SAP155-mediated c-myc suppressor far-upstream element-binding protein-interacting repressor splicing variants are activated in colon cancer tissues

The c-myc transcriptional suppressor, far-upstream element (FUSE)-binding protein (FBP)-interacting repressor (FIR), is alternatively spliced in colorectal cancer tissue (Matsushita et al., Cancer Res 2006). Recently, the knockdown of SAP155 pre-mRNA-splicing factor, a subunit of SF3b, was reported to disturb FIR pre-mRNA splicing and yield FIRexon2, an exon2-spliced variant of FIR, which lacks c-myc repression activity. In the present study, novel splicing variants of FIR, 3 and 4, were also generated by SAP155 siRNA, and these variants were found to be activated in human colorectal cancer tissue. Furthermore, the expression levels of FIR variant mRNA were examined in the peripheral blood of colorectal cancer patients and healthy volunteers to assess its potency for tumor detection. As expected, circulating FIR variant mRNA in the peripheral blood of cancer patients were significantly overexpressed compared to that in healthy volunteers. In particular, the area under the receiving operating characteristic curve of FIR, FIRexon2 or FIRexon2/FIR, was greater than those of conventional carcinoembryonic antigen or carbohydrate antigen 19-9. In addition, FIRexon2 or FIR mRNA expression in the peripheral blood was significantly reduced after operative removal of colorectal tumors. Thus, circulating FIR and FIRexon2 mRNA are potential novel screening markers for colorectal cancer testing with conventional carcinoembryonic antigen and or carbohydrate antigen 19-9. Taken together, our results indicate that overexpression of FIR and its splicing variants in colorectal cancer directs feed-forward or addicted circuit c-myc transcriptional activation. Clinical implications for colorectal cancers of novel FIR splicing variants are also discussed in the present paper.