Alternative splicing of the OCC-1 gene generates three splice variants and a novel exonic microRNA, which regulate the Wnt signaling pathway

• Published in: RNA (Cambridge) 2017-01, Vol.23 (1), p.70-85
• Main Authors: Najafi, Hadi, Soltani, Bahram M, Dokanehiifard, Sadat, Nasiri, Shirzad, Mowla, Seyed Javad
• Format: Article
• Language: English
• Subjects: Adaptor Proteins, Signal Transducing - genetics; Alternative Splicing; Apoptosis; Cell Line, Tumor; Cell Survival; Colorectal Neoplasms - genetics; Disease Progression; Gene Expression Regulation, Neoplastic; Humans; MicroRNAs - genetics; Neoplasm Proteins - genetics; Organ Specificity; Proteins - genetics; Up-Regulation; Wnt Signaling Pathway
• ISSN: 1355-8382; 1469-9001
• DOI: 10.1261/rna.056317.116

The Wnt signaling pathway is hyperactivated in most colorectal cancers (CRC). Finding new regulators of this pathway represents the potential for cancer diagnosis or treatment. OCC-1 was initially reported as an up-regulated gene in colon carcinoma, without knowing its mechanism of action. Here, two novel transcript variants and an exonic microRNA that originated from the OCC-1 gene are reported, showing positive effects on Wnt activity. Up-regulation of the known OCC-1 variant (assigned as OCC-1A/B) was limited to CRC, and its overexpression increased survival of CRC-originated SW480 cells (Wnt ), while resulting in apoptosis of Wnt-suppressed SW480 cells or HeLa cells (Wnt ) detected by PI staining. Immunocytochemistry showed that the OCC-1A/B-encoded peptide was localized to the nucleus, where its overexpression resulted in Wnt signaling up-regulation, detected by TOP/FOPflash assay. The noncoding portion of the OCC-1A/B transcript had a suppressive effect on Wnt activity and had a negative correlation with APPL2 neighboring gene expression. Unlike OCC-1A/B, the novel OCC-1C splice variant had no expression alteration in CRC, and it seemed to encode a smaller peptide with cytoplasmic localization. A 60-nucleotide (nt) fragment containing an AUG start codon is spliced out to produce an OCC-1D noncoding RNA variant. The 60-nt RNA was validated as the precursor of a novel microRNA, which we named miR-ex1 Both OCC-1D and miR-ex1 were coordinately up-regulated in CRC. MiR-ex1 functional analysis revealed that it is targeting the APC2 tumor suppressor gene and is an activator of the Wnt signaling pathway. Overall, the OCC-1 gene is now introduced as a novel Wnt signaling regulator and as a potential therapeutic target.