Screening for nonsense mutations in patients with severe hemophilia A can provide rapid, direct carrier detection

Despite marked genetic heterogeneity in families with hemophilic patients, transition mutations in CG dinucleotides occur frequently. Of 71 CG dinucleotides in the factor VIII cDNA, a C-to-T transition in 12 would lead to a new Stop codon (CGA to TGA). Using restriction enzyme digestion of 11 amplified DNA fragments, seven point mutations were localized among 60 patients with severe hemophilia A. Five were detected as loss of a natural or introduced TaqI site at codons -5, 583, 1941, 2116, and 2209 and were confirmed as CGA (Arg) to TGA (Stop) nonsense mutations by DNA sequencing. A novel C-to-T nonsense mutation was detected as loss of the RsaI site at codon 1966 and confirmed by sequence in two unrelated individuals. Two partial gene deletions were detected as selective failure to amplify exon 1 and exons 15-22, respectively. In an additional (61st) patient who was subsequently found to have mild (instead of severe) hemophilia, digests suggested a mutation in codon 1696. Upon sequencing, this codon contained a novel missense mutation, a C-to-G transversion changing CGA (Arg 1696) to GGA (Gly). In four families with women available for testing, carrier status was rapidly determined by direct screening for the point mutation. In two of three with sporadic occurrences, the mother was a carrier as were two of four sisters. In the other family, the mother and a sister were homozygous for the TaqI cleavage site in their amplified exon 24 fragment, indicating a de novo C-to-T transition in codon 2209 in the patient's factor VIII gene. This final patient's sister was a noncarrier even though by linkage analysis she inherited the same factor VIII gene as her brother.