Breast tumor-initiating cells (BTICs) of triple-negative breast cancer (TNBC) tissues actively repair DNA and are resistant to treatments including chemotherapy, radiotherapy, and targeted therapy. Herein, it is found that a previously reported secreted protein, sclerostin domain containing 1 (SOSTDC1), is abundantly expressed in BTICs of TNBC cells and positively correlated with a poor patient prognosis. SOSTDC1 knockdown impairs homologous recombination (HR) repair, BTIC maintenance, and sensitized bulk cells and BTICs to Olaparib. Mechanistically, following Olaparib treatment, SOSTDC1 translocates to the nucleus in an importin-alpha dependent manner. Nuclear SOSTDC1 interacts with the N-terminus of the nucleoprotein, chromatin helicase DNA-binding factor (CHD1), to promote HR repair and BTIC maintenance. Furthermore, nuclear SOSTDC1 bound to beta-transducin repeat-containing protein (beta-TrCP) binding motifs of CHD1 is found, thereby blocking the beta-TrCP-CHD1 interaction and inhibiting beta-TrCP-mediated CHD1 ubiquitination and degradation. Collectively, these findings identify a novel nuclear SOSTDC1 pathway in regulating HR repair and BTIC maintenance, providing insight into the TNBC therapeutic strategies.

In this study, SOSTDC1 is identified as a potential target mediating BTICs resistance to Olaparib. SOSTDC1 is highly expressed in TNBC tissues and especially in the BTIC population. Nuclear translocation of SOSTDC1 mediated by importin-alpha stabilizes CHD1 protein from ubiquitination, thereby promoting HR repair, BTIC maintenance, and Olaparib resistance in TNBC. image