Safer CRISPR-Cas Alternative? Retroelement-Based Genome Tools

In a Perspective, Stephen Tang and Samuel Sternberg discuss retroelement-based gene editing as a safer alternative to CRISPR-Cas approaches. Precision genome editing technologies have transformed modern biology. Capabilities for programable DNA targeting have improved rapidly, largely due to the development of bacterial RNA-guided CRISPR-Cas systems, which allow precise cleavage of target DNA sequences. However, CRISPR-Cas9 systems generate a DNA double strand break (DSB), which activates cellular DNA repair pathways that can lead to unwanted and complex byproducts, including large chromosomal deletions and translocations, resulting in safety concerns. However, a growing body of research has shown the value of leveraging mobile genetic elements, particularly retroelements, as an alternative to CRISPR-Cas systems. According to Tang and Sternberg, retroelements are a potentially useful tool for targeted genomic engineering because they are endogenous DNA segments that use the host's RNA polymerase to generate an RNA copy of themselves, which is then turned into complementary DNA (cDNA) by a retroelement-encoded reverse transcriptase. This cDNA product is then incorporated into the genome through a range of mechanisms that do not require DSB. Recent investigations into the structure and function of retroelement-encoded protein-RNA complexes have begun to resolve the molecular details of the mechanisms underlying these abilities, "unveiling exciting opportunities to re-engineer them for programmable DNA insertion," write the authors.