Retrotransposon-based mechanisms for transgene addition to the human genome

When human disease arises from a loss of function caused by diverse mutant alleles of the same gene, the patient population could be best served by a clinical therapy that achieves genome safe-harbor supplementation with a functional transgene. Until recently, transgene delivery strategies have shared the disadvantages of induced immune responses and/or genome mutagenesis from untargeted DNA insertion. As a different strategy, several groups recently described the use of retrotransposon proteins to accomplish transgene insertion by RNA-templated cDNA synthesis directly into the genome. In some strategies, gene insertion relies on the retrotransposon protein to bring a transgene-encoding template RNA to the target site. Retrotransposon protein positioning of template RNA for cDNA synthesis minimizes the requirement for RNA base-pairing to target-site DNA. This review presents an overview of RNA-templated DNA synthesis in cells as backdrop for describing recent uses of retrotransposon reverse transcriptases to supplement the human genome.