Emerging roles of RNA:DNA hybrid regulation by mammalian RNase H2 in replication stress and cancer

Replication stress involves the slowing or stalling of the replication fork as DNA is copied during S phase. This stress can drive genomic instability, a cancer hallmark. RNA:DNA hybrids such as R-loops, and single genome-embedded ribonucleotides are significant sources of replication stress. RNA:DNA hybrid homeostasis must therefore be tightly regulated through prevention and removal. Ribonuclease H2 (RNase H2) functions both in R-loop removal and excision of single ribonucleotides from genomic DNA. Recent research has generated new mechanistic insights into the functions of RNase H2 in the replication stress response, and implicated both loss and overexpression of RNase H2 in cancer development and therapy response. These findings help generate new models but also raise new questions. This Review will explore the contribution of RNA:DNA hybrids to replication stress, the involvement of RNase H2 in regulating these structures, and the emerging roles of RNase H2 in replication stress response and cancer.