SAT-331 Elucidating the Role of Breast Cancer Specific GATA3 Mutation in Estrogen Receptor Positive Breast Cancer

Approximately 266,000 women will be newly diagnosed with invasive breast cancer in the US this year, making it the most prevalent cancer in females, resulting in a 1 in 8 lifetime risk. Many of these tumors are of the hormone receptor positive luminal subtype, with 60-80% expressing the estrogen and/or progesterone receptor (ER+/PR+). The transcription factor GATA3 is expressed in the luminal epithelial cells of the mammary gland and is required for mammary development and luminal cell differentiation. Its deletion from the gland results in defects affecting the tightly regulated processes of terminal end bud formation, ductal elongation, and lactation. GATA3 is involved in a positive cross-regulatory loop with ER-alpha expression, and as such, GATA3 is highly expressed in most ER+ breast cancers and is required for breast cancer cell line responses to estradiol. Mutations in GATA3 are observed in approximately 15% of ER+ breast tumors, behind only PIK3CA and TP53 mutation in frequency, and the median tumor onset is eight years earlier than in ER+ tumors without GATA3 mutation. Analysis of TCGA breast tumors illustrated that most mutations cluster to the C-terminal end of GATA3, suggesting a selective advantage for expressing a significant portion of the wild-type protein. Previous studies have relied on xenografts in immunocompromised mice to characterize the effects of GATA3 mutations on tumor biology. We developed a transgenic mouse expressing a hotspot frameshift mutation at GATA3 codon 335 (GATA3(335fs)) in the mammary gland. These mice did not develop spontaneous mammary tumors; however, they do exhibit hyperproliferative phenotypes in the mammary epithelium that are recapitulated in breast cancer cell lines and xenografts also expressing the GATA3(335fs) protein, findings consistent with an active role for mutant GATA3 proteins in regulating mammary cell proliferation. We have used integrated gene expression and ChIP-Seq profiling to demonstrate that these zinc-finger deleted proteins retain the ability to associate with the genome by tethering to complexes associated with FOXA1 and AP-2gamma recognition motifs, where they modulate the expression of adjacent genes. Continuous exposure to a medroxyprogesterone acetate (MPA) depot induces ER+/PR+ mammary tumors at an 80% incidence in wild-type mice with a median latency of one-year. This study follows a cohort of our recently described MMTV-GATA3(335fs) transgenic mouse in a long-term MPA driven mammary tumorigenesis protocol. Given the requirement for GATA3 in ER-alpha expression and the estradiol response, this experimental model will provide insight into the biology of ER driven tumorigenesis and whether and how GATA3 mutation plays a role in this process. This study was supported by the Gundersen Medical Foundation and a Peter T. Rowley Breast Cancer Research Grant from the New York State Dept. of Health.