Identification and characterization of a temperature-sensitive R268H mutation in the human succinyl-CoA:3-ketoacid CoA transferase (SCOT) gene

Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency causes episodic ketoacidosis. We encountered a case of siblings in South Africa in whom a novel homozygous mutation (R268H) was found in genomic DNA. Mutant SCOT protein was very faintly detected in their fibroblasts using immunoblot analysis. Transient expression analysis of R268H mutant cDNA at 37 °C revealed that the R268H mutant protein was clearly detected, as much as 50% wild-type, together with 40% residual SCOT activities, hence R268H was first regarded as not being a disease-causing mutation. Since no other mutation was identified, R268H mutation was re-evaluated by further transient expression analysis. Accumulation of the R268H mutant protein was revealed to be strongly temperature dependent; residual SCOT activities were calculated to be 59.7%, 34%, and 4%, respectively, in expression at 30 °C, 37 °C, and 40 °C in SV40-transformed fibroblasts of GS01(a homozygote of S283X). SCOT activity of the R268H protein was more vulnerable than the wild-type to heat treatment at 50 °C. These results indicated that the R268H mutant protein was clearly more unstable than the wild-type in a temperature-sensitive manner. Furthermore, an analysis of the three-dimensional structure of SCOT showed that the R268H mutation was expected to break a conserved salt bridge between R268 and D52, which would be expected to lead to decreased stability of the protein. Hence we finally concluded that the R268H mutation is a disease-causing one. The stability of mutant protein in transient expression analysis does not always reflect the condition in patients’ fibroblasts.