Current strategy for genetic analysis of haemophilia A families

Carrier detection and prenatal diagnosis of haemophilia A, which was based in the last decade mainly on linkage polymorphism analysis, has been greatly facilitated by the recent discovery that two types of inversion disrupting the factor VIII gene are common mutations observed in 42-48% of severe haemophilia A cases. In this study DNA analysis was performed in 64 unrelated severe haemophilia A patients and 173 women belonging to their families, and in four women from a family with a deceased haemophilic relative whose DNA was unavailable (a total of 177 females from 65 unrelated families). Factor VIII gene inversions were found in 32 out of the 65 families (49%), 29 involving recombination with the distal A gene and three with the proximal A gene. Definitive information regarding carriership of haemophilia was provided to all 81 women belonging to the 32 inversion-positive families, among them one woman previously uninformative for any of the polymorphisms examined, five women who were informative only for extragenic polymorphisms, and four suspected carriers who were relatives of the deceased haemophiliac. In 33 inversion-negative families, 96 females were examined by analysis of the BclI restriction fragment length polymorphism (RFLP) in intron 18 and of the multiallelic dinucleotide repeats in introns 13 and 22, followed by analysis of other intragenic polymorphisms. This procedure yielded an informativity rate of almost 100%. Of the 96 females examined by linkage polymorphism analysis, 78 belonged to 25 families with more than one haemophiliac and 29 of them were obligate carriers. In 47 of the 49 suspected carriers linkage polymorphism analysis enabled definition of carriership based on intragenic polymorphisms. 18 of the 96 females belonged to eight families with sporadic haemophilia cases and only eight of the 18 suspected carriers could be diagnosed by exclusion. In nine pregnant women carrying factor VIII gene inversions, mRNA extracted from chorionic villus samples (CVS) was analysed for factor VIII gene inversion by reverse transcription/polymerase chain reaction (RT/PCR). This procedure enabled rapid prenatal diagnoses in six male fetuses. Taken together, our data indicate that a high rate of informativity and carrier definition is possible by the strategy of first screening for factor VIII gene inversions, and, if none are found, sequential use of highly informative intragenic polymorphisms, followed by less informative intragenic and extragenic polymorphisms.