Dr Liz Caldon

Dr Liz Caldon leads the Replication and Genome Stability Group at the Garvan Institute, NSW. Her focus in the intersection between aberrant growth control in breast cancer and the development of resistance to anti-cancer therapy.

Abstract

BRCA1 mutation stabilises cyclin E1 in breast cancer to create a therapeutically targetable subset

Diar Aziz1, Neil Portman2,3, Kristine Fernandez2, Christine S.L. Lee2, Sarah Alexandrou2, Niantao Deng2, C. Marcelo Sergio2, Dariush Etemadmoghadam4,5, kConFab Investigators4,5, David Bowtell4,5, Paul Waring1, Elgene Lim2,3, C. Elizabeth Caldon2,3

1. Centre for Translational Pathology, Department of Pathology, The University of Melbourne, Parkville, VIC, Australia.
2. The Kinghorn Cancer Centre, Garvan Institute of Medical Research, Darlinghurst, NSW, Australia
3. St. Vincent's Clinical School, Faculty of Medicine, University of New South Wales, Darlinghurst, NSW, Australia
4. Peter MacCallum Cancer Institute, Victorian Comprehensive Cancer Centre, Parkville, VIC, Australia
5. Sir Peter MacCallum Department of Oncology, The University of Melbourne, Melbourne, VIC, Australia

Background:
BRCA1-mutated breast cancers are typically a very aggressive subset that respond poorly to targeted therapies and chemotherapies. High cyclin E1 protein is suspected to co-occur with BRCA1 disruption in these breast cancers.

Aims:
We aimed to demonstrate a mechanistic link between BRCA1 disruption and cyclin E1, and exploit this to develop a new combination therapy for this subset of breast cancer.

Methods:
The kConFab breast cancer cohort was analyzed by immunohistochemistry to identify the relationship between BRCA1/2 disruption and the expression and phosphorylation of cyclin E1. In parallel we used breast cancer cell line models and patient derived xenografts (BRCA1 wildtype and mutant) to determine how Brca1 regulates cyclin E1 expression (western blot, flow cytometry), to assay effects of cyclin E1 expression (colony forming and DNA damage assays), and test potential combination therapies (ComBenefit synergy analysis).

Results:
High cyclin E1 specifically associated with BRCA1 loss in patient samples and not other homologous recombination gene defects, and predicted poor overall survival. BRCA1 disruption by mutation or siRNA in breast cancer cell lines led to cyclin E1 protein stabilisation during the cell cycle. This occurred via loss of cyclin E1-T62 phosphorylation, a site important in cyclin E1 stability, and mutation of the T62 site enhanced survival in colony forming assays. We subsequently confirmed that phospho-T62 was diminished in cancers with BRCA1 mutation across our patient cohort.

Since cyclin E1/CDK2 can protect from DNA damage and cyclin E1 is elevated in BRCA1 disrupted cancers, we hypothesised that CDK2 inhibition sensitises these cancers to PARP inhibition. CDK2 inhibition induced DNA damage and synergised with the PARP inhibitor Rucaparib in BRCA1 mutated cell lines. CDK2 inhibitor/PARP inhibitor treatment of patient-derived xenograft models from Brca1 germline cases show significant reduction of tumour burden when used in combination.

Conclusions:
BRCA1 disruption increases cyclin E1 expression in breast cancer, and BRCA1 status and high cyclin E1 have potential as predictive biomarkers to dictate the therapeutic use of combination CDK inhibitors/PARP inhibitors in breast cancers.