Comparison of phenotypic assessment and the use of two restriction fragment length polymorphisms in the diagnosis of the carrier state in haemophilia B.

Carrier detection in haemophilia B has previously involved pedigree analysis and assessment of the coagulation phenotype. At best using these methods, carrier evaluation may be made with about 80% certainty (Orstavik et al, 1981). With isolation and cloning of the factor IX gene, DNA probes are now available to detect intragenic nucleotide changes. This study uses one such probe which detects restriction fragment length polymorphisms with the restriction endonucleases Taq 1 and Xmn 1. Seventy-eight individuals from eight haemophilia B kindred were initially evaluated for factor IXC and factor IXAg levels. DNA from these individuals was then digested with the two restriction enzymes and after Southern blotting, analysed for restriction fragment length polymorphisms (RFLPs) with the radiolabelled genomic probe. Four kindred proved informative with RFLP linkage assessment. In three families evaluation was possible with both Taq 1 and Xmn 1 polymorphic markers but in the fourth pedigree only Xmn 1 was informative. Using these techniques five potential carriers have been definitively assigned as either normal or carriers of the haemophilia B gene defect. Problems of phenotypic assessment are well illustrated in pedigree 4 where polymorphic linkage positively identified a carrier whose coagulation data were normal.