Isolated complex I deficiency is a rare inborn error of metabolism due to mutations in nuclear or mitochondrial genes encoding subunits or assembly factors of the human mitochondrial complex I (NADH: ubiquinone oxidoreductase) and is characterized by a wide range of manifestations including marked and often fatal lactic acidosis, cardiomyopathy, leukoencephalopathy, pure myopathy and hepatopathy with tubulopathy. Among the numerous clinical phenotypes observed are Leigh syndrome, Leber hereditary optic neuropathy and MELAS syndrome (see these terms).

Acute infantile liver failure resulting from TRMU mutation is a transient disorder of hepatic function. In addition to elevated liver enzymes, jaundice, vomiting, coagulopathy, and hyperbilirubinemia, the presence of increased serum lactate is consistent with a defect in mitochondrial respiratory function. With supportive care, patients who survive the initial acute episode can recover and show normal development (Zeharia et al., 2009). See also transient infantile mitochondrial myopathy (MMIT; 500009), which is a similar disorder. A more severe, permanent disorder with some overlapping features is associated with mitochondrial DNA depletion (251880). See ILFS1 (615438) for information on syndromic infantile liver failure.

Mitochondrial complex IV deficiency nuclear type 22 (MC4DN22) is an autosomal recessive metabolic disorder characterized by neonatal hypertrophic cardiomyopathy, encephalopathy, and severe lactic acidosis with fatal outcome (Wintjes et al., 2021). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.