Biochemical Analysis of a Patient with Fumarate Hydratase Deficiency

Abstract

Fumarate hydratase deficiency (FHD) is a rare autosomal recessive metabolic disease with approximately 100 patients reported worldwide. FHD presents in early infancy and clinical features include developmental delays, brain malformations, hypotonia, poor growth, and seizures. Severely affected individuals often do not survive past early childhood. FHD is caused by two defective copies of the FH gene, which encodes the Krebs cycle enzyme fumarase found in mitochondria and the cytosol. We describe the symptoms, treatment, and biochemical analysis of a patient with FHD. While infantile spasms are common in FHD, the patient presented with epileptic spasms later in childhood. Unexpectedly, the patient responded excellently to lacosamide for her seizures. To biochemically analyze the patient’s two fumarase variants, we introduced each of the mutations (E432Kfs*17 and D65G) into a plasmid harboring the recombinant human FH gene. We purified and characterized the fumarase variants alongside wild‐type fumarase. E432Kfs*17 was extremely defective for both catalysis and oligomerization. The large decrease in catalytic efficiency of E423Kfs*17 is due to a large decrease in turnover (~1500‐fold lower kcat). The novel variant D65G formed homotetramers and had a turnover (kcat) similar to wild‐type, but required ~2‐fold higher substrate concentration to achieve one‐half maximum velocity (2‐fold higher Km). We propose that the relatively mild defect of the D65G variant may have played a role in the patient’s milder clinical severity.Support or Funding InformationThis research was funded by Suffolk University.