Author ORCID Identifier
Date of Graduation
Clinical and Translational Sciences
Doctor of Philosophy (PhD)
polyADP ribose polymerase (PARP) inhibitors are a class of drugs that block the PARP enzymes, involved in the repair of singe-stranded DNA breaks through the base excision repair pathway. PARP inhibition leads to replication-associated double stranded DNA breaks, which are repaired by homologous recombination (HR). In tumors with HR defects (i.e. BRCA mutants), there is a shift to error-prone DNA repair and subsequent genomic instability and cell death.
In 2014, Olaparib became the first FDA-approved PARP inhibitor for the treatment of BRCA-mutant ovarian cancer. In the phase III POLO (Pancreas cancer OLaparib Ongoing) trial presented at the American Society for Clinical Oncology (ASCO) conference in 2019, maintenance therapy with Olaparib significantly delayed the progression of metastatic BRCA mutant pancreatic adenocarcinoma (PDAC) compared to placebo (7.4 months vs 3.8 months). Talazoparib, a second generation PARP inhibitor, is 20-200-fold more efficacious compared to older PARP inhibitors. In 2018, Talazoparib was FDA approved to treat BRCA-mutant, HER2-negative breast cancer and is still in early phase trials for PDAC. While these inhibitors show tremendous promise, not all hereditary PDAC patients respond to PARP inhibitors, and resistance has been observed.
In order to explore resistance to PARP inhibitors in PDAC and I hypothesized that: 1. Genomic alterations are responsible for response and resistance to PARP inhibitor (Talazoparib) treatment in pancreatic patient-derived xenograft (PDX) tumors; and 2. The use of shRNA technology on pancreatic PDX tumors can elucidate synthetic lethal partners to overcome therapeutic resistance to PARP inhibition. To test this hypothesis, I set out to first further characterize the hereditary PDAC population with germline sequencing and then second, find novel resistance mechanisms and potential rational drug combination therapies to overcome this resistance.
I demonstrated the results of a large-scale germline sequencing project of 133 metastatic PDAC patients. In Chapter 2, I show that the incidence of hereditary pancreatic cancer is nearly 20% in this cohort as well as a TCGA validation patient data set. Patients with DNA damage repair gene (ATM, BRCA1, BRCA2, ERCC4, PALB2) alterations have a statistically significant near doubling of overall survival compared to those without mutations (17.9 versus 9.6 months, P = 0.03). I show that although strong family history of multiple breast, ovarian, and pancreatic cancers is associated with improved survival, mutational profile is a better indicator of overall survival.
I tested the efficacy of Talazoparib in BRCA-mutant PDAC PDX models. Using colony formation assays, I show a differential response in BRCA-mutant and wild type cell lines; and found unexpected resistance in one BRCA2-mutant PDX model (PATC55). I found a truncating RIF1 DNA mutation at the BRCA interacting site. Additional shRNA targeted knockdown of RIF1 did not induce resistance, however single cell RNA sequencing of Talazoparib-treated cells did demonstrate high expression levels of SHFM1, known to facilitate Rad51 loading of RPA. Elevated SHFM1 levels has been associated with aggressive breast cancer and platinum resistance and may be playing a part in PARP resistance in hereditary PDAC as well1,2.
Pancreatic cancer is a difficult disease to treat with limited possibility for cure. Some hereditary pancreatic cancers with deficiencies in DNA repair appear sensitive to treatment with PARP inhibitors, although not all cancers respond as expected. This data, provides rationale to pursue drug combination therapies, to mitigate resistance to PARP inhibitors and bring about novel treatment options to hereditary PDAC patients.
PARP inhibitor, hereditary, pancreatic cancer, pancreatic adenocarcinoma, germline, BRCA
Available for download on Tuesday, August 04, 2020