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:
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.
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.
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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. |
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