Post- and prenatal diagnostic methods for the homocystinurias.

[homocystinuria without methylmalonic aciduria]

Diagnosis of the homozygous homocystinurias can be performed by investigations at the metabolite , enzyme and DNA level . The existence of variant forms due to the wide range of genetic variation may result in only small differences in various parameters between controls and affected subjects . 1 . Sulphur amino acid concentrations in plasma , especially total homocysteine , are useful in first line diagnostic investigations . 2 . Cystathionine-beta -synthase (CBS ), methylenetetrahydrofolate reductase (MTHFR ) and methylfolate homocysteine methyltransferase (MFMT ) can be directly assayed in many tissues including fibroblasts (each ) and blood cells (except CBS ). Indirect whole cell assays which measure pathway activity dependent on a particular enzyme can provide useful diagnostic information . 3 . Direct analysis of mutations is available for CBS , MTHFR and recently also for MFMT deficiencies . However the existence of a larger number of very rare , often private , mutations limits the usefulness of this approach in routine diagnosis . The above diagnostic approaches can generally be applied to prenatal diagnosis . Measurement of methylmalonic acid and other metabolites in amniotic fluid by stable isotope dilution / gas chromatography-mass spectrometry is well established for the methylmalonic acidurias . This method has also been applied to combined homocystinuria /methylmalonic aciduria supported by enzyme assays in cultured cells . Total homocysteine measurement in cell free amniotic fluid is also possible , performed so far in 14 cases with two affected fetuses . The indirect assay of methionine formation from [14 C ] labelled formate in intact cultured amniotic fluid cells has been for prenatal diagnosis of the remethylation defects .