A novel type 2A (Group II) von Willebrand disease mutation (L1503Q) associated with loss of the highest molecular weight von Willebrand factor multimers

Von Willebrand disease mutation spectrum and associated mutation mechanisms

Von Willebrand disease (VWD) is a bleeding disorder that is mainly caused by mutations in the multimeric protein von Willebrand factor (VWF). These mutations may lead to deficiencies in plasma VWF or dysfunctional VWF. VWF is a heterogeneous protein and over the past three decades, hundreds of VWF mutations have been identified. In this review we have organized all reported mutations, spanning a timeline from the late eighties until early 2017. This resulted in an overview of 750 unique mutations that are divided over the VWD types 1, 2A, 2B, 2M, 2N and 3. For many of these mutations the disease-causing effects have been characterized in vitro through expression studies, ex vivo by analysis of patient-derived endothelial cells, as well as in animal or (bio)physical models. Here we describe the mechanisms associated with the VWF mutations per VWD type.

Von Willebrand factor processing

Von Willebrand factor (VWF) is a multimeric glycoprotein essential for primary haemostasis that is produced only in endothelial cells and megakaryocytes. Key to VWF's function in recruitment of platelets to the site of vascular injury is its multimeric structure. The individual steps of VWF multimer biosynthesis rely on distinct posttranslational modifications at specific pH conditions, which are realized by spatial separation of the involved processes to different cell organelles. Production of multimers starts with translocation and modification of the VWF prepropolypeptide in the endoplasmic reticulum to produce dimers primed for glycosylation. In the Golgi apparatus they are further processed to multimers that carry more than 300 complex glycan structures functionalized by sialylation, sulfation and blood group determinants. Of special importance is the sequential formation of disulfide bonds with different functions in structural support of VWF multimers, which are packaged, stored and further processed after secretion. Here, all these processes are being reviewed in detail including background information on the occurring biochemical reactions.

Analysis of the storage and secretion of von Willebrand factor in blood outgrowth endothelial cells derived from patients with von Willebrand disease

Patients with von Willebrand disease (VWD) are often heterozygous for a missense mutation in the von Willebrand factor (VWF) gene. Investigating the pathogenic features of VWF mutations in cells directly derived from patients has been challenging. Here, we have used blood outgrowth endothelial cells (BOECs) isolated from human peripheral blood to analyze the storage and secretion of VWF. BOECs showed full endothelial characteristics and responded to Weibel-Palade body (WPB) secretagogues except desmopressin. We examined BOECs derived from a single subject heterozygous for a type 2N mutation (p.Arg854Gln) and from 4 patients with type 1 VWD who were, respectively, heterozygous for p.Ser1285Pro, p.Leu1307Pro, p.Tyr1584Cys, and p.Cys2693Tyr. Compared with normal BOECs, BOECs heterozygous for p.Ser1285Pro, p.Leu1307Pro, or p.Cys2693Tyr showed morphologically abnormal WPB and retention of VWF in the endoplasmic reticulum, whereas BOECs heterozygous for p.Arg854Gln or p.Tyr1584Cys showed normal WPB. The agonist-induced exocytosis of WPB from BOECs and formation of VWF strings on BOECs heterozygous for p.Ser1285Pro, p.Leu1307Pro, or p.Cys2693Tyr, but not for p.Arg854Gln or p.Tyr1584Cys, were reduced. In conclusion, VWD phenotype can be recapitulated in BOECs, and thus BOECs provide a feasible bona fide cell model to study the pathogenic effects of VWF mutations.

von Willebrand factor, shear stress, and ADAMTS13 in hemostasis and thrombosis

von Willebrand factor (VWF), an adhesive glycoprotein whose deficiency is best known for causing bleeding in patients with von Willebrand disease (VWD), is a complex molecule with a myriad of mysterious properties including its dependence on shear stress for adhesive functions. The discovery of ADAMTS13 has provided a critical impetus for understanding the regulation of VWF activity by shear stress. This communication reviews the current knowledge in VWF homeostasis and illustrates how this knowledge may help understand the changes affecting patients with various conditions including thrombotic thrombocytopenic purpura, VWD, hemolytic uremic syndrome, aortic stenosis, and ventricular assist devices.

Identification and functional analysis of a novel von Willebrand factor mutation in a family with type 2A von Willebrand disease

von Willebrand factor (VWF) is essential for normal hemostasis. VWF gene mutations cause the hemorrhagic von Willebrand disease (VWD). In this study, a 9-year-old boy was diagnosed as type 2A VWD, based on a history of abnormal bleeding, low plasma VWF antigen and activity, low plasma factor VIII activity, and lack of plasma high-molecular-weight (HMW) VWF multimers. Sequencing analysis detected a 6-bp deletion in exon 28 of his VWF gene, which created a mutant lacking D1529V1530 residues in VWF A2 domain. This mutation also existed in his family members with abnormal bleedings but not in >60 normal controls. In transfected HEK293 cells, recombinant VWF ΔD1529V1530 protein had markedly reduced levels in the conditioned medium (42±4% of wild-type (WT) VWF, p<0.01). The mutant VWF in the medium had less HMW multimers. In contrast, the intracellular levels of the mutant VWF in the transfected cells were significantly higher than that of WT (174±29%, p<0.05), indicating intracellular retention of the mutant VWF. In co-transfection experiments, the mutant reduced WT VWF secretion from the cells. By immunofluorescence staining, the retention of the mutant VWF was identified within the endoplasmic reticulum (ER). Together, we identified a unique VWF mutation responsible for the bleeding phenotype in a patient family with type 2A VWD. The mutation impaired VWF trafficking through the ER, thereby preventing VWF secretion from the cells. Our results illustrate the diversity of VWF gene mutations, which contributes to the wide spectrum of VWD.

C1272F: a novel type 2A von Willebrand's disease mutation in A1 domain; its clinical significance

Most mutations identified in 2A VWD patients are localized in the A2 domain, although missense substitutions have also been recognized in the A1 domain. We describe a novel heterozygous missense mutation in the A1 domain of VWF gene responsible for type 2A phenotype. Analysis of the complete exon 28 was carried out in a patient and his mother with life-long histories of moderate to severe bleeding and laboratory data of type 2A VWD. The analysis of exon 28 of VWF gene showed a 3815 G → T transversion resulting in C1272F mutation. It is probably associated with a group I mechanism according to patients' clinical symptoms, and, in the case of the propositus, the lack of clinical response to treatment with desmopressin. The mutation was not found in 100 normal alleles. This substitution affected the normal S-S bound between C1272 and C1458, which is involved in A1 loop structure, altering the normal multimerization and function of VWF. The VWFpp/VWF:Ag ratio in the propositus and his mother was >3, suggesting a shortened survival of VWF. We believe it is important to report the complete clinical phenotype corresponding to the new mutation to increase the knowledge in the clinical field.

L1503R is a member of group I mutation and has dominant-negative effect on secretion of full-length VWF multimers: an analysis of two patients with type 2A von Willebrand disease

Type 2A von Willebrand disease (VWD) is characterized by decreased platelet-dependent function of von Willebrand factor (VWF); this in turn is associated with an absence of high-molecular-weight multimers. Sequence analysis of the VWF gene from two unrelated type 2A VWD patients showed an identical, novel, heterozygous T-->G transversion at nucleotide 4508, resulting in the substitution of L1503R in the VWF A2 domain. This substitution, which was not found in 60 unrelated normal individuals, was introduced into a full-length VWF cDNA and subsequently expressed in 293T cells. Only trace amount of the mutant VWF protein was secreted but most of the same was retained in 293T cells. Co-transfection experiment of both wild-type and mutant plasmids indicated the dominant-negative mechanism of disease development; as more of mutant DNA was transfected, VWF secretion was impaired in the media, whereas more of VWF was stored in the cell lysates. Molecular dynamic simulations of structural changes induced by L1503R indicated that the mean value of all-atom root-mean-squared-deviation was shifted from those with wild type or another mutation L1503Q that has been reported to be a group II mutation, which is susceptible to ADAMTS13 proteolysis. Protein instability of L1503R may be responsible for its intracellular retention and perhaps the larger VWF multimers, containing more mutant VWF subunits, are likely to be mal-processed and retained within the cell.

ADAMTS13 and microvascular thrombosis

Interaction between platelet and von Willebrand factor, a circulating adhesive glycoprotein, is essential for hemostasis under the high shear environments of arterioles and capillaries. If unregulated, this interaction may lead to unwarranted platelet thrombosis. ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, number 13), a plasma zinc metalloprotease synthesized primarily in the stellate cells of the liver, cleaves shear stress-activated von Willebrand factor, thereby preventing the occurrence of von Willebrand factor-platelet interaction in the circulation. A profound deficiency of ADAMTS13, due to genetic mutations or autoimmune inhibition, results in intravascular von Willebrand factor platelet aggregation and widespread microvascular thrombosis characteristic of thrombotic thrombocytopenic purpura. Cloning of ADAMTS13 and structure-function analyses of the enzyme are leading to exciting advances in the diagnosis and therapy of this hitherto mysterious disease.

Current concepts in thrombotic thrombocytopenic purpura

Recent advances have demonstrated that thrombotic thrombocytopenic purpura (TTP), characterized by widespread thrombosis in the arterioles and capillaries, is caused by deficiency of a circulating zinc metalloprotease, ADAMTS13. Two types of TTP are recognized: autoimmune TTP, caused by inhibitory antibodies of ADAMTS13, and hereditary TTP, caused by genetic mutations of ADAMTS13. This article reviews the characteristics and function of ADAMTS13, the mechanism by which ADAMTS13 deficiency may lead to thrombosis, and the causes of ADAMTS13 deficiency. It also discusses how the new knowledge may improve the diagnosis and treatment of this previously mysterious disorder.

Impact of mutations in the von Willebrand factor A2 domain on ADAMTS13-dependent proteolysis

Classical von Willebrand disease (VWD) type 2A, the most common qualitative defect of VWD, is caused by loss of high-molecular-weight multimers (HMWMs) of von Willebrand factor (VWF). Underlying mutations cluster in the A2 domain of VWF around its cleavage site for ADAMTS13. We investigated the impact of mutations commonly found in patients with VWD type 2A on ADAMTS13-dependent proteolysis of VWF. We used recombinant human ADAMTS13 (rhuADAMTS13) to digest recombinant full-length VWF and a VWF fragment spanning the VWF A1 through A3 domains, harboring 13 different VWD type 2A mutations (C1272S, G1505E, G1505R, S1506L, M1528V, R1569del, R1597W, V1607D, G1609R, I1628T, G1629E, G1631D, and E1638K). With the exception of G1505E and I1628T, all mutations in the VWF A2 domain increased specific proteolysis of VWF independent of the expression level. Proteolytic susceptibility of mutant VWF in vitro closely correlated with the in vivo phenotype in patients. The results imply that increased VWF susceptibility for ADAMTS13 is a constitutive property of classical VWD type 2A, thus explaining the pronounced proteolytic fragments and loss of HMWM seen in multimer analysis in patients.