An epigenomic approach to therapy for tamoxifen-resistant breast cancer

Tamoxifen has been a frontline treatment for estrogen receptor alpha （ERα）-positive breast tumors in premenopausal women. However, resistance to tamoxifen occurs in many patients. ER still plays a critical role in the growth of breast cancer cells with acquired tamoxifen resistance, suggesting that ERa remains a valid target for treatment of tamoxifen-resistant （Tam-R） breast cancer. In an effort to identify novel regulators of ERa signaling, through a small-scale siRNA screen against histone methyl modifiers, we found WHSC1, a histone H3K36 methyltransferase, as a positive regulator of ERa signaling in breast cancer cells. We demonstrated that WHSC1 is recruited to the ERa gene by the BET protein BRD3/4, and facilitates ERa gene expression. The small-molecule BET protein inhibitor JQ1 potently suppressed the classic ERa signaling pathway and the growth of Tam-R breast cancer cells in culture. Using a Tam-R breast cancer xenograft mouse model, we demonstrated in vivo anti-breast cancer activity by JQ1 and a strong long-lasting effect of combination therapy with JQ1 and the ER degrader fnlvestrant. Taken together, we provide evidence that the epigenomic proteins BRD3/4 and WHSC1 are essential regulators of estrogen receptor signaling and are novel therapeutic targets for treatment of Tam-R breast cancer.