Multiple functions of sushi domain containing 2 (SUSD2) in breast tumorigenesis

Routinely used therapies are not adequate to treat the heterogeneity of breast cancer, and consequently, more therapeutic targets are desperately needed. To identify novel targets, we generated a breast cancer cDNA library enriched for genes that encode membrane and secreted proteins. From this library we identified SUSD2 (Sushi Domain Containing 2), which encodes an 822-amino acid protein containing a transmembrane domain and functional domains inherent to adhesion molecules. Previous studies describe the mouse homolog, Susd2, but there are no studies on the human gene associated with breast cancer. Immunohistochemical analysis of human breast tissues showed weak or no expression of SUSD2 in normal epithelial cells, with the endothelial lining of vessels staining positive for SUSD2. However, staining was observed in pathologic breast lesions and in lobular and ductal carcinomas. SUSD2 interacts with galectin-1 (Gal-1), a 14-kDa secreted protein that is synthesized by carcinoma cells and promotes tumor immune evasion, angiogenesis, and metastasis. Interestingly, we found that localization of Gal-1 on the surface of cells is dependent on the presence of SUSD2. Various phenotype assays indicate that SUSD2 increases the invasion of breast cancer cells and contributes to a potential immune evasion mechanism through induction of apoptosis of Jurkat T cells. Using a syngeneic mouse model, we observed accelerated tumor formation and decreased survival in mice with tumors expressing Susd2. We found significantly fewer CD4 tumor infiltrating lymphocytes in mice with tumors expressing Susd2. Together, our findings provide evidence that SUSD2 may represent a promising therapeutic target for breast cancer.