Impaired Function of a Rare Mutation in the MMUT Gene Causes Methylmalonic Acidemia in a Chinese Patient

Abstract

Methylmalonic acidemia (MMA) is an autosomal recessive metabolic disorder mainly caused by mutations in the methylmalonyl coenzyme A mutase (MCM) gene (MMUT) and leads to the reduced activity of MCM. In this study, a 3-year-old girl was diagnosed with carnitine deficiency secondary to methylmalonic acidemia by tandem mass spectrometry (MS/MS) and gas chromatography/mass spectrometry (GS/MS). Whole-exome sequencing (WES) was performed on the patient and identified two compound heterozygous mutations in MMUT: c.554C>T (p. S185F) and c.729-730insTT (p. D244Lfs ^∗ 39). Bioinformatics analysis predicted that the rare missense mutation of c.554C>T would be damaging. Moreover, this rare mutation resulted in the reduced levels of MMUT mRNA and MMUT protein. Collectively, our findings provide a greater understanding of the effects of MMUT variants and will facilitate the diagnosis and treatment of patients with MMA.

Figure 1

Clinical summary for the MMA family. (a) Family pedigree. An unrelated natural couple who gave birth to the affected child (black arrow denotes the proband). (b) The patient's axial brain MRI (yellow arrowheads show small patches of abnormal signals in the dorsal thalamic region of the left basal ganglia). (c) Sequence analysis of the human MMUT gene. A c.554C>T mutation of the MMUT gene was identified in the proband, and her mother was an asymptomatic heterozygous carrier. Arrows indicate the positions of the rare mutations. A c.729_730insTT mutation of the MMUT gene was identified in the proband, and her father was an asymptomatic heterozygous carrier. Arrows indicate the positions of the mutations.

Figure 2

The conservation of MMUT variants. Multiple sequence alignment of the MCM protein for different species (black arrow denotes the position of the variant) (c.554C>T [p.S185F]).

Figure 3

Effect of MMUT mutations on MMUT protein and mRNA levels in HEK293T cells. (a) The relative mRNA expression of MMUT was normalized to HEK293T cells transfected with the wild-type plasmid. The data collected from three independent experiments were subjected to statistical analysis (^∗P < 0.05; ^∗∗P < 0.01 vs. WT). (b-c) The relative protein expression was normalized to HEK293 T cells transfected with the wild-type plasmid. The data collected from three independent experiments were subjected to statistical analysis (^∗P < 0.05; ^∗∗P < 0.01 vs. WT). WT: wild type; MUT1: c.554C>T (p. S185F); MUT2: c.729–730insTT (p.D244Lfs^∗39).