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829 Understanding BRCA1-Methylation stability to improve PARP inhibitor responses in high-grade serous ovarian carcinomas
  1. Franziska Geissler1,2,
  2. Ksenija Nesic1,2,
  3. Cassandra J Vandenberg1,2,
  4. Olga Kondrashova3,
  5. Fan Zhang4,
  6. Alexander Dobrovic4,
  7. Marc Radke5,
  8. Elisabeth Swisher5,
  9. Joep Vissers6,
  10. Jocelyn Penington1,2,
  11. Sean Grimmond6,
  12. Tony Papenfuss1,
  13. Clare L Scott1,2,7 and
  14. Matthew J Wakefield1,2,7
  1. 1Walter and Eliza Hall Institute for Medical Research, Parkville, Australia
  2. 2Department of Medical Biology, University of Melbourne, Parkville, Australia
  3. 3QIMR Berghofer Medical Research Institute, Brisbane, Australia
  4. 4University of Melbourne, Department of Surgery, Austin Health, Heidelberg, Australia
  5. 5University of Washington, Seattle, USA
  6. 6University of Melbourne Centre for Cancer Research (UMCCR), Parkville, Australia
  7. 7Department of Obstetrics, Gynaecology and Newborn Health, University of Melbourne, Parkville, Australia


Introduction/Background Poly-ADP ribose polymerase inhibitors (PARPi) provide impressive therapeutic responses in carcinomas with deficiencies in the homologous recombination (HR) DNA repair pathway. After BRCA1/2 mutations, epigenetic silencing of BRCA1 is the next most common HR defect in high grade serous ovarian carcinomas (HGSOC), present in 7–17% of cases. BRCA1 promoter hypermethylation (meBRCA1), when homozygous, silences gene expression and correlates with clinical response to PARPi. Loss of meBRCA1 after chemotherapy and PARPi treatment is a reported mechanism of acquired PARPi resistance. Intriguingly, long-term responders to PARPi with stable meBRCA1 have been observed. The identification and exploitation of mechanisms promoting stable meBRCA1, may enable more patients to achieve durable PARPi responses in the clinic.

Methodology Pre-clinical models of meBRCA1 HGSOC were screened for meBRCA1 loss under DNA(cytosine-5)-methyltransferase-1 (DNTM1) inhibitor/PARPi treatment. A genome-wide pooled PARPi-resistance CRISPR screen in WEHI-CS62 was performed.

Results PARPi-responsive HGSOC cell line WEHI-CS62 retains homozygous meBRCA1, even after two months of PARPi pressure within a genome-wide pooled PARPi-resistance CRISPR screen. Treatment with DNMT1 inhibitors, decitabine and GSK-3484862, resulted in a heterogenous methylation pattern of meBRCA1 in this cell line, which was associated with BRCA1 re-expression and PARPi resistance. These results were confirmed in an engineered cell line model of OVCAR8 with homozygous meBRCA1, while treatment of cell line models with other HR defects (e.g. BRCA2 mutations) with DNMT1 inhibitors did not cause PARPi resistance.

Conclusion This study has demonstrated that DNMT1 inhibition leads to PARPi resistance in HGSOC with BRCA1 silencing by promoter methylation. Further validation of these mechanisms in our cohort of meBRCA1 in vitro and in vivo pre-clinical models (cell lines, patient derived xenografts), will help us better understand the mechanisms underpinning BRCA1 epigenetic silencing, and methylation loss under treatment pressure. Improved understanding of mechanisms may inform future combination therapies and guide trials of epigenetic modulating therapies in HGSOC.

Disclosures FG and KN contributed equally.

FG has received research support by the Swiss Cancer Research foundation & Swiss Cancer League [KFS-5445–08-2021]. KN is supported by AACR-AstraZeneca Ovarian Cancer Research Fellowship.

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