Camptothecin targets WRN protein: mechanism and relevance in clinical breast cancer

// Raghavendra A. Shamanna 1 , Huiming Lu 1 , Deborah L. Croteau 1 , Arvind Arora 2 , Devika Agarwal 3 , Graham Ball 3 , Mohammed A. Aleskandarany 2 , Ian O. Ellis 2 , Yves Pommier 4 , Srinivasan Madhusudan 2 and Vilhelm A. Bohr 1 1 Laboratory of Molecular Gerontology, Biomedical Research Center, National Institute on Aging, NIH, Baltimore, Maryland, USA 2 Academic Unit of Oncology, Division of Cancer and Stem Cells, School of Medicine, University of Nottingham, Nottingham, UK 3 School of Science and Technology, Nottingham Trent University, Clifton Campus, Nottingham, UK 4 Developmental Therapeutics Branch and Laboratory of Molecular Pharmacology, National Cancer Institute, Bethesda, Maryland, USA Correspondence to: Vilhelm A. Bohr, email: // Keywords : breast cancer, Werner syndrome, helicase, camptothecin, topoisomerase I Received : November 20, 2015 Accepted : February 09, 2016 Published : March 03, 2016 Abstract Werner syndrome protein (WRN) is a RecQ helicase that participates in DNA repair, genome stability and cellular senescence. The five human RecQ helicases, RECQL1, Bloom, WRN, RECQL4 and RECQL5 play critical roles in DNA repair and cell survival after treatment with the anticancer drug camptothecin (CPT). CPT derivatives are widely used in cancer chemotherapy to inhibit topoisomerase I and generate DNA double-strand breaks during replication. Here we studied the effects of CPT on the stability and expression dynamics of human RecQ helicases. In the cells treated with CPT, we observed distinct effects on WRN compared to other human RecQ helicases. CPT altered the cellular localization of WRN and induced its degradation by a ubiquitin-mediated proteasome pathway. WRN knockdown cells as well as CPT treated cells became senescent and stained positive for senescence-associated β-galactosidase at a higher frequency compared to control cells. However, the senescent phenotype was attenuated by ectopic expression of WRN suggesting functional implication of WRN degradation in CPT treated cells. Approximately 5-23% of breast cancer tumors are known to respond to CPT-based chemotherapy. Interestingly, we found that the extent of CPT-induced WRN degradation correlates with increasing sensitivity of breast cancer cells to CPT. The abundance of WRN decreased in CPT-treated sensitive cells; however, WRN remained relatively stable in CPT-resistant breast cancer cells. In a large clinical cohort of breast cancer patients, we find that WRN and topoisomerase I expression correlate with an aggressive tumor phenotype and poor prognosis. Our novel observations suggest that WRN abundance along with CPT-induced degradation could be a promising strategy for personalizing CPT-based cancer chemotherapeutic regimens.