DNA within human cells is continuously subject to various damaging agents. These agents often cause DNA double-strand breaks (DSBs), which can be repaired by a process known as homology-directed repair (HDR). Proteins encoded by the tumor suppressor genes, Brca1 and Brca2, play an essential role in performing HDR with high-fidelity. Mutations in Brca1 or Brca2 confer an approximately 11-40% lifetime risk of developing ovarian cancer. Approval of olaparib (trade name Lynparza), a medication that inhibits enzymes called poly(ADP-ribose) polymerase (PARP), created a new treatment option for patients with Brca1 or Brca2 mutated advanced ovarian cancer. Inhibition of PARP leads to the accumulation of DSBs, which are preferentially repaired by HDR conducted by BRCA1 and BRCA2. In cancer cells lacking BRCA1/2, DSBs from PARP inhibition can no longer be repaired, leading to selective killing of Brca-mutated cells. A major clinical problem of this promising therapy is the development of drug resistance. One of the most likely mechanisms of resistance to PARP inhibitors (PARPi) is thought to be via BRCA-independent HDR, which is poorly understood. It is unclear what these mechanisms are and how they allow Brca-mutated cells to repair lethal DSBs. The objective of this study is to identify these compensatory, BRCA-independent DNA repair mechanisms and examine how they contribute to PARPi resistance in ovarian cancer.
A major challenge in identifying these BRCA-independent repair pathways is the lack of a suitable model system. Recent studies in the Greenberg lab revealed that during a process called alternative lengthening of telomeres (ALT), HDR occurs in a BRCA-independent manner. Therefore, ALT provides a physiologically relevant system to examine BRCA-independent HDR. Using various biochemical and imaging based assays, we will identify the factors that are essential for BRCA-independent ALT. We hypothesize that factors involved in BRCA-independent ALT are also required for HDR in BRCA-deficient cancer cells and that these factors lead to PARPi resistance by mending the DSBs. To examine this, we have engineered an ovarian cancer cell line resistant to PARPi. Proteins required for BRCA-independent ALT will be knocked down in these resistant cell lines to identify factors, which, when depleted, restore PARPi sensitivity. Additionally, as an independent approach to identify the PAPRi resistance mechanism(s), we will perform a screen using an advance genome editing technology called CRISPR. Using CRISPR, we will identify the genes, which, when knocked out, (i) alter PARPi sensitivity or (ii) selectively kill Brca-mutated cells. Collectively, these studies will help identify the underlying mechanisms of PARPi resistance and will also aid in the discovery of predictive biomarkers to PARPi based therapy. As a therapeutic approach, these resistance mechanisms can be targeted in combination with PARPi to successfully treat BRCA-deficient ovarian cancer.
This grant was made possible through the generous support of Lori Carson, MD, in memory of her mother Christine B. Herring.