Retroviral vector integration in post-transplant hematopoiesis in mice conditioned with either submyeloablative or ablative irradiation

Preclinical safety assessment of modified gamma globin lentiviral vector-mediated autologous hematopoietic stem cell gene therapy for hemoglobinopathies

Previously, we reported the development of a human Aγ-globin gene lentivirus (LV), GbG, which expresses high levels of HbF to correct the sickle cell anemia (SCA) phenotype in the Berkeley SCA mouse model, and then modified the γ-globin gene by substituting glycine at codon 16 with aspartic acid in the Aγ-globin gene to generate GbGM LV. In the present study, we evaluated the long-term safety of human Aγ-globin gene carrying GbGM LV in wild-type mice after primary and secondary transplants of GbGM-modified hematopoietic stem cells (HSC) over 18 months. The safety of the GbGM bone marrow transplant was assessed by monitoring the effects on body weight, hematology, histopathology, malignancy formation, and survival. Mice transplanted with Mock-transduced and spleen focus forming virus (SFFV) γ-retroviral vector (RV)-transduced HSC served as negative and positive controls, respectively. The mean donor-cell engraftment was comparable across Mock, GbGM LV, and SFFV RV groups. There were no significant differences in body weight, clinical signs, immunophenotype, or histopathology in the GbGM-treated mice compared to controls. Four SFFV RV-treated mice, but none of the GbGM-treated mice, developed donor-derived, vector-positive lymphomas as demonstrated by flow cytometry analysis and in situ hybridization. These results highlight the safety of the administration of GbGM LV-modified HSC with long-term follow-up after primary and secondary transplants in mice. This data supported the initiation of phase 1/2 first-in-human SCA clinical trial in the United States.

Quantitative stability of hematopoietic stem and progenitor cell clonal output in rhesus macaques receiving transplants

Autologous transplantation of hematopoietic stem and progenitor cells lentivirally labeled with unique oligonucleotide barcodes flanked by sequencing primer targets enables quantitative assessment of the self-renewal and differentiation patterns of these cells in a myeloablative rhesus macaque model. Compared with other approaches to clonal tracking, this approach is highly quantitative and reproducible. We documented stable multipotent long-term hematopoietic clonal output of monocytes, granulocytes, B cells, and T cells from a polyclonal pool of hematopoietic stem and progenitor cells in 4 macaques observed for up to 49 months posttransplantation. A broad range of clonal behaviors characterized by contribution level and biases toward certain cell types were extremely stable over time. Correlations between granulocyte and monocyte clonalities were greatest, followed by correlations between these cell types and B cells. We also detected quantitative expansion of T cell-biased clones consistent with an adaptive immune response. In contrast to recent data from a nonquantitative murine model, there was little evidence for clonal succession after initial hematopoietic reconstitution. These findings have important implications for human hematopoiesis, given the similarities between macaque and human physiologies.

From bench to bedside: preclinical evaluation of a self-inactivating gammaretroviral vector for the gene therapy of X-linked chronic granulomatous disease

Chronic granulomatous disease (CGD) is a primary immunodeficiency characterized by impaired antimicrobial activity in phagocytic cells. As a monogenic disease affecting the hematopoietic system, CGD is amenable to gene therapy. Indeed in a phase I/II clinical trial, we demonstrated a transient resolution of bacterial and fungal infections. However, the therapeutic benefit was compromised by the occurrence of clonal dominance and malignant transformation demanding alternative vectors with equal efficacy but safety-improved features. In this work we have developed and tested a self-inactivating (SIN) gammaretroviral vector (SINfes.gp91s) containing a codon-optimized transgene (gp91(phox)) under the transcriptional control of a myeloid promoter for the gene therapy of the X-linked form of CGD (X-CGD). Gene-corrected cells protected X-CGD mice from Aspergillus fumigatus challenge at low vector copy numbers. Moreover, the SINfes.gp91s vector generates substantial amounts of superoxide in human cells transplanted into immunodeficient mice. In vitro genotoxicity assays and longitudinal high-throughput integration site analysis in transplanted mice comprising primary and secondary animals for 11 months revealed a safe integration site profile with no signs of clonal dominance.

Transgenic sheep generated by lentiviral vectors: safety and integration analysis of surrogates and their offspring

The safety of HIV-1 based vectors was evaluated during the production of transgenic sheep. Vectors were introduced into the perivitelline space of in vivo derived one-cell sheep embryos by microinjection then transferred into the oviducts of recipient females. At 60-70 days of gestation, a portion of the recipients were euthanized and tissues collected from both surrogates and fetuses. Other ewes were allowed to carry lambs to term. Inadvertent transfer of vector from offspring to surrogates was evaluated in 330 blood and tissue samples collected from 57 ewes that served as embryo recipients. Excluding uterine contents, none of the samples tested positive for vector, indicating that that the vector did not cross the fetal maternal interface and infect surrogate ewes. Evaluating ewes, fetuses and lambs for replication competent lentivirus (RCL); 84 serum samples analyzed for HIV-1 capsid by ELISA and over 600 blood and tissue samples analyzed by quantitative PCR for the VSV-G envelopes revealed no evidence of RCL. Results of these experiments provide further evidence as to the safety of HIV-1 based vectors in animal and human applications.

BCL2A1a over-expression in murine hematopoietic stem and progenitor cells decreases apoptosis and results in hematopoietic transformation

We previously reported the development of a lethal myeloid sarcoma in a non-human primate model utilizing retroviral vectors to genetically modify hematopoietic stem and progenitor cells. This leukemia was characterized by insertion of the vector provirus into the BCL2A1 gene, with resultant BCL2A1 over-expression. There is little information on the role of this anti-apoptotic member of the BCL2 family in hematopoiesis or leukemia induction. Therefore we studied the impact of Bcl2a1a lentiviral over-expression on murine hematopoietic stem and progenitor cells. We demonstrated the anti-apoptotic function of this protein in hematopoietic cells, but did not detect any impact of Bcl2a1a on in vitro cell growth or cell cycle kinetics. In vivo, we showed a higher propensity of HSCs over-expressing Bcl2a1a to engraft and contribute to hematopoiesis. Mice over-expressing Bcl2a1a in the hematologic compartment eventually developed an aggressive malignant disease characterized as a leukemia/lymphoma of B-cell origin. Secondary transplants carried out to investigate the primitive origin of the disease revealed the leukemia was transplantable. Thus, Bcl2a1 should be considered as a proto-oncogene with a potential role in both lymphoid and myeloid leukemogenesis, and a concerning site for insertional activation by integrating retroviral vectors utilized in hematopoietic stem cell gene therapy.

Gene therapy/bone marrow transplantation in ADA-deficient mice: roles of enzyme-replacement therapy and cytoreduction

Gene therapy (GT) for adenosine deaminase-deficient severe combined immune deficiency (ADA-SCID) can provide significant long-term benefit when patients are given nonmyeloablative conditioning and ADA enzyme-replacement therapy (ERT) is withheld before autologous transplantation of γ-retroviral vector-transduced BM CD34+ cells. To determine the contributions of conditioning and discontinuation of ERT to the therapeutic effects, we analyzed these factors in Ada gene knockout mice (Ada(-/-)). Mice were transplanted with ADA-deficient marrow transduced with an ADA-expressing γ-retroviral vector without preconditioning or after 200 cGy or 900 cGy total-body irradiation and evaluated after 4 months. In all tissues analyzed, vector copy numbers (VCNs) were 100- to 1000-fold greater in mice receiving 900 cGy compared with 200 cGy (P < .05). In mice receiving 200 cGy, VCN was similar whether ERT was stopped or given for 1 or 4 months after GT. In unconditioned mice, there was decreased survival with and without ERT, and VCN was very low to undetectable. When recipients were conditioned with 200 cGy and received transduced lineage-depleted marrow, only recipients receiving ERT (1 or 4 months) had detectable vector sequences in thymocytes. In conclusion, cytoreduction is important for the engraftment of gene-transduced HSC, and short-term ERT after GT did not diminish the capacity of gene-corrected cells to engraft and persist.

Identifying viral integration sites using SeqMap 2.0

Retroviral integration has been implicated in several biomedical applications, including identification of cancer-associated genes and malignant transformation in gene therapy clinical trials. We introduce an efficient and scalable method for fast identification of viral vector integration sites from long read high-throughput sequencing. Individual sequence reads are masked to remove non-genomic sequence, aligned to the host genome and assembled into contiguous fragments used to pinpoint the position of integration.

AVAILABILITY AND IMPLEMENTATION

The method is implemented in a publicly accessible web server platform, SeqMap 2.0, containing analysis tools and both private and shared lab workspaces that facilitate collaboration among researchers. Available at http://seqmap.compbio.iupui.edu/.

Gene therapy of chronic granulomatous disease: the engraftment dilemma

The potential of gene therapy as a curative treatment for monogenetic disorders has been clearly demonstrated in a series of recent Phase I/II clinical trials. Among primary immunodeficiencies, gene transfer into hematopoietic stem (HSC)/progenitor cells has resulted in the long-term correction of immune and metabolic defects in treated patients. In most cases, successes were augmented by a recognized biological selection for successfully treated cells in vivo, perhaps even to some extent at the HSC level. In contrast, similar achievements have not turned into reality for immunodeficiencies in which gene-transduced cells lack selective advantages in vivo. This is the case for chronic granulomatous disease (CGD), a primary immunodeficiency, characterized by deficient antimicrobial activity in phagocytic cells. Several attempts to correct CGD by gene transfer in combination with bone marrow conditioning have resulted in low-level long-term engraftment and transient clinical benefits despite high levels of gene marking and high numbers of reinfused cells. This review summarizes the data from clinical trials for CGD and provides some insights into treatment options that may lead to a successful application of gene therapy for CGD.