Summary
Chromosomal instability in cancer cells is associated with changes in chromosome structure and number, and drives tumor heterogeneity and resistance to therapy. The common denominator of chromosomal instability might be the occurrence of replication stress that can induce both structural and numerical chromosomal instability. Replication stress can occur as consequence of non-native DNA structures such as R-loops. Co-transcriptional R-loops are RNA-DNA hybrids with a displaced single stranded DNA, which can accumulate as consequence of transcription-replication conflicts or upon oncogene activation. In our project, we propose to study the contribution of transcription-dependent R-loops to replication stress and chromosomal instability in cancer. In the first funding period, we systematically analyzed the S-phase and mitosis-specific phosphorylation signaling in response to mild replication stress that does not activate cell cycle checkpoints and permits cycling of cells. We identified a phosphorylation signature of mild replication stress and a role of kinase CK2 in the cellular response to cancer relevant replication stress. We found that CK2 phosphorylates Ribonuclease H1 (RNaseH1) in the N-terminal hybrid-binding domain. RNaseH1 acts to digest the RNA moiety in R-loops and thereby can resolve these structures; however, how RNaseH1 is regulated in human cells remains completely unexplored. In our planned project, we will analyze the contribution of transcription-dependent and oncogene-induced R-loops to replication stress, formation of double-strand DNA breaks and chromosomal instability in cancer cells. We will further investigate the phosphorylation signaling in response to oncogene-induced replication stress and compare those signatures to chemically induced mild replication stress. We expect that our results will identify mechanisms and pathways that play a role in R-loop driven replication stress and genomic instability.
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