Genetic diagnosis of haemophilia A of Chinese origin

Genetic diagnosis of haemophilia A has been studied in two aspects. One is to directly identify the mutations in the factor VIII genes of the affected probands, and the other is to examine the usefulness of several intragenic factor VIII markers for gene tracking. Direct mutational analysis by PCR-SSCP (polymerase chain reaction--single-strand conformation polymorphism) has been accomplished previously in 87 haemophilia A patients, accounting for nearly 10% of cases in Taiwan. Of the 87 cases, 46% were with point mutations, short deletions or insertions, and most of the remaining were with gene inversion readily identified by Southern blotting. Further examination of 112 patients has estimated a 33% incidence for gene inversion in all the patients with haemophilia A, or 37% in severe cases. Since the direct mutational detection described above cannot be used in all Chinese families with haemophilia A, genetic markers were also investigated. The two CA repeat markers located at intron 13 (CA-13) and intron 22 (CA-22), respectively, were amplified and analysed simultaneously. Seven different alleles with 18-24 CAs have been identified for CA-13. Alleles of 20 and 21 CAs are the most common and their population frequency was 0.68 and 0.24, respectively. The CA-22 marker contained a repetition of (GT)n(AG)n as was identified in the white European but not in the Canadian population. Alleles with 25 and 26 GT/AGs account for 18% and 75% of this group of samples, respectively. The expected rate of heterozygosity for either CA markers was 68%, although a value of 57% was observed by haplotype analysis, indicating an association of the two repeat markers. Nevertheless, the study of 62 females showed that with the combined use of CA-13 and CA-22 with BclI, approximately 71% would be informative for these markers. This number may increase to 81% if XbaI polymorphism is added. We propose that a better genetic diagnosis procedure for Chinese individuals would be first to look for the inversion mutation, secondly for one of the intragenic markers, and then at the PCR-SSCP analysis.