ANALYSIS OF FACTOR-VIII GENE INVERSION MUTATIONS IN 166 UNRELATED HEMOPHILIA-A FAMILIES - FREQUENCY AND UTILITY IN GENETIC-COUNSELING

Haemophilia A is an X-linked recessive bleeding disorder of variable s everity that is caused by a deficiency of coagulation factor VIII (FVI II). The disease results from mutations in the FVIII gene which are he terogenous both in type and position within the gene. Recently, howeve r, inversion mutations were found to be common to patients with severe disease (Lakich et al., 1993). These mutations result from intrachrom osomal recombinations between DNA sequences in the A gene (located in intron 22 of the FVIII gene) and one of two A genes upstream to the FV III gene. To determine the frequency of these inversions we performed Southern blot analysis on banked DNA from 166 consecutive, unrelated h aemophilia A families previously referred for carrier or prenatal test ing. In 57/166 (34%) families an inversion or other unique mutation wa s detected. The distal and proximal A genes lying upstream to the FVII I gene were involved in 79% and 18% of the mutations, respectively, bu t in 3% of the families the sequences involved in the mutation have no t been identified. In 20/38 (53%) families with severe disease a mutat ion was detected. Interestingly, the relative risk of developing inhib itors in patients with FVIII gene inversions or other 3' mutations det ected by this assay, as compared to patients with no detectable mutati on by this assay, was 3.8. In families for which a mutation is detecte d, direct DNA testing is an accurate and inexpensive alternative to li nkage analysis for prenatal or haemophilia A carrier testing.