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Background  Methylmalonic acidemia is an inherited organic acid metabolic disease. It involves multiple physiological systems and has variable manifestations. The primary causative gene  MMUT  carries wide range of mutations, and one of them, c.1663G > A (p.A555T), is considered to be a rare type, which is seen more frequently in Asian than other populations. So far, little is known about the clinical features of patients carrying this mutation. In the present study, we aimed to define the clinical and biochemical features of the patients with this genotype.    Methods  Among 328  mut  type methylmalonic acidemia patients from multiple hospitals in China, we collected 30 compound heterozygous patients sharing the mutation c.1663G > A (p.A555T) in the  MMUT  gene. Their clinical characteristics and biochemical index were described in detail and compared with methylmalonic acidemia patients without this variant.    Results  Most of these patients were diagnosed via newborn screening (26/30), treated in a timely manner, and kept healthy (24/30). Disease onset occurred in 7 patients. Developmental delay or intellectual impairment occurred in 4 patients. 100% of these patients (29/29) were responsive to Vitamin B12 administration. The blood propionylcarnitine, blood propionylcarnitine/acetylcarnitine ratio, urinary methylmalonic acid, urinary methylcitric acid before and after treatment in c.1663G > A (p.A555T) carrying patients were much lower than those in non-c.1663G > A (p.A555T) carrying patients.    Conclusion  Compared to patients with other mutations in the  MMUT  gene, patients with the c.1663G > A (p.A555T) mutation showed later onset, milder clinical phenotype, lighter biochemical abnormalities, better vitamin B12 responsiveness, lower morbidity, easier metabolic control, and thereby better prognosis. Newborn screening project plays an important role in early diagnosis, treatment, and prognosis of these patients.

orphanet journal of rare diseases

A rare mutation c.1663G &gt; A (p.A555T) in the MMUT gene associated with mild clinical and biochemical phenotypes of methylmalonic acidemia in 30 Chinese patients