Newborn screening for homocystinurias: Recent recommendations versus current practice

PURPOSE

To assess how the current practice of newborn screening (NBS) for homocystinurias compares with published recommendations.

METHODS

Twenty-two of 32 NBS programmes from 18 countries screened for at least one form of homocystinuria. Centres provided pseudonymised NBS data from patients with cystathionine beta-synthase deficiency (CBSD, n = 19), methionine adenosyltransferase I/III deficiency (MATI/IIID, n = 28), combined remethylation disorder (cRMD, n = 56) and isolated remethylation disorder (iRMD), including methylenetetrahydrofolate reductase deficiency (MTHFRD) (n = 8). Markers and decision limits were converted to multiples of the median (MoM) to allow comparison between centres.

RESULTS

NBS programmes, algorithms and decision limits varied considerably. Only nine centres used the recommended second-tier marker total homocysteine (tHcy). The median decision limits of all centres were ≥ 2.35 for high and ≤ 0.44 MoM for low methionine, ≥ 1.95 for high and ≤ 0.47 MoM for low methionine/phenylalanine, ≥ 2.54 for high propionylcarnitine and ≥ 2.78 MoM for propionylcarnitine/acetylcarnitine. These decision limits alone had a 100%, 100%, 86% and 84% sensitivity for the detection of CBSD, MATI/IIID, iRMD and cRMD, respectively, but failed to detect six individuals with cRMD. To enhance sensitivity and decrease second-tier testing costs, we further adapted these decision limits using the data of 15 000 healthy newborns.

CONCLUSIONS

Due to the favorable outcome of early treated patients, NBS for homocystinurias is recommended. To improve NBS, decision limits should be revised considering the population median. Relevant markers should be combined; use of the postanalytical tools offered by the CLIR project (Collaborative Laboratory Integrated Reports, which considers, for example, birth weight and gestational age) is recommended. tHcy and methylmalonic acid should be implemented as second-tier markers.

[Combined methylmalonic acidemia and homocystinuria; a case report]

Combined methylmalonic acidemia and homocystinuria is an inborn error of metabolism of vitamin B12 or cobalamin. It's a rare autosomal recessive disease in which there are several variants depending on the pathogenesis of the metabolic disorder (cblC, cblD, cblF and cblJ). The more frequent and more severe is the cblC variant, which usually manifests in the first months of life, although some cases have been reported at the beginning of adulthood. A proper diagnosis and effective therapeutic approach is fundamental. We report the case of a patient of 18 years with a history of epilepsy who consults for acute renal failure requiring renal replacement therapy and diagnosed with combined methylmalonic acidemia and homocystinuria cblC variant.