Identification and functional characterization of mutations within HADHB associated with mitochondrial trifunctional protein deficiency.

Mitochondrial trifunctional protein (MTP) deficiency is a rare autosomal recessive disorder with several phenotypes. Neuromyopathic form of MTP deficiency is characterized by infantile or juvenile-onset, progressive peripheral neuropathy and rhabdomyolysis. To date, only one Chinese patient harboring homozygous c. 739C>T (p.R247C) in HADHB has been reported. Here, using whole exome sequencing (WES), we identified a compound heterozygote of c.407T>C (p.M136T) and c.421G>A (p.A141T) within HADHB in a Chinese MTP deficiency patient of neuromyopathic form. In vitro cell functional studies were performed to evaluate the effect of mutations on MTP complex expression and subcellular location, which revealed that p.M136T and p.A141T mutations compromised MTP complex stability but not altered subcellular localization, resulting in lower protein level at 37 °C but higher at 30 °C. These results indicated that both mutations were pathogenic through a loss-of-function mechanism and temperature-sensitive leading to their correlation with the mild phenotype. The current study broadens the genetic spectrum of HADHB and highlights the importance of screening fatty acid oxidation deficiency-related gene mutations among patients with intermittent rhabdomyolysis, as in the patient reported here, although extremely rare.