2534 - Disparate Roles for DNA Mismatch Repair Proteins In Radiotherapy and Endocrine Treatment Response In ER+/HER2- Breast Cancer
Presenter(s)
J. T. Dewitt1,2, S. Haricharan1,2, and A. J. Lui2,3; 1Department of Biology, San Diego State University, San Diego, CA, 2UC San Diego Moores Cancer Center, La Jolla, CA, 3Department of Radiation Medicine and Applied Sciences, University of California San Diego, La Jolla, CA
Purpose/Objective(s): Approximately 40% of estrogen receptor positive (ER+) breast cancers eventually learn to live without estrogen and develop resistance to endocrine therapy. Endocrine-resistant breast cancers instead become dependent on other cell signaling pathways for survival and this dependence can be exploited into new targeted therapies. MSH2 (MutS Homolog 2) and MLH1 (MutL Homolog 1) are key components of the DNA mismatch repair complex. We previously found that loss of nuclear expression of MLH1and not MSH2 drives endocrine resistance and occurs in 12-15% of all and at least 30% of endocrine-resistant ER+ breast cancers. We also discovered that MLH1-loss but not MSH2-loss causes a switch from estrogen to HER2-dependence, and sensitizes cells to HER2 inhibition. Alterations in DNA mismatch repair protein expression and activity can also trigger either sensitivity or resistance to radiation, depending on the cancer type and context. We hypothesize that MLH1-loss confers radiation-sensitivity and synergizes with HER2 inhibition to reverse endocrine-resistance in ER+/HER2- breast cancer.
Materials/Methods: We previously published the establishment of MCF7 and T47D cell lines with stable loss of MLH1 and MSH2 expression. Cell lines are also implanted in the 4th mammary fat pad of nude mice and harvested to establish ex-vivo tumor slice organoids. Radiation dose response curves (0 Gy-12 Gy) are generated for T47D shLuc, shMLH1 and shMSH2 cells in vitro and ex-vivo. TUNEL staining, cell counting, cell proliferation assays measure changes in tumor cell survival and proliferation. Multiplex immunofluorescence and immunohistochemistry characterize and contrast changes in HER2 signaling and DNA mismatch repair protein localization and expression.
Results: We report that T47D shMLH1, but not shMSH2 causes endocrine-resistance which can be reversed with HER2 inhibition. Loss of MLH1 is associated with moderate radiation sensitivity while loss of MSH2 drives significant radiation resistance both in vitro and in ex vivo tumor organoids. Proliferation, and survival are reduced in shMLH1 cells after radiation and significantly enhanced in shMSH2 cells. Immunofluorescence indicates key differences in MLH1 localization and expression in each condition. Additional experiments determine whether HER2 inhibition synergizes with radiation in shMLH1 and shMSH2 cells.
Conclusion: Expression and localization of DNA mismatch repair proteins impacts ER+/HER2- breast cancer response to endocrine and radiation therapy. Although MSH2 and MLH1are binding partners in the DNA repair complex, they play disparate roles in estrogen and HER2 signaling pathways. These differences can be exploited to personalize treatment of ER+/HER2- breast cancer.