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).

Genetic defects in the pyruvate dehydrogenase complex are one of the most common causes of primary lactic acidosis in children. Most cases are caused by mutation in the E1-alpha subunit gene on the X chromosome. X-linked PDH deficiency is one of the few X-linked diseases in which a high proportion of heterozygous females manifest severe symptoms. The clinical spectrum of PDH deficiency is broad, ranging from fatal lactic acidosis in the newborn to chronic neurologic dysfunction with structural abnormalities in the central nervous system without systemic acidosis (Robinson et al., 1987; Brown et al., 1994). Genetic Heterogeneity of Pyruvate Dehydrogenase Complex Deficiency PDH deficiency can also be caused by mutation in other subunits of the PDH complex, including a form (PDHXD; 245349) caused by mutation in the component X gene (PDHX; 608769) on chromosome 11p13; a form (PDHBD; 614111) caused by mutation in the PDHB gene (179060) on chromosome 3p14; a form (PDHDD; 245348) caused by mutation in the DLAT gene (608770) on chromosome 11q23; a form (PDHPD; 608782) caused by mutation in the PDP1 gene (605993) on chromosome 8q22; and a form (PDHLD; 614462) caused by mutation in the LIAS gene (607031) on chromosome 4p14.

Mitochondrial complex IV deficiency nuclear type 5 (MC4DN5) is an autosomal recessive severe metabolic multisystemic disorder with onset in infancy. Features include delayed psychomotor development, impaired intellectual development with speech delay, mild dysmorphic facial features, hypotonia, ataxia, and seizures. There is increased serum lactate and episodic hypoglycemia. Some patients may have cardiomyopathy, abnormal breathing, or liver abnormalities, reflecting systemic involvement. Brain imaging shows lesions in the brainstem and basal ganglia, consistent with a diagnosis of Leigh syndrome (see 256000). Affected individuals tend to have episodic metabolic and/or neurologic crises in early childhood, which often lead to early death (summary by Debray et al., 2011). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Combined oxidative phosphorylation deficiency type 3 is an extremely rare clinically heterogenous disorder described in about 5 patients to date. Clinical signs included hypotonia, lactic acidosis, and hepatic insufficiency, with progressive encephalomyopathy or hypertrophic cardiomyopathy.

Four phenotypes comprise the RRM2B mitochondrial DNA maintenance defects (RRM2B-MDMDs): RRM2B encephalomyopathic MDMD, the most severe phenotype, usually manifesting shortly after birth as hypotonia, poor feeding, and faltering growth requiring hospitalization. Subsequent assessments are likely to reveal multisystem involvement including sensorineural hearing loss, renal tubulopathy, and respiratory failure. Autosomal dominant progressive external ophthalmoplegia (adPEO), typically adult onset; other manifestations can include ptosis, bulbar dysfunction, fatigue, and muscle weakness. RRM2B autosomal recessive progressive external ophthalmoplegia (arPEO), a typically childhood-onset predominantly myopathic phenotype of PEO, ptosis, proximal muscle weakness, and bulbar dysfunction. RRM2B mitochondrial neurogastrointestinal encephalopathy (MNGIE)-like, characterized by progressive ptosis, ophthalmoplegia, gastrointestinal dysmotility, cachexia, and peripheral neuropathy. To date, 78 individuals from 52 families with a molecularly confirmed RRM2B-MDMD have been reported.

Autosomal recessive mitochondrial complex III deficiency is a severe multisystem disorder with onset at birth of lactic acidosis, hypotonia, hypoglycemia, failure to thrive, encephalopathy, and delayed psychomotor development. Visceral involvement, including hepatopathy and renal tubulopathy, may also occur. Many patients die in early childhood, but some may show longer survival (de Lonlay et al., 2001; De Meirleir et al., 2003). Genetic Heterogeneity of Mitochondrial Complex III Deficiency Mitochondrial complex III deficiency can be caused by mutation in several different nuclear-encoded genes. See MC3DN2 (615157), caused by mutation in the TTC19 gene (613814) on chromosome 17p12; MC3DN3 (615158), caused by mutation in the UQCRB gene (191330) on chromosome 8q; MC3DN4 (615159), caused by mutation in the UQCRQ gene (612080) on chromosome 5q31; MC3DN5 (615160), caused by mutation in the UQCRC2 gene (191329) on chromosome 16p12; MC3DN6 (615453), caused by mutation in the CYC1 gene (123980) on chromosome 8q24; MC3DN7 (615824), caused by mutation in the UQCC2 gene (614461) on chromosome 6p21; MC3DN8 (615838), caused by mutation in the LYRM7 gene (615831) on chromosome 5q23; MC3DN9 (616111), caused by mutation in the UQCC3 gene (616097) on chromosome 11q12; and MC3DN10 (618775), caused by mutation in the UQCRFS1 gene (191327) on chromosome 19q12. See also MTYCB (516020) for a discussion of a milder phenotype associated with isolated mitochondrial complex III deficiency and mutations in a mitochondrial-encoded gene.

Mitochondrial DNA depletion syndrome-12B is an autosomal recessive mitochondrial disorder characterized by childhood onset of slowly progressive hypertrophic cardiomyopathy and generalized skeletal myopathy resulting in exercise intolerance, and, in some patients, muscle weakness and atrophy. Skeletal muscle biopsy shows ragged-red fibers, mtDNA depletion, and accumulation of abnormal mitochondria (summary by Echaniz-Laguna et al., 2012). For a discussion of genetic heterogeneity of mtDNA depletion syndromes, see MTDPS1 (603041).

Any mitochondrial complex III deficiency in which the cause of the disease is a mutation in the UQCC3 gene.

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.

Combined oxidative phosphorylation deficiency-28 (COXPD28) is a complex autosomal recessive multisystem disorder associated with mitochondrial dysfunction. The phenotype is variable, but includes episodic metabolic decompensation beginning in infancy that can result in mild muscle weakness, cardiorespiratory insufficiency, developmental delay, or even death. Biochemical studies of patient tissues show variable mitochondrial defects, including decreased activities of respiratory chain enzymes (summary by Kishita et al., 2015). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).