Oncogenesis as an Adverse Effect of Gene Replacement Therapy in Hematopoietic Stem Cells

Genetically modified hematopoietic stem cell therapy using gene-modified autologous hematopoietic stem cells has evolved over the last 30 years as an alternative approach to circumvent the limitations of donor availability, risks of excessive regimen related toxicity, prolonged immune suppression and graft-versus-host disease associated with allogeneic hematopoietic cell transplantation. Gene replacement therapy based on viral insertion of transgene into host genome was developed as one of the main methods for gene modification of autologous cells. Unfortunately, many cases of oncogenesis were directly caused by genetically modified hematopoietic stem cell therapy. The purpose of the present review is the description of cases of leukemogenesis in gene replacement therapy in hematopoietic stem cells, elucidation of the causes, and overview of the risk mitigation strategies. It aims to elucidate the main risk factors in gene replacement therapy in hematopoietic stem cells. The insertional mutagenesis leads to activation of proto-oncogenes, mostly LMO2 and MECOM-EVI1. γ-retroviral vectors are dangerous in this case, as they contain long terminal repeats with strong promotor activity and are prone to integration near transcription initiation sites. Therefore, safer self-inactivating lentiviral vectors were developed, with long terminal repeats modified to reduce their promoter activity and with safer integration pattern. Nevertheless, the risk of leukemogenesis remains because the promoter integrated into the transgene expression cassette may still influence nearby gene expression. Another risk factor is monosomy 7, either pre-existing or caused by MECOM-EVI1 activation, which may contribute directly to leukemogenesis. Thus, oncogenesis in HSPC gene replacement therapy does not have a single definitive cause; rather, multiple factors may contribute, and each may be sufficient under specific conditions.