Clinical efficacy of gene-modified stem cells in adenosine deaminase-deficient immunodeficiency.

BACKGROUND: Autologous hematopoietic stem cell transplantation (HSCT) of gene-modified cells is an alternative to enzyme replacement therapy (ERT) and allogeneic HSCT that has shown clinical benefit for adenosine deaminase-deficient (ADA-deficient) SCID when combined with reduced intensity conditioning (RIC) and ERT cessation. Clinical safety and therapeutic efficacy were evaluated in a phase II study. METHODS: Ten subjects with confirmed ADA-deficient SCID and no available matched sibling or family donor were enrolled between 2009 and 2012 and received transplantation with autologous hematopoietic CD34+ cells that were modified with the human ADA cDNA (MND-ADA) γ-retroviral vector after conditioning with busulfan (90 mg/m2) and ERT cessation. Subjects were followed from 33 to 84 months at the time of data analysis. Safety of the procedure was assessed by recording the number of adverse events. Efficacy was assessed by measuring engraftment of gene-modified hematopoietic stem/progenitor cells, ADA gene expression, and immune reconstitution. RESULTS: With the exception of the oldest subject (15 years old at enrollment), all subjects remained off ERT with normalized peripheral blood mononuclear cell (PBMC) ADA activity, improved lymphocyte numbers, and normal proliferative responses to mitogens. Three of nine subjects were able to discontinue intravenous immunoglobulin replacement therapy. The MND-ADA vector was persistently detected in PBMCs (vector copy number [VCN] = 0.1-2.6) and granulocytes (VCN = 0.01-0.3) through the most recent visits at the time of this writing. No patient has developed a leukoproliferative disorder or other vector-related clinical complication since transplant. CONCLUSION: These results demonstrate clinical therapeutic efficacy from gene therapy for ADA-deficient SCID, with an excellent clinical safety profile. TRIAL REGISTRATION: ClinicalTrials.gov NCT00794508. FUNDING: Food and Drug Administration Office of Orphan Product Development award, RO1 FD003005; NHLBI awards, PO1 HL73104 and Z01 HG000122; UCLA Clinical and Translational Science Institute awards, UL1RR033176 and UL1TR000124.

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.

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.