Skewed X-chromosome inactivation in monochorionic diamniotic twin sisters results in severe and mild hemophilia A.

This study describes the genetic mechanisms responsible for the de novo occurrence of severe and mild hemophilia A in monozygotic twin females. Both twins were found to carry a previously known factor VIII mutation (Tyr16Cys) in the heterozygous state which most probably arose in the paternal germ line. Both twins showed concordant skewing of X inactivation toward the maternally derived normal X chromosome, the most severely affected twin exhibiting a higher percentage of inactivation of the normal X chromosome.

Somatic mosaicism in hemophilia A: a fairly common event.

Mutations in the large gene of clotting factor VIII (FVIII) are the most common events leading to severe human bleeding disorder. The high proportion of de novo mutations observed in this gene raises the possibility that a significant proportion of such mutations does not derive from a single germ cell but instead should be attributed to a germline or somatic mosaic originating from a mutation during early embryogenesis. The present study explores this hypothesis by using allele-specific PCR to analyze 61 families that included members who had sporadic severe hemophilia A and known FVIII gene defects.

De novo factor VIII gene intron 22 inversion in a female carrier presents as a somatic mosaicism.

The intron 22 inversion represents the most prevalent factor VIII gene defect in severe hemophilia A, accounting for about 40% of all mutations. It is hypothesized that the inversion mutations occur almost exclusively in germ cells during meiotic cell division by intrachromosomal recombination between 1 of 2 telomeric copies of the Int22h region and its intragenic homologue. The majority of inversion mutations originate in male germ cells, where the lack of bivalent formation may facilitate flipping of the telomeric end of the single X chromosome.