroblasts with culture medium derived from MSC cultures. Investigation of cellular antioxidant defence systems revealed an upregulation of SOD2 (superoxide dismutase 2, mitochondrial) and HO-1 (heme oxygenase 1) in patient-derived cell lines. This adaptive response was normalised upon MSC treatment. Moreover, Seahorse experiments revealed a significant improvement of mitochondrial respiration, indicating a mitigation of the oxidative phosphorylation defect. Experiments with repetitive MSC co-culture at two consecutive time points enhanced this effect. Our study indicates that MSC-based treatment approaches might constitute an interesting option for patients with mitochondrial DNA-encoded mitochondrial diseases. We suggest that this strategy may prove more promising for defects caused by mitochondrial DNA variants compared to nuclear-encoded defects.
2019-04-18 | Donation of mitochondria by iPSC-derived mesenchymal stem cells protects retinal ganglion cells against mitochondrial complex I defect-induced degeneration
Rationale: Retinal ganglion cell (RGC) degeneration is extremely hard to repair or regenerate and is often coupled with mitochondrial dysfunction.Mesenchymal stem cells (MSCs)-based treatment has been demonstrated beneficial for RGC against degeneration.However, underlying mechanisms of MSC-provided RGC protection are largely unknown other than neuroprotective paracrine actions.In this study, we sought to investigate whether mitochondrial donation from induced pluripotent stem cell-derived MSC (iPSC-MSCs) could preserve RGC survival and restore retinal function.Methods: iPSC-MSCs were injected into the vitreous cavity of one eye in NADH dehydrogenase (ubiquinone) Fe-S protein 4 (Ndufs4) knockout (KO) and wild type mice.Phosphate buffer saline (PBS) or rotenone treated iPSC-MSCs were injected as control groups.Retinal function was detected by flash electroretinogram (ERG).Whole-mount immunofluorescence (IF), morphometric analysis, confocal microscopy imaging, polymerase chain reaction (PCR) of the retinas were conducted to investigate mitochondrial transfer from human iPSC-MSCs to mouse retina.Quantitative mouse cytokine arrays were carried out to measure retinal inflammatory response under difference treatments.Results: RGC survival in the iPSC-MSC injected retina of Ndufs4 KO mice was significantly increased with improved retinal function.GFP labelled human mitochondria from iPSC-MSC were detected in the RGCs in the retina of Ndufs4 KO mice starting from 96 hours post injection.PCR result showed only human mitochondrial DNA without human nuclear DNA could be detected in the mouse retinas after iPSC-MSC treatment in Ndufs4 KO mice eye.Quantitative cytokine array analysis showed pro-inflammatory cytokines was also downregulated by this iPSC-MSC treatment.Conclusion: Intravitreal transplanted iPSC-MSCs can effectively donate functional mitochondria to RGCs and protect against mitochondrial damage-induced RGC loss.
2019-03-27 | A Drosophila Mitochondrial Complex I Deficiency Phenotype Array
Mitochondrial diseases are a group of rare life-threatening diseases often caused by defects in the oxidative phosphorylation system. No effective treatment is available for these disorders. Therapeutic development is hampered by the high heterogeneity in genetic, biochemical and clinical spectra of mitochondrial diseases and by limited preclinical resources to screen and identify effective treatment candidates. Alternative models of the pathology are essential to better understand mitochondrial diseases and to accelerate the development of new therapeutics. The fruit fly Drosophila melanogaster is a cost- and time-efficient model that can recapitulate a wide range of phenotypes observed in patients suffering from mitochondrial disorders. We targeted three important subunits of Complex I of the mitochondrial oxidative phosphorylation system with the flexible UAS-Gal4 system and RNA interference: NDUFS4 (ND-18), NDUFS7 (ND-20) and NDUFV1 (ND-51). Using two ubiquitous driver lines at two temperatures, we established a collection of phenotypes relevant to complex I deficiencies. Our data offer models and phenotypes with different levels of severity that can be used for future therapeutic screenings. These include qualitative phenotypes that are amenable to high-throughput drug screening and quantitative phenotypes that require more resources but are likely to have increased potential and sensitivity to show modulation by drug treatment.
2018-03-26 | Modulation of oxidative phosphorylation and redox homeostasis in mitochondrial NDUFS4 deficiency via mesenchymal stem cells.
Disorders of the oxidative phosphorylation (OXPHOS) system represent a large group among the inborn errors of metabolism. The most frequently observed biochemical defect is isolated deficiency of mitochondrial complex I (CI). No effective treatment strategies for CI deficiency are so far available. The purpose of this study was to investigate whether and how mesenchymal stem cells (MSCs) are able to modulate metabolic function in fibroblast cell models of CI deficiency. We used human and murine fibroblasts with a defect in the nuclear DNA encoded NDUFS4 subunit of CI. Fibroblasts were co-cultured with MSCs under different stress conditions and intercellular mitochondrial transfer was assessed by flow cytometry and fluorescence microscopy. Reactive oxygen species (ROS) levels were measured using MitoSOX-Red. Protein levels of CI were analysed by blue native polyacrylamide gel electrophoresis (BN-PAGE). Direct cellular interactions and mitochondrial transfer between MSCs and human as well as mouse fibroblast cell lines were demonstrated. Mitochondrial transfer was visible in 13.2% and 6% of fibroblasts (e.g. fibroblasts containing MSC mitochondria) for human and mouse cell lines, respectively. The transfer rate could be further stimulated via treatment of cells with TNF-α. MSCs effectively lowered cellular ROS production in NDUFS4-deficient fibroblast cell lines (either directly via co-culture or indirectly via incubation of cell lines with cell-free MSC supernatant). However, CI protein expression and activity were not rescued by MSC treatment. This study demonstrates the interplay between MSCs and fibroblast cell models of isolated CI deficiency including transfer of mitochondria as well as modulation of cellular ROS levels. Further exploration of these cellular interactions might help to develop MSC-based treatment strategies for human CI deficiency.
1999-07-01 | Nuclear DNA origin of mitochondrial complex I deficiency in fatal infantile lactic acidosis evidenced by transnuclear complementation of cultured fibroblasts
We have studied complex I (NADH-ubiquinone reductase) defects of the mitochondrial respiratory chain in 2 infants who died in the neonatal period from 2 different neurological forms of severe neonatal lactic acidosis. Specific and marked decrease in complex I activity was documented in muscle, liver, and cultured skin fibroblasts. Biochemical characterization and study of the genetic origin of this defect were performed using cultured fibroblasts. Immunodetection of 6 nuclear DNA–encoded (20, 23, 24, 30, 49, and 51 kDa) and 1 mitochondrial DNA–encoded (ND1) complex I subunits in fibroblast mitochondria revealed 2 distinct patterns. In 1 patient, complex I contained reduced amounts of the 24- and 51-kDa subunits and normal amounts of all the other investigated subunits. In the second patient, amounts of all the investigated subunits were severely decreased. The data suggest partial or extensive impairment of complex I assembly in both patients. Cell fusion experiments between 143B206 ρ° cells, fully depleted of mitochondrial DNA, and fibroblasts from both patients led to phenotypic complementation of the complex I defects in mitochondria of the resulting cybrid cells. These results indicate that the complex I defects in the 2 reported cases are due to nuclear gene mutations.
small molecules
2026-08-07 | Hydroxyhydroquinone as a novel modulator of mitochondrial metabolism with potential to restore CoQ10 homeostasis
Abstract Mitochondrial dysfunction is a hallmark of diverse metabolic and neurodegenerative disorders, often linked to impaired coenzyme Q 10 (CoQ 10 ) homeostasis. Here, we have evaluated the activity of hydroxyhydroquinone (HHQ) as a novel modulator of mitochondrial metabolism. Molecular simulations revealed that HHQ can act as an alternative aromatic substrate for human COQ2 in the CoQ 10 biosynthetic pathway. In cultured cells, HHQ exposure (5.10 − 5 mol.L − 1 ) enhanced complex I activity while maintaining stable ATP levels. HHQ reduced nitric oxide accumulation without altering superoxide dismutase activity, suggesting selective redox modulation. By bypassing the 4-hydroxybenzoic acid (PHBA) pathway, HHQ restores mitochondrial homeostasis and supports aerobic metabolism. These findings highlight HHQ as a small aromatic compound with strong redox potential that may favor metabolic functions driven by CoQ 10 deficiency and mitochondrial dysfunction.
2026-07-28 | Effects of mitochondrial complex I subunit NDUFS5 deficiency on osteogenic differentiation
OBJECTIVE: Mitochondrial complex I sustains oxidative phosphorylation, respiratory adaptation, and mitochondrial homeostasis during osteogenic differentiation. NADH:ubiquinone oxidoreductase core subunit (NDUFS5) is a nuclear-encoded complex I subunit required for complex I assembly and function, but its role in osteoblast differentiation is unknown. This study examined whether NDUFS5 deficiency suppresses osteogenic differentiation through mitochondrial dysfunction and gamma-aminobutyric acid (GABA) shunt-related metabolic disruption. METHODS: NDUFS5 expression was examined in mouse tibial sections and MC3T3-E1 cells during osteogenic induction. Osteogenic differentiation was evaluated by osteogenic marker expression, alkaline phosphatase (ALP) staining, and ALP activity. Mitochondrial morphology and function were evaluated using fluorescence imaging, transmission electron microscopy, and assays of membrane potential, reactive oxygen species, and oxygen consumption. Integrated transcriptomic and metabolomic profiling was performed, and exogenous GABA supplementation was used to test GABA shunt involvement. RESULTS: NDUFS5 expression increased during osteogenic differentiation and was enriched at sites of active bone formation. Ndufs5 deficiency reduced osteogenic marker expression and ALP activity. It also caused mitochondrial swelling, cristae disruption, reduced membrane potential, increased reactive oxygen species, and impaired respiratory function. Integrated analyses indicated GABA shunt disruption, accompanied by reduced intracellular GABA. Exogenous GABA partially restored osteogenic marker expression and ALP activity in Ndufs5-deficient cells. CONCLUSION: NDUFS5 contributes to osteogenic differentiation by maintaining mitochondrial integrity, respiratory function, and GABA shunt-related metabolic homeostasis. These findings identify NDUFS5 as a regulator of metabolic adaptation during osteoblast maturation and suggest partial rescue through GABA replenishment. CLINICAL SIGNIFICANCE: Mitochondrial dysfunction may impair osteoblast differentiation and bone formation. Disruption of NDUFS5-associated mitochondrial metabolism and GABA shunt homeostasis may identify therapeutic targets for improving bone healing and regenerative outcomes.
2026-07-21 | Riboflavin therapy in complex I deficiency: Two new cases of leukoencephalopathy and a systematic literature review.
Complex I (CI) deficiency, the most common biochemical defect in pediatric mitochondrial diseases, presents with diverse phenotypes, including cardiomyopathy, myopathy, Leigh syndrome, and mitochondrial leukoencephalopathy (ML). No curative therapies exist. Riboflavin, a precursor of CI cofactors FMN and FAD, is a potential treatment, but evidence is heterogeneous and formal guidelines are lacking. We retrospectively analyzed two patients with genetically confirmed CI deficiency due to NDUFS1 and NDUFV2 variants, treated with high-dose riboflavin with long-term clinical, biochemical, neurophysiological and MRI follow-up (>16 years). A systematic literature review of riboflavin-responsive CI deficiency was also performed. Both patients presented with early acute psychomotor regression and extensive cavitating white matter lesions. Riboflavin (up to 10 mg/kg/day) was associated with rapid, near-complete neurological recovery, normalization of lactate and evoked potentials, and MRI improvement, stable in time. Our review identified 43 additional riboflavin-responsive CI cases, including cardiomyopathy (n = 16, largely due to ACAD9 variants), myopathy (n = 12, all ACAD9 variants), ML (n = 8, predominantly NDUFV1/NDUFV2 variants), Leigh syndrome (n = 5), MELAS-like presentations (n = 1), and optic atrophy (n = 1). Riboflavin may provide durable benefit across several CI-deficiency phenotypes. Beyond established efficacy in ACAD9-related cardiomyopathy, available evidence supports consideration of therapeutic trials in other phenotypes. Our two cases, supported by long-term follow-up and consistent instrumental data, provide further evidence supporting a potential benefit of riboflavin in ML, complementing eight earlier reports limited by short follow-up and sparse imaging. Variants affecting N-module subunits (NDUFV1, NDUFV2, NDUFS1), depending directly on FMN/FAD, may represent particularly suitable candidates for treatment. Prospective studies are warranted.
2026-06-03 | An Atypical Neurosensory-Predominant Presentation Associated with a Homozygous NDUFS3 Variant: A Diagnostic Challenge Involving Retinal and Hearing Phenotypes.
Mitochondrial complex I deficiency represents a major cause of pediatric mitochondrial disease and is associated with a broad phenotypic spectrum, classically including Leigh syndrome and severe neurodegenerative presentations. Variants in NDUFS3, encoding a core structural subunit of complex I, are rare and most often linked to severe multisystem involvement. However, emerging reports suggest that NDUFS3 variants may be associated with more heterogeneous and atypical clinical manifestations. Here, we describe a child with a neurosensory-predominant phenotype presenting a diagnostic challenge involving retinal and auditory findings in the context of a homozygous NDUFS3 variant. A girl born to consanguineous parents presented with global developmental delay, bilateral digital contractures, impaired visual responses, and progressive hearing difficulties. Ophthalmologic evaluation revealed Stargardt-like maculopathy, while audiologic assessment confirmed bilateral sensorineural hearing loss. Neurological examination showed mild motor delay without neuroregression. Brain magnetic resonance imaging and spectroscopy were unremarkable, and no cardiomyopathy or metabolic crises were observed. Whole-exome sequencing identified a homozygous NDUFS3 c.721G>A (p.Ala241Thr; rs776795187) variant, classified as a variant of uncertain significance and predicted to be deleterious by multiple in silico tools (SIFT, PolyPhen-2, MutationTaster, and CADD). Heterozygous variants were also detected in ABCA4 and HMCN1. Parental segregation analysis was not available at the time of evaluation. Supportive mitochondrial therapy was initiated, and the patient remained clinically stable during long-term follow-up, with nonprogressive neurosensory findings. This case highlights the diagnostic complexity of interpreting atypical neurosensory-predominant phenotypes in mitochondrial disease. While the homozygous NDUFS3 variant may contribute to systemic mitochondrial vulnerability, the coexistence of heterozygous retinal gene variants suggests a possible modifying or oligogenic effect. Rather than defining a distinct monogenic entity, this report underscores the importance of cautious genotype-phenotype correlation and comprehensive genetic evaluation in children presenting with combined retinal and auditory involvement.
2026-04-28 | Coenzyme Q10 and Idebenone Combination for Mitochondrial Electron Transport Chain Enhancement
Coenzyme Q10 enhances electron transport chain efficiency at Complex III, while idebenone acts as an alternative electron acceptor bypassing Complex I dysfunction secondary to LCHAD deficiency. This combination improves ATP synthesis efficiency and reduces accumulation of toxic long-chain 3-hydroxyacyl-CoA intermediates through enhanced mitochondrial respiratory capacity.
proteins
2023-11-16 | Systematic analysis of NDUFAF6 in complex I assembly and mitochondrial disease
Isolated complex I (CI) deficiencies are a major cause of primary mitochondrial disease. Two-thirds of CI deficiencies arise from defects in CI assembly factors (CIAFs) that are not part of the CI holoenzyme. The biochemistry of CIAFs is poorly defined, making their role in CI assembly unclear and confounding interpretation of potential disease-causing genetic variants. To address these challenges, we devised a deep mutational scanning approach to systematically assess the function of thousands of NDUFAF6 genetic variants. Guided by these data, biochemical analyses, and cross-linking mass spectrometry, we discovered that the CIAF NDUFAF6 facilitates incorporation of NDUFS8 into CI and reveal that NDUFS8 overexpression rectifies NDUFAF6 deficiency. Our data further provide experimental support of pathogenicity for seven novel NDUFAF6 variants associated with human pathology and add phenotypic annotation for 5,675 additional variants. Overall, our work defines the function of NDUFAF6 and provides a comprehensive clinical resource for diagnosing NDUFAF6-related diseases.
2023-08-09 | Mitochondrial integrated stress response controls lung epithelial cell fate
Abstract Alveolar epithelial type 1 (AT1) cells are necessary to transfer oxygen and carbon dioxide between the blood and air. Alveolar epithelial type 2 (AT2) cells serve as a partially committed stem cell population, producing AT1 cells during postnatal alveolar development and repair after influenza A and SARS-CoV-2 pneumonia 1–6 . Little is known about the metabolic regulation of the fate of lung epithelial cells. Here we report that deleting the mitochondrial electron transport chain complex I subunit Ndufs2 in lung epithelial cells during mouse gestation led to death during postnatal alveolar development. Affected mice displayed hypertrophic cells with AT2 and AT1 cell features, known as transitional cells. Mammalian mitochondrial complex I, comprising 45 subunits, regenerates NAD + and pumps protons. Conditional expression of yeast NADH dehydrogenase (NDI1) protein that regenerates NAD + without proton pumping 7,8 was sufficient to correct abnormal alveolar development and avert lethality. Single-cell RNA sequencing revealed enrichment of integrated stress response (ISR) genes in transitional cells. Administering an ISR inhibitor 9,10 or NAD + precursor reduced ISR gene signatures in epithelial cells and partially rescued lethality in the absence of mitochondrial complex I function. Notably, lung epithelial-specific loss of mitochondrial electron transport chain complex II subunit Sdhd , which maintains NAD + regeneration, did not trigger high ISR activation or lethality. These findings highlight an unanticipated requirement for mitochondrial complex I-dependent NAD + regeneration in directing cell fate during postnatal alveolar development by preventing pathological ISR induction.
2023-07-03 | Yeast NDI1 reconfigures neuronal metabolism and prevents the unfolded protein response in mitochondrial complex I deficiency
Mutations in subunits of the mitochondrial NADH dehydrogenase cause mitochondrial complex I deficiency, a group of severe neurological diseases that can result in death in infancy. The pathogenesis of complex I deficiency remain poorly understood, and as a result there are currently no available treatments. To better understand the underlying mechanisms, we modelled complex I deficiency in Drosophila using knockdown of the mitochondrial complex I subunit ND-75 (NDUFS1) specifically in neurons. Neuronal complex I deficiency causes locomotor defects, seizures and reduced lifespan. At the cellular level, complex I deficiency does not affect ATP levels but leads to mitochondrial morphology defects, reduced endoplasmic reticulum-mitochondria contacts and activation of the endoplasmic reticulum unfolded protein response (UPR) in neurons. Multi-omic analysis shows that complex I deficiency dramatically perturbs mitochondrial metabolism in the brain. We find that expression of the yeast non-proton translocating NADH dehydrogenase NDI1, which reinstates mitochondrial NADH oxidation but not ATP production, restores levels of several key metabolites in the brain in complex I deficiency. Remarkably, NDI1 expression also reinstates endoplasmic reticulum-mitochondria contacts, prevents UPR activation and rescues the behavioural and lifespan phenotypes caused by complex I deficiency. Together, these data show that metabolic disruption due to loss of neuronal NADH dehydrogenase activity cause UPR activation and drive pathogenesis in complex I deficiency.
2022-03-31 | Severe congenital lactic acidosis and hypertrophic cardiomyopathy caused by an intronic variant in NDUFB7.
Mutations in structural subunits and assembly factors of complex I of the oxidative phosphorylation system constitute the most common cause of mitochondrial respiratory chain defects. Such mutations can present a wide range of clinical manifestations, varying from mild deficiencies to severe, lethal disorders. We describe a patient presenting intrauterine growth restriction and anemia, which displayed postpartum hypertrophic cardiomyopathy, lactic acidosis, encephalopathy, and a severe complex I defect with fatal outcome. Whole genome sequencing revealed an intronic biallelic mutation in the NDUFB7 gene (c.113-10C>G) and splicing pattern alterations in NDUFB7 messenger RNA were confirmed by RNA Sequencing. The detected variant resulted in a significant reduction of the NDUFB7 protein and reduced complex I activity. Complementation studies with expression of wild-type NDUFB7 in patient fibroblasts normalized complex I function. Here we report a case with a primary complex I defect due to a homozygous mutation in an intron region of the NDUFB7 gene.
2021-01-29 | Structural basis for a complex I mutation that blocks pathological ROS production
Abstract Mitochondrial complex I is central to the pathological reactive oxygen species (ROS) production that underlies cardiac ischemia–reperfusion (IR) injury. ND6-P25L mice are homoplasmic for a disease-causing mtDNA point mutation encoding the P25L substitution in the ND6 subunit of complex I. The cryo-EM structure of ND6-P25L complex I revealed subtle structural changes that facilitate rapid conversion to the “deactive” state, usually formed only after prolonged inactivity. Despite its tendency to adopt the “deactive” state, the mutant complex is fully active for NADH oxidation, but cannot generate ROS by reverse electron transfer (RET). ND6-P25L mitochondria function normally, except for their lack of RET ROS production, and ND6-P25L mice are protected against cardiac IR injury in vivo. Thus, this single point mutation in complex I, which does not affect oxidative phosphorylation but renders the complex unable to catalyse RET, demonstrates the pathological role of ROS production by RET during IR injury.
other
2022-01-28 | Oligonucleotide correction of an intronic TIMMDC1 variant in cells of patients with severe neurodegenerative disorder
TIMMDC1 encodes the Translocase of Inner Mitochondrial Membrane Domain-Containing protein 1 (TIMMDC1) subunit of complex I of the electron transport chain responsible for ATP production. We studied a consanguineous family with two affected children, now deceased, who presented with failure to thrive in the early postnatal period, poor feeding, hypotonia, peripheral neuropathy and drug-resistant epilepsy. Genome sequencing data revealed a known, deep intronic pathogenic variant TIMMDC1 c.597-1340A>G, also present in gnomAD (~1/5000 frequency), that enhances aberrant splicing. Using RNA and protein analysis we show almost complete loss of TIMMDC1 protein and compromised mitochondrial complex I function. We have designed and applied two different splice-switching antisense oligonucleotides (SSO) to restore normal TIMMDC1 mRNA processing and protein levels in patients' cells. Quantitative proteomics and real-time metabolic analysis of mitochondrial function on patient fibroblasts treated with SSOs showed restoration of complex I subunit abundance and function. SSO-mediated therapy of this inevitably fatal TIMMDC1 neurologic disorder is an attractive possibility.
gene therapies
2024-12-01 | Proteomics sample preparation v1
Protocol for proteomic analysis and data acquisition used in: "Selective Deficiency of MitochondrialRespiratory Complex I Subunits Ndufs4/6 Causes Tumor Immunogenicity". The paper's abstract is as follows: Cancer cells frequently rewire their metabolism to support proliferation and evade immune surveillance, but little is known about metabolic targets that could increase immune surveillance. Here we show a specific means of mitochondrial respiratory Complex I (CI) inhibition that improves tumor immunogenicity and sensitivity to immune checkpoint blockade (ICB). Targeted genetic deletion of either Ndufs4 or Ndufs6, but not other CI subunits, induces an immune-dependent growth attenuation in melanoma and breast cancer models. We show that deletion of Ndufs4 induces expression of the MHC class-I co-activator Nlrc5 and antigen presentation machinery components, most notably H2-K1. This induction of MHC-related genes is driven by a pyruvate dehydrogenase-dependent accumulation of mitochondrial acetyl-CoA, which leads to an increase in histone H3K27-acetylation within the Nlrc5 and H2-K1 promoters. Taken together, this work shows that selective CI inhibition restricts tumor growth, and that specific targeting of Ndufs4 or Ndufs6, increases T-cell surveillance and ICB responsiveness.
2024-05-09 | Identification of a novel MT-ND3 variant and restoring mitochondrial function by allotopic expression of MT-ND3 gene.
Mitochondrial diseases are caused by nuclear, or mitochondrial DNA (mtDNA) variants and related co-factors. Here, we report a novel m.10197G > C variant in MT-ND3 in a patient, and two other patients with m.10191 T > C. MT-ND3 variants are known to cause Leigh syndrome or mitochondrial complex I deficiency. We performed the functional analyses of the novel m.10197G > C variant that significantly lowered MT-ND3 protein levels, causing complex I assembly and activity deficiency, and reduction of ATP synthesis. We adapted a previously described re-engineering technique of delivering mitochondrial genes into mitochondria through codon optimization for nuclear expression and translation by cytoplasmic ribosomes to rescue defects arising from the MT-ND3 variants. We constructed mitochondrial targeting sequences along with the codon-optimized MT-ND3 and imported them into the mitochondria. To achieve the goal, we imported codon-optimized MT-ND3 into mitochondria in three patients with m.10197G > C and m.10191 T > C missense variants in the MT-ND3. Nuclear expression of the MT-ND3 gene partially restored protein levels, complex I deficiency, and significant improvement of ATP production indicating a functional rescue of the mutant phenotype. The codon-optimized nuclear expression of mitochondrial protein and import inside the mitochondria can supplement the requirements for ATP in energy-deficient mitochondrial disease patients.
2023-06-09 | IQGAP1 mediates the communication between the nucleus and the mitochondria via NDUFS4 alternative splicing
Abstract Constant communication between mitochondria and nucleus ensures cellular homeostasis and adaptation to mitochondrial stress. Anterograde regulatory pathways involving a large number of nuclear-encoded proteins control mitochondrial biogenesis and functions. Such functions are deregulated in cancer cells, resulting in proliferative advantages, aggressive disease and therapeutic resistance. Transcriptional networks controlling the nuclear-encoded mitochondrial genes are known, however alternative splicing (AS) regulation has not been implicated in this communication. Here, we show that IQGAP1, a scaffold protein regulating AS of distinct gene subsets in gastric cancer cells, participates in AS regulation that strongly affects mitochondrial respiration. Combined proteomic and RNA-seq analyses of IQGAP1KO and parental cells show that IQGAP1KO alters an AS event of the mitochondrial respiratory chain complex I (CI) subunit NDUFS4 and downregulates a subset of CI subunits. In IQGAP1KO cells, CI intermediates accumulate, resembling assembly deficiencies observed in patients with Leigh syndrome bearing NDUFS4 mutations. Mitochondrial CI activity is significantly lower in KO compared to parental cells, while exogenous expression of IQGAP1 reverses mitochondrial defects of IQGAP1KO cells. Our work sheds light to a novel facet of IQGAP1 in mitochondrial quality control that involves fine-tuning of CI activity through AS regulation in gastric cancer cells relying highly on mitochondrial respiration.
2021-07-08 | Biallelic mutations in NDUFA8 cause complex I deficiency in two siblings with favorable clinical evolution.
Isolated complex I (CI) deficiency is the most common cause of oxidative phosphorylation (OXPHOS) dysfunction. Whole-exome sequencing identified biallelic mutations in NDUFA8 (c.[293G > T]; [293G > T], encoding for an accessory subunit of CI, in two siblings with a favorable clinical evolution. The individuals reported here are practically asymptomatic, with the exception of slight failure to thrive and some language difficulties at the age of 6 and 9 years, respectively. These observations are remarkable since the vast majority of patients with CI deficiency, including the only NDUFA8 patient reported so far, showed an extremely poor clinical outcome. Western blot studies demonstrated that NDUFA8 protein was strongly reduced in the patients' fibroblasts and muscle extracts. In addition, there was a marked and specific decrease in the steady-state levels of CI subunits. BN-PAGE demonstrated an isolated defect in the assembly and the activity of CI with impaired supercomplexes formation and abnormal accumulation of CI subassemblies. Confocal microscopy analysis in fibroblasts showed rounder mitochondria and diminished branching degree of the mitochondrial network. Functional complementation studies demonstrated disease-causality for the identified mutation as lentiviral transduction with wild-type NDUFA8 cDNA restored the steady-state levels of CI subunits and completely recovered the deficient enzymatic activity in immortalized mutant fibroblasts. In summary, we provide additional evidence of the involvement of NDUFA8 as a mitochondrial disease-causing gene associated with altered mitochondrial morphology, CI deficiency, impaired supercomplexes formation, and very mild progression of the disease.
2020-05-09 | A homozygous variant in NDUFA8 is associated with developmental delay, microcephaly, and epilepsy due to mitochondrial complex I deficiency.
Abstract Mitochondrial complex I deficiency is caused by pathogenic variants in mitochondrial and nuclear genes associated with complex I structure and assembly. We report the case of a patient with NDUFA8‐related mitochondrial disease. The patient presented with developmental delay, microcephaly, and epilepsy. His fibroblasts showed apparent biochemical defects in mitochondrial complex I. Whole‐exome sequencing revealed that the patient carried a homozygous variant in NDUFA8 . His fibroblasts showed a reduction in the protein expression level of not only NDUFA8, but also the other complex I subunits, consistent with assembly defects. The enzyme activity of complex I and oxygen consumption rate were restored by reintroducing wild‐type NDUFA8 cDNA into patient fibroblasts. The functional properties of the variant in NDUFA8 were also investigated using NDUFA8 knockout cells expressing wild‐type or mutated NDUFA8 cDNA. These experiments further supported the pathogenicity of the variant in complex I assembly. This is the first report describing that the loss of NDUFA8, which has not previously been associated with mitochondrial disease, causes severe defect in the assembly of mitochondrial complex I, leading to progressive neurological and developmental abnormalities.
cell therapies
2022-06-21 | Mesenchymal stem cells improve redox homeostasis and mitochondrial respiration in fibroblast cell lines with pathogenic MT-ND3 and MT-ND6 variants.
The most frequent biochemical defect of inherited mitochondrial disease is isolated complex I deficiency. There is no cure for this disorder, and treatment is mainly supportive. In this study, we investigated the effects of human mesenchymal stem cells (MSCs) on skin fibroblast derived from three individuals with complex I deficiency carrying different pathogenic variants in mitochondrial DNA-encoded subunits (MT-ND3, MT-ND6). Complex I-deficient fibroblasts were transiently co-cultured with bone marrow-derived MSCs. Mitochondrial transfer was analysed by fluorescence labelling and validated by Sanger sequencing. Levels of reactive oxygen species (ROS) were measured using MitoSOX Red. Moreover, mitochondrial respiration was analysed by Seahorse XFe96 Extracellular Flux Analyzer. Levels of antioxidant proteins were investigated via immunoblotting. Co-culturing of complex I-deficient fibroblast with MSCs lowered cellular ROS levels. The effect on ROS production was more sustained compared to treatment of patient fibroblasts with culture medium derived from MSC cultures. Investigation of cellular antioxidant defence systems revealed an upregulation of SOD2 (superoxide dismutase 2, mitochondrial) and HO-1 (heme oxygenase 1) in patient-derived cell lines. This adaptive response was normalised upon MSC treatment. Moreover, Seahorse experiments revealed a significant improvement of mitochondrial respiration, indicating a mitigation of the oxidative phosphorylation defect. Experiments with repetitive MSC co-culture at two consecutive time points enhanced this effect. Our study indicates that MSC-based treatment approaches might constitute an interesting option for patients with mitochondrial DNA-encoded mitochondrial diseases. We suggest that this strategy may prove more promising for defects caused by mitochondrial DNA variants compared to nuclear-encoded defects.
2019-04-18 | Donation of mitochondria by iPSC-derived mesenchymal stem cells protects retinal ganglion cells against mitochondrial complex I defect-induced degeneration
Rationale: Retinal ganglion cell (RGC) degeneration is extremely hard to repair or regenerate and is often coupled with mitochondrial dysfunction.Mesenchymal stem cells (MSCs)-based treatment has been demonstrated beneficial for RGC against degeneration.However, underlying mechanisms of MSC-provided RGC protection are largely unknown other than neuroprotective paracrine actions.In this study, we sought to investigate whether mitochondrial donation from induced pluripotent stem cell-derived MSC (iPSC-MSCs) could preserve RGC survival and restore retinal function.Methods: iPSC-MSCs were injected into the vitreous cavity of one eye in NADH dehydrogenase (ubiquinone) Fe-S protein 4 (Ndufs4) knockout (KO) and wild type mice.Phosphate buffer saline (PBS) or rotenone treated iPSC-MSCs were injected as control groups.Retinal function was detected by flash electroretinogram (ERG).Whole-mount immunofluorescence (IF), morphometric analysis, confocal microscopy imaging, polymerase chain reaction (PCR) of the retinas were conducted to investigate mitochondrial transfer from human iPSC-MSCs to mouse retina.Quantitative mouse cytokine arrays were carried out to measure retinal inflammatory response under difference treatments.Results: RGC survival in the iPSC-MSC injected retina of Ndufs4 KO mice was significantly increased with improved retinal function.GFP labelled human mitochondria from iPSC-MSC were detected in the RGCs in the retina of Ndufs4 KO mice starting from 96 hours post injection.PCR result showed only human mitochondrial DNA without human nuclear DNA could be detected in the mouse retinas after iPSC-MSC treatment in Ndufs4 KO mice eye.Quantitative cytokine array analysis showed pro-inflammatory cytokines was also downregulated by this iPSC-MSC treatment.Conclusion: Intravitreal transplanted iPSC-MSCs can effectively donate functional mitochondria to RGCs and protect against mitochondrial damage-induced RGC loss.
2019-03-27 | A Drosophila Mitochondrial Complex I Deficiency Phenotype Array
Mitochondrial diseases are a group of rare life-threatening diseases often caused by defects in the oxidative phosphorylation system. No effective treatment is available for these disorders. Therapeutic development is hampered by the high heterogeneity in genetic, biochemical and clinical spectra of mitochondrial diseases and by limited preclinical resources to screen and identify effective treatment candidates. Alternative models of the pathology are essential to better understand mitochondrial diseases and to accelerate the development of new therapeutics. The fruit fly Drosophila melanogaster is a cost- and time-efficient model that can recapitulate a wide range of phenotypes observed in patients suffering from mitochondrial disorders. We targeted three important subunits of Complex I of the mitochondrial oxidative phosphorylation system with the flexible UAS-Gal4 system and RNA interference: NDUFS4 (ND-18), NDUFS7 (ND-20) and NDUFV1 (ND-51). Using two ubiquitous driver lines at two temperatures, we established a collection of phenotypes relevant to complex I deficiencies. Our data offer models and phenotypes with different levels of severity that can be used for future therapeutic screenings. These include qualitative phenotypes that are amenable to high-throughput drug screening and quantitative phenotypes that require more resources but are likely to have increased potential and sensitivity to show modulation by drug treatment.
2018-03-26 | Modulation of oxidative phosphorylation and redox homeostasis in mitochondrial NDUFS4 deficiency via mesenchymal stem cells.
Disorders of the oxidative phosphorylation (OXPHOS) system represent a large group among the inborn errors of metabolism. The most frequently observed biochemical defect is isolated deficiency of mitochondrial complex I (CI). No effective treatment strategies for CI deficiency are so far available. The purpose of this study was to investigate whether and how mesenchymal stem cells (MSCs) are able to modulate metabolic function in fibroblast cell models of CI deficiency. We used human and murine fibroblasts with a defect in the nuclear DNA encoded NDUFS4 subunit of CI. Fibroblasts were co-cultured with MSCs under different stress conditions and intercellular mitochondrial transfer was assessed by flow cytometry and fluorescence microscopy. Reactive oxygen species (ROS) levels were measured using MitoSOX-Red. Protein levels of CI were analysed by blue native polyacrylamide gel electrophoresis (BN-PAGE). Direct cellular interactions and mitochondrial transfer between MSCs and human as well as mouse fibroblast cell lines were demonstrated. Mitochondrial transfer was visible in 13.2% and 6% of fibroblasts (e.g. fibroblasts containing MSC mitochondria) for human and mouse cell lines, respectively. The transfer rate could be further stimulated via treatment of cells with TNF-α. MSCs effectively lowered cellular ROS production in NDUFS4-deficient fibroblast cell lines (either directly via co-culture or indirectly via incubation of cell lines with cell-free MSC supernatant). However, CI protein expression and activity were not rescued by MSC treatment. This study demonstrates the interplay between MSCs and fibroblast cell models of isolated CI deficiency including transfer of mitochondria as well as modulation of cellular ROS levels. Further exploration of these cellular interactions might help to develop MSC-based treatment strategies for human CI deficiency.
1999-07-01 | Nuclear DNA origin of mitochondrial complex I deficiency in fatal infantile lactic acidosis evidenced by transnuclear complementation of cultured fibroblasts
We have studied complex I (NADH-ubiquinone reductase) defects of the mitochondrial respiratory chain in 2 infants who died in the neonatal period from 2 different neurological forms of severe neonatal lactic acidosis. Specific and marked decrease in complex I activity was documented in muscle, liver, and cultured skin fibroblasts. Biochemical characterization and study of the genetic origin of this defect were performed using cultured fibroblasts. Immunodetection of 6 nuclear DNA–encoded (20, 23, 24, 30, 49, and 51 kDa) and 1 mitochondrial DNA–encoded (ND1) complex I subunits in fibroblast mitochondria revealed 2 distinct patterns. In 1 patient, complex I contained reduced amounts of the 24- and 51-kDa subunits and normal amounts of all the other investigated subunits. In the second patient, amounts of all the investigated subunits were severely decreased. The data suggest partial or extensive impairment of complex I assembly in both patients. Cell fusion experiments between 143B206 ρ° cells, fully depleted of mitochondrial DNA, and fibroblasts from both patients led to phenotypic complementation of the complex I defects in mitochondria of the resulting cybrid cells. These results indicate that the complex I defects in the 2 reported cases are due to nuclear gene mutations.
small molecules
2026-08-07 | Hydroxyhydroquinone as a novel modulator of mitochondrial metabolism with potential to restore CoQ10 homeostasis
Abstract Mitochondrial dysfunction is a hallmark of diverse metabolic and neurodegenerative disorders, often linked to impaired coenzyme Q 10 (CoQ 10 ) homeostasis. Here, we have evaluated the activity of hydroxyhydroquinone (HHQ) as a novel modulator of mitochondrial metabolism. Molecular simulations revealed that HHQ can act as an alternative aromatic substrate for human COQ2 in the CoQ 10 biosynthetic pathway. In cultured cells, HHQ exposure (5.10 − 5 mol.L − 1 ) enhanced complex I activity while maintaining stable ATP levels. HHQ reduced nitric oxide accumulation without altering superoxide dismutase activity, suggesting selective redox modulation. By bypassing the 4-hydroxybenzoic acid (PHBA) pathway, HHQ restores mitochondrial homeostasis and supports aerobic metabolism. These findings highlight HHQ as a small aromatic compound with strong redox potential that may favor metabolic functions driven by CoQ 10 deficiency and mitochondrial dysfunction.
2026-07-28 | Effects of mitochondrial complex I subunit NDUFS5 deficiency on osteogenic differentiation
OBJECTIVE: Mitochondrial complex I sustains oxidative phosphorylation, respiratory adaptation, and mitochondrial homeostasis during osteogenic differentiation. NADH:ubiquinone oxidoreductase core subunit (NDUFS5) is a nuclear-encoded complex I subunit required for complex I assembly and function, but its role in osteoblast differentiation is unknown. This study examined whether NDUFS5 deficiency suppresses osteogenic differentiation through mitochondrial dysfunction and gamma-aminobutyric acid (GABA) shunt-related metabolic disruption. METHODS: NDUFS5 expression was examined in mouse tibial sections and MC3T3-E1 cells during osteogenic induction. Osteogenic differentiation was evaluated by osteogenic marker expression, alkaline phosphatase (ALP) staining, and ALP activity. Mitochondrial morphology and function were evaluated using fluorescence imaging, transmission electron microscopy, and assays of membrane potential, reactive oxygen species, and oxygen consumption. Integrated transcriptomic and metabolomic profiling was performed, and exogenous GABA supplementation was used to test GABA shunt involvement. RESULTS: NDUFS5 expression increased during osteogenic differentiation and was enriched at sites of active bone formation. Ndufs5 deficiency reduced osteogenic marker expression and ALP activity. It also caused mitochondrial swelling, cristae disruption, reduced membrane potential, increased reactive oxygen species, and impaired respiratory function. Integrated analyses indicated GABA shunt disruption, accompanied by reduced intracellular GABA. Exogenous GABA partially restored osteogenic marker expression and ALP activity in Ndufs5-deficient cells. CONCLUSION: NDUFS5 contributes to osteogenic differentiation by maintaining mitochondrial integrity, respiratory function, and GABA shunt-related metabolic homeostasis. These findings identify NDUFS5 as a regulator of metabolic adaptation during osteoblast maturation and suggest partial rescue through GABA replenishment. CLINICAL SIGNIFICANCE: Mitochondrial dysfunction may impair osteoblast differentiation and bone formation. Disruption of NDUFS5-associated mitochondrial metabolism and GABA shunt homeostasis may identify therapeutic targets for improving bone healing and regenerative outcomes.
2026-07-21 | Riboflavin therapy in complex I deficiency: Two new cases of leukoencephalopathy and a systematic literature review.
Complex I (CI) deficiency, the most common biochemical defect in pediatric mitochondrial diseases, presents with diverse phenotypes, including cardiomyopathy, myopathy, Leigh syndrome, and mitochondrial leukoencephalopathy (ML). No curative therapies exist. Riboflavin, a precursor of CI cofactors FMN and FAD, is a potential treatment, but evidence is heterogeneous and formal guidelines are lacking. We retrospectively analyzed two patients with genetically confirmed CI deficiency due to NDUFS1 and NDUFV2 variants, treated with high-dose riboflavin with long-term clinical, biochemical, neurophysiological and MRI follow-up (>16 years). A systematic literature review of riboflavin-responsive CI deficiency was also performed. Both patients presented with early acute psychomotor regression and extensive cavitating white matter lesions. Riboflavin (up to 10 mg/kg/day) was associated with rapid, near-complete neurological recovery, normalization of lactate and evoked potentials, and MRI improvement, stable in time. Our review identified 43 additional riboflavin-responsive CI cases, including cardiomyopathy (n = 16, largely due to ACAD9 variants), myopathy (n = 12, all ACAD9 variants), ML (n = 8, predominantly NDUFV1/NDUFV2 variants), Leigh syndrome (n = 5), MELAS-like presentations (n = 1), and optic atrophy (n = 1). Riboflavin may provide durable benefit across several CI-deficiency phenotypes. Beyond established efficacy in ACAD9-related cardiomyopathy, available evidence supports consideration of therapeutic trials in other phenotypes. Our two cases, supported by long-term follow-up and consistent instrumental data, provide further evidence supporting a potential benefit of riboflavin in ML, complementing eight earlier reports limited by short follow-up and sparse imaging. Variants affecting N-module subunits (NDUFV1, NDUFV2, NDUFS1), depending directly on FMN/FAD, may represent particularly suitable candidates for treatment. Prospective studies are warranted.
2026-06-03 | An Atypical Neurosensory-Predominant Presentation Associated with a Homozygous NDUFS3 Variant: A Diagnostic Challenge Involving Retinal and Hearing Phenotypes.
Mitochondrial complex I deficiency represents a major cause of pediatric mitochondrial disease and is associated with a broad phenotypic spectrum, classically including Leigh syndrome and severe neurodegenerative presentations. Variants in NDUFS3, encoding a core structural subunit of complex I, are rare and most often linked to severe multisystem involvement. However, emerging reports suggest that NDUFS3 variants may be associated with more heterogeneous and atypical clinical manifestations. Here, we describe a child with a neurosensory-predominant phenotype presenting a diagnostic challenge involving retinal and auditory findings in the context of a homozygous NDUFS3 variant. A girl born to consanguineous parents presented with global developmental delay, bilateral digital contractures, impaired visual responses, and progressive hearing difficulties. Ophthalmologic evaluation revealed Stargardt-like maculopathy, while audiologic assessment confirmed bilateral sensorineural hearing loss. Neurological examination showed mild motor delay without neuroregression. Brain magnetic resonance imaging and spectroscopy were unremarkable, and no cardiomyopathy or metabolic crises were observed. Whole-exome sequencing identified a homozygous NDUFS3 c.721G>A (p.Ala241Thr; rs776795187) variant, classified as a variant of uncertain significance and predicted to be deleterious by multiple in silico tools (SIFT, PolyPhen-2, MutationTaster, and CADD). Heterozygous variants were also detected in ABCA4 and HMCN1. Parental segregation analysis was not available at the time of evaluation. Supportive mitochondrial therapy was initiated, and the patient remained clinically stable during long-term follow-up, with nonprogressive neurosensory findings. This case highlights the diagnostic complexity of interpreting atypical neurosensory-predominant phenotypes in mitochondrial disease. While the homozygous NDUFS3 variant may contribute to systemic mitochondrial vulnerability, the coexistence of heterozygous retinal gene variants suggests a possible modifying or oligogenic effect. Rather than defining a distinct monogenic entity, this report underscores the importance of cautious genotype-phenotype correlation and comprehensive genetic evaluation in children presenting with combined retinal and auditory involvement.
2026-04-28 | Coenzyme Q10 and Idebenone Combination for Mitochondrial Electron Transport Chain Enhancement
Coenzyme Q10 enhances electron transport chain efficiency at Complex III, while idebenone acts as an alternative electron acceptor bypassing Complex I dysfunction secondary to LCHAD deficiency. This combination improves ATP synthesis efficiency and reduces accumulation of toxic long-chain 3-hydroxyacyl-CoA intermediates through enhanced mitochondrial respiratory capacity.
proteins
2023-11-16 | Systematic analysis of NDUFAF6 in complex I assembly and mitochondrial disease
Isolated complex I (CI) deficiencies are a major cause of primary mitochondrial disease. Two-thirds of CI deficiencies arise from defects in CI assembly factors (CIAFs) that are not part of the CI holoenzyme. The biochemistry of CIAFs is poorly defined, making their role in CI assembly unclear and confounding interpretation of potential disease-causing genetic variants. To address these challenges, we devised a deep mutational scanning approach to systematically assess the function of thousands of NDUFAF6 genetic variants. Guided by these data, biochemical analyses, and cross-linking mass spectrometry, we discovered that the CIAF NDUFAF6 facilitates incorporation of NDUFS8 into CI and reveal that NDUFS8 overexpression rectifies NDUFAF6 deficiency. Our data further provide experimental support of pathogenicity for seven novel NDUFAF6 variants associated with human pathology and add phenotypic annotation for 5,675 additional variants. Overall, our work defines the function of NDUFAF6 and provides a comprehensive clinical resource for diagnosing NDUFAF6-related diseases.
2023-08-09 | Mitochondrial integrated stress response controls lung epithelial cell fate
Abstract Alveolar epithelial type 1 (AT1) cells are necessary to transfer oxygen and carbon dioxide between the blood and air. Alveolar epithelial type 2 (AT2) cells serve as a partially committed stem cell population, producing AT1 cells during postnatal alveolar development and repair after influenza A and SARS-CoV-2 pneumonia 1–6 . Little is known about the metabolic regulation of the fate of lung epithelial cells. Here we report that deleting the mitochondrial electron transport chain complex I subunit Ndufs2 in lung epithelial cells during mouse gestation led to death during postnatal alveolar development. Affected mice displayed hypertrophic cells with AT2 and AT1 cell features, known as transitional cells. Mammalian mitochondrial complex I, comprising 45 subunits, regenerates NAD + and pumps protons. Conditional expression of yeast NADH dehydrogenase (NDI1) protein that regenerates NAD + without proton pumping 7,8 was sufficient to correct abnormal alveolar development and avert lethality. Single-cell RNA sequencing revealed enrichment of integrated stress response (ISR) genes in transitional cells. Administering an ISR inhibitor 9,10 or NAD + precursor reduced ISR gene signatures in epithelial cells and partially rescued lethality in the absence of mitochondrial complex I function. Notably, lung epithelial-specific loss of mitochondrial electron transport chain complex II subunit Sdhd , which maintains NAD + regeneration, did not trigger high ISR activation or lethality. These findings highlight an unanticipated requirement for mitochondrial complex I-dependent NAD + regeneration in directing cell fate during postnatal alveolar development by preventing pathological ISR induction.
2023-07-03 | Yeast NDI1 reconfigures neuronal metabolism and prevents the unfolded protein response in mitochondrial complex I deficiency
Mutations in subunits of the mitochondrial NADH dehydrogenase cause mitochondrial complex I deficiency, a group of severe neurological diseases that can result in death in infancy. The pathogenesis of complex I deficiency remain poorly understood, and as a result there are currently no available treatments. To better understand the underlying mechanisms, we modelled complex I deficiency in Drosophila using knockdown of the mitochondrial complex I subunit ND-75 (NDUFS1) specifically in neurons. Neuronal complex I deficiency causes locomotor defects, seizures and reduced lifespan. At the cellular level, complex I deficiency does not affect ATP levels but leads to mitochondrial morphology defects, reduced endoplasmic reticulum-mitochondria contacts and activation of the endoplasmic reticulum unfolded protein response (UPR) in neurons. Multi-omic analysis shows that complex I deficiency dramatically perturbs mitochondrial metabolism in the brain. We find that expression of the yeast non-proton translocating NADH dehydrogenase NDI1, which reinstates mitochondrial NADH oxidation but not ATP production, restores levels of several key metabolites in the brain in complex I deficiency. Remarkably, NDI1 expression also reinstates endoplasmic reticulum-mitochondria contacts, prevents UPR activation and rescues the behavioural and lifespan phenotypes caused by complex I deficiency. Together, these data show that metabolic disruption due to loss of neuronal NADH dehydrogenase activity cause UPR activation and drive pathogenesis in complex I deficiency.
2022-03-31 | Severe congenital lactic acidosis and hypertrophic cardiomyopathy caused by an intronic variant in NDUFB7.
Mutations in structural subunits and assembly factors of complex I of the oxidative phosphorylation system constitute the most common cause of mitochondrial respiratory chain defects. Such mutations can present a wide range of clinical manifestations, varying from mild deficiencies to severe, lethal disorders. We describe a patient presenting intrauterine growth restriction and anemia, which displayed postpartum hypertrophic cardiomyopathy, lactic acidosis, encephalopathy, and a severe complex I defect with fatal outcome. Whole genome sequencing revealed an intronic biallelic mutation in the NDUFB7 gene (c.113-10C>G) and splicing pattern alterations in NDUFB7 messenger RNA were confirmed by RNA Sequencing. The detected variant resulted in a significant reduction of the NDUFB7 protein and reduced complex I activity. Complementation studies with expression of wild-type NDUFB7 in patient fibroblasts normalized complex I function. Here we report a case with a primary complex I defect due to a homozygous mutation in an intron region of the NDUFB7 gene.
2021-01-29 | Structural basis for a complex I mutation that blocks pathological ROS production
Abstract Mitochondrial complex I is central to the pathological reactive oxygen species (ROS) production that underlies cardiac ischemia–reperfusion (IR) injury. ND6-P25L mice are homoplasmic for a disease-causing mtDNA point mutation encoding the P25L substitution in the ND6 subunit of complex I. The cryo-EM structure of ND6-P25L complex I revealed subtle structural changes that facilitate rapid conversion to the “deactive” state, usually formed only after prolonged inactivity. Despite its tendency to adopt the “deactive” state, the mutant complex is fully active for NADH oxidation, but cannot generate ROS by reverse electron transfer (RET). ND6-P25L mitochondria function normally, except for their lack of RET ROS production, and ND6-P25L mice are protected against cardiac IR injury in vivo. Thus, this single point mutation in complex I, which does not affect oxidative phosphorylation but renders the complex unable to catalyse RET, demonstrates the pathological role of ROS production by RET during IR injury.
other
2022-01-28 | Oligonucleotide correction of an intronic TIMMDC1 variant in cells of patients with severe neurodegenerative disorder
TIMMDC1 encodes the Translocase of Inner Mitochondrial Membrane Domain-Containing protein 1 (TIMMDC1) subunit of complex I of the electron transport chain responsible for ATP production. We studied a consanguineous family with two affected children, now deceased, who presented with failure to thrive in the early postnatal period, poor feeding, hypotonia, peripheral neuropathy and drug-resistant epilepsy. Genome sequencing data revealed a known, deep intronic pathogenic variant TIMMDC1 c.597-1340A>G, also present in gnomAD (~1/5000 frequency), that enhances aberrant splicing. Using RNA and protein analysis we show almost complete loss of TIMMDC1 protein and compromised mitochondrial complex I function. We have designed and applied two different splice-switching antisense oligonucleotides (SSO) to restore normal TIMMDC1 mRNA processing and protein levels in patients' cells. Quantitative proteomics and real-time metabolic analysis of mitochondrial function on patient fibroblasts treated with SSOs showed restoration of complex I subunit abundance and function. SSO-mediated therapy of this inevitably fatal TIMMDC1 neurologic disorder is an attractive possibility.
Access all drug discovery papers and probability of success in trials forecasts:
Access all drug discovery papers and probability of success in trials forecasts:
Drug Discovery Landscape
4 orphan drug designations for Isolated complex I deficiency.
4 orphan drug designations for Isolated complex I deficiency.
Drug | Therapy type | Regulator | Orphan designation | Approval | Sponsor |
|---|---|---|---|---|---|
2-isopropyl-3H-naphtho[1,2-d]imidazole-4,5-dione | small molecules | FDA | 2018-04-16 | — | Pharming Technologies BV |
(S)-6-hydroxy-2,5,7,8-tetramethyl-N-((R)-piperidin-3-yl)chroman-2-carboxamide hydrochloride | small molecules | FDA | 2014-11-17 | — | Khondrion BV |
modified recombinant mitochondrial transcription factor A (TFAM) containing the mitochondrial transduction domain | proteins | FDA | 2012-08-20 | — | Gencia Corporation |
alpha-tocotrienol quinone | small molecules | FDA | 2010-10-21 | — | PTC Therapeutics, Inc. |
Let's accelerate rare disease drug discovery
Let's accelerate drug discovery
Get access to Explority AI’s forecasts to outperform average preclinical success rates. Whether you’re expanding your R&D pipeline, evaluating a partnership, or simply have a question — we’d love to hear from you.