Replication dependent mechanisms of genome maintenance
Lovejoy, Courtney Alison
The DNA Damage Response (DDR) is an inducible barrier to tumorigenesis at its earliest stages of development. Evidence from cell culture, animal models, and clinical samples reveals DDR activation in response to activation of oncogenes and inactivation of tumor suppressors. These genetic alterations promote replication stress and DNA damage that activates DDR signaling pathways. The DDR restrains the growth of cells with mutated or unstable genomes by preventing cell cycle progression, promoting DNA repair, and inducing senescence or apoptosis. Several genetic alterations that activate DDR pathways have been identified, and recent sequencing efforts indicate hundreds of genes are mutated in breast or colorectal tumors. The heterogenous basis of cancer suggests there are many genes that can contribute to the process of tumorigenesis, and thus an important goal of cancer research is to understand the mechanisms through which genome instability arises and contributes to disease. I identified seventy-four genes with genome maintenance activity using a functional and biologically relevant RNAi screen that monitors the ability of gene-silencing siRNAs to induce DDR activation. Additional analyses identified thirty-five genes that may possess replication-dependent genome maintenance functions, which I confirmed for two genes of interest after further characterization. I demonstrated that DDB1 maintains genome integrity as part of an ubiquitin-ligase complex that mediates the S phase-dependent degradation of the replication-licensing factor CDT1. Proper regulation of CDT1 is critical to prevent re-replication and subsequent double strand break formation that can threaten genome integrity. In collaboration with Xin Xu, I also demonstrated that the genome maintenance function of CINP involves regulation of ATR-dependent DDR signaling. CINP promotes ATR-mediated phosphorylation of CHK1 and maintenance of the G2 checkpoint, in addition to facilitating CDK2-dependent phosphorylation of ATRIP. The seventy-four genome maintenance genes I identified are excellent candidates for the gene function defects that may promote genome instability and DDR activation in pre-cancerous lesions. Further characterization of these genes may provide a greater understanding of the gene functions and cellular pathways that are critical for tumorigenesis.