Molecular basis of cystathionine {beta}-synthase deficiency in pyridoxine responsive and nonresponsive homocystinuria

Cystathionine ß-synthase (CBS) deficiency is an autosomalrecessive disorder associated with multisystem clinical disease.We analyzed PCR amplified products from patients' RNA and genomicDNA. Direct sequencing of the entire coding region of the CBSgene revealed a G-919 to A transition in exon 8, resulting inreplacement of Gly 307 by Ser (G307S) in the protein. The mutationwas detected in one allele of patient L171 of French/Scottishancestry and in both alleles of patient L198 of irish ancestry.Amplifying and sequencing exon 8 from the genomic DNA showedthat both parents of L198 were heterozygotes for G307S. Thepathogenicity of the mutation was demonstrated in an expressionexperiment. The mutant protein was apparently stable in E.collextracts and lacked catalytic activity. Sequencing of exon 8revealed the G307S mutation in five additional families. Allpatients have pyridoxine nonresponsive homocystinuria. We havenow observed this mutation in 9 of 52 apparently unrelated allelesof varied ethnic backgrounds. All 9 are from patients with Celtic(Irlsh/English/Scottish/French) ancestry in either one or bothparents. The G307S mutation was detected in 50% (9 of 18) ofthe Celtic alleles in our series. The second mutation foundin exon 8 is the 1278T mutation, which was described previouslyin one allele of a pyridoxine responsive patient. This missensemutation was detected in one allele of a pyridoxine nonresponsivepatient and in both alleles of a pyridoxine responsive patient.The latter suggests that 1278T is probably associated with pyridoxineresponsiveness.