MRE11-RAD50-NBS1 COMPLEX BLOCKADE TO REVERSE PLATINUM RESISTANCE AND INDUCE SYNTHETIC LETHALITY IN EPITHELIAL OVARIAN CANCERS
Abstract
Resistance to platinum drugs (carboplatin, cisplatin) negatively
impacts patients’ outcomes in ovarian cancer. Platinum-based
chemotherapy cause intra-strand and inter-strand DNA adducts,
which contribute to DSBs during replication. Proficient DNA repair is
a critical mechanism that leads to resistance to platinating agents.
Targeting DNA repair proteins is a promising strategy for ovarian
cancer personalization therapy. The MRE11-RAD50-NBS1 (MRN)
complex is a ‘first responder’ to DNA damage and is critical for repair
of double strand breaks (DSBs) and stalled replication forks. Here
we show that MRE11, RAD50 and NBS1 overexpression at the
transcriptional and protein level is linked to poor survival outcomes
in ovarian cancer patients treated with platinum chemotherapy. Pre-clinically, MRE11, RAD50 and NBS1 depletion promoted platinum
sensitivity in ovarian resistance cell lines. Increased sensitivity was
associated with accumulation of DSBs, cell cycle arrest and
apoptosis. Mirin, a small molecular inhibitor of the 3′ to 5′
exonuclease activity of MRE11 was increased platinum sensitivity in
ovarian resistance cell lines, was correlated with increased DSB
accumulation, cell cycle arrest and apoptotic cells. Mirin was also
synthetically lethal in BRCA2-deficient or XRCC1-deficient ovarian
cancer cells including in 3D-spheroid models as evidenced by DSB
accumulation, cell cycle arrest and increased apoptotic
We
generated a mirin resistant BRCA2-deficient PEO1R cell line which is
cross-resistant to cisplatin and Olaparib (PARP inhibitor) including
3D-spheroid models. PEO1R cells have not restored BRCA2 protein
expression but inactivated 53BP1 and activated DSB repair through
upregulation of MRE11 and other components of HR, NER and MMR
pathways at mRNA and protein levels. Interestingly, PEO1R cells also
overexpressed OCT4 transcription factor which physically interacted
with MRE11 promoting an aggressive cancer stem cell-like
phenotype (characterised by overexpression of CD44, OCT4 &
ZEB1), increased proliferation, spheroid formation, invasion,
epithelial-to-mesenchymal transition (EMT), enhanced DNA repair
expression and reduced apoptosis and increased tumorigenicity in
mice xenograft models.
We conclude that MRE11, RAD50 and NBS1 are key predictive
biomarkers in ovarian cancer, MRE11 is involved in cancer stem cell
regulation and MRE11 targeting may be suitable for clinical
application in DNA repair deficient ovarian cancers.