Long-term outcomes after gene therapy for adenosine deaminase severe combined immune deficiency.

Patients lacking functional adenosine deaminase activity have severe combined immunodeficiency (ADA SCID), which can be treated with ADA enzyme replacement therapy (ERT), allogeneic hematopoietic stem cell transplantation (HSCT), or autologous HSCT with gene-corrected cells (gene therapy [GT]). A cohort of 10 ADA SCID patients, aged 3 months to 15 years, underwent GT in a phase 2 clinical trial between 2009 and 2012. Autologous bone marrow CD34+ cells were transduced ex vivo with the MND (myeloproliferative sarcoma virus, negative control region deleted, dl587rev primer binding site)-ADA gammaretroviral vector (gRV) and infused following busulfan reduced-intensity conditioning. These patients were monitored in a long-term follow-up protocol over 8 to 11 years. Nine of 10 patients have sufficient immune reconstitution to protect against serious infections and have not needed to resume ERT or proceed to secondary allogeneic HSCT. ERT was restarted 6 months after GT in the oldest patient who had no evidence of benefit from GT. Four of 9 evaluable patients with the highest gene marking and B-cell numbers remain off immunoglobulin replacement therapy and responded to vaccines. There were broad ranges of responses in normalization of ADA enzyme activity and adenine metabolites in blood cells and levels of cellular and humoral immune reconstitution. Outcomes were generally better in younger patients and those receiving higher doses of gene-marked CD34+ cells. No patient experienced a leukoproliferative event after GT, despite persisting prominent clones with vector integrations adjacent to proto-oncogenes. These long-term findings demonstrate enduring efficacy of GT for ADA SCID but also highlight risks of genotoxicity with gRVs. This trial was registered at www.clinicaltrials.gov as #NCT00794508.

Gene therapy for adenosine deaminase-deficient severe combined immune deficiency: clinical comparison of retroviral vectors and treatment plans.

We conducted a gene therapy trial in 10 patients with adenosine deaminase (ADA)-deficient severe combined immunodeficiency using 2 slightly different retroviral vectors for the transduction of patients' bone marrow CD34(+) cells. Four subjects were treated without pretransplantation cytoreduction and remained on ADA enzyme-replacement therapy (ERT) throughout the procedure. Only transient (months), low-level (< 0.01%) gene marking was observed in PBMCs of 2 older subjects (15 and 20 years of age), whereas some gene marking of PBMC has persisted for the past 9 years in 2 younger subjects (4 and 6 years). Six additional subjects were treated using the same gene transfer protocol, but after withdrawal of ERT and administration of low-dose busulfan (65-90 mg/m(2)). Three of these remain well, off ERT (5, 4, and 3 years postprocedure), with gene marking in PBMC of 1%-10%, and ADA enzyme expression in PBMC near or in the normal range. Two subjects were restarted on ERT because of poor gene marking and immune recovery, and one had a subsequent allogeneic hematopoietic stem cell transplantation. These studies directly demonstrate the importance of providing nonmyeloablative pretransplantation conditioning to achieve therapeutic benefits with gene therapy for ADA-deficient severe combined immunodeficiency.

Update on gene therapy of inherited immune deficiencies.

Gene therapy has been under development as a way to correct inborn errors for many years. Recently, patients with two forms of inherited severe combined immunodeficiency (SCID), adenosine deaminase and X-linked, treated by three different clinical investigative teams, have shown significant immune reconstitution leading to protective immunity. These advances irrefutably prove the concept that hematopoietic progenitor cell gene therapy can ameliorate these diseases. However, due to proviral insertional oncogenesis, two individuals in one of the X-SCID studies developed T-cell leukemia more than two years after the gene transfer. Depending upon the results of long-term follow-up, the successes together with the side effects highlight the relative merits of this therapeutic approach.

Prolonged pancytopenia in a gene therapy patient with ADA-deficient SCID and trisomy 8 mosaicism: a case report.

A patient with adenosine deaminase-deficient severe combined immune deficiency (ADA-SCID) was enrolled in a study of retroviral-mediated ADA gene transfer to bone marrow hematopoietic stem cells. After the discontinuation of ADA enzyme replacement, busulfan (75 mg/m2) was administered for bone marrow cytoreduction, followed by infusion of autologous, gene-modified CD34+ cells. The expected myelosuppression developed after busulfan but then persisted, necessitating the administration of untransduced autologous bone marrow back-up at day 40. Because of sustained pancytopenia and negligible gene marking, diagnostic bone marrow biopsy and aspirate were performed at day 88. Analyses revealed hypocellular marrow and, unexpectedly, evidence of trisomy 8 in 21.6% of cells. Trisomy 8 mosaicism (T8M) was subsequently diagnosed by retrospective analysis of a pretreatment marrow sample that might have caused the lack of hematopoietic reconstitution. The confounding effects of this preexisting marrow cytogenetic abnormality on the response to gene transfer highlights another challenge of gene therapy with the use of autologous hematopoietic stem cells.

Selective survival of peripheral blood lymphocytes in children with HIV-1 following delivery of an anti-HIV gene to bone marrow CD34(+) cells.

Two HIV-1-infected children on antiretroviral therapy were enrolled into a clinical study of retroviral-mediated transfer of a gene that inhibits replication of HIV-1, targeting bone marrow CD34+ hematopoietic stem/progenitor cells. Two retroviral vectors were used, one encoding a "humanized" dominant-negative REV protein (huM10) that is a potent inhibitor of HIV-1 replication and one encoding a nontranslated marker gene (FX) to serve as an internal control for the level of gene marking. Peripheral blood mononuclear cells (PBMC) containing the huM10 gene or FX gene were detected by quantitative PCR at frequencies of approximately 1/10,000 in both subjects for the first 1-3 months following re-infusion of the gene-transduced bone marrow, but then were at or below the limits of detection (<1/1,000,000) at most times over 2 years. In one patient, a reappearance of PBMC containing the huM10 gene, but not the FX gene, occurred concomitant with a rise in the HIV-1 viral load during a period of nonadherence to the antiretroviral regimen. Unique clones of gene-marked PBMC were detected by LAM-PCR during the time of elevated HIV-1 levels. These findings indicate that there was a selective survival advantage for PBMC containing the huM10 gene during the time of increased HIV-1 load.

Gene therapy for inborn and acquired immune deficiency disorders.

Gene therapy has been under development as a way to correct inborn errors for over 20 years. Immune deficiencies are favorable candidates for gene therapy because of the potential selective advantage of genetically corrected cells in these conditions. Gene therapy for immune deficiencies has been the only application to show incontrovertible benefit in clinical trials to date. Despite the success in treating the underlying disease, there have been two cases of insertional oncogenesis reported in one of these early phase trials. Gene therapy approaches and clinical trials for several inborn as well as acquired immune deficiencies will be reviewed.

Transgenic IL-2 expression in Ewing tumor cell lines after transfection with Starburst dendrimers and cationic liposomes.

Immunotherapy with IL-2-transduced cells requires efficient methods of gene transfer. Nonviral methods have been studied intensively in recent years. This study examined whether tumor and fibroblast cell lines established from Ewing tumor patients could be efficiently transfected with the IL-2 gene. Starburst dendrimers (Superfect), a novel transfection reagent, were chosen for a transfection study and optimal conditions for gene transfer were evaluated. Three Ewing tumor cell lines and 3 fibroblast cell lines obtained from Ewing tumor patients were analyzed. The concentration of IL-2 in the supernatant of transfected cells was measured by ELISA. All three Ewing tumor cell lines transfected by Starburst dendrimers yielded higher IL-2 levels than after lipofection. In contrast to lipofection, expression of IL-2 increased with time and peaked later. In one of three tested fibroblast cell lines, transfection using Superfect yielded higher IL-2 levels. IL-2 production was generally lower in fibroblasts as compared to Ewing tumor cell lines. Given the low toxicity of Superfect reagent and the high efficiency of transfection, this method seems to be ideal for clinical studies on the immunotherapy of Ewing tumors.

Expression from second-generation feline immunodeficiency virus vectors is impaired in human hematopoietic cells.

Vectors based on the feline immunodeficiency virus (FIV) have been developed as an alternative to those based on another lentivirus, human immunodeficiency virus-1 (HIV-1), because of theoretical safety advantages. We compared the efficiency of gene transfer and expression in human and feline hematopoietic progenitors using second-generation HIV-1 and FIV-based vectors. Vector pairs were tested using either human cytomegalovirus or murine phospho-glycerate kinase (PGK) internal promoters and were pseudotyped with the vesicular stomatitis virus G protein (VSV-G). Vector proviral copy numbers were similar in human and feline hematopoietic primary cells and cell lines transduced by HIV-1 or FIV vectors, demonstrating that both vectors are able to transfer genes efficiently to these cell types. HIV-1 vectors were well expressed in human primary hematopoietic cells and cell lines. However, transgene expression from FIV vectors was almost undetectable in human hematopoietic cells. In contrast, the FIV vector was expressed well in primary hematopoietic feline cells and human non-hematopoietic cells, demonstrating that low transgene expression from the FIV vector is a phenomenon specific to human hematopoietic cells. Northern blot analysis demonstrated decreased vector transcript levels in human CEM cells transduced with FIV relative to cells transduced with HIV-1, despite high vector copy numbers. No evidence of vector transcript instability was seen in studies of transduced CEM cells treated with actinomycin D. We conclude that FIV vectors can transfer genes into human hematopoietic cells as effectively as HIV-1 vectors, but that unknown elements in the current FIV backbone inhibit expression from FIV vectors in human hematopoietic cells.