PROJECT SUMMARY / ABSTRACT Invasive lobular carcinoma (ILC) is an understudied subtype of breast cancer. Clinical and epidemiological features of ILC are consistent with an exquisite dependence on estrogen and estrogen receptor alpha (ER), and ILC biomarkers suggest that patients should have good outcomes with anti-estrogen treatment. However, recent studies show that relative to other breast cancers, ILC patients have poorer long-term outcomes, have poorer response to anti-estrogen therapy, and also do not benefit from chemotherapy. New treatments are needed to improve ILC patient outcomes. However, advances are hindered by a fundamental lack of understanding of the distinct biology of ILC, in particular regarding ER signaling. We and others identified de novo anti-estrogen resistance in ILC models, and see that ER in ILC cells is activated by tamoxifen and next- generation ER antagonists. Based on these observations, the limited efficacy of anti-estrogens in treating ILC may be due to differences in ER function in ILC. Our work has identified unique ER functions in ILC, including ILC-specific ER target genes, ER DNA binding, and ER signaling pathways, which in part explain endocrine response and resistance. As a basis for these unique ER functions and anti-estrogen resistance in ILC, we identified mediator of DNA damage checkpoint 1 (MDC1) as a novel transcriptional co-regulator of ER in ILC cells. MDC1 is a cornerstone of DNA damage response, but in ILC, MDC1 acts as an ER co-regulator and is required for ER-driven growth and ER regulation of key target genes, as well as anti-estrogen resistance. Our data indicate that MDC1 regulates ER at least in part through epigenomic remodeling. MDC1 ER co-regulator functions interplay with canonical roles of MDC1 in DNA damage response, implicating these functions in clinical chemoresistance in ILC. The objective of this proposal is to define how MDC1 controls ER-driven gene regulation and how ER:MDC1 mediates DDR functions in ILC cells. Our central hypothesis is that MDC1 is a critical ER co-regulator mediating ER target gene regulation in ILC, creating targetable cross-talk between ER and DNA damage response. In this study, we will: i) Define how ER and MDC1 interact to build a gene regulation complex; ii) Define how ER and MDC1 influence their collective genome interactions; iii) Define the DNA damage response that is associated with ER and MDC1 function. This project will lead to mechanistic understanding of MDC1-driven ER functions in ILC, and link the unique functions of ER in ILC to new precision treatment strategies using DDR inhibitors. Defining the role of MDC1 will also provide insight into the unique ER-dependent etiology of ILC. This proposal will advance translational study of MDC1 by identifying ER:MDC1 biomarkers and therapeutic strategies to target MDC1-related DDR dependencies. Characterizing the role of MDC1 is critical to understand ILC biology and improve outcomes for patients with I...