Isolated complex I deficiency

Isolated complex I deficiency (also known as NADH:ubiquinone oxidoreductase deficiency or mitochondrial complex I deficiency) is the most common enzymatic defect among mitochondrial respiratory chain disorders. Complex I (NADH dehydrogenase) is the largest enzyme complex of the mitochondrial electron transport chain and plays a critical role in cellular energy production. When this complex is deficient, cells cannot produce adequate amounts of ATP, particularly affecting tissues with high energy demands such as the brain, heart, skeletal muscles, and liver. The clinical presentation of isolated complex I deficiency is highly variable, ranging from fatal neonatal lactic acidosis to childhood-onset neurodegenerative disorders. Common clinical phenotypes include Leigh syndrome (subacute necrotizing encephalomyelopathy), leukoencephalopathy, hypertrophic cardiomyopathy, exercise intolerance, and lactic acidosis. Neurological features frequently include developmental delay or regression, seizures, hypotonia, dystonia, optic atrophy, and ataxia. Some patients present with hepatopathy or renal tubular dysfunction. The severity and progression vary widely depending on the specific genetic cause and degree of residual enzyme activity. Isolated complex I deficiency can be caused by mutations in over 30 different genes, including both nuclear DNA-encoded structural subunits and assembly factors (such as NDUFS1, NDUFS2, NDUFS4, NDUFS7, NDUFS8, NDUFV1, NDUFV2, NDUFAF2, ACAD9, among others) as well as mitochondrial DNA-encoded subunits (ND1-ND6, ND4L). There is currently no curative treatment. Management is primarily supportive and may include cofactor supplementation (such as riboflavin, coenzyme Q10, and idebenone), management of lactic acidosis, seizure control, nutritional support, and physical therapy. Prognosis varies considerably but is often poor in severe early-onset forms.

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