Integration and the risk of liver cancer-Is there a real risk?

Adeno-associated virus (AAV)-based gene therapies are in clinical development for haemophilia and other genetic diseases. Since the recombinant AAV genome primarily remains episomal, it provides the opportunity for long-term expression in tissues that are not proliferating and reduces the safety concerns compared with integrating viral vectors. However, AAV integration events are detected at a low frequency. Preclinical studies in mouse models have reported hepatocellular carcinoma (HCC) after systemic AAV administration in some settings, though this has not been reported in large animal models. The risk of HCC or other cancers after AAV gene therapy in clinical studies thus remains theoretical. Potential risk factors for HCC after gene therapy are beginning to be elucidated through animal studies, but their relevance to human studies remains unknown. Studies to investigate the factors that may influence the risk of oncogenesis as well as detailed investigation of cases of cancer in AAV gene therapy patients will be important to define the potential risk of AAV genotoxicity.

Coagulation factor VIII regulates von Willebrand factor homeostasis invivo

BACKGROUND

Coagulation factor VIII (FVIII) and von Willebrand factor (VWF) circulate as a noncovalent complex, but each has its distinct functions. Binding of FVIII to VWF results in a prolongation of FVIII's half-life in circulation and modulates FVIII's immunogenicity during hemophilia therapy. However, the biological effect of FVIII and VWF interaction on VWF homeostasis is not fully understood.

OBJECTIVES

To determine the effect of FVIII in VWF proteolysis and homeostasis in vivo.

METHODS

Mouse models, recombinant FVIII infusion, and patients with hemophilia A on a high dose FVIII for immune tolerance induction therapy or emicizumab for bleeding symptoms were included to address this question.

RESULTS

An intravenous infusion of a recombinant B-domain less FVIII (BDD-FVIII) (40 and 160 μg/kg) into wild-type mice significantly reduced plasma VWF multimer sizes and its antigen levels; an infusion of a high but not low dose of BDD-FVIII into Adamts13+/- and Adamts13-/- mice also significantly reduced the size of VWF multimers. However, plasma levels of VWF antigen remained unchanged following administration of any dose BDD-FVIII into Adamts13-/- mice, suggesting partial ADAMTS-13 dependency in FVIII-augmented VWF degradation. Moreover, persistent expression of BDD-FVIII at ∼50 to 250 U/dL via AAV8 vector in hemophilia A mice also resulted in a significant reduction of plasma VWF multimer sizes and antigen levels. Finally, the sizes of plasma VWF multimers were significantly reduced in patients with hemophilia A who received a dose of recombinant or plasma-derived FVIII for immune tolerance induction therapy.

CONCLUSION

Our results demonstrate the pivotal role of FVIII as a cofactor regulating VWF proteolysis and homeostasis under various (patho)physiological conditions.

Altered cleavage of human factor VIII at the B-domain and acidic region 3 interface enhances expression after gene therapy in hemophilia A mice

BACKGROUND

Variants of human factor VIII (hFVIII) have been developed to further understand the structure and function of hFVIII and improve gene-based therapeutics. We have previously characterized several hFVIII variants of the furin cleavage site (1645-1648) with improved secretion. We have also identified a second cleavage site in the acidic region 3 (a3) (1657-1658) that becomes the primary hFVIII intracellular cleavage position in the absence of the furin site. We tested a hypothesis that modification of this site may confer additional functional advantages to hFVIII.

OBJECTIVES

The aim of this study was to conduct the biochemical and functional characterization of hFVIII variants of the furin cleavage site, the a3 cleavage site, or in combination, both in vitro and in vivo after AAV mediated gene therapy.

METHODS

Recombinant hFVIII variants of the furin cleavage site (hFVIII-Δ3), the a3 cleavage site (hFVIII-S1657P/D1658E [SP/DE]), or in combination (hFVIII-Δ3-SP/DE) were purified and characterized in vitro and in vivo.

RESULTS

Recombinant hFVIII-Δ3, hFVIII-SP/DE, and hFVIII-Δ3-SP/DE variants all had comparable specific activity to B-domain deleted (BDD) hFVIII. Hemophilia A mice tolerant to hFVIII did not develop immune responses to hFVIII after protein challenge with these variants or after adeno-associated virus (AAV) delivery. Following AAV delivery, hFVIII-Δ3-SP/DE resulted in expression levels that were 2- to 5-fold higher than those with hFVIII-BDD in hemophilia A mice.

CONCLUSION

The novel hFVIII-Δ3-SP/DE variant of the furin and a3 cleavage sites significantly improved secretion compared with hFVIII-BDD. This key feature of the Δ3-SP/DE variant provides a unique strategy that can be combined with other approaches to further improve factor VIII expression to achieve superior efficacy in AAV-based gene therapy for hemophilia A.

Evaluating the state of the science for adeno-associated virus integration: An integrated perspective

On August 18, 2021, the American Society of Gene and Cell Therapy (ASGCT) hosted a virtual roundtable on adeno-associated virus (AAV) integration, featuring leading experts in preclinical and clinical AAV gene therapy, to further contextualize and understand this phenomenon. Recombinant AAV (rAAV) vectors are used to develop therapies for many conditions given their ability to transduce multiple cell types, resulting in long-term expression of transgenes. Although most rAAV DNA typically remains episomal, some rAAV DNA becomes integrated into genomic DNA at a low frequency, and rAAV insertional mutagenesis has been shown to lead to tumorigenesis in neonatal mice. Currently, the risk of rAAV-mediated oncogenesis in humans is theoretical because no confirmed genotoxic events have been reported to date. However, because insertional mutagenesis has been reported in a small number of murine studies, there is a need to characterize this genotoxicity to inform research, regulatory needs, and patient care. The purpose of this white paper is to review the evidence of rAAV-related host genome integration in animal models and possible risks of insertional mutagenesis in patients. In addition, technical considerations, regulatory guidance, and bioethics are discussed.

Preclinical assessment of an optimized AAV-FVIII vector in mice and non-human primates for the treatment of hemophilia A

Extensive clinical data from liver-mediated gene therapy trials have shown that dose-dependent immune responses against the vector capsid may impair or even preclude transgene expression if not managed successfully with prompt immune suppression. The goal of this preclinical study was to generate an adeno-associated viral (AAV) vector capable of expressing therapeutic levels of B-domain deleted factor VIII (FVIII) at the lowest possible vector dose to minimize the potential Risk of a capsid-mediated immune response in the clinical setting. Here, we describe the studies that identified the investigational agent SPK-8011, currently being evaluated in a phase 1/2 study (NCT03003533) in individuals with hemophilia A. In particular, the potency of our second-generation expression cassettes was evaluated in mice and in non-human primates using two different bioengineered capsids (AAV-Spark100 and AAV-Spark200). At 2 weeks after gene transfer, primates transduced with 2 × 1012 vg/kg AAV-Spark100-FVIII or AAV-Spark200-FVIII expressed FVIII antigen levels of 13% ± 2% and 22% ± 6% of normal, respectively. Collectively, these preclinical results validate the feasibility of lowering the AAV capsid dose for a gene-based therapeutic approach for hemophilia A to a dose level orders of magnitude lower than the first-generation vectors in the clinic.

A long-term study of AAV gene therapy in dogs with hemophilia A identifies clonal expansions of transduced liver cells

Nine dogs with hemophilia A were treated with adeno-associated viral (AAV) gene therapy and followed for up to 10 years. Administration of AAV8 or AAV9 vectors expressing canine factor VIII (AAV-cFVIII) corrected the FVIII deficiency to 1.9-11.3% of normal FVIII levels. In two of nine dogs, levels of FVIII activity increased gradually starting about 4 years after treatment. None of the dogs showed evidence of tumors or altered liver function. Analysis of integration sites in liver samples from six treated dogs identified 1,741 unique AAV integration events in genomic DNA and expanded cell clones in five dogs, with 44% of the integrations near genes involved in cell growth. All recovered integrated vectors were partially deleted and/or rearranged. Our data suggest that the increase in FVIII protein expression in two dogs may have been due to clonal expansion of cells harboring integrated vectors. These results support the clinical development of liver-directed AAV gene therapy for hemophilia A, while emphasizing the importance of long-term monitoring for potential genotoxicity.

Functionalized lipid-like nanoparticles for in vivo mRNA delivery and base editing

Messenger RNA (mRNA) therapeutics have been explored to treat various genetic disorders. Lipid-derived nanomaterials are currently one of the most promising biomaterials that mediate effective mRNA delivery. However, efficiency and safety of this nanomaterial-based mRNA delivery remains a challenge for clinical applications. Here, we constructed a series of lipid-like nanomaterials (LLNs), named functionalized TT derivatives (FTT), for mRNA-based therapeutic applications in vivo. After screenings on the materials, we identified FTT5 as a lead material for efficient delivery of long mRNAs, such as human factor VIII (hFVIII) mRNA (~4.5 kb) for expression of hFVIII protein in hemophilia A mice. Moreover, FTT5 LLNs demonstrated high percentage of base editing on PCSK9 in vivo at a low dose of base editor mRNA (~5.5 kb) and single guide RNA. Consequently, FTT nanomaterials merit further development for mRNA-based therapy.

Origins and organization of the NHLBI State of the Science Workshop: Generating a national blueprint for future research on factor VIII inhibitors

INTRODUCTION

The major complication of protein replacement therapy for haemophilia A is the development of anti-FVIII antibodies or inhibitors that occur in 25%-30% of persons with severe haemophilia A. Alternative therapeutics such as bypassing agents or immune tolerance induction protocols have additional challenges and are not always effective.

AIM

Assemble a National Heart, Lung and Blood Institute (NHLBI) State of the Science (SOS) Workshop to generate a national blueprint for research on inhibitors to solve the problem of FVIII immunogenicity.

METHODS

An Executive Steering Committee was formed in October 2017 to establish the scientific focus and Scientific Working Groups for the SOS Workshop in May 2018. Four working groups were assembled to address scientific priorities in basic, translational and clinical research on inhibitors.

RESULTS

Working Group 1 was charged with determining the scientific priorities for clinical trials to include the integration of non-intravenous, non-factor therapeutics including gene therapy into the standard of care for people with haemophilia A with inhibitors. Working Group 2 established the scientific priorities for 21st-century data science and biospecimen collection for observational inhibitor cohort studies. The scientific priorities for acquiring an actionable understanding of FVIII immunogenicity and the immunology of the host response and FVIII tolerance were developed by Working Group 3. Working Group 4 designed prospective pregnancy/birth cohorts to study FVIII immunogenicity, inhibitor development and eradication.

CONCLUSION

The NHLBI SOS Workshop generated a focused summary of scientific priorities and implementation strategies to overcome the challenges of eradicating and preventing inhibitors in haemophilia A.

Apoptotic effects of platelet factor VIII on megakaryopoiesis: implications for a modified human FVIII for platelet-based gene therapy

BACKGROUND

Ectopically expressed B-domainless factor VIII in megakaryocytes is stored in α-granules, is effective in a number of murine hemostatic models, and is protected from circulating inhibitors. However, this platelet (p) FVIII has different temporal-spatial availability from plasma FVIII, with limited efficacy in other murine hemostatic models.

OBJECTIVES AND METHODS

We sought to improve pFVIII hemostatic efficacy by expressing canine (c) FVIII, which has higher stability and activity than human (h) FVIII in FVIII(null) mice.

RESULTS AND CONCLUSIONS

We found that pcFVIII was more effective than phFVIII at restoring hemostasis, but peak pcFVIII antigen levels were lower and were associated with greater megakaryocyte apoptosis than phFVIII. These new insights suggest that pFVIII gene therapy strategies should focus on enhancing activity rather than levels. We previously showed that modification of the PACE/furin cleavage site in hFVIII resulted in secretion of hFVIII primarily as a single-chain molecule with increased biological activity. In megakaryocytes, this variant was expressed at the same level as phFVIII with a lentiviral bone marrow transplant approach to reconstitute FVIII(null) mice, but was more effective, resulting in near-normal hemostasis in the cremaster laser injury model. These studies may have implications for pFVIII gene therapy in hemophilia A.

Light chain of factor VIII is sufficient for accelerating cleavage of von Willebrand factor by ADAMTS13 metalloprotease

We previously demonstrated that coagulation factor VIII (FVIII) accelerates proteolytic cleavage of von Willebrand factor (VWF) by A disintegrin and metalloprotease with thrombospondin type 1 repeats (ADAMTS13) under fluid shear stress. In this study, the structural elements of FVIII required for the rate-enhancing effect and the biological relevance of this cofactor activity are determined using a murine model. An isolated light chain of human FVIII (hFVIII-LC) increases proteolytic cleavage of VWF by ADAMTS13 under shear in a concentration-dependent manner. The maximal rate-enhancing effect of hFVIII-LC is ∼8-fold, which is comparable with human full-length FVIII and B-domain deleted FVIII (hFVIII-BDD). The heavy chain (hFVIII-HC) and the light chain lacking the acidic (a3) region (hFVIII-LCΔa3) have no effect in accelerating VWF proteolysis by ADAMTS13 under the same conditions. Although recombinant hFVIII-HC and hFVIII-LCΔa3 do not detectably bind immobilized VWF, recombinant hFVIII-LC binds VWF with high affinity (K(D), ∼15 nM). Moreover, ultra-large VWF multimers accumulate in the plasma of fVIII(-/-) mice after hydrodynamic challenge but not in those reconstituted with either hFVIII-BDD or hFVIII-LC. These results suggest that the light chain of FVIII, which is not biologically active for clot formation, is sufficient for accelerating proteolytic cleavage of VWF by ADAMTS13 under fluid shear stress and (patho) physiological conditions. Our findings provide novel insight into the molecular mechanism of how FVIII regulates VWF homeostasis.

Muscle Gene Therapy for Hemophilia

Muscle-directed gene therapy for hemophilia is an attractive strategy for expression of therapeutic levels of clotting factor as evident from preclinical studies and an early phase clinical trial. Notably, local FIX expression by AAV-mediated direct intramuscular injection to skeletal muscle persists for years. Development of intravascular delivery of AAV vector approaches to skeletal muscle resulted in vector in widespread areas of the limb and increased expression of FIX in hemophilia B dogs. The use of FIX variants with improved biological activity may provide the opportunity to increase the efficacy of these approaches. Studies for hemophilia A are less developed at this point, but utilizing transgenes that improve hemostasis independent of FIX and FVIII has potential therapeutic application for both hemophilia A and B. Continuous monitoring of humoral and T cell responses to the transgene and AAV capsid in human trials will be critical for the translation of these promising approaches for muscle gene therapy for hemophilia.

Animal models of hemophilia

The X-linked bleeding disorder hemophilia is caused by mutations in coagulation factor VIII (hemophilia A) or factor IX (hemophilia B). Unless prophylactic treatment is provided, patients with severe disease (less than 1% clotting activity) typically experience frequent spontaneous bleeds. Current treatment is largely based on intravenous infusion of recombinant or plasma-derived coagulation factor concentrate. More effective factor products are being developed. Moreover, gene therapies for sustained correction of hemophilia are showing much promise in preclinical studies and in clinical trials. These advances in molecular medicine heavily depend on availability of well-characterized small and large animal models of hemophilia, primarily hemophilia mice and dogs. Experiments in these animals represent important early and intermediate steps of translational research aimed at development of better and safer treatments for hemophilia, such a protein and gene therapies or immune tolerance protocols. While murine models are excellent for studies of large groups of animals using genetically defined strains, canine models are important for testing scale-up and for long-term follow-up as well as for studies that require larger blood volumes.

Diverse IgG subclass responses to adeno-associated virus infection and vector administration

Humoral immune responses occur following exposure to Adeno-associated virus (AAV) or AAV vectors. Many studies characterized antibody responses to AAV, but human IgG subclass responses to AAV have not been previously described. In this study, IgG subclass responses were examined in serum samples of normal human subjects exposed to wild-type AAV, subjects injected intramuscularly with AAV vectors and subjects injected intravascularly with AAV vectors. A diversity of IgG subclass responses to AAV capsid were found in different subjects. IgG1 was found to be the dominant response. IgG2, IgG3, and IgG4 responses were also observed in most normal human subjects; IgG2 and IgG3 each represented the major fraction of total anti-AAV capsid IgG in a subset of normal donors. Subjects exposed to AAV vectors showed IgG responses to AAV capsid of all four IgG subclasses. IgG responses to AAV capsid in clinical trial subjects were inversely proportional to the level of pre-existing anti-AAV antibody and independent of the vector dose. The high levels of anti-AAV capsid IgG1 can mask differences in IgG2, IgG3, and IgG4 responses that were observed in this study. Analysis of IgG subclass distribution of anti-AAV capsid antibodies indicates a complex, non-uniform pattern of responses to this viral antigen. J. Med. Virol. 81:65-74, 2009. (c) 2008 Wiley-Liss, Inc.

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.

Persistent expression of hF.IX After tolerance induction by in utero or neonatal administration of AAV-1-F.IX in hemophilia B mice

The major complication associated with protein replacement therapy currently used in the treatment of hemophilia B (HB) is the development of antibodies to the infused human Factor IX (hF.IX). We hypothesized that vector-mediated expression of hF.IX, either at a prenatal stage or early in life may lead to tolerance to hF.IX and long-term transgene expression. Fetal, neonatal, and adult F.IX-deficient mice were injected with AAV-1-hF.IX, and the hF.IX levels as well as antibodies to hF.IX in the circulation were assayed. In utero injection followed by postnatal re-administration of adeno-associated virus 1 (AAV-1) vector achieved persistent expression of hF.IX in all animals, with no cellular or humoral immune response to F.IX. Similar results were seen after initial injection in neonatal mice followed by re-administration, whereas all mice injected at the adult stage developed antibodies to hF.IX. In contrast, after administration of AAV-2-hF.IX in the neonatal period, antibodies to hF.IX were formed in all the injected animals. We conclude that in utero or neonatal-stage injection of AAV-1-hF.IX can lead to long-term expression and absence of immune response. The differences in immune response between the AAV-1 and AAV-2 groups suggests that tolerance may be related to differences in bio-distribution, timing of expression, and/or the initial levels of hF.IX expression. This supports the concept of a narrow "window of opportunity" for tolerance induction.

CD8(+) T-cell responses to adeno-associated virus capsid in humans

Hepatic adeno-associated virus (AAV)-serotype 2 mediated gene transfer results in transgene product expression that is sustained in experimental animals but not in human subjects. We hypothesize that this is caused by rejection of transduced hepatocytes by AAV capsid-specific memory CD8(+) T cells reactivated by AAV vectors. Here we show that healthy subjects carry AAV capsid-specific CD8(+) T cells and that AAV-mediated gene transfer results in their expansion. No such expansion occurs in mice after AAV-mediated gene transfer. In addition, we show that AAV-2 induced human T cells proliferate upon exposure to alternate AAV serotypes, indicating that other serotypes are unlikely to evade capsid-specific immune responses.

Major erythrocyte membrane protein genes in EKLF-deficient mice

OBJECTIVES

Mice deficient in the transcription factor erythroid Krüppel-like factor, KLF1 (EKLF) die approximately 14.5 days postcoitum of anemia, attributed to decreased expression of the beta-globin gene. The objectives of this study were to rescue EKLF-deficient embryos with mice expressing gamma-globin from beta-spectrin or ankyrin promoters and to characterize expression of the major erythrocyte membrane genes in EKLF-deficient cells.

METHODS

Transgenic beta-spectrin/gamma-globin or ankyrin/gamma-globin mice were bred onto EKLF-deficient and wild-type backgrounds. Animals were genotyped, gamma-globin mRNA levels measured, and hemoglobin electrophoresis performed. Steady-state mRNA levels and transcriptional rates of the major erythrocyte membrane protein genes were assayed.

RESULTS

beta-spectrin/gamma-globin or ankyrin/gamma-globin mice on EKLF-deficient and wild-type backgrounds had identical levels of gamma-globin mRNA, indicating EKLF-independence of these promoters. gamma-Globin expression improved globin chain imbalance, but hemolysis was not improved and no live-born EKLF-deficient/(A)gamma-globin mice were obtained. Circulating erythroid cells from EKLF-deficient/(A)gamma-globin embryos exhibited hemolysis reminiscent of that seen in patients with severe erythrocyte membrane defects. Levels of beta-spectrin, ankyrin, and band 3 mRNA, but not alpha-spectrin, were decreased in EKLF-deficient fetal liver RNA. In a run-on assay, levels of transcription of the ankyrin and band 3 genes were decreased in EKLF-deficient fetal liver nuclei.

CONCLUSIONS

These results indicate that the EKLF-responsive regions of the ankyrin and beta-spectrin genes are outside their promoters and that EKLF is necessary for full transcriptional activity of the ankyrin and band 3 genes; the results also provide additional evidence that defects in addition to beta-globin deficiency, including an abnormal erythrocyte membrane, contribute to the anemia and embryonic lethality in EKLF-deficient mice.

Identification of mouse AAV capsid-specific CD8+ T cell epitopes

Adeno-associated virus has been developed for use as a gene transfer vector. To understand the impact of AAV capsid-specific CD8(+) T cells on AAV-mediated gene transfer, we identified CD8(+) T cell epitopes for AAV-2 and AAV-8 capsid in C57BL/6 (H-2(b) MHC haplotype) and BALB/c (H-2(d) MHC haplotype) mice. Mice of both the H-2(b) and the H-2(d) haplotypes recognized epitopes on AAV-2 and AAV-8 capsid. T cells from H-2(b) mice recognized an epitope that was conserved between AAV-2 and AAV-8 capsid. Cross-reactivity of AAV-specific CD8(+) T cells induced by different AAV serotypes may have important implications for gene transfer. Identification of these epitopes will facilitate studies of immune response to AAV capsid in mouse models.

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)

Erythroid expression of the human alpha-spectrin gene promoter is mediated by GATA-1- and NF-E2-binding proteins

alpha-Spectrin is a highly expressed membrane protein critical for the flexibility and stability of the erythrocyte. Qualitative and quantitative defects of alpha-spectrin are present in the erythrocytes of many patients with abnormalities of red blood cell shape including hereditary spherocytosis and elliptocytosis. We wished to determine the regulatory elements that determine the erythroid-specific expression of the alpha-spectrin gene. We mapped the 5' end of the alpha-spectrin erythroid cDNA and cloned the 5' flanking genomic DNA containing the putative alpha-spectrin gene promoter. Using transfection of promoter/reporter plasmids in human tissue culture cell lines, in vitro DNase I footprinting analyses, and gel mobility shift assays, an alpha-spectrin gene erythroid promoter with binding sites for GATA-1- and NF-E2-related proteins was identified. Both binding sites were required for full promoter activity. In transgenic mice, a reporter gene directed by the alpha-spectrin promoter was expressed in yolk sac, fetal liver, and erythroid cells of bone marrow but not adult reticulocytes. No expression of the reporter gene was detected in nonerythroid tissues. We conclude that this alpha-spectrin gene promoter contains the sequences necessary for low level expression in erythroid progenitor cells.

Erythrocyte ankyrin promoter mutations associated with recessive hereditary spherocytosis cause significant abnormalities in ankyrin expression

Ankyrin defects are the most common cause of hereditary spherocytosis (HS). In several kindreds with recessive, ankyrin-deficient HS, mutations have been identified in the ankyrin promoter that have been proposed to decrease ankyrin synthesis. We analyzed the effects of two mutations, -108T to C and -108T to C in cis with -153G to A, on ankyrin expression. No difference between wild type and mutant promoters was demonstrated in transfection or gel shift assays in vitro. Transgenic mice with a wild type ankyrin promoter linked to a human (A)gamma-globin gene expressed gamma-globin in 100% of erythrocytes in a copy number-dependent, position-independent manner. Transgenic mice with the mutant -108 promoter demonstrated variegated gamma-globin expression, but showed copy number-dependent and position-independent expression similar to wild type. Severe effects in ankyrin expression were seen in mice with the linked -108/-153 mutations. Three transgenic lines had undetectable levels of (A)gamma-globin mRNA, indicating position-dependent expression, and four lines expressed significantly lower levels of (A)gamma-globin mRNA than wild type. Two of four expressing lines showed variegated gamma-globin expression, and there was no correlation between transgene copy number and RNA level, indicating copy number-independent expression. These data are the first demonstration of functional defects caused by HS-related, ankyrin gene promoter mutations.

Development of a stable retrovirus vector capable of long-term expression of gamma-globin mRNA in mouse erythrocytes

Gene therapy for patients with hemoglobin disorders such has been hampered by the inability of retrovirus vectors to transfer globin genes and the locus control region (LCR) into hematopoietic stem cells without rearrangement. In addition, the expression from intact globin gene vectors has been variable in red blood cells as a result of position effects and retrovirus silencing. We hypothesized that by substituting the globin gene promoter for the promoter of another gene expressed in red blood cells, we could generate stable retrovirus vectors that would express globin at sufficient levels to treat hemoglobinopathies. Transgenic mice containing the human ankyrin (Ank) gene promoter fused to the human gamma-globin gene showed position-independent, copy number-dependent expression of a linked gamma-globin mRNA. We generated a "double-copy" Ank/A gamma-globin retrovirus vector that transferred two copies of the Ank/A gamma-globin gene into target cells. Stable gene transfer was observed in primary primary mouse progenitor cells and long-term repopulating hematopoietic stem cells. Expression of Ank/A gamma-globin mRNA in mature red blood cells was approximately 8% of the level of mouse alpha-globin mRNA. We conclude that this novel retrovirus vector may be valuable for treating a variety of hemoglobinopathies by gene therapy if the level of expression can be further increased.

Functional requirements for phenotypic correction of murine beta-thalassemia: implications for human gene therapy

As initial human gene therapy trials for beta-thalassemia are contemplated, 2 critical questions important to trial design and planning have emerged. First, what proportion of genetically corrected hematopoietic stem cells (HSCs) will be needed to achieve a therapeutic benefit? Second, what level of expression of a transferred globin gene will be required to improve beta-thalassemic erythropoiesis? These questions were directly addressed by means of a murine model of severe beta-thalassemia. Generation of beta-thalassemic mice chimeric for a minority proportion of genetically normal HSCs demonstrated that normal HSC chimerism levels as low as 10% to 20% resulted in significant increases in hemoglobin (Hb) level and diminished extramedullary erythropoiesis. A large majority of the peripheral red cells in these mice were derived from the small minority of normal HSCs. In a separate set of independent experiments, beta-thalassemic mice were bred with transgenic mice that expressed different levels of human globins. Human gamma-globin messenger RNA (mRNA) expression at 7% of the level of total endogenous alpha-globin mRNA in thalassemic erythroid cells resulted in improved red cell morphology, a greater than 2-g/dL increase in Hb, and diminished reticulocytosis and extramedullary erythropoiesis. Furthermore, gamma-globin mRNA expression at 13% resulted in a 3-g/dL increase in Hb and nearly complete correction of red cell morphology and other indices of inefficient erythropoiesis. These data indicate that a significant therapeutic benefit could be achieved with expression of a transferred globin gene at about 15% of the level of total alpha-globin mRNA in patients with severe beta-thalassemia in whom 20% of erythroid precursors express the vector genome.

Long-term expression of gamma-globin mRNA in mouse erythrocytes from retrovirus vectors containing the human gamma-globin gene fused to the ankyrin-1 promoter

Gene therapy for patients with hemoglobin disorders has been hampered by the inability of retrovirus vectors to transfer globin genes and their cis-acting regulatory sequences into hematopoietic stem cells without rearrangement. In addition, the expression from intact globin gene vectors has been variable in red blood cells due to position effects and retrovirus silencing. We hypothesized that by substituting the globin gene promoter for the promoter of another gene expressed in red blood cells, we could generate stable retrovirus vectors that would express globin at sufficient levels to treat hemoglobinopathies. Recently, we have shown that the human ankyrin (Ank) gene promoter directs position-independent, copy number-dependent expression of a linked gamma-globin gene in transgenic mice. We inserted the Ank/(A)gamma-globin gene into retrovirus vectors that could transfer one or two copies of the Ank/(A)gamma-globin gene to target cells. Both vectors were stable, transferring only intact proviral sequences into primary mouse hematopoietic stem cells. Expression of Ank/(A)gamma-globin mRNA in mature red blood cells was 3% (single copy) and 8% (double copy) of the level of mouse alpha-globin mRNA. We conclude that these novel retrovirus vectors may be valuable for treating a variety of red cell disorders by gene replacement therapy including severe beta-thalassemia if the level of expression can be further increased.

A minimal ankyrin promoter linked to a human gamma-globin gene demonstrates erythroid specific copy number dependent expression with minimal position or enhancer dependence in transgenic mice

In red blood cells ankyrin (ANK-1) provides the primary linkage between the erythrocyte membrane skeleton and the plasma membrane. We have previously demonstrated that a 271-bp 5'-flanking region of the ANK-1 gene has promoter activity in erythroid, but not non-erythroid, cell lines. To determine whether the ankyrin promoter could direct erythroid-specific expression in vivo, we analyzed transgenic mice containing the ankyrin promoter fused to the human (A)gamma-globin gene. Sixteen of 17 lines expressed the transgene in erythroid cells indicating nearly position-independent expression. We also observed a significant correlation between the level of Ank/(A)gamma-globin mRNA and transgene copy number. The level of Ank/(A)gamma mRNA averaged 11% of mouse alpha-globin mRNA per gene copy at all developmental stages. The addition of the HS2 enhancer from the beta-globin locus control region to the Ank/(A)gamma-globin transgene resulted in Ank/(A)gamma-globin mRNA expression in embryonic and fetal erythroid cells in six of eight lines but resulted in absent or dramatically reduced levels of Ank/(A)gamma-globin mRNA in adult erythroid cells in eight of eight transgenic lines. These data indicate that the minimal ankyrin promoter contains all sequences necessary and sufficient for erythroid-specific, copy number-dependent, position-independent expression of the human (A)gamma-globin gene.

A human beta-spectrin gene promoter directs high level expression in erythroid but not muscle or neural cells

beta-Spectrin is an erythrocyte membrane protein that is defective in many patients with abnormalities of red blood cell shape including hereditary spherocytosis and elliptocytosis. It is expressed not only in erythroid tissues but also in muscle and brain. We wished to determine the regulatory elements that determine the tissue-specific expression of the beta-spectrin gene. We mapped the 5'-end of the beta-spectrin erythroid cDNA and cloned the 5'-flanking genomic DNA containing the putative beta-spectrin gene promoter. Using transfection of promoter/reporter plasmids in human tissue culture cell lines, in vitro DNase I footprinting analyses, and gel mobility shift assays, a beta-spectrin gene erythroid promoter with two binding sites for GATA-1 and one site for CACCC-related proteins was identified. All three binding sites were required for full promoter activity; one of the GATA-1 motifs and the CACCC-binding motif were essential for activity. The beta-spectrin gene promoter was able to be transactivated in heterologous cells by forced expression of GATA-1. In transgenic mice, a reporter gene directed by the beta-spectrin promoter was expressed in erythroid tissues at all stages of development. Only weak expression of the reporter gene was detected in muscle and brain tissue, suggesting that additional regulatory elements are required for high level expression of the beta-spectrin gene in these tissues.

Substitution of the human beta-spectrin promoter for the human agamma-globin promoter prevents silencing of a linked human beta-globin gene in transgenic mice

During development, changes occur in both the sites of erythropoiesis and the globin genes expressed at each developmental stage. Previous work has shown that high-level expression of human beta-like globin genes in transgenic mice requires the presence of the locus control region (LCR). Models of hemoglobin switching propose that the LCR and/or stage-specific elements interact with globin gene sequences to activate specific genes in erythroid cells. To test these models, we generated transgenic mice which contain the human Agamma-globin gene linked to a 576-bp fragment containing the human beta-spectrin promoter. In these mice, the beta-spectrin Agamma-globin (betasp/Agamma) transgene was expressed at high levels in erythroid cells throughout development. Transgenic mice containing a 40-kb cosmid construct with the micro-LCR, betasp/Agamma-, psibeta-, delta-, and beta-globin genes showed no developmental switching and expressed both human gamma- and beta-globin mRNAs in erythroid cells throughout development. Mice containing control cosmids with the Agamma-globin gene promoter showed developmental switching and expressed Agamma-globin mRNA in yolk sac and fetal liver erythroid cells and beta-globin mRNA in fetal liver and adult erythroid cells. Our results suggest that replacement of the gamma-globin promoter with the beta-spectrin promoter allows the expression of the beta-globin gene. We conclude that the gamma-globin promoter is necessary and sufficient to suppress the expression of the beta-globin gene in yolk sac erythroid cells.

Amphotropic or gibbon ape leukemia virus retrovirus binding and transduction correlates with the level of receptor mRNA in human hematopoietic cell lines

The low level of amphotropic retrovirus mediated gene transfer into human hematopoietic stem cells (HSC) has been an impediment to gene therapy for hematopoietic diseases (1). We have previously shown that mouse and human HSC have low levels of the mRNA encoding PiT-2, the amphotropic retrovirus receptor. We hypothesized that the low level of PiT-2 mRNA was responsible for the low frequency of transduction of HSC by amphotropic retroviral vectors (2). In this study we compared the level of PiT-2 and PiT-1, the Gibbon Ape Leukemia Virus receptor (GaLV), in 5 human tissue culture cell lines. PiT-2 and PiT-1 mRNA levels were highest in K562 cells and lowest in HL60 cells. In hematopoietic cell lines, the level of PiT-2 or PiT-1 mRNA correlated directly with retrovirus binding and transduction with the appropriate (amphotropic or GaLV) retrovirus vector. The level of expression of PiT-2 and PiT-1 mRNA could be increased by treatment of HL60 cells with either PMA or Interleukin-1alpha. The increase in the level of PiT-2 and PiT-1 mRNA correlated with increased transduction with both amphotropic and GaLV retroviral vectors. We conclude that the improved transduction was a direct effect of the increased levels of receptor mRNA and unrelated to changes in the cell cycle status.

Distal 8p deletion (8)(p23.1): an easily missed chromosomal abnormality that may be associated with congenital heart defect and mental retardation

We describe the clinical manifestations and molecular cytogenetic analyses of three patients with a similar distal deletion of chromosome 8. Each child had mild developmental delay and subtle minor anomalies. Two had cardiac anomalies but no other major congenital anomalies were present. High resolution G and R banding showed in all three patients del(8)(p23.1), but the breakpoint in case 1 was distal to 8p23.1, in case 2 was in the middle of 8p23.1, and in case 3 proximal to 8p23.1. Fluorescence in situ hybridization (FISH) studies with a chromosome 8 paint probe confirmed that no other rearrangement had occurred. FISH with a chromosome 8-specific telomere probe indicated that two patients had terminal deletions. Chromosome analysis of the parents of case 1 and mother of case 2 were normal; the remaining parents were not available for study. Thirteen individual patients including the three in this study, and three relatives in one family with del(8)(p23.1), have been reported in the past 5 years. Major congenital anomalies, especially congenital heart defects, are most often associated with a breakpoint proximal to 8p23.1. Three patients were found within a 3-year period in this study and five cases were found within 4 years by another group, indicating that distal 8p deletion might be a relatively common chromosomal abnormality. This small deletion is easily overlooked (i.e., cases 1 and 2 were reported as normal at amniocentesis) and can be associated with few or no major congenital anomalies.