DNA double-strand breaks (DSBs) pose a serious threat to cell viability and genome stability. The two major DSB repair pathways that have been studied extensively are homologous recombination (HR) and non- homologous end-joining (NHEJ). Understanding the detailed regulation of DNA repair pathway choice will help us design better therapy for cancer patients. For example, a major breakthrough in the targeted treatment of BRCA1-mutant cancers is the use of PARP inhibitors (PARPi), which display synthetic lethality with BRCA1 deficiency. Unfortunately, resistance to PARPi therapy is a major clinical problem in BRCA1-mutant cancers. Recent studies showed that loss of another DNA repair protein, 53BP1 renders these BRCA1-deficient cells or tumors HR repair capacity and resistant to PARPi, suggesting that 53BP1 and BRCA1 compete with each other to influence the pathway choice for DSB repair. Since 53BP1 lacks enzymatic activities that would be directly implicated in DNA repair, we thus decided to further explore the 53BP1-dependent pathway, focusing on the identification of downstream effector proteins that directly participate in DNA repair. Excitingly, we uncovered a nuclease Artemis as a PTIP-binding protein. Artemis has endonuclease activity and is a known component of the NHEJ pathway. Our preliminary data demonstrated that Artemis is a major nuclease that acts downstream of 53BP1- dependent DNA repair pathway. Just like 53BP1, loss of Artemis expression in BRCA1-deficient cells leads to PARPi resistance. However, several important questions remain unanswered. We need to further elucidate the underlying mechanisms how the 53BP1-dependent pathway acts in DNA repair and antagonizes HR functions. Importantly, how to effectively treat BRCA1-deficient tumors that become resistant to PARPi is an urgent clinical question. The following Specific Aims are proposed to address these important questions.
Aim 1. Determine the mechanism by which 53BP1-dependent NHEJ repair pathway counteracts HR repair pathway, which is critical for resistance to PARP inhibition.
Aim 2. Explore strategies to overcome PARPi resistance for breast cancer treatment.

Public Health Relevance

A major breakthrough in the targeted treatment of BRCA mutant cancers is the use of PARP inhibitors (PARPi), which display 'synthetic lethality' with BRCA deficiency. Notably, loss of 53BP1 expression in cells endows BRCA1-deficient cells resistance to PARPi, indicating that 53BP1 competes with BRCA1 to influence the DNA repair pathway choice; however, 53BP1 is adaptor protein lacking the enzymatic activities that would be directly involved in DNA repair. In this application, we showed that the 53BP1/PTIP pathway facilitates the accumulation of Artemis at the sites of DNA damage, and prevents end resection and HR repair, thereby renders BRCA1-deficient cells resistance to PARPi, and will further study the underlying mechanisms to develop new therapies for breast cancer patients who are resistance to PARPi.

National Institute of Health (NIH)
National Cancer Institute (NCI)
Exploratory/Developmental Grants (R21)
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Special Emphasis Panel (ZCA1)
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Forry, Suzanne L
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University of Texas MD Anderson Cancer Center
United States
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Zhang, Aili; Peng, Bo; Huang, Ping et al. (2017) The p53-binding protein 1-Tudor-interacting repair regulator complex participates in the DNA damage response. J Biol Chem 292:6461-6467
Chen, Zhen; Tran, Mykim; Tang, Mengfan et al. (2016) Proteomic Analysis Reveals a Novel Mutator S (MutS) Partner Involved in Mismatch Repair Pathway. Mol Cell Proteomics 15:1299-308
Wang, Jiadong; Aroumougame, Asaithamby; Lobrich, Markus et al. (2014) PTIP associates with Artemis to dictate DNA repair pathway choice. Genes Dev 28:2693-8