Poster Presentation 29th Lorne Cancer Conference 2017

Uncovering mechanisms of PARP inhibitor resistance in high-grade serous ovarian carcinoma patient-derived xenografts (#235)

Ksenija Nesic 1 , Olga Kondrashova 1 , Elizabeth Lieschke 1 , Gwo Yaw Ho 1 2 , Elizabeth M Swisher 3 , Maria L Harrell 3 , Neil Johnson 4 , Yifan Wang 4 , Matthew Wakefield 1 , Clare Scott 1 5
  1. Walter and Eliza Hall Institute of Medical Research, Parkville, VIC, Australia
  2. Peter MacCallum Cancer Center, Melbourne, VIC, Australia
  3. University of Washington, Seattle, WA, United States of America
  4. Fox Chase Cancer Center, Philadelphia, Pennsylvania, United States of America
  5. Royal Women’s Hospital, Melbourne, VIC, Australia

High-grade serous ovarian carcinoma (HGSOC) is the most common subtype of ovarian cancer, and is characterized by frequent loss of homologous recombination (HR) DNA repair pathway proteins, such as BRCA1 and BRCA2. Despite the promising efficacy of PARP inhibitors (PARPi) in treating HR Deficient (HRD) cancers, less than 50% of BRCA1/2-defective cancers have a durable response. Multiple mechanisms of PARPi resistance have been proposed, though few have been documented in the clinic. Our unique HGSOC patient-derived xenograft (PDX) cohort allows us to study these mechanisms in an in vivo system closely reflecting that of a patient1. This project will explore the spectrum and prevalence of PARPi resistance mechanisms in HGSOC, how they arise, and approaches to overcome them. 

Secondary mutations that correct primary HR defects are a well-documented mechanism of PARPi resistance in the clinic2,3. Four HRD PDX from our cohort were screened for secondary HR gene mutations using targeted Next-Generation Sequencing (NGS). PDX were from chemotherapy-naïve patients (#19 and #148), a matched post-chemotherapy biopsy (#19B), and a biopsy following multiple lines of chemotherapy and a PARPi clinical trial (#23133). No secondary mutations were detected in the NGS screen, ruling this out as the mechanism of resistance in these PDX.

Another recently described mechanism of PARPi resistance in cases with deleterious BRCA1 exon 11 mutations is increased expression of alternative BRCA1 isoform missing part of exon 11 (Δ11q)4. Quantitative PCR was used to compare levels of canonical and Δ11q BRCA1 transcripts in treated and un-treated PDX tumours from PARPi sensitive and resistant sub-lineages of BRCA1 11q mutant PDX #56. The qPCR screen did not reveal higher expression of the Δ11q isoform compared to canonical BRCA1, however the plasticity of this resistance mechanism has not been established and warrants further investigation.

Given the non-durable responses to PARPi seen in these PDX, other mechanisms of PARPi resistance will continue to be explored. Defining and curating PARPi resistance mechanisms in this cohort will provide better insight into which patients are most and least likely to derive benefit from PARPi treatment, and may help us discover more effective therapies for PARPi resistant cancer patients.

 

  1. Topp, Monique D., et al. "Molecular correlates of platinum response in human high-grade serous ovarian cancer patient-derived xenografts." Molecular oncology 8.3 (2014): 656-668.
  2. Norquist, Barbara et al. “Secondary Somatic Mutations Restoring BRCA1/2 Predict Chemotherapy Resistance in Hereditary Ovarian Carcinomas.” Journal of Clinical Oncology 29.22 (2011): 3008–3015. PMC. Web. 16 Dec. 2016.
  3. Barber, Louise J., et al. "Secondary mutations in BRCA2 associated with clinical resistance to a PARP inhibitor." The Journal of pathology 229.3 (2013): 422-429.
  4. Wang, Yifan, et al. "The BRCA1-Δ11q Alternative Splice Isoform Bypasses Germline Mutations and Promotes Therapeutic Resistance to PARP Inhibition and Cisplatin." Cancer research 76.9 (2016): 2778-2790.