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Complex II is, as in Complex I, caused by mutations in nDNA. This mutation is defined as a “direct hit" in the genes that encode subunits of respiratory chains complexes. It affects the enzyme succinate CoQ reductase which is responsible for the transfer of electrons by the reduction of succinate to fumarate in the electron chain pathway (see Table C-2). Deficiency of complex II is characterized by highly variable phenotypic expression. The clinical features include encephalomyopathy, failure to thrive, severe developmental delay, muscle hypotonia, lethargy, respiratory failure, ataxia, and myoclonic seizures. The presence of lactic acidosis is common. The most frequent clinical condition is Leigh syndrome. See Table C-4.

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Table C-4 Summary of Mitochondrial Syndromes and Common Clinical Manifestations
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Succinate CoQ Reductase Deficiency; Succinate Dehydrogenase.

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If we consider only cases with documented mutations, complex II deficiency appears to be a rare cause of mitochondrial disorders.

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Inheritance is usually autosomal recessive. Deficiency of complex II is characterized by highly variable phenotypic expression. It can be caused by mutation in the nuclear-encoded SDHA gene on chromosome 5p.

Cheam EW, Critchley LA. Anesthesia for a child with complex I respiratory chain enzyme deficiency. J Clin Anesth 10:524, 1998.  [PubMed: 9793822]

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