Identification of a Therapeutically Targetable JAK-STAT Enriched Androgen Receptor (AR) and AR Splice Variant Positive Triple Negative Breast Cancer Subtype [project3]

Triple-negative breast cancer (TNBC) is an aggressive breast cancer subtype with no targeted therapeutics. The luminal androgen receptor (LAR), one of the six TNBC subtypes, constitutes 15% of TNBC and is enriched for AR and AR-target genes. Here, we show that a cohort of TNBC not only expresses full-length AR (AR-FL) at a much higher rate (~80%) than previously reported, but also expresses AR splice variants (AR-SVs) (~20%), subclassifying the LAR-TNBC subtype into LAR-FL, LAR-AR-SV, or dual positive. Higher AR and AR-SV expression and corresponding aggressive phenotypes were observed predominantly in specimens obtained from African American women. RNA sequencing of LAR TNBC specimens indicates an enrichment of hallmark interferon, JAK-STAT, and androgen signaling pathways, which were exclusive to AR-expressing epithelial cancer cells as demonstrated by spatial genomics. LAR and LAR-AR-SV TNBC cell proliferation, orthotopic cell line and patient-derived xenograft, and patient-derived tumor explants growth were inhibited by AR N-terminal domain (NTD)-binding selective AR degrader (SARD) that irreversibly inhibits AR or by the JAK inhibitor ruxolitinib. Subsequent biochemical analysis suggests that STAT1 is an AR coactivator, and SARD or ruxolitinib blocks this coactivation. Collectively, our work identifies and characterizes a pharmacologically targetable and previously unreported TNBC subtype predominantly in African American women and identifies a growth-promoting interaction between AR and JAK-STAT signaling. Overall design: MDA-MB-453 cells were implanted in female NSG mice. Once the tumors reach 100-200 mm3, the animals were randomized and treated with vehicle or 60 mg/kg/day UT-105 or enzalutamide orally. Tumor volume was measured twice weekly. At the end of the study (28 days of treatment) the animals were sacrificed, tumors isolated, and weighed. RNA was extracted from the tumors and sequenced.