Mutational and expressional analyses of SPOP, a candidate tumor suppressor gene, in prostate, gastric and colorectal cancers

Mounting evidence exists that alterations of ubiquitination processes are involved in cancer pathogenesis. Speckle-type POZ protein (SPOP) is a key adaptor for Cul3-based ubiquitination process. Recent studies reported that SPOP may be a tumor suppressor gene (TSG) and somatic mutation of SPOP was detected in prostate cancer (PCA). The aim of this study was to see whether alterations of SPOP protein expression and somatic mutation of SPOP gene are features of cancers. In this study, we analyzed SPOP somatic mutation in 45 gastric (GC), 45 colorectal cancer (CRC) and 45 PCA by single-strand conformation polymorphism (SSCP). Also, we analyzed SPOP protein expression in 60 GC, 60 CRC and 60 PCA by immunohistochemistry. Overall, we detected three somatic missense mutations of SPOP gene in the coding sequences (p.Ser14Leu, p.Tyr87Cys and p.Phe133Leu). The mutations were observed in two PCA and one CRC. Of note, the p.Phe133Leu was a recurrent mutation reported in an earlier study. In the immunohistochemistry, SPOP protein was expressed in normal gastric, colonic and prostate epithelial cells, whereas it was lost in 30% of GC, 20% of CRC and 37% of PCA. Our data indicate that loss of SPOP expression was common in GC, CRC and PCA, but somatic mutation of SPOP in this study was rare in these tumors. Also, the data provide a possibility that loss of expression of SPOP gene might play a role in cancer pathogenesis by altering TSG functions of SPOP.