MedGen

Isolated complex I deficiency is the most common enzymatic defect of the oxidative phosphorylation disorders (McFarland et al., 2004; Kirby et al., 2004). It causes a wide range of clinical disorders, ranging from lethal neonatal disease to adult-onset neurodegenerative disorders. Phenotypes include macrocephaly with progressive leukodystrophy, nonspecific encephalopathy, hypertrophic cardiomyopathy, myopathy, liver disease, Leigh syndrome (see 256000), Leber hereditary optic neuropathy (535000), and some forms of Parkinson disease (see 556500) (Loeffen et al., 2000; Pitkanen et al., 1996; Robinson, 1998). Genetic Heterogeneity of Complex I Deficiency Mitochondrial complex I deficiency shows extreme genetic heterogeneity and can be caused by mutation in nuclear-encoded genes or in mitochondrial-encoded genes. There are no obvious genotype-phenotype correlations, and inference of the underlying basis from the clinical or biochemical presentation is difficult, if not impossible (summary by Haack et al., 2012). However, the majority of cases are caused by mutations in nuclear-encoded genes (Loeffen et al., 2000; Triepels et al., 2001). Complex I deficiency resulting from mutation in nuclear-encoded genes include MC1DN1, caused by mutation in the NDUFS4 gene (602694); MC1DN2 (618222), caused by mutation in the NDUFS8 gene (602141); MC1DN3 (618224), caused by mutation in the NDUFS7 gene (601825); MC1DN4 (618225), caused by mutation in the NDUFV1 gene (161015); MC1DN5 (618226), caused by mutation in the NDUFS1 gene (157655); MC1DN6 (618228), caused by mutation in the NDUFS2 gene (602985); MC1DN7 (618229), caused by mutation in the NDUFV2 gene (600532); MC1DN8 (618230), caused by mutation in the NDUFS3 gene (603846); MC1DN9 (618232), caused by mutation in the NDUFS6 gene (603848); MC1DN10 (618233), caused by mutation in the NDUFAF2 gene (609653); MC1DN11 (618234), caused by mutation in the NDUFAF1 gene (606934); MC1DN12 (301020), caused by mutation in the NDUFA1 gene (300078); MC1DN13 (618235), caused by mutation in the NDUFA2 gene (602137); MC1DN14 (618236), caused by mutation in the NDUFA11 gene (612638); MC1DN15 (618237), caused by mutation in the NDUFAF4 gene (611776); MC1DN16 (618238), caused by mutation in the NDUFAF5 gene (612360); MC1DN17 (618239), caused by mutation in the NDUFAF6 gene (612392); MC1DN18 (618240), caused by mutation in the NDUFAF3 gene (612911); MC1DN19 (618241), caused by mutation in the FOXRED1 gene (613622); MC1DN20 (611126), caused by mutation in the ACAD9 gene (611103); MC1DN21 (618242), caused by mutation in the NUBPL gene (613621); MC1DN22 (618243), caused by mutation in the NDUFA10 gene (603835); MC1DN23 (618244), caused by mutation in the NDUFA12 gene (614530); MC1DN24 (618245), caused by mutation in the NDUFB9 gene (601445); MC1DN25 (618246), caused by mutation in the NDUFB3 gene (603839); MC1DN26 (618247), caused by mutation in the NDUFA9 gene (603834); MC1DN27 (618248), caused by mutation in the MTFMT gene (611766); MC1DN28 (618249), caused by mutation in the NDUFA13 gene (609435); MC1DN29 (618250), caused by mutation in the TMEM126B gene (615533); MC1DN30 (301021), caused by mutation in the NDUFB11 gene (300403); MC1DN31 (618251), caused by mutation in the TIMMDC1 gene (615534); MC1DN32 (618252), caused by mutation in the NDUFB8 gene (602140); MC1DN33 (618253), caused by mutation in the NDUFA6 gene (602138); MC1DN34 (618776), caused by mutation in the NDUFAF8 gene (618461); MC1DN35 (619003), caused by mutation in the NDUFB10 gene (603843); MC1DN36 (619170), caused by mutation in the NDUFC2 gene (603845); MC1DN37 (619272), caused by mutation in the NDUFA8 gene (603359); MC1DN38 (619382), caused by mutation in the DNAJC30 gene (618202); and MC1DN39 (620135), caused by mutation in the NDUFB7 gene (603842). Complex I deficiency with mitochondrial inheritance has been associated with mutation in 6 mitochondrial-encoded components of complex I: MTND1 (516000), MTND2 (516001), MTND3 (516002), MTND4 (516003), MTND5 (516005), MTND6 (516006). Most of these patients have a phenotype of Leber hereditary optic neuropathy (LHON; 535000) or Leigh syndrome. Features of complex I deficiency may also be caused by mutation in other mitochondrial genes, including MTTS2 (590085). [from OMIM]

Mitochondrial DNA (mtDNA)-associated Leigh syndrome and NARP (neurogenic muscle weakness, ataxia, and retinitis pigmentosa) are part of a continuum of progressive neurodegenerative disorders caused by abnormalities of mitochondrial energy generation. Leigh syndrome (or subacute necrotizing encephalomyelopathy) is characterized by onset of symptoms typically between ages three and 12 months, often following a viral infection. Decompensation (often with elevated lactate levels in blood and/or CSF) during an intercurrent illness is typically associated with psychomotor retardation or regression. Neurologic features include hypotonia, spasticity, movement disorders (including chorea), cerebellar ataxia, and peripheral neuropathy. Extraneurologic manifestations may include hypertrophic cardiomyopathy. About 50% of affected individuals die by age three years, most often as a result of respiratory or cardiac failure. NARP is characterized by proximal neurogenic muscle weakness with sensory neuropathy, ataxia, and pigmentary retinopathy. Onset of symptoms, particularly ataxia and learning difficulties, is often in early childhood. Individuals with NARP can be relatively stable for many years, but may suffer episodic deterioration, often in association with viral illnesses. [from GeneReviews]

Leber hereditary optic neuropathy (LHON) typically presents in young adults as bilateral, painless, subacute visual failure. The peak age of onset in LHON is in the second and third decades of life, with 90% of those who lose their vision doing so before age 50 years. Very rarely, individuals first manifest LHON in the seventh and eighth decades of life. Males are four to five times more likely to be affected than females, but neither sex nor mutational status significantly influences the timing and severity of the initial visual loss. Neurologic abnormalities such as postural tremor, peripheral neuropathy, nonspecific myopathy, and movement disorders have been reported to be more common in individuals with LHON than in the general population. Some individuals with LHON, usually women, may also develop a multiple sclerosis-like illness. [from GeneReviews]