Identification of genome safe harbor loci for human gene therapy based on evolutionary biology and comparative genomics
Gene transfer into CD34+ hematopoietic stem and progenitor cells (HSPCs) involving integrating viral vectors has unpredictable outcomes including potential adverse events like leukemogenesis, resulting from insertional mutagenesis. Therefore, identifying and characterizing genome safe harbor (GSH) sites where exogenous gene material can be safely integrated into adult progenitor and stem cells is critically important for therapeutic gene addition. Here, we present a novel approach to identify new GSH sites based on a proven system of stable transgene insertion: the evolutionarily conserved integration of parvovirus DNA into the germlines of host species. From a dataset of 199 unique endogenous parvovirus (EPV) integration events identified in host species genomes, 102 loci were mapped to the human genome with 17 being experimentally evaluated as GSHs in primary human CD34+ HSPCs. Nine promising GSHs resulted in cells edited using nucleofection alone or in combination with rAAV transduction. Of the nine GSH sites, six edited loci displayed sustained transgene expression in both erythroid and immune phenotypes while three clearly exhibited immune branch specific-regulation. Following this approach, additional GSH sites are likely to emerge from the remaining mapped loci for gene addition in hematopoietic stem and progenitor cells. Since it is unlikely that the GSH-lineage-restricted transgene expression is exclusive to hematopoietic stem cells, this approach extends the options for gene knock-ins while reducing the risks of insertional mutagenesis, unpredictable expression profiles, effects on differentiation, and increasing therapeutic effects.