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Keywords:

  • genetics;
  • von Willebrand disease;
  • von Willebrand factor

Abstract

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

Summary.  von Willebrand disease (VWD), the most common inherited bleeding disorder in humans, is characterised by a prolonged bleeding time due to quantitative and/or functional deficits of von Willebrand factor (VWF), a huge multimeric protein. Given the large size and complexity of the protein, the many functions of VWF, for example, binding to collagen, to platelet GPIb, and to FVIII, the localisation of these binding sites in different VWF domains, as well as the dependence on a high molecular weight multimer structure for proper function, VWF is prone to quantitative and very heterogeneous structural and functional defects. Comprehensive clinical and laboratory phenotypic description of patients with VWD in correlation to the genotype has considerably increased our knowledge on this disorder and the physiology and pathophysiology of VWF. This article focuses on the phenotype/genotype relationship in VWD and the context of VWD types and subtypes with particular VWF domains.


von Willebrand disease and von Willebrand factor

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

von Willebrand disease (VWD) is considered the most common inherited bleeding disorder and is characterised mainly by mucosal bleeding manifestations. Patients with severe forms of VWD may also suffer from joint, muscle and central nervous system bleeding. VWD is caused by quantitative and/or functional deficits of von Willebrand factor (VWF), a huge multimeric protein with a key role in platelet-dependent primary haemostasis. In a simplified model, plasma VWF binds to collagen in the subendothelial matrix after an injury of the vessel wall. Thereafter, it expands its structure under shear flow, thereby exposing multiple binding sites for platelet GPIb as the initial event in platelet plug formation. In this situation, high molecular weight multimers (HMWM) are the most active forms of VWF. Additionally, VWF binds coagulation Factor VIII (FVIII) and protects it against inactivation in the circulation.

Classification of VWD

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

Six types of VWD have been defined: VWD type 1 (OMIM ID #193400) is characterised by low levels of a functionally normal VWF and dominant inheritance. VWD type 3 (OMIM ID #277480) is defined as severe VWD with VWF:Ag levels < 1% both in plasma and platelets of patients and recessive inheritance. In most cases, heterozygous mutation carriers of VWD type 3 are unaffected. Structural and functional defects of VWF (OMIM ID #613554) are the most heterogeneous types of VWD both in their clinical presentations and their molecular pathogenic mechanisms [1]. The most common such VWD is type 2A, which is associated with platelet dependent functional deficits due to significant reduction or absence of VWF HMWM. Type 2M VWD shows a similar phenotype, although VWF HMWM are present and not significantly reduced. Classical type 2B may show the same multimer pattern as type 2A with enhanced VWF proteolysis and is diagnosed phenotypically by Ristocetin induced platelet aggregation (RIPA) at lower than normal assay concentrations of Ristocetin. Finally type 2N represents forms with defective VWF:FVIII binding (VWF:FVIIIB), thereby mimicking haemophilia A. With the exception of types 2N and 2B, which are phenotypically and genetically rather well defined, this classification allows only a very simplified view on the variety of molecular mechanisms hidden in particular behind type 2A and type 2M as well as type 1.

Biosynthesis and structure of VWF

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

VWF is synthesised as Pre-Pro-VWF of 2813 amino acids (aa) with a signal peptide of 22 aa directing the transport to the endoplasmatic reticulum where two Pro-VWF monomers dimerise at their carboxy-terminal cysteine knot like (CK) domains. This process seems to be quantitative, resulting in further transport of only dimers to the Golgi and Post-Golgi compartment where VWF-dimers polymerise to multimers at their Cysteine-rich D3 domains to different sizes (multimers) of up to more than 10 000 kDa [2]. The propeptide of 741 aa which has an important function in catalysing the multimerisation process, most probably by its two CGLC disulphide isomerase consensus sequences [3] is cleaved off at the Furin cleavage site, but remains non-covalently bound and directs the mature protein to the Weibel Palade bodies for storage and regulated release [4,5]. A proportion of VWF is not stored, but rather follows a constitutive secretion pathway [6]. During its synthesis, VWF is also heavily glycosylated, which is a prerequisite for its function and integrity, but to date, is only poorly understood with respect to its functional consequences.

Structural and functional domains of VWF

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

VWF consists of multiple domains as a result of exon shuffling during the evolutionary process [7]. Binding studies and also identification of disease mutations and their pathogenic nature were the tools to identify functional and structural domains of VWF (Fig. 1). Structural domains as defined in the context of this article are involved in the post-translational processing of VWF, most importantly the CK domain for VWF dimerisation and the D3 domain for VWF dimer multimerisation. The latter process is dependent on the VWF propeptide, which harbours a CGLC sequence in both the D1 and D2 domain emphasising that these domains are also structurally important. It is clear that mutations in the structural domains causing defects of dimerisation and multimerisation also cause functional deficiencies since reduced concentrations or even more lack of HMWM significantly impair VWF binding to GPIb (VWF:GPIbB) and to collagen (VWF:CB) (Fig. 1).

image

Figure 1.  Structural and functional domains of VWF in correlation to different VWD types and subtypes and VWF multimers. Yellow triangles refer to mutation clusters causing type 2A due to defects of dimerisation (CK domain) or multimerisation (D1, D2, D3 domains); light green triangles point to mutation clusters causing enhanced proteolysis by ADAMTS13 in type 2B (A1 domain) and type 2A phenotype IIA (A2 domain); green triangles indicate multimers with a ‘smeary’ appearance that are designated either type 2M or type 1 depending on the mutant VWF functional properties; the blue triangle points to VWF:CB defects in the A3 domain and the red triangle to FVIII binding defects (type 2N).

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Functional VWF domains confer important properties to VWF for example, FVIII binding by the D’ domain [8], GPIb binding by the VWF A1 [9] domain and collagen binding by the VWF A1 and A3 domains [10–14]. Mutations in these domains cause isolated functional deficiencies, which are diagnosed by specific functional tests.

The functional properties of most of the carboxy-terminal domains, namely D4-B1-3-C1-2, are not known to date. It has been shown that platelet αIIbβIII can bind via the RGD sequence in the VWF C1 domain [15–17] suggesting that this binding site could also be clinically important. However, to date, no naturally occurring respective mutations have been identified in patients with VWD, which questions an important physiological role of this sequence in VWF-mediated haemostasis. According to experimental data, the C1 and C2 domains were suggested to be involved in VWF binding to fibrin under high shear forces, independent of the RGD sequence [18], however a clinical significance has not been shown yet. Similarly, the three B domains and the D4 domain have not been assigned to a particular function yet, except that a VWF carboxy-terminal fragment including the D4 domain is regarded as an additional binding site for ADAMTS13 [19].

These domains as well as the C domains are cysteine-rich, which may suggest structural importance. Interestingly, the D4 domain harbours the same CGLC disulphide isomerase sequence as D1, D2 and D3 but, in contrast to the other domains, this has no impact on VWF multimerisation [R. Schneppenheim, U. Budde, unpublished data.] Possibly, this carboxy-terminal cysteine rich part of VWF might be involved in shear-induced disulphide bonding as proposed by others [20] which could be instrumental for the formation of VWF higher order networks observed under high shear stress [21]. Besides the limited knowledge of the function of individual domains, even less is known about their interaction with neighbouring domains in their particular functional properties, except regarding possible shear-dependent influences of the D’-D3 domains on the A1/GPIb binding site activation [22].

Formal genetics of VWD

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

VWD was initially been described as an autosomal dominant trait [23], which has been confirmed for most of the affected families. However, severe VWD type 3 is inherited in a recessive fashion and consequently more often diagnosed in populations with consanguineous marriages. Other examples of recessive inheritance are a particular subtype of VWD type 2A, the phenotype IIC with a lack of HMWM, and VWD type 2N characterised by defective VWF:FVIIIB, respectively. In families with more than 1 defect, complex inheritance patterns may result in heterogeneous clinical presentations of the family members.

Molecular genetics of VWD

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

The VWF cDNA sequence was published in 1985 [24]. The first reported mutations were large homozygous deletions in patients with VWD type 3 detected by Southern Blotting [25,26] These 253 kb deletions were recently mapped and shown to involve the VWF neighbouring gene ANO2, thereby affecting the olfactory sense in a homozygous patient [27,28]. The introduction of PCR and fast sequencing techniques then allowed the identification of point mutations in the very large VWF gene with its 51 coding exons. The different mutations are collected onto the ISTH-SSC VWF Online Database maintained at the University of Sheffield [http://www.vwf.group.shef.ac.uk/]. According to the different main types of VWD various molecular mechanisms can be assumed.

VWD type 3

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

A number of point mutations, most of them truncating mutations, were identified in VWD type 3 by complete sequencing of the VWF coding exons. However, a significant number of missense mutations have also been detected. They represent interesting molecular mechanisms ranging from defective intracellular transport and secretion with intracellular retention to severe defects of either dimerisation (CK domain) [29] or multimerisation with mutations in the D1, D2 [30–32] and D3 domain [32] (Fig. 1). The mutation p.C1169W in the D3 domain, compound-heterozygous with the most common VWD type 3 mutation c.2435delC, seems to correlate with a very rapid clearance of VWF from the circulation, since after a significant response to desmopressin, VWF:Ag in the patient’s plasma fell very rapidly to the initially very low level of < 5 IU dL−1 [H. Lenk, personal communication].

Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

Mutations in VWD type 2 attracted much interest because of their particular structural and functional implications that defined the laboratory phenotype. They can be grouped according to their pathomechanism.

Mutations affecting dimerisation

Dimerisation defects are localised in the CK domain at the carboxy-terminal of VWF. They are found in some patients with VWD type 3 but mainly in patients with VWD type 2A, phenotype IID [33,34]. A particular mutation p.C2754W causes a severe dimerisation defect and VWD type 3 [29]. Multimer analysis typically shows additional bands between the individual triplets, representing multimers with an odd number of monomers (Fig. 1).

Mutations affecting multimerisation

Disorders of further polymerisation of the dimers to multimers at their amino-terminal end are caused by two different pathogenetic mechanisms. First, mutations in the D3 domain itself, which contains the cysteines that are directly or indirectly involved in polymerisation can prevent or impair multimerisation [35]. This correlates with a relative reduction in HMWM, and occasionally also with their loss. Because of reduced proteolysis through the VWF-cleaving protease ADAMTS13, the individual multimers do not have the typical triplet structure of normal plasma VWF but merely a broader central band containing sub bands close to the actual central band (Fig. 1). This specific phenotype corresponds to the previous types IIE, IIF and IIH. They are now grouped under type 2A.

The second pathogenic mechanism of a multimerisation defect is due to mutations in the D1 and/or D2 domain of the VWF propeptide. Each of the two domains that are proteolytically cleaved from mature VWF contains a CGLC amino acid sequence considered to be a consensus sequence of disulphide isomerases [3]. Both regions apparently have a catalytic role in amino-terminal multimerisation. Homozygous or compound heterozygous mutations in the D1-D2 domain correlate with the phenotype of the former type IIC [36,37], which is characterised by a lack of large and occasionally also medium-sized multimers. A triplet structure is completely absent, indicating greatly reduced proteolysis (Fig. 1). A particular interesting mutation p.N528S not only causes the IIC phenotype but also blocks the presumable binding site for the VWF propeptide, probably through creating a new N-linked glycosylation site, thereby preventing the intracellular transport of HMWM to Weibel Palade bodies and to α-granules of platelets [38]. The IIC phenotype is also included among VWD type 2A. In most cases the mode of inheritance is recessive, unlike other VWD type 2A phenotypes. Some more severe mutations in that region correlate with VWD type 3.

Mutations causing impaired secretion of VWF HMWM

Besides cysteine mutations in the D3 domain [35,39], mutations in the A1 and A2 domain have been correlated with impaired intracellular transport of VWF HMWM that cause VWD type 2A. They are called group 1 mutations [40]. However, it has been shown that group 1 mutations in the A2 domain in addition also confer increased susceptibility for the ADAMTS13 protease, indicated by the pronounced proteolytic sub bands of individual VWF oligomers in multimer analysis [41] (see below).

Mutations causing increased ADAMTS13 mediated proteolysis

Type 2A  Lack of HMWM can also be caused by increased susceptibility of mutant VWF to the VWF cleaving protease ADAMTS13. Particular mutations (group 2) in the VWF A2 domain [41–43] alter the A2 structure and expose the cleavage site without the necessity of shear forces. The resulting multimer pattern lacks HMWM and is characterised by enhanced proteolytic sub bands. (Fig. 1). The difference between group 1 and group 2 mutants is that the latter are initially expressed with a normal multimer pattern but which deteriorates quickly by proteolysis in the circulation, whereas the group 1 mutants constitutively lack HMWM.

Type 2B  A very similar multimer pattern is seen in patients with VWD type 2B. Enhanced proteolysis and decreased HMWM are secondary to gain of function mutations in the VWF A1 domain [44–46]. These mutants have a constitutively accessible binding site for platelet GPIb [47]. However, they do not correlate with thromboembolism but rather with a significant bleeding disorder as a result of thrombocytopenia and lack of HMWM. A possible pathomechanism could involve spontaneous binding of platelets to VWF in the circulation without their activation, thereby ‘setting sail’ in the blood flow, expanding the VWF molecule and exposing the cleavage site in the A2 domain to ADAMTS13.

Mutations affecting VWF:GPIbB

A few mutations in the VWF A1 domain cause defective VWF:GPIbB although HMWM are present [48,49]. This fulfils the criteria for the diagnosis of VWD type 2M. In most of the respective patients at least minor aberrations of the VWF multimer pattern are visible and only very few patients have completely normal multimers. One such example is a patient with the mutation p.V1409F, normal VWF multimers but selectively decreased VWF:GPIbB and no response of this parameter after infusion of desmopressin although all other parameters responded very well [50] (Fig. 2). However, such a phenotype is certainly very rare and in general, the differentiation of type 2M from type 1 or type 2A remains a matter of debate [51,52].

image

Figure 2.  VWD type 2M. Normal response to desmopressin regarding VWF:Ag, VWF:CB and FVIII:C, and almost no response of VWF:RCo in a patient with normal multimers and the mutation p.Y1409F. The correction of the PFA100 closure time is probably due to the significant increase in VWF:CB. [50] with permission.

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A mutation in the A1 domain, p.D1472H, which causes significantly decreased VWF:RCo without correlation to a bleeding phenotype represents a common allele of a single nucleotide polymorphism (SNP) in the African American population of 63%, which is much less frequent in Caucasians (17%). This variant is the sole reason for a generally decreased VWF:RCo in African American compared with Caucasian controls [53]. The lack of correlation of the H1472 allele with bleeding manifestations and with a direct VWF:GPIbB assay questions the clinical validity of the VWF:RCo assay.

Mutations affecting VWF:CB

To date several individuals have been reported with specific defects of VWF:collagen type I and type III binding, but the clinical significance of this condition is not clear up to now. The first such family with a mutation in the VWF A3 domain had only mild bleeding symptoms [54] but several other individuals with this defect had alternative reasons for bleeding, or had no bleeding tendency at all [55]. Recent studies have shown that some mutations in the A3 domain associated with bleeding cause defective binding of collagen type I and type III but not of type VI which was bound normally to the VWF A1 domain [56]. Also of interest is the good correction of the tail bleeding time by hydrodynamic injection of mutant VWF cDNA into VWF−/− mice with mutations causing defective VWF:CB of the VWF A3 domain in humans [57]. These equivocal results challenge the assumption of the A3 domain as most important site for collagen binding of VWF, rather favouring the A1 domain for this position.

Mutations affecting VWF:FVIIIB

The VWF:FVIIIB defect or type VWD type 2N [58] was previously often misdiagnosed as haemophilia A [59]. In cases of female haemophilia A or obviously autosomal recessive instead of X-linked inheritance of FVIII deficiency, VWD type 2N should be considered as an alternative diagnosis, and a FVIII binding assay should be performed. The majority of the mutations are found in exons 18 through 20, coding for the VWF D’ domain [59–62], and some also in the D3 domain that may also be associated with aberrant VWF multimers [63,64]. The phenotype may vary considerably depending on the mutations involved. The most severe but rare mutation has been reported to be p.E787K [59] with an associated FVIII activity as low as 1%, and the mildest mutation which qualifies as a polymorphism due to its allele frequency of > 0.01 in the normal Western European population is p.R854Q [59,65].

VWD type 1

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

VWD type 1, the most frequent type, but with generally milder clinical symptoms, was initially of low interest. This changed with the initiation of two comprehensive studies on patients and their families with VWD type 1, namely the EU funded study MCMDM-1VWD on patients historically diagnosed with VWD type 1 in expert laboratories [66] and a Canadian cohort study [67]. Mutation analysis in these families confirmed previous assumptions that the mutation spectrum of VWD type 1 is completely different from VWD type 3, which implies that the clinically milder form VWD type 1 is not just merely due to heterozygous VWD type 3 mutations. In the European study, only few VWD type 3 mutations, among them the most common VWD type 3 mutation c.2435delC, were identified as being contributive to the VWD type 1 phenotype by compound-heterozygosity with another mutation [66]. Despite their prior classification as VWD type 1, a large proportion of patients (38%) showed abnormalities in their multimer distribution or oligomer composition and in all of them mutations were identified. Some of them had to be re-classified as having VWD type 2A. Among 93 patients (62%) of those with completely normal multimers, mutations were identified in only 51 individuals. No mutations were identified in the remaining 42 patients with normal multimers, suggesting the contribution of other gene loci to the phenotype of VWD type 1 [68]. The mutation spectrum partly overlapped with VWD type 2A phenotype IIE [35] and VWD type 2M [69] which points to the problem of inter-individual phenotypic differences in spite of the same VWF genotype, or problems in the performance of non-standardised multimer analysis. Multimer abnormalities that were more frequently observed featured a smeary appearance of VWF oligomers and in some cases faster or slower migrating electrophoretic bands, which correlated with particular mutations involving cysteine residues. Most of these mutations were located in the carboxy-terminal or in the A1 domain of VWF. Besides decreased synthesis and intracellular retention of mutant VWF, respectively, another interesting mechanism causing VWD type 1 is increased clearance of mutant VWF from the circulation. This mechanism applies to the classical VWD type 1 Vicenza mutation p.R1205H with very low VWF:Ag [70,71] and probably other mutations causing VWD type 1, and to cysteine mutations in the D3 domain and at the VWF carboxy-terminal [72].

Indications for genetic testing

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

As pointed out in this article, mutation analysis of VWF has helped to understand the structure-function relationship of this complex protein. However, it is clear that the effort invested in research cannot currently be translated in normal VWD patients’ care. Therefore, to date molecular genetic testing will not be applied as a routine, although the advantage to obtain an unequivocal diagnosis in most cases is attractive [73,74]. Certainly, families with severe VWD type 3 have an indication for genetic testing as a basis for genetic counselling, since unaffected heterozygous carriers cannot be identified by phenotypic laboratory parameters alone, similar to the situation in haemophilia. This may also apply to patients with more severe forms of VWD type 2N and VWD type 2A phenotype IIC that are both recessively inherited. In other cases genetic testing can be helpful in establishing the correct diagnosis when conventional laboratory parameters are inconclusive.

Outlook

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References

Phenotype/genotype studies are valuable tools to elucidate the various structural and functional requirements of VWF to fulfil its role in primary haemostasis. However, the diagnostic tools available in the routine clinical laboratory are very limited and most assays are performed under static conditions or in the presence of non-physiologic reagents like Ristocetin, which may produce spurious results. VWF is a protein that is activated but also regulated by shear flow. It is therefore necessary to develop simple routine method devices for measuring VWF function under shear flow conditions to mimic the situation of blood flow at the vessel wall. Such methods will fuel further research on supposed additional functional properties of several other VWF domains, the function of which is not yet apparent. This will probably also open new opportunities to develop alternative means of manipulating the process of primary haemostasis either to prevent bleeding or thromboembolism.

References

  1. Top of page
  2. Abstract
  3. von Willebrand disease and von Willebrand factor
  4. Classification of VWD
  5. Biosynthesis and structure of VWF
  6. Structural and functional domains of VWF
  7. Formal genetics of VWD
  8. Molecular genetics of VWD
  9. VWD type 3
  10. Structural and functional defects of VWF (VWD types 2A, 2B, 2M, 2N)
  11. VWD type 1
  12. Indications for genetic testing
  13. Outlook
  14. Disclosure of Conflict of Interests
  15. References
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