PROJECT SUMMARY/ABSTRACT About 10-20% of breast cancers are triple-negative breast cancers (TNBC), which have a poorer prognosis and worse clinical outcome than other types of BC. Due to the lack of target therapies, chemotherapy remains the mainstay of treatment of TNBC, but acquired resistance often develops followed by metastasis and mortality. This supplemental project is closely relevant to the ongoing research proposed in the parent grant R01CA260357 and significantly extends the scope of the current project. Our recent study demonstrated that LSD1 depletion significantly reduced the incidence of BRCA1-deficiency induced mouse mammary gland tumors. Based on this finding, we hypothesize that LSD1 overexpression is essential for breast cancer initiation and development due to BRCA1 loss. In Aim 1, we will validate our preliminary findings and characterize the functional role of LSD1 in BRCA1 mutation mediated TNBC progression. We will also investigate if LSD1 overexpression contributes to PARPi resistance. Furthermore, we will explore the in vivo roles of LSD1 in tumor progression using xenograft and mammary gland BRCA1 deficient TNBC mouse models. Cumulative evidence has suggested that epigenetic silencing of a subset of cytotoxic T-cell lymphocyte (CTL) attracting chemokines (e.g. Th1-type CXCL9 and CXCL10) plays an important role in shaping the tumor-promoting immune environment. However, the exact epigenetic mechanisms underlying chemokine silencing are still unclear. Our recent studies showed that LSD1- mediated silencing of CTL chemokines is associated with H3K4 demethylation at proximal promoter regions. Tissue Factor Pathway Inhibitor 2 (TFPI2), a tumor suppressor gene, was identified as a key LSD1 targeted gene. Induction of TFPI2 by LSD1 inhibition downregulates activity of matrix metalloproteinases (MMPs) and promotes stability of chemokines, leading to enhanced CD8+ T-cell infiltration in tumors. Based on these new findings, we hypothesize that overexpression of LSD1 facilities TNBC progression through promoting immune suppressive environment. In Aim 2, we will characterize the crosstalk between epigenetic changes and TNBC immune environment and further elucidate the molecular basis of LSD1-medicated chemokine silencing in promoting TNBC progression. The proposed studies are expected to provide key preclinical data, novel mechanistic insight, and rationales to address mechanistic and clinical associations between epigenetic defects and TNBC tumor environment and tackles a significant clinical problem. Thus, this project has high scientific significance and clinical relevance.