Screening and gene mutations analysis for neonates with methylmalonic aciduria

Objective To investigate the characteristics of neonatal methylmalonic aciduria (MMA) regarding clinical manifestations, laboratory findings, gene mutations, treatments and prognosis. Methods Acylcarnitine levels in blood samples of 207 308 neonates born from January 2016 to December 2017 in Xuzhou were detected by liquid chromatography tandem mass spectrometry and the abnormal results were further confirmed by detecting organic acids in urine samples with gas chromatography-mass spectrometry and gene sequencing analysis. Patients with isolated MMA were treated with dietary control and levocarnitine, while those complicated by homocysteinemia were treated with vitamin B12, levocarnitine, glycine betaine and calcium folinate. Clinical manifestations, laboratory findings, imaging features, genotypes, treatments and prognosis of patients with MMA were retrospectively analyzed. Paired sample t-test was applied for statistical analysis. Results MMA was eventually diagnosed in 12 patients, among which three were isolated MMA and nine were complicated by homocysteinemia. The three isolated MMA cases failed to response to vitamin B12 treatment without any symptoms on diagnosis. However, vitamin B12 was effective for the other nine patients, among which four had no clinical symptoms on diagnosis and five had manifestations such as slow response, recurrent vomiting, poor feeding, dyspnea, anemia and jaundice. Abnormal results of cranial MRI included bilateral basal ganglia damage, enlarged extracranial space, ventriculomegaly and changes in white matter. All patients underwent genetic analysis and three were found with MUT gene mutations and nine with MMACHC gene mutations. MUT gene mutations were classified into five types, including c.1106G>A, c.1880A>G, c.441T>A, c.581C>T and c.1741C>T. Eight types of MMACHC gene mutations were identified, including c.609G>A, c.658_660delAAG, c.482G>A, c.1A>G, c.567dupT, c.80A>G, c.276+1G>A and c.228_231delTGAC. Two mutations, c.276+1G>A and c.228_231delTGAC, were novel mutations. The most common mutation in MMACHC gene was c.609G>A, followed by c.658_660delAAG and c.482G>A. One of the isolated MMA patients died after refusing treatments and the other two showed significant decrease in serum propionylcarnitine, propionylcarnitine to acetylcarnitine ratio, serum homocysteine and methylmalonic acid and methylcitric acid in urine after treatment. Moreover, of the two patients who were alive at follow-up, one experienced normal growth and development and the other suffered from growth retardation. The ratio of propionylcarnitine to acetylcarnitine and the levels of serum propionylcarnitine, serum homocysteine and methylmalonic acid and methylcitric acid in urine were significantly decreased in the nine patients with MMA complicated by homocystinuria after one month of treatment [0.88±0.35 vs 0.13±0.05, (7.12±1.90) μmol/L vs (3.18±1.08) μmol/L, (136.48±38.14) μmol/L vs (34.41±17.33) μmol/L, 103.51±69.62 vs 5.35±2.15 and 7.95±6.88 vs 1.02±0.48, t=-6.166, -6.687, -12.941, -4.208 and -3.015, respectively, all P<0.05]. Two deaths, three asymptomatic and four psychomotor retardation patients were reported during follow-up. Conclusions Newborn screening with liquid chromatography tandem mass spectrometry is important for early diagnosis of MMA. MMACHC gene defects are the main causes of MMA in Xuzhou area and the predominant one is c.609G>A mutation. Prognosis of MMA might be related to disease type, age of onset and patient's reactivity to vitamin B12. Key words: Amino acid metabolism, inborn errors; Carrier proteins; Methylmalonyl-CoA mutase; Mutation; Chromatography, liquid; Tandem mass spectrometry