ALBERT

Description: High-capacity adenoviral (HC-Ad) vectors expressing B-domain–deleted human or canine factor VIII from different liver-specific promoters were evaluated for gene therapy of hemophilia A. Intravenous administration of these vectors into hemophilic FVIII-deficient immunodeficient SCID mice (FVIIIKO-SCID) at a dose of 5 × 109 infectious units (IU) resulted in efficient hepatic gene delivery and long-term expression of supraphysiologic FVIII levels (exceeding 15 000 mU/mL), correcting the bleeding diathesis. Injection of only 5 × 107 IU still resulted in therapeutic FVIII levels. In immunocompetent hemophilic FVIII-deficient mice (FVIIIKO), FVIII expression levels peaked at 75 000 mU/mL but declined thereafter because of neutralizing anti-FVIII antibodies and a cellular immune response. Vector administration did not result in thrombocytopenia, anemia, or elevation of the proinflammatory cytokine interleukin-6 (IL-6) and caused no or only transient elevations in serum transaminases. Following transient in vivo depletion of macrophages before gene transfer, significantly higher and stable FVIII expression levels were observed. Injection of only 5 × 106 HC-Ad vectors after macrophage depletion resulted in long-term therapeutic FVIII levels in the FVIIIKO and FVIIIKO-SCID mice. Intravenous injection of an HC-Ad vector into a hemophilia A dog at a dose of 4.3 × 109 IU/kg led to transient therapeutic canine FVIII levels that partially corrected whole-blood clotting time. Inhibitory antibodies to canine FVIII could not be detected, and there were no signs of hepatotoxicity or of hematologic abnormalities. These results contribute to a better understanding of the safety and efficacy of HC-Ad vectors and suggest that the therapeutic window of HC-Ad vectors could be improved by minimizing the interaction between HC-Ad vectors and the innate immune system.

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: 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: A major recent discovery from large-scale sequencing studies was that over half of Myelodysplastic Syndromes (MDS) patients harbor mutations in splicing factor (SF) genes. SF mutations are the most common class of mutations in MDS and occur early in the course of the disease. These strongly suggest that SF mutations are key to the pathogenesis of MDS and can provide new therapeutic opportunities. However, identifying the downstream effects of SF mutations that are critical for the development of MDS presents a big challenge due to the cellular and genetic heterogeneity of primary patient samples, the unavailability of immortalized cell lines harboring SF mutations in the native genomic context and the limited conservation of alternative splicing isoforms between mice and humans. We previously showed that SF-mutant induced pluripotent stem cells (iPSCs) generated from MDS patients recapitulate key features of the disease upon differentiation into hematopoietic lineages, including cellular phenotypes (increased cell death, decreased clonogenicity and dysplastic morphology), sensitivity to splicing modulating drugs and the altered RNA binding specificity of mutant SFs (Chang et al. Stem Cell Reports, 2018). To further investigate the effects of SF mutations, we used CRISPR to introduce each of the 3 main canonical SF mutations (SRSF2 P95L, SF3B1 K700E, U2AF1 S34F) in the same normal iPSC line N-2.12 that we previously derived and extensively characterized in terms of pluripotency, genetic integrity and hematopoietic differentiation potential. The derivative iPSC lines contain the 3 SF mutations in isogenic conditions in the context of a diploid genome, in a heterozygous state, with both the normal and mutant alleles expressed at physiological and equal levels. To uncover potential new therapeutic targets and gain insights into the downstream effects of SF mutations, we set up CRISPR knockout (KO) lethality screens in hematopoietic progenitor cells (HPCs) derived from these SF-mutant iPSCs. We began with a gRNA library containing 224 gRNAs targeting 57 kinase genes (4 gRNAs per gene). The library was assembled and packaged in a lentiviral backbone also expressing GFP. Cas9 together with mCherry was expressed from a separate lentiviral vector. iPSCs were differentiated along the hematopoietic lineage, transduced on day 11, coinciding with the onset of the emergence of CD34+/CD45+ HPCs, and further cultured for up to day 27 to allow "dropout" of lethal genes, read out by next-generation sequencing (NGS). We titrated the lentiviral vectors to obtain transduction efficiency of nearly 100% for the Cas9 vector and up to 40-50% for the gRNA library (in order to obtain the highest percentage of cells harboring a single gRNA) in 500,000 HPCs, to ensure representation of the library of at least 500 cells per gRNA. To avoid population bottlenecks, we ensured that at least 500,000 cells were present in the culture at all times. All library gRNAs were present in the transduced cells and their distribution correlated tightly with that of the lentiviral supernatant. Technical repeats of independently prepared DNA samples, independent PCR reactions and independently generated NGS reads showed high reproducibility and absence of batch effects. The library was screened in 3 independent clones harboring each of the 3 canonical SF mutations, as well as in 3 clones of the parental normal line. This design allows the identification of potential convergent genes or pathways downstream of the 3 SF mutations and exclusion of non-synthetically lethal targets (which would also drop out in the isogenic normal cells). CRISPR scores were calculated as the average of the log of the final vs initial abundance of all gRNAs per gene, and showed a distribution consistent with the expectation that the majority of the gRNAs do not have a major impact on cell viability. Experiments with evidence of random genetic drifts from the CRISPR scores distribution were excluded from the analyses. Initial hits, defined as kinase genes with targeting gRNAs consistently depleted in SF-mutant lines of all 3 genotypes, but not in the normal isogenic cells are being validated with individual gene knockout and small molecule inhibition. In parallel, we are setting up CRISPR screens in expandable HPCs (eHPCs) derived from iPSCs. The latter can be expanded in culture for several weeks and could enable screening of larger or even genome-wide gRNA libraries. Disclosures No relevant conflicts of interest to declare.

Description: Two helper-dependent (HD) adenoviral vectors encoding a canine factor VIII B-domain–deleted transgene (cFVIII) were constructed and evaluated in 4 hemophilia A dogs. One vector was regulated by the cytomegalovirus (CMV) promoter (HD-CMV-cFVIII), while the other vector contained a tissue-restricted promoter comprised of the human FVIII proximal promoter with an upstream concatemer of 5 hepatocyte nuclear factor 1 binding sites (HD-HNF-cFVIII). We detected no toxicity at low dose (5 × 1011 vp/kg), but at higher vector doses (〉 1 × 1012 vp/kg) transient hepatotoxicity and thrombocytopenia were observed. Low-level increases in FVIII activity were detected in all 3 HD-HNF-cFVIII–treated dogs, which corresponded with decreased whole blood clotting times. None of the animals receiving the HD-HNF-cFVIII vector developed FVIII inhibitors, and in 1 of the 3 animals, FVIII activity was sustained for over 6 months after treatment. One animal, which received the HD-CMV-cFVIII vector, achieved peak levels of FVIII above 19 000 mU/mL, but FVIII activity disappeared within 1 week, coincident with the development of a potent anti–canine FVIII antibody response. This study supports previous demonstrations of improved safety using HD gene transfer and suggests that these vectors can provide transient FVIII expression with minimal, acute toxicity in the absence of inhibitor formation.

Description: Liver gene transfer is a highly sought goal for the treatment of inherited and infectious diseases. Lentiviral vectors (LVs) have many desirable properties for hepatocyte-directed gene delivery, including the ability to integrate into nondividing cells. Unfortunately, upon systemic administration, LV transduces hepatocytes relatively inefficiently compared with nonparenchymal cells, and the duration of transgene expression is often limited by immune responses. Here, we investigated the role of innate antiviral responses in these events. We show that administration of LVs to mice triggers a rapid and transient IFNαβ response. This effect was dependent on functional vector particles, and in vitro challenge of antigen-presenting cells suggested that plasmacytoid dendritic cells initiated the response. Remarkably, when LVs were administered to animals that lack the capacity to respond to IFNαβ, there was a dramatic increase in hepatocyte transduction, and stable transgene expression was achieved. These findings indicate that, even in the setting of acute delivery of replication-defective vectors, IFNs effectively interfere with transduction in a cell-type–specific manner. Moreover, because disabling a single component of the innate/immune network was sufficient to establish persistent xenoantigen expression, our results raise the hope that the immunologic barriers to gene therapy are less insurmountable than expected.

Description: A longstanding goal for the treatment of hemophilia B is the development of a gene transfer strategy that can maintain sustained production of clotting factor IX (F.IX) in the absence of an immune response. To this end, we have sought to use lentiviral vectors (LVs) as a means for systemic gene transfer. Unfortunately, initial evaluation of LVs expressing F.IX from hepatocyte-specific promoters failed to achieve sustained F.IX expression in hemophilia B mice due to the induction of an anti-F.IX cellular immune response. Further analysis suggested that this may be a result of off-target transgene expression in hematopoietic-lineage cells of the spleen. In order to overcome this problem, we modified our vector to contain a target sequence for the hematopoietic-specific microRNA, miR-142-3p. This eliminated off-target expression in hematopoietic cells, and enabled sustained gene transfer in hemophilia B mice for more than 280 days after injection. Treated mice had more than 10% normal F.IX activity, no detectable anti-F.IX antibodies, and were unresponsive to F.IX immunization. Importantly, the mice survived tail-clip challenge, thus demonstrating phenotypic correction of their bleeding diathesis. This work, which is among the first applications to exploit the microRNA regulatory pathway, provides the basis for a promising new therapy for the treatment of hemophilia B.

Description: We previously showed that incorporating target sequences for the hematopoietic-specific microRNA miR-142 into an antigen-encoding transgene prevents antigen expression in antigen-presenting cells (APCs). To determine whether this approach induces immunologic tolerance, we treated mice with a miR-142–regulated lentiviral vector encoding green fluorescent protein (GFP), and subsequently vaccinated the mice against GFP. In contrast to control mice, no anti-GFP response was observed, indicating that robust tolerance to the transgene-encoded antigen was achieved. Furthermore, injection of the miR-142–regulated vector induced a population of GFP-specific regulatory T cells. Interestingly, an anti-GFP response was observed when microRNA miR-122a was inserted into the vector and antigen expression was detargeted from hepatocytes as well as APCs. This demonstrates that, in the context of lentiviral vector-mediated gene transfer, detargeting antigen expression from professional APCs, coupled with expression in hepatocytes, can induce antigen-specific immunologic tolerance.

Description: Recent studies in gene transfer suggest that the innate immune system plays a significant role in impeding gene therapy. In this review, we examine factors that might influence the recruitment and activation of the innate system in the context of gene therapy. We have adopted a novel model of immunology that contends that the immune system distinguishes not between self and nonself, but between what is dangerous and what is not dangerous. In taking this perspective, we provide an alternative and complementary insight into some of the failures and successes of current gene therapy protocols.