Intravascular recovery of VWF and FVIII following intraperitoneal injection and differences from intravenous and subcutaneous injection in mice.

Intravenous infusion studies in humans suggest that both von Willebrand factor (VWF) and factor VIII (FVIII) remain intravascular in contrast to other coagulation proteins. We explored whether infusion of VWF and FVIII by either intraperitoneal (i.p.) or subcutaneous (s.c.) injection would result in efficient absorption of these large proteins into the vascular circulation. FVIII(null) or VWF(null) mice were infused with plasma-derived or recombinant VWF and/or FVIII by i.p., s.c., or intravenous (i.v.) injection. Both VWF and FVIII were absorbed into the blood circulation after i.p. injection with a peak between 2 and 4 h at levels similar to those observed in mice infused intravenously. In contrast, neither VWF nor FVIII was detected in the plasma following s.c. injection. Although i.v. injection achieved peak plasma levels quickly, both human VWF and FVIII rapidly decreased during the first 2 h following i.v. injection. Following both i.v. and i.p. infusion of VWF, the multimeric structure of circulating VWF was similar to that observed in the infusate. These results demonstrate that both VWF and FVIII can be efficiently absorbed into the blood circulation following i.p., but not s.c. injection, indicating that i.p. administration could be an alternative route for VWF or FVIII infusion.

Lentivirus-mediated platelet-derived factor VIII gene therapy in murine haemophilia A.

BACKGROUND: Previous studies from our laboratory have demonstrated that lineage-targeted synthesis of factor VIII (FVIII) under the direction of the platelet-specific integrin alphaIIb gene promoter (2bF8) can correct the murine haemophilia A phenotype even in the presence of high titer inhibitory antibodies in a transgenic mouse model. OBJECTIVE: In this study, we assessed the efficacy of using a genetic therapy approach to correct haemophilia A in FVIII-deficient (FVIII(null)) mice by transplantation of bone marrow (BM) transduced with a lentivirus (LV)-based gene transfer cassette encoding 2bF8. RESULTS: Functional FVIII activity (FVIII:C) was detected in platelet lysates from treated mice and the levels were similar to 2bF8 heterozygous transgenic mice. Mice transplanted with 2bF8 LV-transduced BM survived tail clipping and we did not detected inhibitory or non-inhibitory FVIII antibodies over the period of this study (11 months). Furthermore, BM transferred from the primary transplant recipients into FVIII(null) secondary recipients demonstrated sustained platelet-FVIII expression leading to correction of the haemophilia A phenotype showing that gene transfer occurred within long-term repopulating haematopoietic stem cells. CONCLUSIONS: These results demonstrate that ectopic expression of FVIII in platelets by lentivirus-mediated bone marrow transduction/transplantation may be a promising strategy for gene therapy of haemophilia A in humans.

An efficient method to successively introduce transgenes into a given genomic locus in the mouse.

BACKGROUND: Expression of transgenes in mice requires transcriptional regulatory elements that direct expression in a chosen cell type. Unfortunately, the availability of well-characterized promoters that direct bona-fide expression of transgenes in transgenic mice is limited. Here we described a method that allows highly efficient targeting of transgenes to a preselected locus in ES cells. RESULTS: A pgk-LoxP-Neo cassette was introduced into a desired genomic locus by homologous recombination in ES cells. The pgk promoter was then removed from the targeted ES cells by Cre recombinase thereby restoring the ES cells' sensitivity to G418. We demonstrated that transgenes could be efficiently introduced into this genomic locus by reconstituting a functional Neo gene. CONCLUSION: This approach is simple and extremely efficient in facilitating the introduction of single-copy transgenes into defined genomic loci. The availability of such an approach greatly enhances the ease of using endogenous regulatory elements to control transgene expression and, in turn, expands the repertoire of elements available for transgene expression.

Targeting platelet GPIbalpha transgene expression to human megakaryocytes and forming a complete complex with endogenous GPIbbeta and GPIX.

Bernard-Soulier Syndrome (BSS) is a severe congenital platelet disorder that results from a deficiency of the platelet membrane glycoprotein (GP) Ib/IX complex that is composed of four subunits (GPIbalpha, GPIbbeta, GPIX, and GPV). Mutations in either GPIbalpha, GPIbbeta, or GPIX can result in BSS with many of the known mutations occurring in GPIbalpha. In this study, we have developed a gene therapy strategy to express hemagglutinin (HA)-tagged GPIbalpha in megakaryocytes and potentially correct a hereditary deficiency. To direct GPIbalpha expression in megakaryocytic lineage cells, we designed a GPIbalpha cassette where human GPIbalpha cDNA was placed under control of the megakaryocytic/platelet-specific alphaIIb promoter and inserted into a lentiviral vector. Human CD34+ peripheral blood cells (PBC) and Dami cells were transduced with alphaIIb-HA-GPIbalpha-WPT virus. Flow cytometry analysis demonstrated that 50.1% of the megakaryocytes derived from CD34+ stem cells and 97.3% of Dami cells were transduced and expressed transgene GPIbalpha protein. Immunoprecipitation with Western blot analysis demonstrated that transgene protein associated with endogenous GPIbbeta and GPIX proteins. To address further the lineage-specific expression of the alphaIIb-HA-GPIbalpha construct, three cell lines, Dami, AtT-20 and HepG2, were transfected with GPIbalpha expression plasmids and analyzed by confocal microscopy. The results demonstrated that among these three cell lines, the tissue-specific alphaIIb promoter was active only in Dami cells. Thus, GPIbalpha can be efficiently and specifically expressed in the megakaryocytic compartment of hematopoietic cells and the transgene product associates with endogenous GPIbbeta and GPIX forming a complete complex. This strategy could potentially be utilized for gene therapy of BSS.

Cloning and expression of canine glycoprotein Ibalpha.

The interaction of the glycoprotein (GP) Ib-IX-V complex with von Willebrand factor (vWF) is critical in initiation of haemostasis and thrombosis through platelet adhesion to damaged endothelium. The binding site for vWF resides within the GPIbalpha subunit of the complex. To further define the physiological function of platelet GPIbalpha we cloned and expressed the canine GPIbalpha cDNA. A canine platelet cDNA library was constructed and screened with a randomly primed 32P-labeled 1041-base-pair restriction fragment of the human GPIbalpha cDNA. Analysis of 23 clones demonstrated that the canine GPIbalpha cDNA is 2530 nucleotides in length and includes a short 5' untranslated segment of 42 nucleotides followed by a signal peptide of 16 amino acids, a mature peptide of 645 amino acids and a 3' noncoding region of 455 nucleotides. A single intron of 142 nucleotides, 6 nucleotides upstream from the ATG translation initiation codon was identified in the canine gene in a similar location to that present in the human gene. Chinese hamster ovary cells that stably express human GPIbbeta and GPIX were transfected with the canine GPIbalpha cDNA. Canine GPIbalpha was expressed on the surface of these cells and bound vWF in the presence of botrocetin. The binding of vWF was inhibited by an anti-vWF human monoclonal antibody known to inhibit vWF binding to GPIbalpha. The results of this investigation will allow the development of reagents to study the physiological function of GPIbalpha in an animal model.

Non-myeloablative conditioning with busulfan before hematopoietic stem cell transplantation leads to phenotypic correction of murine Bernard-Soulier syndrome.

BACKGROUND: Bernard-Soulier syndrome (BSS) is an inherited bleeding disorder characterized by macrothrombocytopenia. Platelet transfusion is used for the management of bleeding, but repeated transfusion often results in alloimmunization. We have recently shown phenotypic correction of murine BSS (GPIbα(null) ) using lethal radiation conditioning followed by hematopoietic lentivirus-mediated gene transfer. OBJECTIVES: For application of gene therapy to treatment of human patients, it is important to minimize treatment-related side effects. The objective of this study is to model a clinically relevant non-myeloablative hematopoietic stem cell (HSC) transplantation strategy. METHODS: Using transplantation of bone marrow (BM) HSCs from transgenic mice that express hGPIbα (hGPIbα(tg+/+) ), we sought to (i) determine the percentage of hGPIbα(tg+/+) HSCs required for therapeutic benefit, (ii) evaluate the efficacy of non-myeloablative conditioning using busulfan, and (iii) test the ability of anti-thymocyte globulin (ATG) to prevent/reduce undesirable immune responses. RESULTS: Transplantation of 10-20% hGPIbα(tg+/+) BM HSCs mixed with GPIbα(null) BM HSCs into irradiated GPIbα(null) mice was sufficient to correct bleeding time (n = 5). Transplantation of hGPIbα(tg+/+) BM HSCs into busulfan-conditioned GPIbα(null) mice corrected bleeding time in 21 of 27 recipients. Antibody response to hGPIbα and immune-mediated thrombocytopenia was documented in eight of 27 recipients, suggesting immunogenicity of hGPIbα in busulfan-conditioned GPIbα(null) mice. However, these antibodies disappeared without treatment within 30 weeks after transplantation. A combination of busulfan plus ATG conditioning successfully prevented antibody development and significantly increased therapeutic engraftment. CONCLUSION: A conditioning regimen of busulfan in combination with ATG could potentially be used in non-myeloablative autologous gene therapy in human BSS.

Contribution of platelet vs. endothelial VWF to platelet adhesion and hemostasis.

BACKGROUND: von Willebrand factor (VWF) is a glycoprotein that plays an important role in primary hemostasis. VWF is synthesized and stored in endothelial cells (ECs) and megakaryocytes/platelets. Plasma VWF is primarily derived from ECs and is generally believed to be essential for hemostasis. VWF synthesized in megakaryocytes is stored in platelet α-granules, from which it is released following platelet activation. The relative contribution of VWF stored in ECs or megakaryocytes/platelets or present in plasma to hemostasis is not clear. OBJECTIVES: We investigated whether EC-derived VWF plays the major role in hemostasis while the contribution of platelet-derived VWF is negligible, or if platelet-derived VWF also significantly contributes to hemostasis. METHODS AND RESULTS: Mice expressing VWF only in ECs (EC-VWF) or platelets (Plt-VWF) were created by reciprocal bone marrow transplantation between C57BL/6J (WT) and VWF knockout mice (VWF-/-). Plasma VWF levels in EC-VWF were similar to WT. Plt-VWF mice had a trace amount of VWF in their plasma while VWF levels in platelet lysate were comparable to WT. Tail bleeding time was normal in EC-VWF. Interestingly, Plt-VWF showed partially corrected bleeding time and significantly decreased blood loss volume compared with VWF-/-. Adhesion of platelets perfused over immobilized collagen under shear stress was significantly higher in both EC-VWF and Plt-VWF compared with VWF-/-. CONCLUSION: VWF synthesized in ECs is sufficient to support hemostasis in VWF-/- mice, and VWF produced in megakaryocytes/platelets can also contribute to hemostasis in the absence of EC-derived VWF.

Lentivirus-mediated platelet gene therapy of murine hemophilia A with pre-existing anti-factor VIII immunity.

BACKGROUND: The development of inhibitory antibodies, referred to as inhibitors, against exogenous factor VIII in a significant subset of patients with hemophilia A remains a persistent challenge to the efficacy of protein replacement therapy. Our previous studies using the transgenic approach provided proof-of-principle that platelet-specific expression could be successful in treating hemophilia A in the presence of inhibitory antibodies. OBJECTIVE: To investigate a clinically translatable approach for platelet gene therapy of hemophilia A with pre-existing inhibitors. METHODS: Platelet FVIII expression in preimmunized FVIII(null) mice was introduced by transplantation of lentivirus-transduced bone marrow or enriched hematopoietic stem cells. FVIII expression was determined with a chromogenic assay. The transgene copy number per cell was quantitated with real-time PCR. Inhibitor titer was measured with the Bethesda assay. Phenotypic correction was assessed by the tail clipping assay and an electrolytically induced venous injury model. Integration sites were analyzed with linear amplification-mediated PCR. RESULTS: Therapeutic levels of platelet FVIII expression were sustained in the long term without evoking an anti-FVIII memory response in the transduced preimmunized recipients. The tail clip survival test and the electrolytic injury model confirmed that hemostasis was improved in the treated animals. Sequential bone marrow transplants showed sustained platelet FVIII expression resulting in phenotypic correction in preimmunized secondary and tertiary recipients. CONCLUSIONS: Lentivirus-mediated platelet-specific gene transfer improves hemostasis in mice with hemophilia A with pre-existing inhibitors, indicating that this approach may be a promising strategy for gene therapy of hemophilia A even in the high-risk setting of pre-existing inhibitory antibodies.

Factor VIII inhibitors: von Willebrand factor makes a difference in vitro and in vivo.

BACKGROUND: The important association between von Willebrand factor (VWF) and factor VIII (FVIII) has been investigated for decades, but the effect of VWF on the reactivity of FVIII inhibitory antibodies, referred to as inhibitors, is still controversial. OBJECTIVE: To investigate the interaction among VWF, FVIII and FVIII inhibitory antibodies. METHODS: Three sources of inhibitors were used for in vitro studies, including the plasma from immunized VWF(null) FVIII(null) mice, purified plasma IgG from human inhibitor patients, or human monoclonal antibody from inhibitor patients' B-cell clones. Inhibitors were incubated with recombinant human FVIII (rhFVIII) either with or without VWF. The remaining FVIII activity was determined by chromogenic assay and inhibitor titers were determined. For in vivo studies, inhibitors and rhFVIII were infused into FVIII(null) or VWF(null) FVIII(null) mice followed by a tail clip survival test. RESULTS: VWF has a dose-dependent protective effect on FVIII, limiting inhibitor inactivation of FVIII in both mouse and human samples. A preformed complex of VWF with FVIII provides more effective protection from inhibitors than competitive binding of antibodies and VWF to FVIII. The protective effect of VWF against FVIII inactivation by inhibitors was further confirmed in vivo by infusing inhibitors and FVIII into FVIII(null) or VWF(null) FVIII(null) mice followed by a tail clip survival test. CONCLUSION: Our results demonstrate that VWF exerts a protective effect, reducing inhibitor inactivation of FVIII, both in vitro and in vivo.

von Willebrand factor storage and multimerization: 2 independent intracellular processes.

The von Willebrand factor propeptide, vW AgII, has been shown to be required for the formation of vWF multimers and sorting of vWF to storage granules; whether these 2 processes are independent events has been unclear. Chimeric constructs of human and canine vWF were developed to further define these processes and to determine whether they are independent intracellular events. Cells expressing only mature vWF (Deltapro) produced vWF dimers that were not stored in AtT-20 cells; whereas the expression of vW AgII alone resulted in vW AgII granular storage. Expression of vW AgII in trans with Deltapro resulted in the multimerization of vWF and colocalized storage of vW AgII and vWF. Expression of canine vW AgII in trans or cis with human Deltapro resulted in the multimerization of human vWF, with no storage of human vWF but with normal storage of canine vW AgII. This dissociation of functions indicates that the signals for multimerization of vWF are different from the signals for trafficking of vWF to storage and demonstrates that vWF storage and multimerization are 2 independent intracellular processes. vW AgII contains the signal(s) required for trafficking to storage, and only through interaction with vW AgII is vWF chaperoned into granules. (Blood. 2000;96:1808-1815)

The defective interaction between von Willebrand factor and factor VIII in a patient with type 1 von Willebrand disease is caused by substitution of Arg19 and His54 in mature von Willebrand factor.

In this report we describe the further investigation of the von Willebrand factor (vWF)/FVIII interaction in a type 1 von Willebrand disease patient characterized by discrepant VIII:C levels as determined by one-stage and two-stage VIII:C assays. A solid-phase binding assay shows that this patient's plasma vWF is moderately defective in capturing recombinant FVIII. Sequence analysis of the FVIII-binding domain encoded by the vWF mRNA of the affected individual identified mutations in both vWF alleles. In allele A, the mutations C2344T and T2451A result in the substitution of Trp for Arg19 (R19W) and of G1n for His54 (H54Q) in mature vWF, respectively. This allele also contains a reported polymorphism (A2365G, Thr26Ala). Allele B, which is underexpressed at the RNA level, contains a one-nucleotide deletion in the FVIII-binding domain (delta G2515) that results in the premature termination of translation. Analysis of the binding of FVIII by full-length vWF transiently expressed in COS-7 cells confirms that the combined R19W and H54Q substitutions are the cause of the defective vWF/FVIII interaction in this patient. The FVIII-binding defect of vWF containing either mutation alone is approximately half that of the double mutant, which suggests that the effect of these mutations is additive. The mutant proteins are recognized equally well by vWF monoclonal antibodies MBC105.4, 32B12, and 31H3, which block the binding of FVIII by vWF, indicating that amino acids Arg19, Thr26, and His54 are not critical residues in the epitopes of these antibodies.

Expression of human factor VIII under control of the platelet-specific alphaIIb promoter in megakaryocytic cell line as well as storage together with VWF.

Hemophilia A, which results in defective or deficient factor VIII (FVIII) protein, is one of the genetic diseases that has been addressed through gene therapy trials. FVIII synthesis does not occur in normal megakaryocytes. In hemophilia patients who have inhibitors to FVIII activity, megakaryocytes could be a protected site of FVIII synthesis and subsequent release. Since von Willebrand factor (VWF) is a carrier protein for FVIII, we hypothesize that by directing FVIII synthesis to megakaryocytes, it would traffick together with VWF to storage in megakaryocyte alpha-granules and the platelets derived from these cells. Such synthesis would establish a protected, releasable alpha-granule pool of FVIII together with VWF. When platelets are activated in a region of local vascular damage, FVIII and VWF could potentially be released together to provide improved local hemostatic effectiveness. To direct FVIII expression to the megakaryocyte lineage, we designed a FVIII expression cassette where the human B-domain deleted FVIII cDNA was placed under the control of the megakaryocytic/platelet-specific glycoprotein IIb (alphaIIb) promoter. We demonstrated by means of a functional FVIII activity assay that the biosynthesis of FVIII occurred normally in Dami cells transfected with FVIII. FVIII production was higher when driven by the alphaIIb promoter compared to the CMV promoter, and was increased about 8-fold following PMA treatment of the transfected Dami cells. Immunofluorescence staining of the transfected cells showed that FVIII stored together with VWF in the granules. The data indicate that the megakaryocytic compartment of hematopoietic cells may represent a potential target of gene therapy for hemophilia A-especially in those patients who have developed inhibitors to plasma FVIII.

Abnormal binding of factor VIII is linked with the substitution of glutamine for arginine 91 in von Willebrand factor in a variant form of von Willebrand disease.

von Willebrand factor (vWf) is a multimeric plasma glycoprotein that functions in hemostasis as the initiator of platelet adhesion to damaged blood vessels and as the carrier of Factor VIII (FVIII). Montgomery et al. (Montgomery, R.R., Hathaway, W.E., Johnson, J., Jacobsen, L., and Muntean, W. (1982) Blood 60, 201-207) reported a variant of von Willebrand disease characterized by the abnormal interaction between FVIII and a defective vWf. To identify the molecular basis of this abnormal interaction, we isolated platelet RNA from members of one of the affected families and determined the nucleotide sequence of the FVIII-binding domain encoded by the vWf mRNA. A single G to A transition at nucleotide 2561 was linked with disease expression and results in the substitution of Gln for Arg91 in mature vWf. A restriction fragment containing this mutation was introduced into a full-length vWf expression vector, and both wild type and mutant vWf were expressed in COS-7 cells. In a solid-phase binding assay, expressed vWf was captured with anti-vWf monoclonal antibody AVW1 and then incubated with 6.25-400 milliunits of recombinant FVIII. After washing, vWf-bound FVIII activity was determined with a chromogenic assay. Mutant vWf showed reduced binding of FVIII compared with wild type, suggesting that the substitution of Gln for Arg91 is the likely basis for the abnormal vWf/FVIII interaction in this von Willebrand disease variant.