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.

Combined oxidative phosphorylation deficiency-13 (COXPD13) is an autosomal recessive multisystem disorder resulting from mitochondrial dysfunction. Affected individuals develop severe neurologic impairment in the first months of life, including hypotonia, abnormal dystonic movements, hearing loss, poor feeding, global developmental delay, and abnormal eye movements. Brain imaging shows signal abnormalities in putamen, basal ganglia, caudate nuclei, or corpus callosum, as well as delayed myelination. Analysis of patient tissues shows multiple defects in enzymatic activities of the mitochondrial respiratory chain, although some tissues may show normal values since tissue expression of the mitochondrial defect and metabolic needs of specific tissues are variable (summary by Vedrenne et al., 2012). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).