Targeting DNA Repair Mechanisms in MYC-Driven Cancer

Abstract

Genetic instability is a hallmark of cancer and contributes to tumorigenesis, since it can lead to acquisition of the remaining cancer hallmarks. Consistent with this, DNA damage is detected at early stages of tumor development, specifically in some premalignant human tumors. The source of this DNA damage is, in part, due to oncogene-induced replication stress. Replication stress is defined as slowing of DNA synthesis or stalling of the replication fork, and can lead to accumulation of genomic damage if not resolved or repaired efficiently. Oncogene activation has been identified as one source of replication stress due to the influence these proteins have on the cell cycle. For example overexpression of C-MYC, one of the most highly amplified oncogenes in human cancer, leads to deregulated cyclin dependent kinase (CDK) activity, a shortened G1, and premature entry into S-phase of the cell cycle. These effects can lead to replication stress, and it has previously been shown that the DNA damage response (DDR) is activated upon C-MYC overexpression. With the growing knowledge of the cellular consequences of oncogene overexpression, there is great interest in identifying the proteins required for repair of oncogene-induced DNA damage. The Mre11/Rad50/Nbs1 (MRN) DNA repair complex has roles in all known DNA DSB repair pathways. Previous studies have shown MRN plays important roles in resolving stalled replication forks, and promoting fork restart. This thesis is focused on investigating the role of the MRE11 DNA nuclease in tumorigenesis and specifically in cells that overexpress the MYC oncogene. The studies in my thesis utilize a mouse model of spontaneous lymphoma associated IgH:Myc translocations leading to C-MYC or N-MYC overexpression. These mice harbor gene targeted mutations in Artemis, the V(D)J recombination DNA nuclease, and p53, and have a strong predisposition to early onset pro-B lymphoma. Interestingly, we found B cell specific deletion of MRE11 or inactivation of MRE11 nuclease activity completely suppressed pro-B lymphomagenesis. Although MRE11 has been implicated in the DSB repair pathway proposed to generate some translocations, we provide evidence that shows MRE11 is not required for the generation of specific IgH:Myc translocations. Based on these data, we hypothesize that MRE11 is required during repair of oncogene-induced DNA damage and inactivation of MRE11 leads to catastrophic genomic instability and cell death. To address these questions, the impact of pharmacologic inhibition of MRE11 nuclease activities in cells that overexpress C-MYC was examined. I observed MRE11 exonuclease activity, and not endonuclease activity, is critical for survival in these cells. I demonstrate that pharmacological inhibition of MRE11 exonuclease activity results in increased DNA damage, decreased cellular survival, and increased apoptosis specifically in cells that overexpress C-MYC. The findings presented in my thesis provide promising mechanistic preclinical evidence in support of inhibiting MRE11 exonuclease activity to therapeutically target MYC-driven and replication stress-associated cancers.