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.