Identification of Novel Transmembrane Proteins in Scirrhous-Type Gastric Cancer by the <b><i>Escherichia coli</i></b> Ampicillin Secretion Trap (CAST) Method: <b><i>TM9SF3</i></b> Participates in Tumor Invasion and Serves as a Prognostic Factor

<b><i>Objective:</i></b> Scirrhous-type gastric cancer (GC) is highly aggressive and has a poor prognosis due to rapid cancer cell infiltration accompanied by extensive stromal fibrosis. The aim of this study is to identify genes that encode transmembrane proteins frequently expressed in scirrhous-type GC. <b><i>Methods:</i></b> We compared <i>Escherichia coli</i> ampicillin secretion trap (CAST) libraries from 2 human scirrhous-type GC tissues with a normal stomach CAST library. By sequencing 2,880 colonies from scirrhous CAST libraries, we identified a list of candidate genes. <b><i>Results:</i></b> We focused on the TM9SF3 gene because it has the highest clone count, and immunohistochemical analysis demonstrated that 46 (50%) of 91 GC cases were positive for TM9SF3, which was observed frequently in scirrhous-type GC. TM9SF3 expression showed a significant correlation with the depth of invasion, tumor stage and undifferentiated GC. There was a strong correlation between TM9SF3 expression and poor patient outcome, which was validated in two separate cohorts by immunostaining and quantitative RT-PCR, respectively. Transient knockdown of the TM9SF3 gene by siRNA showed decreased tumor cell-invasive capacity. <b><i>Conclusion:</i></b> Our results indicate that TM9SF3 might be a potential diagnostic and therapeutic target for scirrhous-type GC.