A viable mouse model of factor X deficiency provides evidence for maternal transfer of factor X

BACKGROUND

Activated factor X (FXa) is a vitamin K-dependent serine protease that plays a pivotal role in blood coagulation by converting prothrombin to thrombin. There are no reports of humans with complete deficiency of FX, and knockout of murine F10 is embryonic or perinatal lethal.

OBJECTIVE

We sought to generate a viable mouse model of FX deficiency.

METHODS

We used a socket-targeting construct to generate F10-knockout mice by eliminating F10 exon 8 (knockout allele termed F10(tm1Ccmt), abbreviated as '-'; wild-type '+'), and a plug-targeting construct to generate mice expressing a FX variant with normal antigen levels but low levels of FX activity [4-9% normal in humans carrying the defect, Pro343-->Ser, termed FX Friuli (mutant allele termed F10(tm2Ccmt), abbreviated as F)].

RESULTS

F10 knockout mice exhibited embryonic or perinatal lethality. In contrast, homozygous Friuli mice [F10 (F/F)] had FX activity levels of approximately 5.5% (sufficient to rescue both embryonic and perinatal lethality), but developed age-dependent iron deposition and cardiac fibrosis. Interestingly, F10 (-/F) mice with FX activity levels of 1-3% also showed complete rescue of lethality. Further study of this model provides evidence supporting a role of maternal FX transfer in the embryonic survival.

CONCLUSIONS

We demonstrate that, while complete absence of FX is incompatible with murine survival, minimal FX activity as low as 1-3% is sufficient to rescue the lethal phenotype. This viable low-FX mouse model will facilitate the development of FX-directed therapies as well as investigation of the FX role in embryonic development.

A viable mouse model of factor X deficiency provides evidence for maternal transfer of factor X

BACKGROUND

Activated factor X (FXa) is a vitamin K-dependent serine protease that plays a pivotal role in blood coagulation by converting prothrombin to thrombin. There are no reports of humans with complete deficiency of FX, and knockout of murine F10 is embryonic or perinatal lethal.

OBJECTIVE

We sought to generate a viable mouse model of FX deficiency.

METHODS

We used a socket-targeting construct to generate F10-knockout mice by eliminating F10 exon 8 (knockout allele termed F10(tm1Ccmt), abbreviated as '-'; wild-type '+'), and a plug-targeting construct to generate mice expressing a FX variant with normal antigen levels but low levels of FX activity [4-9% normal in humans carrying the defect, Pro343-->Ser, termed FX Friuli (mutant allele termed F10(tm2Ccmt), abbreviated as F)].

RESULTS

F10 knockout mice exhibited embryonic or perinatal lethality. In contrast, homozygous Friuli mice [F10 (F/F)] had FX activity levels of approximately 5.5% (sufficient to rescue both embryonic and perinatal lethality), but developed age-dependent iron deposition and cardiac fibrosis. Interestingly, F10 (-/F) mice with FX activity levels of 1-3% also showed complete rescue of lethality. Further study of this model provides evidence supporting a role of maternal FX transfer in the embryonic survival.

CONCLUSIONS

We demonstrate that, while complete absence of FX is incompatible with murine survival, minimal FX activity as low as 1-3% is sufficient to rescue the lethal phenotype. This viable low-FX mouse model will facilitate the development of FX-directed therapies as well as investigation of the FX role in embryonic development.

The enhancing effects of the light chain on heavy chain secretion in split delivery of factor VIII gene

Coagulation factor VIII (FVIII) is secreted as a heterodimer consisting of a heavy chain (HC) and a light chain (LC), which can be expressed independently and reassociate with recovery of biological activity. Because of the size limitation of adeno-associated virus (AAV) vectors, a strategy for delivering the HC and LC separately has been developed. However, the FVIII HC is secreted 10-100-fold less efficiently than the LC. In this study, we demonstrated that the F309S mutation and enhanced B-domain glycosylations alone are not sufficient to improve FVIII HC secretion, which suggested a role of the FVIII LC in regulating HC secretion. To characterize this role of the FVIII LC, we compared FVIII HC secretion with and without the LC via post-translational protein trans-splicing. As demonstrated in vitro, ligation of the LC to the HC significantly increased HC secretion. Such HC secretion increases were also confirmed in vivo by hydrodynamic injection of FVIII intein plasmids into hemophilia A mice. Moreover, similar enhancement of HC secretion can also be observed when the LC is supplied in trans, which is probably due to the spontaneous association of the HC and the LC in the secretion pathway. In sum, enhancing the secretion of the FVIII HC polypeptide may require the proper association of the FVIII LC polypeptide in cis or in trans. These results may be helpful in designing new strategies to improve FVIII gene delivery.

Modulation of tolerance to the transgene product in a nonhuman primate model of AAV-mediated gene transfer to liver

Adeno-associated virus (AAV)-mediated gene transfer of factor IX (F.IX) to the liver results in long-term expression of transgene in experimental animals, but only short-term expression in humans. Loss of F.IX expression is likely due to a cytotoxic immune response to the AAV capsid, which results in clearance of transduced hepatocytes. We used a nonhuman primate model to assess the safety of AAV gene transfer coupled with an anti-T-cell regimen designed to block this immune response. Administration of a 3-drug regimen consisting of mycophenolate mofetil (MMF), sirolimus, and the anti-IL-2 receptor antibody daclizumab consistently resulted in formation of inhibitory antibodies to human F.IX following hepatic artery administration of an AAV-hF.IX vector, whereas a 2-drug regimen consisting only of MMF and sirolimus did not. Administration of daclizumab was accompanied by a dramatic drop in the population of CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs). We conclude that choice of immunosuppression (IS) regimen can modulate immune responses to the transgene product upon hepatic gene transfer in subjects not fully tolerant; and that induction of transgene tolerance may depend on a population of antigen-specific Tregs.

A prospective study on the prevalence and risk factors for neonatal thrombocytopenia and platelet alloimmunization among 9332 unselected Brazilian newborns

BACKGROUND

Neonatal thrombocytopenia (NT) occurs in 0.5 to 0.9% of unselected Caucasian newborns. However, the prevalence of this complication in other populations is unknown. In this study the prevalence/causes of NT was determined in Brazilian newborns, a population characterized by admixture among Indigenous, Africans, and Caucasians.

STUDY DESIGN

A prospective study was carried out in a 3-year period, to determine the prevalence and causes of thrombocytopenia in cord blood samples. Genotyping for HPA 1-5 systems was performed in pairs of mother/neonates with and without thrombocytopenia. All mothers with genotypic mismatches from each group were tested for HPA-specific antibody using the MAIPA technique to identify alloimmunization.

RESULTS

Platelet counts <100 x 10(9)/L were detected in 1.5% of 9,332 unselected newborns. In 0.15%, platelet count was <50 x 10(9)/L. Clinically significant bleeding was rare. Underlying diseases were present in 48% of the thrombocytopenic cases. HPA 1-5 system genotype mismatches occurred in 50% of gestations, but did not predict the risk for thrombocytopenia. Notably, mismatched genotypes for HPA-5 were slightly increased in the thrombocytopenic group. The presence of anti-HPA-5b antibodies was observed in 0.05% of unselected pregnancies, but increased to 12% among mothers of neonates with thrombocytopenia and mismatched genotype (N = 51).

CONCLUSIONS

Neonatal thrombocytopenia is common among Brazilian newborns at rates comparable with those described among Caucasians. These data suggest that screening for genotypic HPA mismatch, followed by an HPA-specific immunoassay system, particularly for the HPA-5 system, among mothers of newborns with thrombocytopenia in our population would allow the identification of pregnancies at risk of alloimmune thrombocytopenia.

Three novel mutations in the activin receptor-like kinase 1 (ALK-1) gene in hereditary hemorrhagic telangiectasia type 2 in Brazilian patients

Hereditary hemorrhagic telangiectasia (HHT) or Osler-Rendu-Weber disease is a systemic fibrovascular dysplasia with an autosomal dominant inheritance pattern. Mutations in two genes, endoglin and ALK-1, are known to cause HHT, both of which mediate signaling by transforming growth factor beta ligands in vascular endothelial cells. Ten patients were analyzed. Diagnosis of HHT was carried out by means of family history, recurrent bleeding, and the presence of multiple telangiectases lesions. Conformation-sensitive gel electrophoresis analyses with consistent abnormal migration patterns were cloned and sequenced using the MegaBace 1000 DNA automated analyzer. Three novel mutations were identified in the coding sequence of the ALK-1 gene in five patients and their families, which demonstrated clinical manifestations of HHT type 2. These mutations included a G insertion and a T deletion of single base pairs in exons 3 and 7, as well as missense mutations in exons 7 and 8 of the ALK-1 gene. These data indicate that loss-of-function mutations in a single allele of the ALK1 locus are sufficient to contribute to defects in maintaining endothelial integrity. We suggest the high rate of mutation detection and the small size of the ALK-1 gene make genomic sequencing a viable diagnostic test for HHT2.

Evidence of multiyear factor IX expression by AAV-mediated gene transfer to skeletal muscle in an individual with severe hemophilia B

In a phase I study, administration of an AAV2-FIX vector into the skeletal muscle of eight hemophilia B subjects proved safe and achieved local gene transfer and FIX expression for at least 10 months after vector injection, the last time point assessed by muscle biopsy. In hemophilia B dogs we have demonstrated FIX in both muscle biopsies and circulation >4 years following AAV2-FIX injection. Because circulating FIX levels remained less than 1% of normal in human subjects from the study, the duration of AAV2-mediated transgene expression in humans is unknown. We sought to determine if FIX gene transfer and expression persisted locally at injection sites. Muscle biopsies were obtained from one subject 3.7 years following treatment and revealed transgene FIX DNA and protein by quantitative PCR, DNA fluorescence in situ hybridization, and immunohistochemistry for FIX. These results demonstrate, for the first time, multiyear FIX expression by AAV2 vector in humans and suggest that improved muscle delivery provides effective treatment for protein deficiencies or muscle-specific diseases.

Inadvertent Germline Transmission of AAV2 Vector: Findings in a Rabbit Model Correlate with Those in a Human Clinical Trial

The risk of germline transmission of vector sequences in humans is a major safety concern, because the enrollment of subjects of reproductive age in early-phase clinical trials of gene transfer continues to increase. In a study of adult men with hemophilia B, adeno-associated virus serotype 2 (AAV2) delivered to the liver via the hepatic artery resulted in unexpected transient vector dissemination to the semen. Here we report that intravenous AAV2 injection in rabbits proved a useful model to assess biologic parameters of vector dissemination to the semen. Detectable vector sequences in semen disappeared in a dose-dependent and time-dependent fashion. AAV infectious particles were present only as long as day 4 after injection and were undetectable thereafter. The kinetics of vector clearance was faster in the semen fractions enriched for motile sperm than in the total semen. In addition, increased frequency of semen sampling accelerated the clearance of vector sequences from semen. Long-term follow-up, spanning hundreds of spermatogenesis cycles, showed that there was no recurrence of detectable vector sequences in semen, thus reducing the probability of inadvertent transduction of early spermatogonia not committed to differentiation at the time of vector injection. We conclude that AAV2 presents minimal germline transmission risk for humans.

Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response

We have previously shown that a single portal vein infusion of a recombinant adeno-associated viral vector (rAAV) expressing canine Factor IX (F.IX) resulted in long-term expression of therapeutic levels of F.IX in dogs with severe hemophilia B. We carried out a phase 1/2 dose-escalation clinical study to extend this approach to humans with severe hemophilia B. rAAV-2 vector expressing human F.IX was infused through the hepatic artery into seven subjects. The data show that: (i) vector infusion at doses up to 2 x 10(12) vg/kg was not associated with acute or long-lasting toxicity; (ii) therapeutic levels of F.IX were achieved at the highest dose tested; (iii) duration of expression at therapeutic levels was limited to a period of approximately 8 weeks; (iv) a gradual decline in F.IX was accompanied by a transient asymptomatic elevation of liver transaminases that resolved without treatment. Further studies suggested that destruction of transduced hepatocytes by cell-mediated immunity targeting antigens of the AAV capsid caused both the decline in F.IX and the transient transaminitis. We conclude that rAAV-2 vectors can transduce human hepatocytes in vivo to result in therapeutically relevant levels of F.IX, but that future studies in humans may require immunomodulation to achieve long-term expression.

Factor V Leiden improves in vivo hemostasis in murine hemophilia models

The role of factor V Leiden (FVL) as a modifier of the severe hemophilia phenotype is still unclear. We used mice with hemophilia A or B crossed with FVL to elucidate in vivo parameters of hemostasis. Real-time thrombus formation in the microcirculation was monitored by deposition of labeled platelets upon laser-induced endothelial injury using widefield microscopy in living animals. No thrombi formed in hemophilic A or B mice following vascular injuries. However, hemophilic mice, either heterozygous or homozygous for FVL, formed clots at all injured sites. Injection of purified activated FV into hemophilic A or B mice could mimic the in vivo effect of FVL. In contrast to these responses to a laser injury in a microvascular bed, FVL did not provide sustained hemostasis following damage of large vessels in a ferric chloride carotid artery injury model, despite of the improvement of clotting times and high circulating thrombin levels. Together these data provide evidence that FVL has the ability to improve the hemophilia A or B phenotype, but this effect is principally evident at the microcirculation level following a particular vascular injury. Our observations may partly explain the heterogeneous clinical evidence of the beneficial role of FVL in hemophilia.

The inhibitory effects of anticoagulation on in vivo gene transfer by adeno-associated viral or adenoviral vectors

Identifying factors that influence gene transfer efficacy is critical for a successful gene-based clinical study. Here we demonstrate that in vivo AAV-2-mediated gene transfer is efficiently inhibited by unfractionated heparin, but not by a heparin preparation containing mainly low-molecular-weight forms (LMWH). Surprisingly, inhibitors of thrombin or factor Xa (F.Xa) significantly reduced AAV-2 transduction in a dose-dependent manner. These effects were independent of the vector promoter, transgene, or strain of mice. Expression by alternate AAV serotypes 5 and 8 was not affected by anticoagulant drugs, which suggests an AAV-2-specific effect. Moreover, AAV-2-mediated gene expression was diminished in mice with deficiency in thrombin generation (factor IX deficiency) and enhanced in mice with procoagulant phenotype due to factor V Leiden. In addition, inhibitors of F.Xa diminished adenovirus-mediated gene expression. These results demonstrated that coagulation activity itself is critical to ensure optimal viral vector transduction. Since intravascular delivery of vectors often requires the use of anticoagulants, the use of LMWH appears to be safe. These observations are of relevance for approaches using AAV-2 or adenoviral vectors, especially in early phase studies designed to identify the minimum therapeutic doses.

Factor IX variants improve gene therapy efficacy for hemophilia B

Intramuscular injection of adeno-associated viral (AAV) vector to skeletal muscle of humans with hemophilia B is safe, but higher doses are required to achieve therapeutic factor IX (F.IX) levels. The efficacy of this approach is hampered by the retention of F.IX in muscle extracellular spaces and by the limiting capacity of muscle to synthesize fully active F.IX at high expression rates. To overcome these limitations, we constructed AAV vectors encoding F.IX variants for muscle- or liver-directed expression in hemophilia B mice. Circulating F.IX levels following intramuscular injection of AAV-F.IX-K5A/V10K, a variant with low-affinity to extracellular matrix, were 2-5 fold higher compared with wild-type (WT) F.IX, while the protein-specific activities remained similar. Expression of F.IX-R338A generated a protein with 2- or 6-fold higher specific activity than F.IX-WT following vector delivery to skeletal muscle or liver, respectively. F.IX-WT and variant forms provide effective hemostasis in vivo upon challenge by tail-clipping assay. Importantly, intramuscular injection of AAV-F.IX variants did not trigger antibody formation to F.IX in mice tolerant to F.IX-WT. These studies demonstrate that F.IX variants provide a promising strategy to improve the efficacy for a variety of gene-based therapies for hemophilia B.

Platelet glycoprotein Ibalpha polymorphisms modulate the risk for myocardial infarction

Platelet glycoprotein Iba (GPIba) gene polymorphisms have been reported to affect the risk of developing coronary heart disease. Here, within the GPIba gene, we determine the association between the variable number of tandem repeats (VNTR), the -5C/T Kozak sequence dimorphism, and the human platelet antigen (HPA)-2 polymorphisms with occurrence of myocardial infarction (MI). Patients (n=180) presenting survivors of MI were compared to 180 controls matched by age, gender, and race. Carriers of VNTR-CD genotype had a 2-fold higher risk for MI compared to controls. The prevalence of VNTR-BC was lower among patients than among controls (P=.007). These data are in agreement with recent reports of increased plug formation by human platelets containing VNTRCD but no other VNTR genotypes. Among patients, the number of vessels severely occluded was greater among carriers of the D-allele (P=.019) or VNTR-CD (P=.026) and lower among carriers of the C-allele (P=.003) or VNTR-CC (P=.0009) compared to non-carriers of these alleles. No influence was seen with the Kozak or HPA-2 polymorphisms. Determination of VNTR of the GPIba gene may prove useful for identifying high-risk individuals for MI.

Characterization of mild coagulation factor VII deficiency: activity and clearance of the Arg315Trp and Arg315Lys variants in the Cys310-Cys329 loop (c170s)

BACKGROUND AND OBJECTIVES

Arginine 315 in factor VII (FVII) belongs to a solvent-exposed loop involved in direct interaction with the co-factor (tissue factor, TF), in transmission of TF-induced effects and potentially in FVIIa inactivation. Natural FVII variants at position 315 provide peculiar models for structure-function studies.

DESIGN AND METHODS

We characterized a mild coagulation FVII deficiency associated with reduced FVII activity (26%) and antigen (67%). Mutations were searched by FVII gene sequencing. FVII variants were created by mutagenesis of FVII cDNA and characterized through expression in HEK293 cells followed by functional studies. FVII antigen in media was estimated by immunoassay while FVII activity was assessed by prothrombin-time based and FXa generation assays. FVII variants were injected into mice to investigate their recovery and half-life. One-way ANOVA was used to test statistical significance.

RESULTS

The patient was double heterozygous for a novel R315W mutation and for the R304Q substitution (FVII Padua) previously demonstrated to impair TF binding. The recombinant 315W-FVII was normally expressed in medium but showed a markedly reduced coagulant function (52%) and activity towards factor X (FX) in plasma (34%). Moreover, the 315W-FVII showed significantly decreased recovery of the protein (20%) and a slightly shorter half-life (8.6 min) as compared to wt-FVII (50% and 10.7 min). We also studied the conservative R315K change that was responsible for low recovery (20%) and a decreased half-life (7 min) of a FVII variant with virtually normal FVII antigen and activity levels.

INTERPRETATION AND CONCLUSIONS

These findings suggest a dual role of R315 for FVII function and clearance, and indicate that substitutions at this position have appreciable effects on human FVII biology, compatible with residual FVII function and thus with mild FVII deficiency.

Novel hemophilia B mouse models exhibiting a range of mutations in the Factor IX gene

Animal models have been critical to the development of novel therapeutics in hemophilia. A deficiency of current murine models of hemophilia B is that they are all due to gene deletions, a type of mutation that is relatively rare in the human hemophilia population. We generated mice with a range of mutations in the Factor IX (F.IX) gene; these more faithfully reflect the types of mutations that cause disease in the human population. Transgenic mice expressing either wild-type human F.IX (hF.IX), or F.IX variants with premature translation termination codons, or missense mutations, under the control of the murine transthyretin promoter, were generated and crossed with mice carrying a large deletion of the murine F.IX gene. Gene copy number, F.IX transcript levels in the liver, intrahepatocyte protein expression, and circulating levels of F.IX protein in the mice were determined and compared with data generated by transient transfection assays using the same F.IX variants. Mice were injected with a viral vector expressing hF.IX and displayed a range of immune responses to the transgene product, depending on the underlying mutation. These new mouse models faithfully mimic the mutations causing human disease, and will prove useful for testing novel therapies for hemophilia. (Blood. 2004;104:2767-2774)

Regional intravascular delivery of AAV-2-F.IX to skeletal muscle achieves long-term correction of hemophilia B in a large animal model

In earlier work, we showed that adeno-associated virus-mediated delivery of a Factor IX gene to skeletal muscle by direct intramuscular injection resulted in therapeutic levels of circulating Factor IX in mice. However, achievement of target doses in humans proved impractical because of the large number of injections required. We used a novel intravascular delivery technique to achieve successful transduction of extensive areas of skeletal muscle in a large animal with hemophilia. We provide here the first report of long-term (> 3 years, with observation ongoing), robust Factor IX expression (circulating levels of 4%-14%) by muscle-directed gene transfer in a large animal, resulting in essentially complete correction of the bleeding disorder in hemophilic dogs. The results of this translational study establish an experimental basis for clinical studies of this delivery method in humans with hemophilia B. These findings also have immediate relevance for gene transfer in patients with muscular dystrophy.