Newborn screening and genetic analysis of methylmalonic acidemia in Shaanxi province / 中华围产医学杂志

ABSTRACT

Objective:To analyze the incidence, biochemical and molecular characteristics, and gene mutation spectrum of neonatal methylmalonic acidemia (MMA) in Shaanxi province.Methods:This study involved 146 152 newborns undergoing neonatal screening for methylmalonic acidemia by tandem mass spectrometry in Northwest Women's and Children's Hospital from January 2014 and December 2019. Clinical manifestations and follow-up data of newborns diagnosed with MMA and their acylcarnitine profiles and gene mutations were analyzed. According to whether they had elevated homocysteine or not, these patients were divided into two groups, the complicated group and the isolated MMA group. The control neonates were those excluded from having methylmalonic acid by re-examination. Kruskal-Wallis and Mann-Whitney U test was conducted for statistical analysis. Results:(1) Twenty-one cases of MMA were confirmed with an incidence of 1/6 960, including 11 cases (52.4%) of isolated MMA (isolated MMA group) and 10 (47.6%) complicated by elevated homocysteine (complicated group). Eight patients in the isolated group had symptoms within one month after birth, mainly feeding difficulties, vomiting, drowsiness, poor response and infection, and five died. Patients in the complicated group were all diagnosed before developing typical clinical symptoms, and no developmental abnormalities were reported during follow-up. (2) Blood propionyl carnitine and its ratios to acetylcarnitine and free carnitine in the isolated MMA and complicated groups were higher than those in the control group [ M (min-max), 9.26 (3.70-37.78) μmol/L and 7.27 μmol/L (3.58-13.62 μmol/L) vs 4.51 μmol/L (1.48-8.69 μmol/L), H=23.239; 1.12 (0.32-2.43) and 0.74 (0.36-1.90) vs 0.25 (0.09-0.45), H=47.061; 0.94 (0.12-1.92) and 0.56 (0.18-1.03) vs 0.17 (0.06-0.38), H=36.868; all P<0.001]. The blood methionine level in the complicated group was significantly lower than that in the isolated MMA group [7.64 μmol/L (3.40-19.25 μmol/L) vs 24.22 μmol/L (10.73-56.55 μmol/L), U=3.000, P<0.001]. (3) All 21 patients carried complex heterozygous mutations or homozygous mutations in pathogenic genes, including 15 distinct MMUT mutations and 13 distinct MMACHC mutations. In the isolated MMA group, the most common mutation was c.323G>A (p.Arg108His) in the MMUT gene with a positive rate of 13.6%, and an unreported mutation, c.1676+11A>G, with unidentified clinical significance, was also found. The most common mutations in the complicated group were c.609G>A (p.Trp203Ter) and c.567dupT (p.Ile190fs) in the MMACHC gene, and the positive rates were both 20.0%. Moreover, two unreported variants, c.430-2A>C and c.648_650delAGA (p.216_217delSEinsS), were detected and suspected to be pathogenic. Conclusions:MMA is not uncommon in Shaanxi province. Children with isolated MMA tend to be more severe clinically. The identification of hotspot mutations, including c.609G>A (p.Trp203Ter) and c.567dupT (p.Ile190fs) in MMACHC gene and c.323G>A (p.Arg108His) in MMUT gene, provides a foundation for further genetic screening, counseling, and prenatal diagnosis, and is conducive to reduce the mortality and disability rate of neonatal MMA.