E-Poster Presentation 34th Lorne Cancer Conference 2022

BRCA1 D11q isoform expression impacts PARP inhibitor responses in ovarian carcinoma patient derived xenografts (#209)

Ksenija Nesic 1 2 , Cassandra J Vandenberg 1 2 , Olga Kondrashova 3 , John J Krais 4 , Yifan Wang 4 , Erica Huelsmann 4 , Elizabeth Lieschke 1 2 , Gwo Yaw Ho 1 5 , Marc R Radke 6 , Elizabeth M Swisher 6 , Neil Johnson 4 , Matthew J Wakefield 1 2 7 , Clare L Scott 1 2 7
  1. Walter and Eliza Hall Institute of Medical Research, Parkville, VIC, Australia
  2. University of Melbourne, Parkville, VIC, Australia
  3. QIMR Berghofer Medical Research Institute, Brisbane, QLD, Australia
  4. Fox Chase Cancer Center, Philadelphia, Pennsylvania, United States of America
  5. Monash University, Clayton, VIC, Australia
  6. University of Washington, Seattle, WA, United States of America
  7. Royal Women’s Hospital, Melbourne, VIC, Australia

Background: Acquired PARP inhibitor (PARPi) resistance in High Grade Serous Ovarian Carcinoma (HGSOC) can occur via secondary mutations in BRCA1/2 that restore homologous recombination DNA repair. However, certain BRCA1 splice isoforms can also contribute to PARPi resistance in HGSOC. This includes the D11q isoform of BRCA1, where deleterious mutations in the 11q region of BRCA1 (exon 10) are spliced out resulting in a truncated but partially functional BRCA1 protein. Here we analysed five HGSOC Patient Derived Xenografts (PDX) with BRCA1 exon 10 mutations, where alternative isoform expression correlated with platinum and PARPi responses in vivo.

 

Methods: Five PDX, derived from chemo-naive and pre-treated patients, were studied. PDX #032, #049 and #217 were generated from tumours of patients who had received prior PARPi therapy. PDX #206 was derived from a patient who had received prior platinum chemotherapy, and PDX #056 from a chemo-naive patient. PDX were treated with PARPi rucaparib (300-450mg/kg) and cisplatin (4mg/kg). D11q isoform expression was assessed using two-step RT-qPCR and Western Blotting. DNA sequencing was performed using the BROCA Cancer Risk Panel.

 

Results: PARPi resistant PDX #032 and #049 had high D11q expression relative to D11q-high cell line UWB1.289, and the PARPi-responsive PDX models #206 and #56. Interestingly, PDX #049 harboured a second BRCA1 exon 10 donor splice site mutation, shown previously to enhance splicing and increase abundance of the D11q isoform. PDX #217 also harboured a secondary splicing BRCA1 mutation, that was driving expression of the D11 isoform (missing all of exon 10).

 

Conclusions: Our work in PDX provides further evidence that BRCA1 D11q expression can impact on PARPi and platinum responses, and is a clinically relevant mechanism of PARPi resistance in a subset of patients with BRCA1 mutations. Whilst high D11q expression can be associated with secondary BRCA1 splice site mutations (PDX #049 and #217), it can arise independently (PDX #032). Thus, BRCA1 D11q isoform expression should be considered a potential mechanism of acquired PARPi-resistance in patients with BRCA1 exon 10 mutations.