RAD18 opposes transcription-associated genome instability through FANCD2 recruitment
DNA replication is a vulnerable time for genome stability maintenance. Intrinsic stressors, as well as oncogenic stress, can challenge replication by fostering conflicts with transcription and stabilizing DNA:RNA hybrids. RAD18 is an E3 ubiquitin ligase for PCNA that is involved in coordinating DNA damage tolerance pathways to preserve genome stability during replication. In this study, we show that RAD18 deficient cells have higher levels of transcription-replication conflicts and accumulate DNA:RNA hybrids that induce DNA double strand breaks and replication stress. We find that these effects are driven in part by failure to recruit the Fanconi Anemia protein FANCD2 at difficult to replicate and R-loop prone genomic sites. While RAD18 is important for FANCD2 localization after splicing inhibition or aphidicolin treatment, it is comparatively less important during interstrand crosslink repair. Thus, we highlight a RAD18 dependent pathway promoting FANCD2-mediated suppression of R-loops and transcription-replication conflicts. AUTHOR SUMMARYGenome instability, a state in which cells acquire mutations more quickly, drives cancer initiation and progression. DNA is normally protected from such mutations by a host of specialized factors that recognize and repair potentially damaging circumstances. Here we describe under-appreciated links between a protein called RAD18, which recognizes potentially damaging stress, and a phenomenon called a transcription-replication conflict. If not repaired such conflicts can lead to DNA breaks. We find that RAD18 plays an important role in recruiting another repair protein called FANCD2 to sites of transcription-replication conflicts. By doing so, RAD18 protects the human genome from damage when it is faced with stresses such as those seen in cancer cells. This work provides new insight to how the FANCD2 protein is brought to potentially damaging sites in our genomes.