Human RNaseH2 subunit upregulation counteracts oncogene- and chemotherapy-induced replication stress

RNaseH2 is a heterotrimeric endoribonuclease that resolves RNA:DNA hybrids and mis-incorporated ribonucleotides, which are implicated in DNA replication stress and cancer development and potential therapeutic targets. Individual RNaseH2 subunit mRNA or protein levels are found elevated in some cancers, but little is known about the mechanisms behind and impact of RNaseH2 subunit upregulation. We report that RNaseH2 subunits are upregulated at the protein level in response to replication stress induced by oncogenes and chemotherapy drugs hydroxyurea, gemcitabine and camptothecin in human cancer and non-cancer cell lines. We show that inducible overexpression of the RNaseH2B subunit increases the levels of the whole active RNaseH2 complex and can counteract drug-induced RNA:DNA hybrid formation. RNaseH2B overexpression reduces replication fork stalling induced by camptothecin and hydroxyurea but can itself increase RNA:DNA hybrid levels and cause replication stress. RNaseH2 subunit levels do not strongly impact survival of chemotherapy drug treatments but may have more subtle effects on genomic instability and cytoplasmic DNA signalling. In contrast, increased RNaseH2 subunit levels in presence of oncogenic HRAS are required to limit not only RAS-induced replication fork stalling but also cell death. Our findings shed new light on the functions of RNaseH2 and suggest that upregulation of RNaseH2 subunits may be an important aspect of replication stress responses in cancer.