Diseases of the tricarboxylic acid cycle (TCA cycle) constitute a group of rare human diseases that affect core mitochondrial metabolism. The Fumarase deficiency is caused by impairment of the fumarate hydratase enzyme. The symptoms of the disorder include developmental delay, severe mental retardation, language impairment, seizures and dysmorphic facial features. The succinate dehydrogenase deficiency [DS:H02005] affects mitochondrial complex II, which links the TCA cycle with the electron transport chain. The phenotype is highly variable and can include Leigh syndrome, leukodystrophy, cardiomyopathy and mental and motor skill deterioration. The alpha-ketoglutarate dehydrogenase deficiency is extremely rare and characterised by encephalopathy and hyperlactatemia resulting in death in early childhood.
