Type 2M Von Willebrand Disease Research Articles

Biologic response to subcutaneous and intranasal therapy with desmopressin in a large Amish kindred with Type 2M von Willebrand disease

The aim of this study was to characterize the adequacy and longevity of biological response to desmopressin (DDAVP) in a large Amish kindred of Type 2M von Willebrand disease (VWD) possessing C-to-T transition at nucleotide 4120 in exon 28 of A1 domain of von Willebrand factor (VWF) gene. Response to both intranasal (Stimate) and subcutaneous DDAVP administration was assessed. Rise in ristocetin cofactor activity (VWF:RCo) > or = 40% at 90-min post-Stimate and 1-2 h after subcutaneous DDAVP was defined as initial response; response longevity was assessed only after subcutaneous dosing by measuring VWF:RCo levels at time-points 1, 2, 4 and 6 h. Eleven patients (five males, six females; age range: 20-56 years) participated in intranasal and 9/11 (four males, five females) in subcutaneous testing. Baseline haemostatic profiles included: VWF:RCo < 15%, VWF:Ag < 40% and normal VWF multimers. Initial response was comparable by both intranasal (6/11; 54.5%) and subcutaneous (4/9; 44%) routes; sustained response (VWF:RCo > 40% for 2 h) was observed in only one in nine (11%) patients tested. Median VWF:RCo peak levels after intranasal (40%) and subcutaneous (39%) routes were equivalent. Peak VWF:Ag levels were significantly higher after subcutaneous than intranasal DDAVP (94% vs. 54%; P = 0.03). Area under the curve for VWF:RCo was significantly decreased (170 microg h mL(-1)) compared with VWF:Ag (471 microg h mL(-1)) and FVIII:C (624.60 microg h mL(-1)). This study suggests that in this population: (i) intra-individual DDAVP response is consistent with subcutaneous and intranasal administration; and (ii) extending DDAVP challenge test up to at least 6 h is required to characterize adequacy and longevity of biologic response prior to using DDAVP as a sole haemostatic intervention.

A Tiered Management Program To Conserve Replacement Therapy Resources for a Rural Population of Women with Type 2M von Willebrand Disease during Labor and Delivery.

Introduction: Type 2M Von Willebrand Disease (VWD) results from a mutation in the Von Willebrand factor (VWF) gene resulting in decreased or absent binding to platelet glycoprotein Ib. Bleeding episodes in type 2M VWD are variable, but can be severe or life-threatening; effective treatment requires administration of exogenous normal VWF. Pregnant women with type 2M VWD require close monitoring to prevent adverse bleeding events. Following a fatal post-partum hemorrhage in a woman from a large kindred with type 2M VWD, routine prophylactic doses of VWF were administered both pre- and postpartum to women with this disorder. This population is uninsured and utilized product is donated for their care. To determine if the amount of VWF could be safely reduced, a tiered risk stratification and replacement therapy program was developed to manage women with type 2M vWD during labor, delivery and the peuperium. We report the results of this tiered risk stratification in terms of the total product utilized and the rate of bleeding experienced.Methods: Between Jan–Mar 2007, we followed 7 Amish women from a large kindred with type 2M VWD (mutation 4120 C-->T on exon 28). This rural community has limited access to sophisticated hematologic monitoring and is uninsured. Women were risk stratified into 1 of 3 tiers (T) based on their personal or immediate family history of bleeding with childbirth. A nurse provided peripartum monitoring via home visits on postpartum days 1, 3, 5, 10 with a portable hematocrit (Hct) machine. Women in T1 were at lowest risk and given VWF replacement only in the event of excessive bleeding or postpartum Hct <30; T2 (intermediate risk) were given a single prophylactic VWF replacement dose immediately prior to delivery then monitored as T1; T3 received prophylactic VWF dose immediately prior to delivery, at 12 hours postpartum, then monitored by serial Hct per the other tiers. Women requiring Cesarean section were placed into T3 but treated with further VWF replacement due to the surgical intervention.Results: The 7 women fell into the following tiers; T1: 4; T2: 2; T3: 1. Of the 4 women in T1, none experienced excessive bleeding and no VWF therapy was required. In the 2 women in T2, there were no bleeding events requiring additional VWF. Postpartum Hct remained >30 on all evaluations in T1 and 2. One woman was delivered by elective caesarean section for complicated twin pregnancy, and given VWF prior to surgery and once daily for three consecutive postoperative days. She had no bleeding complications, and maintained Hct >30. Compared with the prior method of required pre- and post-partum prophylactic VWF administration regardless of tier or Hct, this pilot protocol safely decreased VWF usage in women assigned to T1 and 2 without adverse bleeding outcome or the need for transfusion.Conclusions: Women with type 2M VWD can be safely managed with a tiered risk stratification system and close postpartum clinical/Hct monitoring that utilizes conservative VWF replacement. For T1 and 2 this lead to a reduction in VWF administration of 83% compared with the prior method of mandatory pre- and postpartum VWF administration. This further translated into a substantial cost savings while still achieving safe hemostatic outcomes. Women at highest risk continue to receive prophylactic VWF pre- and postpartum to prevent serious hemorrhagic events.

Common VWF Haplotypes in Normal African-Americans and Caucasians Recruited into the ZPMCB-VWD and Their Impact on VWF Laboratory Testing.

Von Willebrand disease (VWD) is a common bleeding disorder that has been reported to affect up to 1% of the population. Diagnostic testing for VWD relies on specific tests of von Willebrand factor (VWF) that include VWF antigen (Ag) and VWF ristocetin cofactor activity (RCo). Variability in these tests, especially in the RCo, has the potential to affect diagnosis of VWD. Clinically, the RCo/Ag ratio is used to identify patients with type 2M VWD, of which 35% of our local type 2M index cases were of African-American descent. As part of the ZPMCB-VWD, a large study of both healthy controls and patients with VWD, we evaluated VWF Ag, RCo, multimers, EU bleeding score, and VWF gene sequencing to look for mutations and/or common polymorphisms (SNPs) that might contribute to RCo measurement. Healthy controls completed a computerized version of the EU bleeding score and provided blood for clinical VWF testing. Since platelet VWF binding primarily involves the A1 domain of VWF, exon 28 gene sequencing was analyzed and common SNPs were identified, particularly in African-Americans (AA) with altered RCo/Ag ratios. Statistical comparisons were performed using t-tests. For the AA control group, the presence of specific exon 28 SNPs, including I1380V, N1435S, and D1472H, correlated with a low RCo/Ag. In controls, the 3 SNPs occurred together in 22% of AA and 1.5% of Caucasians. For AA controls with all 3 SNPs, the mean Ag was 155, RCo 115, and RCo/Ag 0.77, while for AA without the 3 SNPs, the mean Ag was 129, RCo 129, and RCo/Ag 1.01. The difference in RCo/Ag ratio was significant (p<0.001). In comparison, the Caucasian controls without the 3 SNPs had a mean Ag of 108, RCo of 115, and RCo/Ag of 1.08. When only D1472H was considered, a significant difference in RCo/Ag ratio was noted for both African-American and Caucasian controls with D1472H (present in 63% of AA and 24% of Caucasian controls). For AA controls with D1472H, the mean RCo/Ag ratio was 0.82 (range 0.42–1.16) compared to 1.01 for those without the SNP (p<0.001). The Caucasian controls with D1472H had a mean ratio of 0.87 (range 0.57–1.17) compared to 1.08 for those without the SNP (p<0.001). EU Bleeding Score was not significantly affected by either race or SNP status. All Caucasian controls with D1472H were heterozygous, while 14% of the African-American controls were homozygous. The mean RCo/Ag for the homozygous controls was 0.71, compared to 0.86 for the AA heterozygous controls (p<0.025). In order to determine if the exon 28 SNPs intrinsically altered the measurement of VWF, the 3 SNPs were engineered into recombinant VWF and expressed in HEK293T cells. VWF Ag and RCo were performed on the purified expressed VWF. The RCo/Ag ratios for the recombinant expressed VWF were decreased for 1380V and 1472H, while the construct containing all 3 SNPs showed an even lower ratio (59% of wild-type RCo/Ag ratio). These data suggest that specific exon 28 SNPs may contribute to low RCo/Ag ratios, especially in the African-American population. Since the RCo assay involves ristocetin binding to VWF, mutations (and polymorphisms) in VWF may affect the measurement of “VWF activity” by this assay and might not reflect true hemorrhagic risk.

Challenges in defining type 2M von Willebrand disease: results from a Canadian cohort study.

In order to better characterize the genotype-phenotype correlation in type 2M von Willebrand disease (VWD), we sequenced the coding region for the mature subunit of the von Willebrand factor (VWF) gene (exons 18-52, including exon/intron boundaries) in 16 index cases originally submitted to the Canadian Type 1 VWD Study as type 1 VWD, but reclassified as type 2M VWD on the basis of phenotype (excessive mucocutaneous bleeding and von Willebrand factor: antigen (VWF:Ag) and/or von Willebrand factor: ristocetin cofactor (VWF:RCo) between 0.05 and 0.50 IU mL(-1) on at least two occasions and RCo/Ag ratio < 0.6 and no loss of high molecular weight multimers). Available family members (16 affected, 23 unaffected and six unknown) were sequenced for identified mutations. We identified eight different missense mutations (R854Q, T1054M, R1315C, R1374C, R1374H, L1382P, S2179F, and T2647M) within these 16 families. We were significantly more likely to identify a VWF mutation in cases with RCo/Ag ratios < 0.50 (P < 0.05, chi-squared test). Importantly, every index case with an RCo/Ag ratio < 0.40 (4/4 index cases) had a mutation identified within the A1 domain, in contrast to 1/12 cases with an RCo/Ag ratio > 0.40. Difficulties with the standardization of the VWF:RCo may be responsible for the heterogeneity in cases with RCo/Ag ratios between 0.40 and 0.60. The genotype-phenotype correlation for cases with RCo/Ag ratios < 0.40 is clear. On the basis of our results, the phenotypic definition of type 2M VWD may need to be more stringent, and should be the subject of an international standardization initiative.

Von Willebrand Factor/Factor VIII Concentrate (Humate-P®) Is Safe and Effective in the Prevention of Perioperative Bleeding in Patients with Very Low von Willebrand Factor: Ristocetin Cofactor (VWF:RCo).

Optimal dosing to prevent excessive surgical bleeding in von Willebrand Disease (VWD) was investigated in an open-label study of replacement therapy with a von Willebrand factor/factor VIII concentrate (VWF/FVIII, Humate-P®). This analysis focused on the subset of patients with very low VWF, defined as those with baseline VWF:RCo levels of <12 IU/dL. Of a total of 35 patients, 17 had very low levels of VWF:RCo; of these 17 “severe” VWD patients, 12 had type 3 VWD, one each had types 1, 2A or 2B, and 2 had type 2M VWD. Previous pharmacokinetic data in each patient was utilized to calculate pre-operative and immediate postoperative doses to raise plasma VWF:RCo and FVIII to 80–100 IU/dL for major surgery and 50–60 IU/dL for minor and oral surgery; subsequent doses were adjusted based on VWF:RCo levels. Eleven patients underwent major surgery including orthopedic, gynecologic and plastic surgery, and multiple tooth extractions; 4 had minor surgery and 2 had single tooth extractions. Hemostatic efficacy was assessed by investigators as excellent, good, moderate/poor, or none immediately after the surgery, 24 hours after the last VWF/FVIII infusion and 14 days post-op. Expected and actual estimated blood loss (EBL) was compared, transfusions recorded and adverse events (AEs) documented. The median loading dose was higher among subjects with severe VWD types (71 IU/kg, range 39 to 135) compared with non-severe VWD (44 IU/kg, range 17 to 121). Subjects with severe VWD types also used higher total doses (median: 280 IU/kg, range 63 to 859) and had longer treatment duration (6 days, range 1–26) than subjects with non-severe VWD types (median total dose: 208 IU/kg, range 79 to 1699; and treatment duration: 4.5 days, range 2–19). Hemostasis was rated as effective (good or excellent efficacy) in 15/17 (88.2%) patients immediately postoperatively, in 17/17 (100%) patients 24 hours after the last infusion (primary endpoint), and in 17/17 (100%) patients 14 days postoperatively. A bleeding related serious AE occurred in one patient; she had hemorrhage post-hysteroscopic resection of uterine fibroids followed by hysterectomy; actual EBL exceeded expected EBL and hemostatic efficacy was considered moderate/poor immediately post-op but good/excellent at the other time points. Three other surgery-related hemorrhagic events were mild in the severe VWD types; in the non-severe VWD patients, 1 severe, 1 moderate and 2 mild hemorrhagic adverse events were reported; none of these were considered to be related to poor efficacy of the drug. No thromboembolic complications or changes in viral titers were observed in the study. We conclude that patients with very low baseline VWF levels can safely undergo both major and minor surgery with VWF/FVIII concentrate when dosing is calculated to achieve and maintain hemostatic VWF levels based on VWF:RCo monitoring. It is important to base therapeutic decisions on the severity of disease as assessed by baseline plasma VWF and FVIII levels as well as VWD type.

The interaction of von Willebrand factor-A1 domain with collagen: mutation G1324S (type 2M von Willebrand disease) impairs the conformational change in A1 domain induced by collagen.

It is established that the A3 domain in von Willebrand factor (VWF) contains the major collagen-binding site. However, there are conflicting reports describing the capacity of the A1 domain to interact with collagen types I and III. In this study, we have used recombinant VWF-A1 polypeptides, as well as conformation-specific monoclonal antibodies (mAb), to analyze the A1-collagen interaction. The A1 domain bound to collagen with K(d) approximately 8.0 nm and this binding was blocked by the mAb 6G1, which blocks the interaction between ristocetin and VWF. In addition, collagen-bound A1 protein was able to support flow-dependent adhesion of platelets, demonstrating that the binding sites for collagen and glycoprotein (GP)Ib are different. Analysis with two conformation-specific mAb demonstrated that the structure of the A1 domain changed as a result of the binding to collagen. In contrast, the antibodies failed to detect conformational change in the G1324S mutant (type 2M von Willebrand disease). Thus, direct binding to collagen induces a change in the structural conformation within the VWF-A1 domain, and the G1324S substitution prevents this conformational change. This study has shown that the isolated A1 domain can simultaneously bind to collagen and platelet GPIb, supporting platelet adhesion under high-flow conditions. In addition, this study has used mAb to demonstrate that the binding of the isolated A1 domain or full-length VWF to collagen is accompanied by a conformational change in A1 domain.

Von Willebrand disease R1374C: Type 2A or 2M? A challenge to the revised classification. High frequency in the northwest of Spain (Galicia)

Patients initially diagnosed with type 1 von Willebrand disease (VWD) have been reclassified as type 2 after a more exhaustive analysis in several studies. Our study's objectives were (1) to reanalyze patients that were previously diagnosed as type 1 to achieve a more accurate diagnosis and (2) to compare the von Willebrand factor (VWF) ristocetin cofactor assay (VWF:RCo) and the VWF collagen binding assay (VWF:CB) in order to evaluate the possibility of replacing the former assay with the latter in the diagnosis of VWD. Twenty-one patients from two large unrelated families and 104 normal controls were studied. VWF:Ag, VWF:RCo, FVIII coagulant activity (FVIII:C), bleeding time (BT), PFA(100), and multimeric analysis of VWF were tested. Genetic analysis by sequencing exon 28 on the VWF gene was also carried out. Patients presented lower levels of VWF:Ag and VWF:RCo, a dissociation between VWF:RCo/VWF:Ag, and the presence of all sizes of multimers in plasma VWF. The results for VWF:CB varied depending on the type of collagen used. The genetic analysis showed that the mutation R1374C is responsible for type 2M VWD. A high frequency of the R1374C mutation is observed in northwestern Spain (Galicia). Some types of 2M VWD are misdiagnosed as type 1 VWD. The VWF:CB (with type I collagen) assay was unable to discriminate defective platelet binding of the R1374C VWF. This confirms that VWF:CB cannot substitute for VWF:RCo, and both should be tested when diagnosing VWD.

Gene conversions are a common cause of von Willebrand disease

von Willebrand disease (VWD), the most common inherited bleeding disorder, is very heterogeneous, both in its phenotype and genotype. One particular molecular mechanism of VWD is due to recombination events between the true gene and its pseudogene on chromosome 22. We assessed the frequency and extension of such events in 50 multi-ethnic index patients with severe VWD type 3 and in five index patients with VWD type 2M Vicenza. One additional unclassified patient had been diagnosed with possible VWD in Russia solely on a clinical basis. Gene conversions, previously thought to be rare events, were identified in >10% of our study population: in six multi-ethnic patients with severe VWD type 3, in one patient with VWD type 2M Vicenza and the Russian patient was finally diagnosed with VWD type 2B New York/Malmoe. Our results suggest a significant contribution of this particular molecular mechanism to the manifestation of VWD. The location of the gene conversions, their extension and their occurrence as homozygous, compound heterozygous or heterozygous mutations determines the resulting phenotype.

Increased Clearance Explains the Ultra-Large Multimers in Von Willebrand Disease Type 2M Vicenza; Lessons from Recombinant VWF Vicenza and Modeling of Multimer Catabolism.

The pathogenesis of VWD Vicenza has remained elusive. VWD Vicenza is characterized by low plasma but normal platelet VWF concentration, the presence of ultra-large plasma multimers, and a heterozygous R1205H mutation. VWF Vicenza is reported to have a decreased half-life in the circulation. When we expressed rVWF Vicenza R1205H in 293T cells, it was secreted with wild type efficiency and multimer distribution, suggesting that the primary defect is accelerated clearance. To evaluate this hypothesis, we developed a pharmacokinetic model of VWF multimer catabolism. The initial assumptions are: 1. Secretion occurs at a fixed rate with the initial “ultra-large” multimer distribution seen in platelet alpha granules. 2. Cleavage of multimers occurs with a probability p that increases with increasing multimer size. 3. Clearance occurs with a time constant determined by the plasma half life and is independent of multimer size. Modeled multimer distributions were compared to those determined experimentally for patient plasma samples. The effects of DDAVP infusion also were modeled and compared to published data (Casonato et al, Blood 2002; 99:180). For p = 7.5 x 10−4 min−1 and a half life of 12 h, the modeled multimer patterns were comparable to the observed steady-state distribution of normal VWF. Decreasing the half life to 2 hours produces a low plasma concentration of “ultra-large” multimers typical of VWD Vicenza without a change in any other parameter. Conversely, increasing the probability of cleavage by only thirty percent produces typical VWD 2A multimer distributions. The model also reproduces the triplet patterns of normal and type 2A VWF. Finally, the DDAVP simulations reproduced the time course of VWF plasma concentrations and multimer distributions of DDAVP-treated patients. We conclude that increased clearance alone can explain the ultra-large multimer distribution of VWD Vicenza. Similar modeling should allow the estimation of VWF cleavage and clearance rates in other variants of VWD and in other clinical situations including thrombotic thrombocytopenic purpura. [Display omitted] [Display omitted]

Type 2M von Willebrand disease: correlation of phenotype with genotype?

Type 2M von Willebrand disease: correlation of phenotype with genotype?

A new mutation, S1285F, within the A1 loop of von Willebrand factor induces a conformational change in A1 loop with abnormal binding to platelet GPIb and botrocetin causing type 2M von Willebrand disease.

We report the identification of a new mutation in exon 28 of the von Willebrand factor (VWF) gene in two related patients with type 2M von Willebrand disease (VWD). The molecular abnormality changes the Ser 1285 to Phe within the A1 loop of VWF. The S1285F mutation was reproduced by site-directed mutagenesis on the full-length VWF cDNA. The mutated recombinant VWF (rVWF), F1285rVWF, and the hybrid, S/F1285rVWF, were expressed in COS-7 cells. F1285rVWF exhibited a slight decrease of high-molecular-weight multimers and markedly reduced ristocetin- or botrocetin-induced binding of VWF to platelets in association with a decreased binding to botrocetin. The hybrid S/F1285rVWF showed a normal multimeric profile and bound to platelets in a similar way to the patients' plasma VWF, in the presence of ristocetin or botrocetin. Thus, the new S1285F mutation within the A1 loop was responsible for the type 2M VWD observed in these patients, and was involved in the binding of VWF to botrocetin and to platelet glycoprotein Ib (GPIb). Three anti-VWF monoclonal antibodies, with conformational epitopes within the A1 loop but distinct GPIb binding inhibitory properties, showed a different interaction with F1285rVWF. These results indicate that the S1285F substitution alters the folding of the A1 loop and prevents the correct exposure of the VWF binding sites to botrocetin and GPIb.

Time for a redefinition of type 2m von willebrand disease

The recent paper by Riddell et al (2002) was interesting and timely. However, the results were entirely predictable without any experimentation because of preselection of patients and the currently used definition of type 2M von Willebrand disease (VWD). Both the ristoctin cofactor (VWF:RCo) assay and the collagen-binding (VWF:CB) assay will be sensitive to loss of von Willebrand factor (VWF) protein and especially to the loss of the higher-molecular-weight (HMW) multimers. The current definition of type 2M VWD is ‘decreased platelet-dependent function that is not caused by the absence of HMW-VWF multimers’. Defects in the VWF glycoprotein Ib binding site will lead to loss of platelet-dependent function, cause type 2M VWD by this definition, and be detected by the VWF:RCo (but not the VWF:CB). Defects in the VWF collagen binding site will lead to loss of collagen-dependent function, cause a currently unclassified form of VWD, and be detected by the VWF:CB (but not the VWF:RCo). I suggest that ‘platelet-dependent’ is removed from the definition of type 2M VWD. Type 2M can then be subtyped as type 2Mp – decreased platelet-dependent function (the old 2M), or as type 2Mc – decreased collagen-dependent function. The VWF:RCo detects only the former and the VWF:CB only the latter. That type 2Mc exists is clear (Ribba et al, 2001). We will only know the relative importance of type 2Mp and type 2Mc when the VWF:CB is used as widely as the VWF:RCo.

Identification of a New Type 2M von Willebrand Disease Mutation also at Position 1324 of von Willebrand Factor

Type 2M von Willebrand disease (VWD) refers to variants with decreased platelet-dependent function that is not associated with the loss of high molecular weight (HMW) von Willebrand factor (VWF) multimers. This category includes the so-called "phenotype B" responsible for inexistent ristocetin-induced but normal botrocetin-induced binding of VWF to platelet glycoprotein lb. The missense mutation G1324S was identified in the first patient reported to display "phenotype B". We report here on the identification in four members of a French family of a missense mutation also affecting this glycine residue but changing it into an alanine residue. These individuals are heterozygous for this mutation and two of them display an additional quantitative VWF deficiency resulting from a stop codon at position 2470. After transient transfection in Cos-7 cells, the mutated recombinant protein harbouring the G1324A substitution was shown to exhibit normal multimers and inexistent ristocetin-induced but normal botrocetin-induced binding to GPIb, confirming the classification of this new mutation as a type 2M VWD mutation.

P-3 A new candidate type 2M Von Willebrand disease mutation in an asymptomatic patient with absent ristocetin but normal botrocetin activity and a daughter with type 3 VWD

P-3 A new candidate type 2M Von Willebrand disease mutation in an asymptomatic patient with absent ristocetin but normal botrocetin activity and a daughter with type 3 VWD

P-5 An evaluation of genetic analysis in the diagnosis of type 2A, 2B and 2M von Willebrand disease

P-5 An evaluation of genetic analysis in the diagnosis of type 2A, 2B and 2M von Willebrand disease

Type 2M von Willebrand disease variant characterized by abnormal von Willebrand factor multimerization

We describe a von Willebrand disease (VWD) variant characterized by low plasma and platelet von Willebrand factor (VWF), impaired ristocetin-induced VWF binding to platelet glycoprotein Ib (GPIb), and abnormal VWF multimer pattern not associated with the absence of large forms. A C-to-T transition at nucleotide 4120 in exon 28 of the VWF gene was found; this mutation introduces a cysteine at the codon for Arg 611 of mature VWF. In addition to the decreased factor VIII (FVIII) and VWF levels, ristocetin-induced platelet aggregation (RIPA) was almost absent, and VWF ristocetin cofactor activity (VWF:RCo) was significantly more decreased than VWF antigen. The patients (mother and son) also showed a defect in VWF collagen-binding activity. Plasma VWF multimers were decreased, with no limit in the size of large forms, and the normal discontinuous multimer organization was replaced by a diffuse smear, especially detectable in the large forms. This picture was emphasized by 1-deamino-8-D -arginine vasopressin (DDAVP) infusion, so that the abnormal VWF multimers appeared to have a molecular weight higher than those present in, or released by, human umbilical vein endothelial cells. DDAVP also increased FVIII and VWF levels but did not normalize the GPIb-dependent VWF functions expressed as RIPA and VWF:RCo. We include this variant in type 2M VWD, focusing on the abnormality in GPIb-dependent VWF function. We advance that this defect depends on the mutation in the GPIb binding domain of VWF rather than the abnormal VWF multimer pattern. (J Lab Clin Med 2001;137:70-6)

Mapping the Glycoprotein Ib-binding Site in the von Willebrand Factor A1 Domain

The von Willebrand factor (vWF) mediates platelet adhesion to exposed subendothelium at sites of vascular injury. It does this by forming a bridge between subendothelial collagen and the platelet glycoprotein Ib-IX-V complex (GPIb). The GPIb-binding site within vWF has been localized to the vWF-A1 domain. Based on the crystal structure of the vWF-A1 domain (Emsley, J., Cruz, M., Handin, R., and Liddington, R. (1998) J. Biol. Chem. 273, 10396-10401), we introduced point mutations into 16 candidate residues that might form all or part of the GPIb interaction site. We also introduced two mutations previously reported to impair vWF function yielding a total of 18 mutations. The recombinant vWF-A1 mutant proteins were then expressed in Escherichia coli, and the activity of the purified proteins was assessed by their ability to support flow-dependent platelet adhesion and their ability to inhibit ristocetin-induced platelet agglutination. Six mutations located on the front and upper anterior face of the folded vWF-A1 domain, R524S, G561S, H563T, T594S/E596A, Q604R, and S607R, showed reduced activity in all the assays, and we suggest that these residues form part of the GPIb interaction site. One mutation, G561S, with impaired activity occurs in the naturally occurring variant form of von Willebrand's disease-type 2M underscoring the physiologic relevance of the mutations described here.

DDAVP is not a panacea for children with bleeding disorders.

DDAVP is not a panacea for children with bleeding disorders.

Von Willebrand Disease Type 2M “Vicenza” in Italian and German Patients: Identification of the First Candidate Mutation (G3864A; R1205H) in 8 Families

Von Willebrand disease type 2M "Vicenza" (VWD 2M V) is characterised by autosomal dominant inheritance, low von Willebrand factor (VWF) and the presence of "supranormal" multimers in plasma. This specific phenotype has been described in Italian and recently also in German patients. The molecular defect is linked to the VWF gene. However, no specific mutations have been identified until now. We analysed the complete coding region and adjacent intron sequences of the VWF gene in Italian families in comparison to German families with VWD 2M V by a PCR-based mutation screening, combined with SSC- and heteroduplex-analysis of exons 2 through 52, followed by direct sequencing. We identified the first heterozygous candidate mutation (G3864A; R1205H) in all affected members of the 7 Italian families and in 1 German patient but not in the unaffected family members nor on 100 chromosomes of normal subjects, suggesting a causal relationship between the mutation and the phenotype. Haplotype identity, with minor deviations in one Italian family, suggests a common but not very recent genetic origin of R1205H.

Type 2M von Willebrand Disease: F606I and I662F Mutations in the Glycoprotein Ib Binding Domain Selectively Impair Ristocetin- but not Botrocetin-Mediated Binding of von Willebrand Factor to Platelets

von Willebrand disease (vWD) is a common, autosomally inherited, bleeding disorder caused by quantitative and/or qualitative deficiency of von Willebrand factor (vWF). We describe two families with a variant form of vWD where affected members of both families have borderline or low vWF antigen levels, normal vWF multimer patterns, disproportionately low ristocetin cofactor activity, and significant bleeding symptoms. Whereas ristocetin-induced binding of plasma vWF from affected members of both families to fixed platelets was reduced, botrocetin-induced platelet binding was normal. The sequencing of genomic DNA identified unique missense mutations in each family in the vWF exon 28. In Family A, a missense mutation at nucleotide 4105T → A resulted in a Phe606Ile amino acid substitution (F606I) and in Family B, a missense mutation at nucleotide 4273A → T resulted in an Ile662Phe amino acid substitution (I662F). Both mutations are within the large disulfide loop between Cys509 and Cys695 in the A1 domain that mediates vWF interaction with platelet glycoprotein Ib. Expression of recombinant vWF containing either F606I or I662F mutations resulted in mutant recombinant vWF with decreased ristocetin-induced platelet binding, but normal multimer structure, botrocetin-induced platelet binding, collagen binding, and binding to the conformation-sensitive monoclonal antibody, AvW-3. Both mutations are phenotypically distinct from the previously reported variant type 2MMilwaukee-1 because of the presence of normal botrocetin-induced platelet binding, collagen binding, and AvW-3 binding, as well as the greater frequency and intensity of clinical bleeding. When the reported type 2M mutations are mapped on the predicted three-dimensional structure of the A1 loop of vWF, the mutations cluster in one region that is distinct from the region in which the type 2B mutations cluster.