Safety and efficacy of retroviral and lentiviral vectors for gene therapy

Abstract

In hematopoietic stem progenitor cell (HSPC) gene therapy (GT) applications for the treatment of genetic diseases, retroviral vectors (RVs) are used to efficiently transduce and integrate therapeutic genes in the genome of patient-derived HSPCs, which, upon reinfusion, reconstruct the entire hematopoietic system and restore the correct hematopoietic functions, or deliver the therapeutic factor to different tissues. However, in initial HSPC-GT clinical trials using early-generation γ-RVs, vector insertions near proto-oncogenes triggered their overexpression and induced leukemia in some of the transplanted patients. These unexpected adverse events have prompted the development of highly sensitive preclinical assays to test the genotoxic potential of different GT vector types and designs, and the development of powerful PCR-based techniques, combined with next generation sequencing (NGS) and bioinformatics analyses, have allowed to study integration sites (ISs) present in leukemic and dominant expanding cells, identify the genes targeted by insertions and to investigate the clonal composition of complex vector-marked cell populations. The positive safety data obtained from the testing in highly sensitive preclinical models and, successively, in clinical trials, of the more advanced lentiviral vectors (LVs) with self-inactivating (SIN) long terminal repeats (LTRs), have reduced the concerns related to insertional mutagenesis, encouraging the adoption of this vector platform in GT protocols for the treatment of many other diseases. Nevertheless, the evidences collected from genotoxicity assays and a b-thalassemia clinical trial, during which a vector-driven clonal dominance event has occurred, point to the fact that even SIN LVs insertions are not entirely neutral, and thus to the importance of a continuous effort to improve both the design of GT vectors, and the sensitivity of preclinical assays aimed at assessing their residual genotoxicity.