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At a glance

Complex III is located within the inner membrane of the mitochondrium and is the second enzyme in the electron transport chain of the oxidative phosphorylation process. The enzyme ubiquinol-cytochrome-c oxidoreductase catalyzes the electron transfer from succinate and nicotinamide adenine dinucleotide linked dehydrogenases to cytochrome c within the respiratory chain. The clinical features usually include dementia, progressive ataxia, predominantly proximal muscle weakness, areflexia, extensor plantar responses, and concomitant nonspecific myo- and neuropathic changes in muscles. External ophthalmoplegia, ptosis, and cardiomyopathy are often present. The most frequently associated clinical condition is ☞Leber Hereditary Optic Neuropathy (LHON).


Ubiquinone-cytochrome Oxidoreductase; Coenzyme Q-Cytochrome C Reductase Deficiency.



Genetic inheritance

Inheritance is usually autosomal recessive. Mutations in the BCS1L (BCS1 homolog, Ubiquinol-Cytochrome-c Reductase Complex Chaperone) gene located on chromosome 2q35 or in the UQCRB (Ubiquinol-Cytochrome c Reductase-Binding Protein) gene on chromosome 8q22.1 have been demonstrated. The BCS1L mutations are associated with renal tubulopathy, encephalopathy, and liver failure.

Other condition to be considered

  • GRACILE Syndrome: An acronym that stands for Growth Retardation, Amino aciduria, Cholestasis, Iron overload, Lactic acidosis, Early death Syndrome (Finnish Lethal Neonatal Metabolic Syndrome; Finnish Lactic Acidosis with Hepatic Hemosiderosis; Fellman Syndrome), is also caused by mutations in the BCS1L gene, but displays a different phenotype.

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