Significance of ACADM mutations identified through newborn screening of MCAD deficiency in Japan

• MCAD mutations of Japanese patients are quite different from those of Caucasians.
• Their allele-specific activities are compared with Caucasian common mutations.
• 985A > G and 199C > T can be standards to judge novel mutations in newborn screening.
• MCAD activities in lymphocytes correlate well with allele-specific activities.
• Screening of non-Caucasian newborns requires careful genetic and enzymatic study.

BackgroundSince the first case was detected in 2000, there has been a remarkable increase in Japanese patients diagnosed with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. Genetic analysis has revealed a spectrum of mutations that is quite different from those observed in Caucasian populations. In 2014, Japan initiated nationwide newborn screening (NBS) for MCAD using tandem mass spectrometry (MS/MS). It is an urgent issue to assess the risk of acute metabolic decompensation from the respective novel mutations found thus far.MethodsTo evaluate the pathogenic effect of each mutation, we established a eukaryotic cell expression system and prepared 11 mutant proteins identified in five symptomatic patients and eight MS/MS-NBS-positive newborns, as well as two common Caucasian mutations, p.K329E (c.985G > A) and p.Y67H (c.157C > T) for comparison.ResultsThe expression of four mutant proteins (p.Q45R, p.P92L, p.P128X and p.Y397N) were severely impaired, whereas the others expressed normally, as did p.K329E and p.Y67H. Based on their dehydrogenase activities toward n-octanoyl-CoA, we determined three mutations (p.R53C, p.R281S and p.G362E) to be disease-causing, two mutations having (p.R17H and p.M274V) to be of marginal risk, and two mutations (p.K271E and p.I416T) as benign. Their allele-specific activities were as a whole in accordance with those estimated from the results of measurement in peripheral blood mononuclear cells.ConclusionAs most of the mutations detected in the Japanese population are unique, prudent genetic and enzymatic analysis is essential to precisely evaluate the latent risk of clinical onset for screening-positive newborns.