The reverse transcriptase domain of prime editors contributes to DNA repair in mammalian cells

Reverse transcriptase (RT) has been shown to play a role in double-strand break repair in bacteria, yet the impact of the RT component of prime editors (PEs) on normal mammalian cellular functions is unclear. Here we show that overexpressed RT or PE increases short insertions and diminishes homology-directed repair following Cas9 cleavage at multiple loci in multiple cell lines. Live-cell imaging shows that RT and PEs are rapidly recruited to laser-induced DNA damage sites and promote endogenous repair, independent of known DNA damage sensors. Interestingly, RT-mCherry partially impairs green fluorescent protein-PARP1 recruitment. A compact PE without an RNase H domain shows reduced DNA repair activity and may therefore be more suitable for clinical application. These data reveal a role for untethered RT or the RT domain of PEs in the repair of chromosomal breaks, calling for evaluation of the long-term effect of PEs and retroviral RT in mammalian cells.