CARRIER DETECTION AND PRENATAL DIAGNOSIS IN HAEMOPHILIA A BY GENE ANALYSIS

Restriction fragment length polymorphism (RFLP) analysis has been used to perform family studies, including prenatal diagnosis, in 21 haemophilia A kindred.Two intragenomic RFLPs were studied in conjunction with one linked RFLP. The intragenomic BgII RFLP,situation 3' to exon 26 was detected with cDNA probe C (Genetics Institute) giving bands of 20kb (17% of X chromosomes) and 5kb (83%), and the intragenomic Bell RFLP, situated 3' to exon 18, was detected with the genomic DNA probe pi 14.12 from Genentech. The frequency of this RFLP in the local population was 23% (1.1 kb allele) and 77% (0.88kb allele). The linked probe DXS15 (DX13) was used to detect a Bglll RFLPwith alleles of 5.8kb (45%) and 2.8kkb (55%). A recombination rate of approximately 5% has been estimated between the factor VIII and DXS15 loci

Carrier studies were performed in 15 kindreds. 25 obligate carriers were identified and of these, 20 were potentially informative (heterozygous and phase known) for at least 1 RFLP (9 for Bgll, 9 for Bell and 7 for BgIII). 34 possible carriers were studied, of which 13 were diagnosed as normal (6 by BgII, 6 by Bell and 5 by BgIII). 17 were diagnosed as carriers (2 by Bgll, 12 by Bell and 10 by Bglll) and diagnosis was not possible in a further 4 cases. Of these diagnosed as carriers 3 were non-informative for all RFLPs, and 14informative for at least one RFLP (3 by Bgll, 8 by Bell and 8 with BgIII).Prenatal diagnosis was attempted by analysis of DNA extracted by chorionic villussampling in 6 cases of male fetuses at risk of havinghaemophilia A. 1 fetus was diagnosed as being affected (Bell) and was electively terminated. Three otherfetuses were diagnosed as normal by the BgIII/DXS15 RFLP, but the two intragenomic RFLPs were non-informative. Because of the possibility of a crossover allthree patients opted for mid-trimester fetoscopy andmeasurement of fetal factor VIII at Kings College Hospital, London (Dr Reuben Mibashan), where the diagnoses were confirmed. In the 4th case a normal fetus was diagnosed by the Bgll RFLP analysis, but a spontaneous abortion at 12 weeks prevented confirmation of this result. In the final case of twin male fetuses, none of the RFLPs was informative and both were diagnosed as normal by fetal blood sampling at fetoscopy.