VARIABLE HYPERHOMOCYSTEINAEMIA PHENOTYPE IN HETEROZYGOTES FOR THE GLY307SER MUTATION IN CYSTATHIONINE BETA-SYNTHASE

Background: A deficiency of cystathionine beta-synthase (CBS) activity is the most frequent cause of homocystinuria, an autosomal recessive disease with multiple clinical manifestations. Mutations in the CBS ge ne have been reported in several patients with homocystinuria. Aims: T o establish the molecular basis of CBS deficiency in a female patient with pyridoxine non-responsive homocystinuria, and to apply the findin gs to genetic screening of her family members. Methods: The entire cod ing region of the CBS cDNA was amplified by PCR and used for direct se quence analysis. Mutant alleles were confirmed by direct sequence anal ysis of PCR-amplified genomic DNA, and by a combination of single stra nd conformation polymorphism and temperature gradient gel electrophore sis analysis. Results: The proband was homozygous for a G(9I9)A base t ransition which predicts the substitution of serine for glycine at cod on 307 in the CBS protein (G307S). The parents (both of Irish backgrou nd) were heterozygotes for the C307S allele, while an asymptomatic sib ling had normal CBS sequence. Plasma homocysteine, assessed after an o ral methionine load, indicated the mother clearly had mo derate hyperh omocysteinaemia, whereas the father had normal concentrations of homoc ysteine. This is the first report of a normal methionine load test in a proven heterozygote for a CBS mutation which causes severe homocysti nuria in the homozygote. Other factor(s) may have contributed to hyper homocysteinaemia in the mother. The G307S allele has been reported in other patients and appears to be a common allele among families of Cel tic origin.