Long-Term Clinical and Molecular Follow-up of Large Animals Receiving Retrovirally Transduced Stem and Progenitor Cells: No Progression to Clonal Hematopoiesis or Leukemia

Abstract

There has been significant progress toward clinically relevant levels of retroviral gene transfer into hematopoietic stem cells (HSC), and the therapeutic potential of HSC-based gene transfer has been convincingly demonstrated in children with severe combined immunodeficiency syndrome (SCID). However, the subsequent development of leukemia in two children with X-linked SCID who were apparently cured after transplantation of retrovirally corrected CD34+ cells has raised concerns regarding the safety of gene therapy approaches utilizing integrating vectors. Nonhuman primates and dogs represent the best available models for gene transfer safety and efficacy and are particularly valuable for evaluation of long-term effects. We have followed 42 rhesus macaques, 23 baboons, and 17 dogs with significant levels of gene transfer for a median of 3.5 years (range 1-7) after infusion of CD34+ cells transduced with retroviral vectors expressing marker or drug-resistance genes. None developed abnormal hematopoiesis or leukemia. Integration site analysis confirmed stable, polyclonal retrovirally marked hematopoiesis, without progression toward mono- or oligoclonality over time. These results suggest that retroviral integrations using replication-incompetent vectors, at copy numbers achieved using standard protocols, are unlikely to result in leukemogenesis and that patient- or transgene-specific factors most likely contributed to the occurrence of leukemia in the X-SCID gene therapy trial.

ATP-binding-cassette transporters in hematopoietic stem cells and their utility as therapeutical targets in acute and chronic myeloid leukemia

“…[28][29][30] However, when extrapolating these experiments to a primate model using competitive repopulation assays and long-term follow-up of hematopoiesis, no detectable adverse consequences have been observed. 31,32 In line with this, MDR1 induction in hematopoietic cells in clinical trials, in an attempt to protect these cells for chemotherapeutical damage, has not resulted in abnormal hematopoiesis 33,34 and marrow of adult mdr1a/1bÀ/À mice shows normal generation, function and multi-tissue trafficking of primitive hematopoietic cells. 14 Together these studies failed to demonstrate a role for ABCB1 in steady-state hematopoiesis.…”

Retroviral vector integration deregulates gene expression but has no consequence on the biology and function of transplanted T cells

“…The occurrence of leukemia in patients treated by gene therapy for X-linked severe combined immunodeficiency (X-SCID) has been correlated with insertional activation of a T cell proto-oncogene (9). No malignancy has been reported so far in other preclinical (10) or clinical (11,12) trials, suggesting the existence of specific risk factors in the X-SCID case (13). In-depth analysis on the consequences of retroviral vector integration is therefore necessary, to provide risk-benefit assessments in different biological and clinical contexts.…”

Cloning and Functional Analysis of the Rhesus Macaque ABCG2 Gene

“…Certainly, differences between the small and large animal have been observed with respect to the first of the ABC transporters to be tested in both models. The myeloproliferative syndrome observed in mice after transplantation of hematopoietic stem cells transduced with MDR1 (15) was not observed in our rhesus macaque model (36,37). Differences in gene transfer efficiency into mouse versus rhesus HSCs might explain this result, because the oligoclonal expanded stem cell population found in mice showed high proviral copy number, and expression at high levels may have been required to produce the phenotype.…”