ALBERT

Description: Von Willebrand Factor (VWF) is a large multimeric glycoprotein that mediates platelet adhesion to the damaged blood vessel wall and subsequent platelet aggregation at the site of vascular injury. The size of VWF multimers is regulated by the metalloprotease, ADAMTS13. Alterations in VWF sequence can lead to an increase or decrease in ADAMTS13-mediated cleavage, resulting in either a loss or increase in high molecular weight VWF multimers, respectively. With the availability of a VWF knockout mouse, variant forms of VWF can be evaluated in vivo in terms of their contribution to hemostasis and thrombosis. In these studies, we have taken into account the significant differences in VWF-GPIb binding and ADAMTS13 cleavage efficiency seen between mice and humans and have also assumed that functionally important residues are likely to be conserved between species. With these considerations in mind, the protocols described in this report utilize mouse-exclusive reagents. Previous reports of correction of the VWF KO phenotype using hydrodynamic gene delivery have shown contradictory results for correction of the bleeding time and blood volume loss as well as a grossly abnormal multimer structure of the rescued VWF protein, due initially to an inadvertent C799R mutation and latterly to factors possibly related to the site of VWF synthesis. In this study, we compare wild type VWF clearance to a cleavage site knockout, Y1605A/M1606A, a type 2A Von Willebrand Disease (VWD) mutation, R1597W, and the common type 1 VWD mutation, Y1584C. The murine R1597W variant exhibited increased ADAMTS13-mediated cleavage (0.36- fold ADAMTS13 concentration), and the Y1605A/M1606A variant greatly decreased cleavage in vitro (〉100-fold ADAMTS13 concentration). VWF KO mice, 7-10 weeks old, were injected with recombinant murine VWF (200U/kg) produced in HEK293 cells. VWF antigen levels (VWF:Ag), multimers, and complete blood counts (CBCs) were performed. Compared to the wild type infused protein (T1/2=33.2 minutes), the Y1605A/M1606A and R1597W mutant proteins show faster clearance (T1/2=17.7 minutes, p

Description: Hemophilia A is an excellent candidate disorder for the use of gene therapy as a treatment modality. To date, although lentiviral delivery of the factor VIII (FVIII) transgene has the potential to provide sustained therapeutic correction of the hemophilia A phenotype, this has not been achieved in adult animals because of the anti-FVIII immune response. We have used lentiviral vectors to deliver the canine FVIII transgene to hemophilia A neonates and although no anti-FVIII immune response occurred, and indeed the treated mice displayed long-term tolerance to the canine FVIII antigen, this strategy did not provide sustained therapeutic levels of plasma FVIII. To overcome these limitations, we modified our lentiviral vector and the protocol for viral delivery to enhance transduction of hepatocytes and direct transgene expression away from antigen presenting cells. We engineered lentiviral vectors that encode the B-domain deleted canine FVIII cDNA under the transcriptional control of either a non-viral ubiquitous promoter or two different liver-restricted promoters. However, no plasma FVIII was detected in any of the adult hemophilia A mice after intravenous injection of the various lentiviral vectors because of an anti-canine FVIII immune response. An alternate pseudotype (GP64) was used to enhance transduction of hepatocytes and a target sequence for a hematopoietic-specific microRNA was incorporated into the transgene to prevent FVIII expression in antigen presenting cells that may arise from promoter trapping. When hemophilia A mice received intravenous infusions of these modified vectors, where the cFVIII trangene is under the control of either of the liver-restricted promoters, all treated mice (n=4) showed sustained FVIII expression (mean FVIII levels 28.2±2.4 mU/mL) for more than 150 days (last time analyzed) without developing anti-FVIII antibodies. Moreover, temporary depletion of Kuppfer cells prior to viral administration resulted in a 3-fold elevation of levels of plasma FVIII (mean FVIII levels 83.3±2.1mU/mL; n=4). Analysis of the biodistribution of the integrated FVIII transgene and expression of canine FVIII mRNA indicate an enhanced restriction of FVIII expression in hepatocytes with the use of the modified lentiviral vectors. These results demonstrate, for the first time, the long-term therapeutic potential of modified lentiviral vectors for treating adult pre-clinical animal models of hemophilia A.

Description: Abstract 2217 In order to evaluate the possible role of abnormalities of von Willebrand factor in the hemostatic defects seen in indivdiuals with chronic kidney disease (CKD), a cohort study was performed evaluating pre- and post-dialysis levels of von Willebrand factor (VWF), VWF multimer profiles and levels of its cleaving protease, ADAMTS-13. There were 57 subjects (31 males, 26 females) enrolled with CKD with a mean age of 75 years (range 60 – 90). Subjects with known vascular disease were recruited; 49 (86%) had documented ischemic heart disease, 16 (29%) had cerebrovascular disease and 17 (31%) had peripheral vascular disease. A little over half had diabetes mellitus (30 subjects or 54%), 37 (67%) were on antiplatelet therapy and 7 (13%) were chronically anticoagulated with warfarin. Blood samples were drawn immediately pre- and again post-dialysis and all results were compared with a group of age-matched normal controls (Table 1). As has been previously reported, VWF antigen levels (VWF:Ag) and VWF functional activity as measured by the ristocetin cofactor assay (VWF:RCo) were higher in the pre-dialysis samples compared with controls, and both levels were increased even further following dialysis. Additionally, the percentage of high molecular weight VWF multimers (% HMWM) were significantly increased in the pre-dialysis samples compared with controls. This is a novel finding, and the level of % HMWM seen in the subjects is similar to what has been reported in individuals with Thrombotic Thrombocytopenic Purpura (TTP). This difference decreased following dialysis, potentially due to the effect of shear stress on VWF and the resultant proteolytic processing, however still remained significantly higher when compared with controls. ADAMTS-13 functional activity was lower in the subjects compared with controls, providing a possibly explanation for the increase in % HMWM. IL-6 levels are higher in subjects compared with controls. IL-6, which is an inflammatory cytokine known to be increased in patients with CKD, has been previously reported as a marker of inactivation of ADAMTS-13. Two years after enrollment, follow up of the subjects revealed that 22 had died, 17 from documented cardiovascular events. Higher VWF levels at the time of enrollment significantly correlated with risk of death (p=0.041) during the study period. The increase in % HMWM suggests that a “TTP-like phenotype” may also be playing a role. Taken together, these data suggest that both quantitative and qualitative abnormalities of VWF contribute to the risk of thrombotic death in chronic kidney disease. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 22 Introduction: Type 1 von Willebrand disease (VWD) is caused by mutations that result in moderate decreases in VWF antigen (VWF:Ag is 5–50% of normal levels) and a mild bleeding phenotype. The common recurrent VWF missense mutation Y1584C is associated with mildly decreased VWF:Ag levels, increased ADAMTS13 cleavage, as well as a possible increase in clearance. The Vicenza mutation, R1205H, exhibits a more severe phenotype (VWF:Ag ∼10%) and accelerated clearance. Although well described in patients and through in vitro studies, extensive controlled in vivo investigation of these mutations has yet to be performed. In this study, we compared both Y1584C and R1205H to wild type VWF using hydrodynamic gene delivery of mouse VWF and ADAMTS13 transgenes in the VWF knockout mouse to determine the pathological mechanisms associated with these variants. Methods: Hydrodynamic injections were performed using 100 μ g wild type (WT) or mutant mouse Vwf cDNA in Ringer's solution in 7–9 week old C57Bl6 VWF knockout mice, replacing plasma VWF. Co-injections with mouse Adamts13 cDNA were also performed. Mice were sampled at days 2, 5, 8, and then weekly. Mouse plasma was analyzed for complete blood counts, VWF:Ag, VWF propeptide, and VWF multimer structure. Thrombotic injury was induced using ferric chloride injury to the arterioles of the cremaster in VWF knockout mice expressing VWF:Ag levels from 0.5–2 U/ml. Platelets were labeled with Rhodamine-6G to evaluate platelet accumulation. Time to stable vessel occlusion and platelet accumulation by relative fluorescence intensity were compared. Results: Hydrodynamic injection caused no adverse events in any animals. Complete blood count values were unaffected for both variants compared to WT. Initial high VWF:Ag values at day 2 were similar for WT VWF (25.4 ± 2.5 U/ml, n= 12, mean U/ml±SEM, n) and Y1584C (26.8 ± 5.5, n= 10), but R1205H levels were 36% lower (16.3 ± 2.1, n= 10). Lower VWF:Ag levels were demonstrated in both “homozygous” and “heterozygous” forms for both type 1 mutations from days 14–42, when VWF expression plateaus. R1205H VWF:Ag was 34.3 ± 5.9% of WT (P 〈 0.001) and “heterozygous” 1:1 ratio R1205H/WT co-delivery was 27.5 ± 4.7% (p 〈 0.001). Y1584C was 29.4 ± 7.5% of WT (P 〈 0.001), and Y1584C/WT was 51.1 ± 4.6% (p 〈 0.001). VWF propeptide to VWF:Ag ratios (days 2–42) demonstrate that R1205H mouse VWF had an increased clearance rate (165.4 ± 13.5%, p 〈 0.001), while Y1584C was normal (97.1 ± 6.8 %, P 〉 0.05) compared to WT (100.0 ± 10.0%). The R1205H mutation showed no significant difference in multimer structure by mean multimer band numbers (days 2 to 42, 93 ± 16%, n = 4, P 〉 0.05) to wild type VWF (100 ± 12%, n = 4). In contrast, Y1584C had a significant decrease (66 ± 18%, n = 4, P 〈 0.001). This effect was exaggerated by co-delivery of mouse ADAMTS13 for Y1584C, but not R1205H. Y1584C showed reduced thrombus formation in a ferric chloride injury model while R1205H demonstrated similar thrombogenic activity to wild type VWF. Mean occlusion times were WT = 29.9 ± 2.1 minutes, n = 8, R1205H = 29.1 ± 4.0, n = 8 (p 〉 0.05), and Y1584C = 38.7 ± 1.1, n = 9 (p = 0.001). Total platelet accumulation was decreased for Y1584C (83.6 ± 6.3%, p = 0.043), but was similar for R1205H (103 ± 6.3, P = 0.72) and WT (100 ± 5.3%). Conclusions: This study demonstrates that these two type 1 VWD mutations have a strong observable effect in the VWF knockout mouse model. R1205H exhibits a large decrease in VWF:Ag levels and evidence of accelerated clearance with R1205H. However, there is no alteration in multimer structure and apparently normal participation in a thrombosis model. Y1584C, in contrast, shows a loss of high molecular weight multimers that is exacerbated by the additional expression of ADAMTS13, indicating that ADAMTS13 cleavage is increased. Y1584C also has an initially high VWF:Ag level that was less than WT levels from day 14 onward, but shows no alteration in clearance, suggesting that there is a biosynthetic defect. Y1584C shows a significant defect in the arteriolar thrombosis model, presenting a Type 2A VWD-like phenotype in the mouse model, which is more severe than the human phenotype. This study has elucidated several novel mechanistic details for these two mutations and highlights that the pathogenic aspects of type 1 VWD can be recapitulated in the VWF knockout hydrodynamic injection model. Disclosures: No relevant conflicts of interest to declare.

Description: Hemophilia is an excellent candidate disorder for the use of gene therapy as a treatment modality. However, significant obstacles have been encountered with systemic delivery of viral vectors that have prevented sustained expression of the therapeutic protein. Investigation of alternative gene therapy strategies for hemophilia that enhance safety and facilitate long-term, therapeutic levels of the transgene product is imperative. In this study, we evaluated an ex vivo gene therapy strategy for hemophilia A. Circulating endothelial cell progenitors (blood outgrowth endothelial cells - BOECs) were isolated from canine and mouse blood and transduced with a third generation self-inactivating lentiviral vector encoding the canine FVIII transgene under the transcriptional control of either the CMV promoter or an endothelial cell-specific regulatory element. Transduced BOECs were injected either intravenously (IV) or subcutaneously mixed with Matrigel (SC+Matrigel) into NOD/SCID mice. Canine FVIII antigen levels were assayed at weekly intervals using an Asserachrom VIII:Ag ELISA that detects canine FVIII against a background of normal murine FVIII levels in the NOD/SCID mice. The mean FVIII antigen levels in mice injected with BOECs at 3 weeks following treatment were 37.5 mU/mL and 105.8mU/mL, for IV and SC+Matrigel administration, respectively. These FVIII antigen levels were sustained up to 12 weeks at therapeutic levels (21.3mU/mL and 21.7mU/mL, for IV and SC+Matrigel administration respectively). To evaluate if the observed loss of FVIII expression by 12 weeks post-treatment resulted from transcriptional silencing of the viral promoter, the CMV promoter was replaced with the endothelial cell-specific thrombomodulin (TM) promoter and transduced BOECs were implanted SC with Matrigel. In contrast to results from the CMV-regulated transgene, sustained therapeutic levels of FVIII have been documented for the duration of the study with the TM-regulated construct (34.3 mU/mL at 3 weeks and 22.5 mU/mL at 20 weeks) Immunostaining at 18 weeks after SC implantation of the transduced BOECs, shows that these cells still express FVIII and von Willebrand Factor. Biodistribution analysis by flow cytometry and quantitative PCR demonstrated that SC-implanted BOECs were retained inside the scaffold and were not detected at any other anatomic site. These results indicate that genetically-modified endothelial progenitors implanted in a SC scaffold can provide sustained therapeutic levels of FVIII and are a promising safe delivery vehicle for gene therapy of hemophilia. Currently, these engineered cells have been implanted into immunocompetant mice and FVIII levels are being assessed.

Description: Type 1 VWD is the mild to moderate reduction of VWF levels. This study examined the mechanisms underlying 2 common type 1 VWD mutations, the severe R1205H and more moderate Y1584C. In vitro biosynthesis was reduced for both mutations in human and mouse VWF, with the effect being more severe in R1205H. VWF knockout mice received hydrodynamic injections of mouse Vwf cDNA. Lower VWF antigen levels were demonstrated in both homozygous and heterozygous forms for both type 1 mutations from days 14-42. Recombinant protein infusions and hydrodynamic-expressed VWF propeptide to antigen ratios demonstrate that R1205H mouse VWF has an increased clearance rate, while Y1584C is normal. Recombinant ADAMTS13 digestions of Y1584C demonstrated enhanced cleavage of both human and mouse VWF115 substrates. Hydrodynamic-expressed VWF shows a loss of high molecular weight multimers for Y1584C compared with wild-type and R1205H. At normal physiologic levels of VWF, Y1584C showed reduced thrombus formation in a ferric chloride injury model while R1205H demonstrated similar thrombogenic activity to wild-type VWF. This study has elucidated several novel mechanisms for these mutations and highlights that the type 1 VWD phenotype can be recapitulated in the VWF knockout hydrodynamic injection model.

Description: Key PointsThe robustness of the VWF:collagen-binding assay is confirmed in a comprehensive evaluation of VWD collagen-binding defects. Collagen binding by VWF, GPVI, and α2β1 have major albeit overlapping functions in primary hemostasis.

Description: Abstract 23 Introduction: Type 2B von Willebrand disease (2B VWD), a qualitative variant of VWD, is characterized by thrombocytopenia and loss of high molecular weight multimers. 2B VWF has both an increased affinity for platelet glycoprotein Ibα (GPIbα) and increased susceptibility to ADAMTS13-mediated cleavage. In order to determine whether the loss of high molecular weight (HMW) multimers associated with 2B VWD occurs due to the increased ADAMTS13 susceptibility or due to the increased binding to platelet GPIbα, we created a modified type 2B mouse model, in which the mice expressed ADAMTS13-insensitive 2B VWF. Methods: Three common type 2B VWD mutations (R1306W, V1316M, and R1341Q) were independently introduced into the mouse Vwf (mVWF) cDNA sequence containing the ADAMTS13 cleavage site knockout, Y1605A/M1606A (CSKO). Recombinant mutant mVWF was produced using transient transfection of HEK293T cells. ADAMTS13 digestion of the recombinant mVWF was performed using a Tris-Urea system. Platelet binding affinity was assessed using a botrocetin-induced platelet-binding ELISA assay. 8–9 week old C57Bl6 VWF KO mice were hydrodynamically injected with 100 μ g of naked plasmid DNA containing the liver specific ET promoter and wildtype (WT) or mutant mouse Vwf DNA. The mice were sampled weekly; complete blood counts, VWF:Ag levels, VWFpp levels, and VWF multimer structure were examined. Results: Following transient transfections, mutant and wildtype mVWF were secreted at similar levels, with a full range of VWF multimer sizes. Recombinant 2B/CSKO mVWF is markedly ADAMTS13 cleavage insensitive, with 〉80 U mADAMTS13 required for 50% cleavage (WT mVWF requires 1 U mADAMTS13). Similar to 2B VWF, 2B/CSKO mVWF showed enhanced platelet binding affinity in the presence of botrocetin. Hydrodynamic injections of the naked expression plasmids did not result in adverse events in the mice. Mean 2B/CSKO platelet counts at day 7 were significantly reduced compared to WT platelet counts (149 and 370, respectively). At day 14, only the mice expressing V1316M/CSKO remained thrombocytopenic, with platelet counts similar to those seen with the V1316M mutation alone (170 and 171, respectively). By day 14, the mice expressing 2B/CSKO VWF had significantly lower mean VWF:Ag levels than WT mice or mice expressing the CSKO alone (see table below). The mean VWFpp/VWF:Ag ratios were significantly increased in the 2B/CSKO mice when compared to the WT mice (1.53 and 1.00, respectively). There was no loss of HMW multimers in the mice expressing R1306W/CSKO and R1341Q/CSKO and, indeed, these mice showed significantly more multimer bands than WT mice (days 7 to 21; 14.5±0.6 and 13.9±1.2 multimer bands versus 10.2±0.5 bands for WT). In marked contrast, the mice expressing V1316M/CSKO VWF showed complete loss of high molecular weight multimers, similar to mice expressing the same 2B VWF mutation alone (8.3±1.8 and 7.7±0.9 multimer bands, respectively). Conclusions: Mice expressing R1306W/CSKO and R1341Q/CSKO VWF have an absence of thrombocytopenia and show the presence of supranormal high molecular weight VWF multimer bands. These results suggest that enhanced ADAMTS13-mediated cleavage plays an important role in the type 2B phenotype associated with these mutations. In contrast, low V1316M/CSKO VWF:Ag was associated with both thrombocytopenia and loss of HMW multimers, demonstrating that enhanced GPIbα binding affinity is the predominant mechanism associated with this mutation. Thus, the importance of ADAMTS13 susceptibility in 2B VWD is mutation-dependent, and ADAMTS13-mediated cleavage plays an important role in enhancing the severity of the type 2B VWD phenotype associated with the R1306W and R1341Q mutations. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 3578 Poster Board III-515 Ex vivo delivery of therapeutic transgenes provides an additional level of safety as it avoids systemic administration of viral vectors. Our group has shown that autologous blood outgrowth endothelial cells (BOECs) transduced with a lentiviral vector delivery system containing the FVIII transgene is a promising gene therapy strategy for hemophilia A. We have shown that subcutaneous implantation of factor (F) VIII-expressing BOECs in a murine model of hemophilia A can produce therapeutic levels of FVIII that are sustained for more than 6 months. However, to improve the levels of FVIII expression and cell viability we wanted to evaluate the omentum as an alternative site for BOEC implantation. Initially this strategy was evaluated in two normal dogs. One and three months after delivery of the cells, immunostaining of biopsies from the injection sites showed the presence of viable cells expressing FVIII and evidence of neovascularization. To evaluate the efficacy of this strategy, a hemophilia A dog received 5×108 transduced autologous BOECs that expressed high levels of FVIII in vitro (1.5 IU/106 cells/24hrs). We used autologous fibrinogen as a vehicle for the cells along with canine endothelial growth factors (VEGF and bFGF). For the implantation procedure the dog received prophylaxis with canine cryoprecipitate transfusions. FVIII antigen levels (FVIII:Ag) of between 20 and 50 ng/mL continue to be detected in the plasma 8 months post-implantation, indicating that these cells remain viable and express persistent high levels of FVIII over an extended period of time. However, two weeks after the procedure, the dog developed an anti-FVIII immune response comprising both inhibitory and non-inhibitory antibodies, and therefore no FVIII coagulant activity (FVIII:C) was detected. With a view to preventing the development of the anti-FVIII immune response, we used immunosuppression with cyclophosphamide in two additional hemophilia A dogs. Each of these dogs received 5 × 108 transduced autologous BOECs. In place of fibrinogen, these cells were delivered in a gel comprised of synthetic, heparin-binding peptide-amphiphiles (HBPA) and heparan sulfate, along with canine VEGF and bFGF. The peptide gel prolongs the activity of these growth factors and protects them from proteolysis, enhancing their angiogenic activity. HBPA gel has been shown to increase vascularization of cell transplant sites, which should improve BOEC survival in the omentum. The procedure of implanting the genetically modified BOECs was completed without complications in all hemophilia A dogs and we are continuing to evaluate the efficacy of this strategy. Disclosures: Hulvat: Nanotope Inc.: Employment.