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  1. Mitochondrial complex III, the gateway to the cytochrome segment of the respiratory chain, conveys electrons from several dehydrogenase-associated quinone pools to cytochrome c. The mammalian complex III monomer (bc1 complex; ubiquinol cytochrome c reductase; EC contains up to 11 subunits in heart and liver, of which only one, cytochrome b, is encoded by the mitochondrial genome. In its native form, complex III is dimeric, being closely associated in varying proportions with complexes I and IV to form the respirasome.

  2. Isolated complex III deficiency is a rare condition. Using functional tests of mitochondria, partial deficiencies of complex III are difficult to detect.

  3. Depending upon the causal gene, complex III deficiency can be maternally inherited, transmitted as an autosomal recessive trait or sporadic.

  4. A number of distinct phenotypes have been ascribed to mutations in the cytochrome bgene of mitochondrial DNA (MIM 516020). These include exercise intolerance, mitochondrial encephalomyopathy, infantile histiocytoid cardiomyopathy, Parkinsonism/MELAS overlap syndrome, optic neuropathy and susceptibility to obesity.

  5. Primary complex III deficiency (MIM #124000) has also been associated with mutations in four nuclear genes: BCS1L, UQCRB, UQCRQ and TTC19.

  6. BCS1L maps to chromosome 2q33 and encodes an assembly factor for complex III. BCS1L mutations can produce at least three autosomal recessively-inherited clinical syndromes. Patients were first identified in 4 consanguineous families, presenting with multivisceral neonatal-onset disease featuring proximal renal tubulopathy, liver involvement, and encephalopathy. The related GRACILE syndrome (growth retardation, aminoaciduria, cholestasis, iron overload, lactic acidosis, early death) features severe intrauterine growth retardation, Fanconi-type tubulopathy, cholestasis with liver hemosiderosis, severe lactic acidosis and early death. Björnstad syndrome is characterized by congenital sensorineural hearing loss and pili torti (trichokinesis), a condition in which hair shafts are brittle, irregularly flattened and twisted 180 degrees from their normal axis.

  7. UQCRB maps to chromosome 8q22 and encodes an 11 kDa ubiquinone-binding protein of complex III. A homozygous frameshift UQCRB mutation was described in a single patient, associated with hypoglycemia and lactic acidosis.

  8. UQCRQ, on chromosome 5q31, encodes the ubiquinol-cytochrome c reductase 9.5 kDa complex III subunit. Only one family with UQCRQ mutations has been reported. Multiple members were affected. Clinical features include autosomal recessive severe psychomotor retardation, extrapyramidal signs including dystonia and athetosis, ataxia, mild axial hypotonia and marked global dementia.

  9. Mutations in TTC19, a putative complex III assembly factor that maps to chromosome 17, have been described in individuals exhibiting slowly progressive childhood-onset encephalopathy or adult onset subacute neurodegenerative disease with cerebellar ataxia, pyramidal signs, hearing loss and basal ganglia hyperintensities on cerebral magnetic resonance imaging.

  10. Other reported patients with isolated complex III deficiency are not clinically distinguishable patients with other mitochondrial respiratory chain deficiencies.

  11. Complex III deficiency can occur in combination with deficiencies of other respiratory chain complexes, as a consequence for instance of mitochondrial DNA depletion syndromes or of mutations in transfer RNA genes of mitochondrial DNA.

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