IDENTIFICATION OF 2 POINT MUTATIONS IN THE VONWILLEBRAND-FACTOR GENE OF 3 FAMILIES WITH THE NORMANDY VARIANT OF VONWILLEBRAND DISEASE

Plasma von Willebrand factor (vWf) is a multidomain multimerized glycoprotein which has a dual role in haemostasis: it promotes platelet adhesion to subendothelium and is the carrier of blood coagulation factor VIII (FVIII). We previously characterized a functional defect of vWf, limited to its ability to bind FVIII, in two families whose affected members have the same phenotype that mimics mild haemophilia A and was tentatively named von Willebrand's disease (vWD) 'Normandy'. A homozygous point mutation C --> T converting Thr 28 to Met in mature vWf subunit was identified in one of these patients who was born of third-cousin parents. In the present studies we report two unrelated new cases of vWD 'Normandy' and characterize, using the analysis of the vWf gene intron 40 region containing a variable number of tandem repeats, the recessive inheritance of the disease in two affected families without known consanguinity. Exons 18-24 of the vWf gene encoding for the first 311 amino acids of mature vWf subunit were amplified by the polymerase chain reaction method and sequenced. Two new missense mutations, both corresponding to a C --> T transition and predicting respectively an Arg 53 --> Trp and an Arg 91 --> Gln substitution, were characterized. The three patients from family 1 were homozygous for the first-mentioned mutation while the patient from family 3 was homozygous for the second. The patient from family 2 was found a compound heterozygote for the two mutations. None of the two point mutations reported, both destroying a MspI restriction site, could be detected in DNA from 50 normal controls screened by restriction endonuclease analysis. Our data show that different mutations may be found in patients with the 'Normandy' phenotype. The mutations characterized so far are all localized on the N-terminal region of mature vWf subunit, within or near the major FVIII binding domain, and some of them occur within the epitope of monoclonal antibodies inhibiting the vWf/FVIII interaction. These observations suggest a causal relationship between these mutations and the vWD 'Normandy' phenotype.