Targeting DNA damage repair pathways in breast and ovarian cancers
Publication Date
March 15, 2020
Creator
Abstract
Background
DNA damage repair pathways are responsible for repairing DNA lesions and thus maintaining genomic integrity. Deficit in the DNA damage response is one of the hallmarks of cancers. Thus, drugs targeting DNA repair relying on this concept are rapidly under development. Targeting PARP1 for synthetic lethality is a novel strategy for BRCA germ-line mutated breast and ovarian cancers. However, BRCA germ-line mutations are rare and reactivation of BRCA mediated pathways may result in eventual resistance to PARP1 inhibitor therapy. Hence, developing alternative synthetic lethality approaches is an exciting area for future research.
Methods
In this study, we investigated XRCC1 and PARP1 expression in an invasive breast cancer cohort (n=1011) and pre-invasive ductal carcinoma in situ (DCIS) (n=776). We evaluated Olaparib and Talazoparib (PARP inhibitors) sensitivity in a panel of XRCC1 deficient and proficient breast cancer cell lines. In the MCF10-DCIS model we evaluated XRCC1 down regulation and epithelial to mesenchymal transition phenotype. We explored XRCC1 KO and Olaparib sensitivity. In the ovarian cancer cohort (n=525) we investigated the correlations between XRCC1 as well as PARP1 expression and clinicopathological outcomes. XRCC1 KO and Olaparib/Talazoparib sensitivity were tested in platinum sensitive and platinum resistant ovarian cancer cell lines in 2D and 3D models. Additionally, polb and PARG protein expression and correlation with clinicopathological outcomes were investigated in our ovarian cancer cohort. We evaluated polb depletion and platinum sensitivity in platinum-sensitive and platinum-resistant ovarian cancer cell lines. We tested polb small molecule inhibitors (Pamoic acid & NSC666719) toxicity in BRCA2 deficient cells. We assessed Olaparib and PARG inhibitor (PDD00017273) lethality in polb deficient ovarian cancer cells.
Results
Low XRCC1/high PARP1 expression is associated with aggressive phenotype and poor breast cancer prognosis. In DCIS XRCC1 downregulation is an early event in human breast cancer pathogenesis. In XRCC1 deficient cell lines, Olaparib and Talazoparib were synthetically lethal in XRCC1 deficient breast cancer cell lines as well as MCF10-DCIS cells. PARPi induced cell cycle arrest at S-phase or G2/M phases in XRCC1 deficient breast cancer cells. Cell cycle arrest was dependent on p53 status. Additionally, PARP inhibition selectively induced DNA damage and apoptosis in XRCC1 deficient cells. In MCF10-DCIS XRCC1 knock out induced epithelial to mesenchymal transition phenotype. In 3D spheroids PARP inhibition abolished spheroid forming capacity in MCF10-DCIS(XRCC1_KO) but not in control cells. In ovarian cancers, high XRCC1 or high PARP1 levels were associated with aggressive phenotype and were significantly linked to poor progression free survival (PFS). PARP inhibitors remained selectively toxic in XRCC1 deficient platinum- sensitive ovarian cancer cell lines. PARPi cytotoxicity was associated with DSBs formation, cell cycle arrest and apoptosis in XRCC1 deficient cells. Polb expression was a predictor of platinum sensitivity in ovarian cancer patients. Polb depletion not only increased platinum sensitivity but also reduced invasion and migration in ovarian cancer cell lines. Polb small molecule inhibitors were selectively toxic in BRCA2 deficient cells in 2D as well as 3D spheroids models. High polb and high PARG co-expression correlated with poor PFS as well as overall survival in ovarian cancer. PARG inhibitor but not Olaparib were selectively toxic in polb deficient ovarian cancer cells. Polb-PARG co-expression adversely impacted survival in patients.
Conclusions
PARP1 targeting is an attractive synthetic lethality strategy and chemoprevention in XRCC1 deficient breast cancers and pre-invasive DCIS lesions. In ovarian cancers XRCC1 deficiency is a biomarker for PARP inhibitor sensitivity. Targeting PARP1 in XRCC1 deficient ovarian cancers is an alternative synthetic lethality approach for PARPi in clinics. Similarly,, our data demonstrates that development of polb inhibitors could be of clinical benefit to ovarian cancer patients. Polb depletion can re-sensitize ovarian cancer patients to platinum. Polb targeting in BRCA2 deficient ovarian cancers is an attractive synthetic lethality. PARG inhibitors could have a clinical application in targeting Polb deficient ovarian tumours. Our results could have a valuable therapeutic potential in personalizing breast and ovarian cancers’ therapy.
Item Type
ethesis
Thesis Type
PhD
Subjects (LC)
Associated Schools / Departments
School of Medicine
eprints ID
59701
UoN Repository URI
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Examined. PhD corrected thesis_Reem Ali
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