Cyclin E1 protein is stabilized in BRCA1 mutated breast cancers leading to synergy between CDK2 and PARP inhibitors

Basal-like breast cancers (BLBC) are aggressive breast cancers that respond poorly to targeted therapies and chemotherapies. In order to define therapeutically targetable subsets of BLBC we examined two markers: cyclin E1 and BRCA1 loss. In high grade serous ovarian cancer (HGSOC) these markers are mutually exclusive, and define therapeutic subsets. We tested the same hypothesis for BLBC. Using a BLBC cohort enriched for BRCA1 loss, we identified convergence between BRCA1 loss and high cyclin E1 expression, in contrast to HGSOC in which CCNE1 amplification drives increased cyclin E1 gene expression. Instead, BRCA1 loss stabilized cyclin E1 during the cell cycle. Using siRNA we showed that BRCA1 loss leads to stabilization of cyclin E1 by reducing phospho-cyclin E1-T62, and conversely the overexpression of BRCA1 increased phospho-T62. Mutation of cyclin E1-T62 to alanine increased cyclin E1 stability. We showed that tumors with high cyclin E1/BRCA1 mutation in the BLBC cohort had decreased phospho-T62, supporting this hypothesis. Since cyclin E1/CDK2 protects cells from DNA damage and cyclin E1 is elevated in BRCA1 mutant cancers, we hypothesized that CDK2 inhibition would sensitize these cancers to PARP inhibition. CDK2 inhibition induced DNA damage and synergized with PARP inhibitors to reduce cell viability in BRCA1 mutated cell lines. Treatment of BLBC patient-derived xenograft models with combination PARP and CDK2 inhibition led to tumor regression and increased survival. We conclude that BRCA1 status and high cyclin E1 have potential as predictive biomarkers to dictate the therapeutic use of combination CDK inhibitors/PARP inhibitors in BLBC.