63. Long-Term Constitutive Expression of the Therapeutic Gene, IL2RG, Causes T-Cell Lymphomas in a Gene Therapy Model for XSCID

Abstract

Top of pageAbstract The successful gene therapy clinical trials, where the immune function of patients with X-linked severe combined immune deficiency (XSCID) was restored, demonstrated the utility for vector mediated gene transduction of hematopoietic stem cells in the treatment of a life threatening disease. However, the occurrence of T-cell leukemias in three patients, with the suspected oncogenic event resulting from upregulation of expression of the known oncogene, LMO2, by insertional mutagenesis, has led to the re- evaluation of the safety of this procedure. To model the gene therapy adverse events and to better understand the mechanism of generation of leukemia in the patients, we expressed the therapeutic gene, IL2RG, used to treat XSCID patients, in combination with and without LMO2 in a murine model of XSCID, and followed the fates of transplanted mice up to 1.8 years post transplantation. In this study, we unexpectedly found that the therapeutic gene IL2RG alone is a major contributor to the genesis of T-cell lymphomas in mice, with a prevalence of 33%, n=15. We found that IL2RG|[minus]|/|[minus]| mice transplanted with IL2RG vector transduced IL2RG|[minus]|/|[minus]| cells or similarly transduced wild type cells developed the T-cell lymphomas. As expected, T-cell lymphomas were also detected in the positive control LMO2 transduced mice with a prevalence of 50%, n=12, with cell phenotype similar to those previously published. In contrast, none of the mock transduced mice (n=16) nor LV-GFP vector transduced mice (n=17) developed the T-cell lymphoma in this study. These findings were generated from 7 independent experiments totaling 99 mice ranging from 71 to 91 weeks posttransplant. Interestingly, the average time of onset of the T-cell lymphomas in IL2RG transduced mice was faster than the LMO2 transduced mice, 10.1 months versus 12.2 months post transplant, p=0.034. The T-cell lymphoma manifested itself as a gross thymic mass, which showed neoplastic cells with a CD3/B220 double positive phenotype, as well as larger CD4|[minus]|/CD8+ blast-like cells, which were also predominant in the spleen. Vector integration site analysis showed lymphomic cells to be of clonal origin with similar integration site profiles being present in the thymuses, bone marrow cells, and spleens of individual mice. Dual transgene expression of IL2RG and LMO2 did not significantly accelerate lymphomagenesis. Our results suggest a re-evaluation of the current view that IL2RG has minimal oncogenic properties and offer caution that current gene therapy trials for XSCID may pose increased risks to patients than previously anticipated. Furthermore, these results may explain why leukemia has been seen with such prevalence in XSCID trials when gene therapy trials for other diseases have not shown any such side effects.