A case of N⁵, N¹⁰-methylenetetrahydrofolate reductase deficiency found by a pilot newborn screening for hypomethioninemia

Abstract

Current newborn screening (NBS) in Japan detects homocystinuria type 1 by elevated levels of methionine (Met), which is insufficient to detect homocystinuria types 2 and 3 that are associated with hypomethioninemia. We have conducted a pilot NBS for inborn errors of cobalamin metabolism in Hiroshima area of Japan since 2018, using the ratio of propionylcarnitine (C3) to Met (C3/Met) and low Met level as markers. We found a newborn female with low Met (9.54 μmol/L, cutoff <10 μmol/L) without elevated C3 or C3/Met in a dried blood specimen collected 4 days post-birth. Second-tier tests revealed slightly elevated total homocysteine (tHcy) (15.7 μmol/L, cutoff ≥5.0 μmol/L) but normal methylmalonic acid (MMA) levels. At 20 days of age, plasma tHcy rose to 52.0 μmol/L, with slightly elevated MMA (3.9 μmol/L, cutoff ≥1.0 μmol/L) and decreased serum vitamin B₁₂ (VB₁₂: 146 pg/mL, reference range 197–771 pg/mL). Gene panel analysis of related disorders detected two MTHFR variants: p.G149V (c. 447delinsTT) and p.P65 L (c.193T>C). 5,10-methylenetetrahydrofolate reductase (MTHFR) activity in lymphocytes and in a dried blood specimen were 56.27 μU/mg protein (8.40% of mean control value) and 50.19 mmol/min/L (11.5% of mean control value), respectively. On the basis of a diagnosis of MTHFR deficiency associated with mild VB₁₂ deficiency, we initiated betaine and mecobalamin administration. The successful detection of this mild case of MTHFR deficiency suggests a good sensitivity of our pilot NBS for the disease, while the combination of second-tier tHcy and MMA measurements is required to retain an appropriate level of specificity.