Impaired mammary tumor formation and metastasis by the point mutation of a Smad3 linker phosphorylation site

Triple-negative breast cancer (TNBC) is often aggressive and metastatic. Transforming growth factor-β acts as a tumor-promoter in TNBC. Smad3, a major downstream effector protein in the TGF-β signaling pathway, is regulated by phosphorylation at several sites. The functional significance of the phosphorylation of the linker region in Smad3 is poorly understood for TNBC. Among the four sites in the Smad3 linker region, threonine-179 (T179) appears to be unique as it serves as the binding site for multiple WW-domain-containing proteins upon phosphorylation, suggesting that this phosphorylation is a key for Smad3 to engage other pathways.Using genome editing, we introduced for the first time a knock-in (KI) mutation in the endogenous Smad3 gene in IV2, a lung-tropic subline of the human MDA-MB-231 TNBC cell line. In the resulting cell line, the Smad3 T179 phosphorylation site is replaced by non-phosphorylatable valine (T179V) with the mutation in both alleles.The T179V KI reduced cell growth rate and mammosphere formation. These phenomena were accompanied by a significant upregulation of p21^Cip1 and downregulation of c-Myc. The T179V KI also reduced cell migration and invasion in vitro. In the mouse xenograft models, the T179V KI markedly reduced the establishment of primary tumor in the mammary fat pad and the lung metastasis.Our results using gene editing indicate the cancer-promoting role of Smad3 T179 phosphorylation in the human TNBC cells. Our findings highly suggest that controlling this phosphorylation may have therapeutic potential for TNBC.