Amplification Of The Whsc1l1 Oncogene Regulates Expression And Estrogen-Independent Activation Of Er Alpha In Sum-44 Breast Cancer Cells And Is Associated With Era Over Expression In Breast Cancer

The 8p11-p12 amplicon occurs in approximately 15% of breast cancers and occurs almost exclusively in aggressive luminal B type tumors. Our lab and other labs have identified the WHSC1L1 oncogene as a driving oncogene from this region with potent transforming activity. In the present studies, we found that over expression of WHSC1L1 is linked to over expression of the ESR1 and ERα protein in the SUM-44 breast cancer cell line, and also in primary human breast cancer specimens. Knockdown of WHSC1L1, and particularly the short isoform of WHSC1L1, had a dramatic effect on ESR1 mRNA and ERα protein levels. SUM-44 cells do not require exogenous estrogen for continuous growth in vitro; however these cells are dependent on ERα expression as determined from ESR1 knockdown experiments, and potent growth inhibition and ERα degradation following exposure to the selective estrogen receptor degrader (SERD) fulvestrant. ChIP-Seq experiments utilizing ERα antibodies demonstrated potent ERα binding to chromatin in SUM-44 cells under estrogen-free conditions. ERα bound to ERE and FOXA1 binding motifs under estrogen-free conditions and regulated expression of number of well-known estrogen responsive genes. Short term treatment with estradiol enhanced binding of ERα to chromatin and influenced expression of many of the same genes expressed and to which ERα was bound under estrogen-free conditions. Finally, knockdown of WHSC1L1 in SUM-44 cells resulted in loss of ERα binding to chromatin under estrogen-free conditions; however treatment estradiol restored ERα binding to chromatin at key estrogen-response elements and genes. These results indicate the SUM-44 cells are a good model for a subset of luminal B breast cancers that have the 8p11-p12 amplicon, over express the WHSC1L1 oncogene, and over express ERα that is independent of estrogen for binding to chromatin and regulation of gene expression. Dependence on ERα activity for growth and survival of breast cancer cells but independence of estradiol is a major cause of breast cancer mortality as such cells become non-responsive to current hormonally based therapies. Citation Format: Ethier SP, Irish J, Mills J, Ivey B, Hardiam G, Wilson R, Domkowski A, Guest S. Amplification of the WHSC1L1 oncogene regulates expression and estrogen-independent activation of ERα in SUM-44 breast cancer cells and is associated with ERα over expression in breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P6-03-04.