Background: Isolated methylmalonic acidemia is a rare autosomal recessive metabolic disorder mostly caused by mutations in the methylmalonyl coenzyme A mutase (MCM) gene (MUT). This study aimed to verify whether missense mutations in MUT in Chinese patients affect the stability and enzymatic activity of MCM.

Methods: Eight Chinese patients were identified with novel mutations. Plasmids carrying the wild-type and mutated MUT cDNA were constructed and transfected into HEK293T cells for functional analyses. The expression and activity of MCM were determined by western blot and ultra-performance liquid chromatography, respectively.

Results: All patients had high levels of blood propionylcarnitine and urinary methylmalonyl acid. By the end of the study, two patients were lost to follow-up, three died, and three survived with mental retardation. Compared to the wild-type protein, the expression levels of all missense mutations of in vitro MCM protein were decreased (P<0.05) except those for I597R, and the MCM activity of the mutations was reduced in a permissive assay.

Conclusion: The missense mutations L140P, A141T, G161V, W309G, I505T, Q514K, I597R and G723D affected the stability and enzymatic activity of MCM, indicating that they had a disease-causing capacity.