Carrier Detection in Severe (Type III) von Willebrand Disease Using Two Intragenic Restriction Fragment Length Polymorphisms

The molecular biology of von Willebrand disease

von Willebrand disease (VWD) is a common autosomally inherited bleeding disorder associated with mucosal or trauma-related bleeding in affected individuals. VWD results from either a quantitative or qualitative deficiency of von Willebrand factor (VWF)--a glycoprotein with essential roles in primary haemostasis and as a carrier of coagulation factor VIII (FVIII) in the circulation. In recent years the identification of mutations in the VWF gene in patients with VWD has improved our understanding of the structure and function of the VWF protein, and has illustrated the importance of specific regions of VWF for its interaction with other components of the vasculature. The underlying genetic lesions and associated molecular pathology have been identified in many cases of type 2A, type 2B, type 2M, type 2N and type 3 VWD. However in the most common variant, type 1 VWD, the causative molecular defect is unknown in the large majority of cases. In the absence of an understanding of the molecular pathology underlying type 1 VWD, precise diagnosis and classification of this common disorder remains problematic.

Congenital von Willebrand disease type 3: clinical manifestations, pathophysiology and molecular biology

von Willebrand disease type 3 is the most severe form of this condition. Patients present with a moderate-to-severe bleeding tendency. The plasma von Willebrand factor level in these patients is very low or undetectable. Although rare, von Willebrand disease type 3 is of major interest because of its severe clinical presentation, the need for replacement therapy and the risk of occurrence of alloantibodies after the infusion of plasma concentrates. The inheritance of type 3 disease is typically autosomal recessive. The parents are often consanguineous, although compound heterozygous inheritance does occur. The molecular basis of von Willebrand disease type 3 has recently been studied in detail, several molecular defects being identified. This chapter will focus on the clinical and molecular aspects of type 3 von Willebrand disease.

von Willebrand disease

Considerable progress has been made in characterizing the specific molecular defects responsible for the heterogeneous disorder known as von Willebrand disease (VWD). A large number of molecular defects have been identified and precise characterization may now be possible in the majority of type 2A, type 2B, type 2N, and potentially also type 3 VWD cases. However, the most common variant, type 1 VWD, still remains a major challenge. Continued progress in this area will improve our understanding of the pathogenesis of VWD and lead to more rapid and precise diagnosis and classification for this common disorder. The problems of incomplete VWD penetrance and poor diagnostic sensitivity and accuracy for the currently available clinical laboratory tests provide strong incentives for the development of DNA-based diagnostics. In addition, prenatal diagnosis is now possible either at the level of single point mutations (for some subtypes) or by RFLP analysis (assuming linkage to the von Willebrand factor [VWF] gene) and will probably be applied with increasing frequency for VWD type 3 (17, 133, 175). Understanding the molecular basis of VWD also has important implications for VWF structure and function and is helping to define critical binding domains within the VWF molecule. Insights gained from these studies may eventually lead to improved therapeutic approaches not only for VWD, but also for a variety of other genetic and acquired hemorrhagic and thrombotic disorders.

A novel modifier gene for plasma von Willebrand factor level maps to distal mouse chromosome 11

Type 1 von Willebrand disease (VWD), characterized by reduced levels of plasma von Willebrand factor (VWF), is the most common inherited bleeding disorder in humans. Penetrance of VWD is incomplete, and expression of the bleeding phenotype is highly variable. In addition, plasma VWF levels vary widely among normal individuals. To identify genes that influence VWF level, we analyzed a genetic cross between RIIIS/J and CASA/Rk, two strains of mice that exhibit a 20-fold difference in plasma VWF level. DNA samples from F2 progeny demonstrating either extremely high or extremely low plasma VWF levels were pooled and genotyped for 41 markers spanning the autosomal genome. A novel locus accounting for 63% of the total variance in VWF level was mapped to distal mouse chromosome 11, which is distinct from the murine Vwf locus on chromosome 6. We designated this locus Mvwf for "modifier of VWF." Additional genotyping of as many as 2407 meioses established a high resolution genetic map with gene order Cola1-Itg3a-Ngfr-Mvwf/Gip-Hoxb9-Hoxb1++ +-Cbx'rs2-Cox5a-Gfap. The Mvwf candidate interval between Ngfr and Hoxb9 is approximately 0.5 centimorgan (cM). These results demonstrate that a single dominant gene accounts for the low VWF phenotype of RIIIS/J mice in crosses with several other strains. The pattern of inheritance suggests a gain-of-function mutation in a unique component of VWF biosynthesis or processing. Characterization of the human homologue for Mvwf may have relevance for a subset of type 1 VWD cases and may define an important genetic factor modifying penetrance and expression of mutations at the VWF locus.

von Willebrand disease in children and adolescents

The term von Willebrand disease includes many bleeding disorders caused by abnormalities of vWF. Frequent or severe bleeding may be indicative of vWD or other bleeding conditions. Primary care practitioners need to be familiar with vWD and evaluate possibly affected individuals with appropriate laboratory studies. Patients with vWD should be educated about their disorder and preventive measures to limit its effect. Medications are available that can treat or prevent bleeding complications for most patients with vWD. Intervention with blood products is occasionally necessary.

Detection of a molecular defect in 40 of 44 patients with haemophilia B by PCR and denaturing gradient gel electrophoresis

Oligonucleotides were computer designed to amplify by the polymerase chain reaction (PCR) the coding region, splice junctions, 112 bp of the 5' flanking region and 279 bp surrounding the polyadenylation site of the factor IX gene for analysis by denaturing gradient gel electrophoresis (DGGE). Forty-four unselected haemophilia B patients were studied of whom 24 had severe haemophilia and 20 had a mild to moderate form of the disease. Potential mutations were identified in 40 (91%) of the 44 cases. A defect could not be detected in three severe and one mild haemophiliac by DGGE analysis and direct sequencing of all the PCR fragments from these patients revealed no nucleotide alteration supporting the DGGE results. A total of 37 point mutations, two complete gene deletions and a duplication of 26 bp were found. The 37 point mutations included 35 single nucleotide substitutions, a deletion and an insertion of one nucleotide. The 35 single nucleotide substitutions included 26 missense mutations, seven nonsense mutations, a G (-6) to A transition in the promoter region and a G (30154) to A transition within the donor splice site of the last intron. Fifteen of these nucleotide substitutions involved CpG dinucleotides. Fifteen point mutations were found at codons where nucleotide substitutions had not been detected before. An insertion of a single nucleotide T at position 6370 and deletion of a G at nucleotide 30845 resulted in frameshift mutations creating stop codons at amino acid positions -2 and 250, respectively. A duplication of 26 bp (17747-17772) in exon V was found in a severe haemophilia patient resulting in a termination codon in exon VI. The detection of the mutation by the combined use of PCR, DGGE and direct sequencing was important for carrier diagnosis of 20 families with no prior history of haemophilia B.

Defects in type IIA von Willebrand disease: a cysteine 509 to arginine substitution in the mature von Willebrand factor disrupts a disulphide loop involved in the interaction with platelet glycoprotein Ib-IX

Type IIA von Willebrand disease (vWD) is characterized by the loss of high and intermediate weight multimers of von Willebrand factor (vWF) from plasma. The 3' end of exon 28 in the vWF gene from four type IIA vWD patients was amplified by the polymerase chain reaction, cloned and sequenced. Sequencing identified two potential missense mutations resulting in the amino acid substitutions Arg 834-->Gln and Glu 875-->Lys in the mature vWF subunit within an area of vWF where mutations in type IIA vWD have been reported. Neither of these amino acid substitutions was found in over 100 normal alleles tested by allele specific oligonucleotide hybridization. A polymorphism (Val 802-->Leu) was identified in another patient. Other areas of exon 28 were analysed by denaturing gradient gel electrophoresis (DGGE) and DNA from one patient demonstrated an irregular DGGE pattern on the 5' end of the exon. Sequencing demonstrated an amino acid substitution of an arginine for cysteine at position 509 adjacent to an area of vWF where defects associated with type IIB vWD have been found. This substitution was not found in 100 normal chromosomes tested by restriction enzyme digestion. The Cys 509-->Arg substitution eliminates an intramolecular disulphide bridge formed by Cys 509 and Cys 695 which is important to maintain the configuration of vWF functional domains that interact with platelet glycoprotein Ib-IX.

Characterization of the pseudogenic and genic homologous regions of von Willebrand factor

The homologous pseudogenic and genic regions of von Willebrand factor (vWF) were studied in DNA from a patient with homozygous deletion of vWF genes and compared with a normal control. This analysis indicates informative restriction patterns for the investigation of restriction fragment length polymorphisms (RFLPs) and gene lesions, and for molecular cloning. A useful new genic XbaI RFLP was found and characterized. A large BgIII fragment of the pseudogenic region was cloned and mapped, and single sequences (9 kb) were used as probes. Corresponding genic and pseudogenic fragments, which contain exons 23-28, and specific restriction patterns were identified, including a new polymorphic TaqI site that was mapped in the gene. A cloned fragment contains the 5' boundary of the pseudogene and recognizes an additional and unknown homologous sequence in the genome. The chromosomal localization of the vWF pseudogene and of the breakpoint cluster region (BCR) gene were compared by 'in situ' hybridization: overlapping patterns were detected. The cloning, characterization and mapping of the pseudogenic region improves the analysis of this portion of chromosome 22 affected by several somatic and constitutional alterations, and also of the corresponding genic region on chromosome 12.

An infrequent DNA polymorphism associated with severe von Willebrand's disease

Genomic DNA of six unrelated Dutch patients with severe von Willebrand's disease (vWD) was submitted to restriction fragment length polymorphism analysis. We observed a strong association between a 36 kb allele detected by a partial complementary DNA probe (pvWF 1100) and the restriction enzyme XbaI with severe von Willebrand's disease. This 36 kb allele is rare (allele frequency of 7%) both in the general population and in patients with autosomal dominant types of von Willebrand's disease. Three of our six patients were found to be homozygous for this allele while two others were heterozygous. The association of this rare XbaI allele with severe vWD enables carrier detection and prenatal diagnosis in these families. The high frequency (67%) of the 36 kb allele observed in this patient group raises the possibility that a subgroup of patients with severe vWD has a genetic defect with a common origin.

von Willebrand factor and its disorders: an overview of recent molecular studies

von Willebrand factor (vWF) is a plasma protein with multiple functions in haemostasis. The vWF gene, located on chromosome 12p2.1, encodes a primary gene product of 2813 amino acids. Post-translational modification, assembly and secretion of vWF are highly complex. The pro vWF promoter is covalently linked by intermolecular disulphide bonds to form a dimer of MW approximately 440 kDa. This then polymerises to form multimers ranging in MW from 1-20 x 10(6). Simultaneously the pro piece of vWF is cleaved, releasing a 741 amino acid peptide known as vW Ag II from the polymerised protomers. Two distinct secretion pathways are found in the endothelial cell, a regulated pathway with storage in Weibl-Palade bodies and a constitutive pathway. Platelets store vWF in their alpha-granules. Mature vWF participates in platelet adhesion, spreading and aggregation and is a carrier of factor VIII, protecting the latter from degradation. Disorders of vWF are highly diverse. At least 20 subtypes of von Willebrand's disease have been described to date, based on features of the vWF present in or absent from patients plasma and platelets. Some patients have reduced amounts of apparently normal vWF whilst others have clearly abnormal vWF with aberrant structure and function. Rare patients virtually or completely lack vWF. The genetic and structural basis of some of these abnormalities is just beginning to emerge. This article outlines the molecular biology and physiology of vWF, and reviews some recent progress on the molecular pathology and genetics of von Willebrand's disease.

von Willebrand factor and platelet function

vWF is an adhesive protein that binds to two distinct platelet glycoproteins, GP Ib and GP IIb-IIa complex. Its interaction with GP Ib is primarily responsible for platelet adhesion to the subendothelium. The current model is that vWF binds to collagen and/or another component of the subendothelium, after which a conformational change in the vWF molecule exposes the GP Ib binding site. This interaction may not only promote the initial attachment of platelets to the subendothelium but also play a role in thrombus formation through exposure of GP IIb-IIIa to which vWF and fibrinogen can bind. The second important function of vWF is to be a carrier for F. VIII, protecting it from degradation and playing a role in its activation by thrombin. Circulating vWF has a complex multimeric structure that ranges in Mrs from 0.5 to 20 x 10(6) Daltons. The basic subunit has a Mr of 270 kDa. Amino acid sequencing of vWF demonstrated that the basic subunit or mature vWF is made up of 2050 amino acids. Molecular cloning of the vWF cDNA revealed that the primary transcript consists of 8900 base pairs that encode for 2813 amino acids, including a 22 amino acid signal peptide and a propolypeptide of 741 amino acids, called vWF antigen II. Recent studies on the expression of recombinant vWF molecules indicate that the propolypeptide is involved in the multimerization of vWF. The domains on the vWF molecule involved in the interactions of vWF with GP Ib, GP IIb-IIIa, collagen, F. VIII and heparin have been localized to varying extents. It is anticipated that peptide analysis and recombinant DNA techniques, such as in vitro mutagenesis, will further define the structural requirements of these binding domains. vWF is synthesized in a cell-specific manner by endothelial cells and megakaryocytes. It undergoes a complex intracellular biosynthesis involving transcription of a 200 kb gene, splicing out more than 42 introns, translation of a 8900 bp mRNA, glycosylation, disulphide bond formation, sulphatation, multimerization and proteolytic cleavage. The molecule can be secreted in a constitutive or regulated manner upon perturbation of the endothelial cells with physiological and non-physiological secretagogues. The mechanisms that control the synthesis of vWF should be an exciting area of further research. vWD is probably the most common of all congenital disorders of haemostasis. It is an extremely heterogeneous syndrome involving quantitative or qualitative disorders of vWF.(ABSTRACT TRUNCATED AT 400 WORDS)