Author ORCID Identifier
Date of Graduation
Doctor of Philosophy (PhD)
Khandan Keyomarsi, Ph.D.
Junjie Chen, Ph.D.
Sendurai Mani, Ph.D.
Jian Kuang, Ph.D.
Chandra Bartholomeusz, M.D., Ph.D.
Pawel Mazur, Ph.D.
Low molecular weight cyclin E (LMW-E) are oncogenic forms of cyclin E that are post translationally generated by neutrophil elastase (NE) mediated cleavage of the 50 KDa full-length cyclin E1 (FL-cycE, encoded by CCNE1gene). The resultant N-terminus deleted (40 amino acids) form of LMW-E is detected in breast cancer cells and tumor tissues, but not in normal mammary epithelial cells or adjacent normal tissues. Unlike FL-cycE, LMW-E drives mammary epithelial cell transformation in human cells and spontaneous mammary tumor formation in transgenic mouse models, but the oncogenic mechanisms of LMW-E and its unique function(s) independent of FL-cycE are not fully understood. It is currently assumed that LMW-E drives the tumorigenic process by promoting G1/S cell cycle transition and accelerating mitotic exit. Biochemical features such as longer protein half-life, higher affinity to its kinase partner CDK2, and resistance to endogenous CDK inhibitors such as p21 and p27 all promote the tumorigenic ability of LMW-E. Clinical studies in breast cancer reveal that overexpression of LMW-E predicts recurrence and poor survival in breast cancer patients independent of molecular subtype, Ki67 status, nodal status, or tumor grade, suggesting LMW-E may be driving breast cancer development independent of its role in cell proliferation.
In the current study, we tested the hypothesis that LMW-E promotes genomic instability by deregulating DNA replication and damage repair. We generated immortalized pre-cancerous human mammary epithelial cells (hMECs) engineered to express doxycycline inducible LMW-E or FL-cycE in CCNE1 knock-out background. We found that, unlike LMW-E, FL-cycE overexpression led to DNA replication stress and DNA damage accumulation, resulting in reduced cell viability. LMW-E overexpression, on the other hand, promoted cell survival under replication stress, resulting in persistent genomic instability. RNA-sequencing results showed LMW-E but not FL-cycE overexpression enhanced DNA replication and damage repair pathways. Molecularly, LMW-E but not FL-cycE strongly interacted with CDC6, bound to chromatin, and facilitated replication stress tolerance by upregulating pre-replication complex assembly. LMW-E also mediated DNA repair by upregulating the levels of RAD51 and C17orf53, showing a dominant repairing effect over DNA damage induced by FL-cycE. Moreover, targeting the replication stress response pathway ATR-CHK1-RAD51 with small molecule inhibitors significantly decreased viability of LMW-E overexpressing hMECs and breast cancer cells. Lastly, we showed that positive LMW-E status was associated with genomic instability in tumors from a cohort of 725 patients diagnosed with early-stage breast cancer, further supporting our hypothesis that LMW-E promotes genomic instability to fuel breast cancer development.
Collectively, our findings delineated a novel role for LMW-E in breast tumorigenesis mediated by replication stress tolerance and genomic instability, providing novel therapeutic strategies for LMW-E overexpressing breast cancers.
Low molecular weight cyclin E, cell cycle, DNA replication, DNA damage repair, genomic instability, copy number variation, CCNE1, CDC6, ATR-CHK1-RAD51, breast cancer