Fumarase deficiency is an autosomal recessive encephalopathy affecting both the mitochondrial and the cytosolic enzymes.

A 7-month-old boy died in a demented state after a clinical history characterized by generalized seizures, psychomotor deterioration, and fumaric aciduria. We found a marked deficiency of both mitochondrial and cytosolic fumarases in skeletal muscle, brain, cerebellum, heart, kidney, liver, and cultured fibroblasts. Fumarase activities were 30 to 50% compared with controls in both mitochondria and cytosol from cultured fibroblasts of the parents. Antifumarase cross-reacting material was present in negligible amounts in the patient's tissues. Our data indicate that this disease is an autosomal recessive encephalopathy, due to a single mutation affecting the gene encoding both forms of the enzyme.