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At a Glance

Complex II Disease is caused by mutations in nuclear DNA. These mutations are defined as a “direct hit” to the genes that encode subunits of respiratory chains complexes. They affect the enzyme Succinate dehydrogenase, which is responsible for the transfer of electrons by reduction of succinate to fumarate in the electron chain pathway (see Table C-1). 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. Lactic acidosis is common. The most frequent clinical condition is ☞Leigh Syndrome.


Succinate CoQ Reductase Deficiency; Succinate Dehydrogenase Deficiency.


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

Genetic inheritance

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.

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