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. The presence of somatic mosaicisms of varying degrees (0.2%-25%) could be shown in 8 (13%) of the 61 families and has been confirmed by a mutation-enrichment procedure. All mosaics were found in families with point mutations (8 [25%] of 32 families). In the subgroup of 8 families with CpG transitions, the percentage with mosaicism increased to 50% (4 of 8 families). In contrast, no mosaics were observed in 13 families with small deletions/insertions or in 16 families with intron 22 inversions. Our data suggest that mosaicism may represent a fairly common event in hemophilia A. As a consequence, risk assessment in genetic counseling should include consideration of the possibility of somatic mosaicism in families with apparently de novo mutations, especially families with the subtype of point mutations.

von Willebrand disease (vWD) is a frequent and heterogeneous bleeding disorder which is characterized by quantitative and/or qualitative abnormalities of von Willebrand factor (vwF). vwF is a high molecular weight (HMw), multimeric glycoprotein, which carries factor VIII (FVIII) and mediates platelet adhesion and aggregation ar high shear rates (l,Z). The vwF gene, of 178 kb, contains 52 exons. A precursor protein, pre-pro-vWF, containing four types of repeating domains (A to D), is synthesized in endothelial cells and megakaryocytes (Fig. 1). Disulphide bridges involved in the dimerization and multim erization of vWF are located in the C-terminal part and D3 domain, respectively. vWF multimers are composed of mature vWF subunits of 270 kDa corresponding to the 2050 C-terminal aa of pre-pro-vWF. These subunits contain two remarkable disulphide loops of l g5 aa in Al and ,{3 domains, an important proteolytic site within ,{2 domain, between aa 842 and 843, and an RGD sequence in Cl domain. The functions of vWF are related to the presence along the 270 kDa subunit of a series of binding sites for FVIII, collagen, platelet glycoproteins (GP) GPIb and GPIIb/IIIa (Fig.1). The multimerization of vWF is also essential for its role in platelet adhesion and aggregation.

A large number of different mutations in the factor VIII (F8) gene have been identified as a cause of haemophilia A. This compilation lists known single base-pair substitutions, deletions and insertions in the F8 gene and reviews the status of the inversional events which account for a substantial proportion of mutations causing severe haemophilia A.