Carrier detection in hemophilia using pedigree analysis coagulation tests and DNA probes

Hemophilia A and B are hereditary X-linked recessive bleeding disorders due to an anomaly or absence of the gene coding for coagulation factors VIII or IX. Until recently, carrier detection was performed on standard pedigree analysis and clotting factor assays. Due to lyonisation, the results obtained by these methods were only probabilistic. Recombinant DNA procedures have now been applied to the identification of molecular defects and carrier detection in inherited diseases. Because of the great heterogeneity of the molecular defects in hemophilia, the diagnosis of carrier status has to be made by the study of restriction fragment length polymorphisms (RFLP) genetically linked to factor VIII or factor IX genes. In a large number of cases, gene probing provides certain diagnosis. We studied some 300 individuals belonging to 70 families with hemophilia A or B. We used two probes to explore hemophilia A: an intragenic probe, p114.12, which detects an RFLP with the enzyme BclI and the extragenic polymorphic probe, St 14, which reveals an RFLP with the enzyme TaqI. For hemophilia B a genomic probe comprising exons b, c, d was used to detect an RFLP linked to a TaqI site. Despite the risk of recombination due to its extragenic location, the St 14 probe proved to be very useful because of the high informativity obtained in the families with hemophilia A. In contrast, the low informativity of the factor IX probe necessitates a search for other RFLPs in or near the factor IX gene. A comparison of the different methods used for carrier detection showed the possibility of misdiagnosis when using only pedigree analysis and biologic data and the improved certainty of diagnosis by gene probing.