Characterization of factor VIII gene inversions using a non-radioactive detection method: a survey of 102 unrelated haemophilia A families from northern France.

Intrachromosomal rearrangements of the long arm of chromosome X, between gene A (F8A) in intron 22 of the factor VIII gene and one of its two telomeric copies, are responsible for about half of the severe cases of haemophilia A. A group of 98 unrelated patients from Northern France with moderate to severe haemophilia A was screened for this gene inversion using a non-radioactive Southern blotting method. Whereas none of the 18 moderately affected patients presented the FVIII gene rearrangement, gene inversion was found in 38 (48%) of the 80 severe haemophilia A patients. Recombinations involving the distal copy of gene A (group 1) were more frequent (79%) than those involving the proximal copy (group 2). Individual variation in the number of gene A copies on the X chromosome probably explains an alternative Southern blot profile, referred to as group 3 inversion, which was observed in one of our patients. In the severely affected patients, neither the prevalence of inhibitor development nor the frequency of sporadic cases differed significantly in the group presenting gene inversion as compared to the group without chromosomal rearrangement. Study of four families where no patient was available enabled in one case direct carrier detection and prenatal diagnosis in the absence of an affected member. The Southern blotting technique described in the present work is relevant to about 50% of cases of severe haemophilia A, can be performed without use of a radiolabelled probe and represents a major advance in the diagnosis of the disease.