A new asymptomatic case of methylmalonic acidemia (MMA) identified by MS/MS newborn screening

A newborn screening pilot study using tandem mass spectrometry has been set up since 2003. To date more than 15000 newborns have been screened. We reported a newborn affected by benign MMA identified by MS/MS. The analysis of derivatized acylcarnitines and amino acids in dried blood spot was performed using PerkinElmer NeoGram MS2 Kit. The analysis of patient’s blood sample obtained at 3 days of life showed an elevation of propionylcarnitine (C3: 4.38 μM) slightly below the cut-off value (4.6 μM) and a C3/C2 ratio higher than cutoff ( C3/C2 0.20, cutoff 0.18), therefore a repeat of blood spot collection was requested. This second sample, obtained at the age of 28 days, showed a marked increase of C3 and C3/C2. The organic acid analysis in urine demonstrated an increase of methylmalonic acid. The (hydroxy)cobalamin administration (loading) test was carried out and no significant reduction of methylmalonic acid excretion in urine was found. Therefore a deficiency of methylmalonyl-CoA mutase (EC 5.4.99.2) enzyme was suspected. Molecular genetic analysis was performed on the 13 exons and intron- exon boundaries of methylmalonyl-CoA mutase gene (c.DNA NM_00025; g.DNA NT_007592) were analyzed by direct sequencing. The patient resulted compound heterozygote for two already described mutations: N219Y (c.655A>T) and R694W (c.2080C>T). The mutations were confirmed in the parents. The N219Y is a quite frequent mutation in MMA (19% alleles in Caucasians). It was already described associated to severe phenotype (mut0) with low or absent residual enzymatic activity. The Asn 219 is a conserved amino acid and is located at the fourth β strand of the substrate binding (α/β)8 barrel. Modelling analysis suggests that this mutation gives impaired folding and/or poor stability of the protein. The R264W mutation was associated to a milder phenotype (mut-). This residue is located in the protein pocket binding dimethylbenzimidazole portion of cobalamin molecule. Being the protein an homodimer, we suppose that the compound heterozigosity mut0/mut- gives enough molecules with residual enzymatic activity to manifest a milder phenotype. Conclusions: 1- Benign MMAs can be identified using MS/MS in newborn screening. MMA disorders may not produce significant concentrations of C3 and will not be detected. The evaluation of C3/C2 ratio is important to reduce the number of false positive and false negative results. 2- Molecular analysis could be an important tool to predict clinical phenotype 3-The early detection and therapy have had a favorable effect on prevention of metabolic decompensation. However, a long-term study is necessary to assess whether the early detection and intervention may improve the outcome.