Abstract 3054: Sushi domain containing 2 (SUSD2): A plasma membrane protein on breast cancer cells that interacts with galectin-1
Authors: Watson Allison P. , Gubbels Jennifer A. A. , Egland Kristi A. 
Published: June 2012

Abstract
Tumor-specific immunotherapies use the binding specificity of antibodies to target and kill cancer cells. For example, the anti-ErbB2 monoclonal antibody, TrastuzuMab (or Herceptin), has shown clinical efficacy for the treatment of metastatic breast cancers with ErbB2 overexpression. Because only 25-30% of human breast cancers overexpress ErbB2, there is a great need for the identification of more breast tumor-specific immunotherapy targets that are present on cancer cells but are minimally expressed in normal essential tissues.
To identify novel immunotherapy targets, we generated a cDNA library enriched for breast cancer genes that encode membrane and secreted proteins. From this library we identified a new breast cancer gene, SUSD2 (Sushi Domain Containing 2), which encodes an 820 amino acid protein containing a transmembrane domain and several functional domains inherent to adhesion molecules. Previous studies describe the mouse homolog, mSVS-1, but there are no previous studies on the human gene associated with breast cancer. Reverse-Transcriptase PCR analysis using total RNA from frozen breast cancer samples and various cancer cell lines showed that SUSD2 is expressed in 19 of 24 breast tumors and all but one cancer cell line. Confocal microscopy revealed that SUSD2 localizes to the plasma membrane of breast cancer cells. Paraffin-embedded blocks of tissue were collected from bilateral mastectomies, with one block containing tumor tissue and one from the contralateral normal breast. Immunohistochemistry (IHC) analysis was performed using an anti-SUSD2 antibody. IHC showed weak or no expression of SUSD2 in normal epithelial cells, with the endothelial lining of vessels staining positive for SUSD2. However, strong staining was observed in various pathological breast lesions and in lobular and ductal carcinomas.
To identify proteins that interact with SUSD2, immunoprecipitation followed by western immunoblot analysis, confocal microscopy, and flow cytometry were used. All methods confirmed that SUSD2 interacts with Galectin-1, a 14-kDa protein that is synthesized by carcinoma cells and secreted into the stroma surrounding tumors. Galectin-1 has been shown to contribute to tumor evasion of immune responses by inducing apoptosis of activated T cells, while resting T cells also bind to Galectin-1, but do not undergo apoptosis. A stable MDA-MB-231 cell line overexpressing SUSD2 was generated. In plate-based cell adhesion assays, MDA-MB-231 (SUSD2) cells bound with higher affinity to peripheral blood mononuclear cells (PBMCs), yet were more protected from PBMC-mediated killing compared to the vector control cell line.
Future studies will explore the relationship of SUSD2 and Galectin-1 in breast cancer, and their role in immune escape. Because of its location on the plasma membrane and its high expression in breast cancer, SUSD2 may be used as a possible therapeutic target.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3054. doi:10.1158/1538-7445.AM2011-3054