Abstract P6-04-17: Role of HOXB13 in modulating estrogen signaling in breast cancer cells

Background: The ratio of HOXB13 (H) and IL17BR (I) gene expression levels in hormone receptor-positive (HR+) primary early stage breast tumors define a predictive index (H/I) that, combined with the Molecular Grade Index (MGI), constitute the Breast Cancer Index (BCI). The BCI score (MGI+H/I prognostic model) provides individualized risk of overall (0-10 years) and specifically late (5-10 years) distant recurrence, and BCI (H/I) predicts the benefit of extended endocrine therapy (EET). Hormone receptor responses are dependent on genome-wide hormone receptor binding patterns. In hormone-dependent cancers, nuclear receptor binding patterns are frequently reprogrammed by other transcription factors. For example, in breast cancer cells, GATA3 has been shown to reprogram estrogen receptor (ER) binding and to alter gene expression. Similarly, HOXB13 was previously shown to reprogram genome-wide binding of the androgen receptor (AR) during prostate cancer tumorigenesis. To assess whether HOXB13 expression is associated with regulation of ER binding, global ER binding data in highly HOXB13-expressing, tamoxifen-resistant breast cancer cells to the ER binding pattern in HOXB13-low, tamoxifen-sensitive breast cancer cells were compared. The study objective was to understand whether HOXB13 contributes to ER reprogramming in early-stage breast cancer. Methods: Global ER, HOXB13 and FOXA1 DNA binding patterns and the enhancer activation mark histone H3 lysine 27 acetylation (H3K27ac) were characterized in BT474 cells by chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-Seq). To compare ER binding in highly HOXB13-expressing, tamoxifen-resistant BT474 cells to published ER binding data in HOXB13-low, tamoxifen-sensitive MCF-7 breast cancer cells, we downloaded published ER, FOXA1, GATA3 and H3K27ac ChIP-seq data from MCF-7 cells. After aligning reads with Bowtie2, peak calling and data integration was performed using HOMERv4.10. Results: Analysis of highly HOXB13-expressing BT474 cells identified a large number of HOXB13 binding sites, which overlapped with approximately half of the ER binding sites in these cells. Many of these binding sites were co-bound by FoxA1, a general pioneer factor that has been shown to also colocalize with HoxB13 at reprogrammed AR binding sites in human prostate tumors. Interestingly, most of these locations are distinct from the genomic binding sites of ER in HOXB13-low MCF-7 cells, where ER frequently binds to MCF-7-specific sites in conjunction with GATA3. Conclusion: Our preliminary evidence suggests that HOXB13 influences the estrogen receptor binding pattern in human breast cancer cells and that ER binding sites differ between HOXB13 high- and low-expressing cell lines, which may underlie their differential responses to endocrine therapy. These results will be compared to ongoing experiments assessing the effects of conditional expression or loss of HOXB13 on ER binding and function in HOXB13-negative and HOXB13-positive cells, respectively. Citation Format: Kai Treuner, Michael Hayes, Christopher Benner, Catherine Schnabel, Sven Heinz. Role of HOXB13 in modulating estrogen signaling in breast cancer cells [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-17.