Retinoic acid-induced 2 (RAI2) is a novel tumor suppressor, and promoter region methylation of RAI2 is a poor prognostic marker in colorectal cancer

Background: Reduced expression of retinoic acid-induced 2 (RAI2) was found in breast cancer. The regulation and function of RAI2 in human colorectal cancer (CRC) remain unclear. Methods: Eight CRC cell lines and 237 cases of primary CRC were analyzed. Methylation-specific PCR (MSP), flow cytometry, xenograft mouse model, and shRNA technique were employed. Results: RAI2 was completely methylated in RKO, LOVO, and HCT116 cells; partially methylated in HT29 cells; and unmethylated in SW480, SW620, DLD1, and DKO cells. RAI2 was methylated in 53.6% (127/237) of primary colorectal cancer. Methylation of RAI2 was significantly associated with gender (P < 0.001), TNM stage (P < 0.001), and lymph node metastasis (P < 0.001). Analyzing by the Kaplan-Meier method, methylation of RAI2 was significantly associated with poor 5-year overall survival (OS) (P = 0.0035) and 5-year relapse-free survival (RFS) (P = 0.0062). According to Cox proportional hazards model analysis, RAI2 methylation was an independent poor prognostic marker for 5-year OS (P = 0.002) and poor 5-year RFS (P = 0.022). RAI2 suppressed cell proliferation, migration, and invasion and induced cell apoptosis in CRC. In addition, RAI2 inhibited AKT signaling in CRC cells and suppressed human CRC cell xenograft growth in mice. Conclusion: RAI2 is frequently methylated in human CRC, and the expression of RAI2 is regulated by promoter region methylation. Methylation of RAI2 is an independent poor prognostic marker of CRC. RAI2 suppresses CRC cell growth both in vitro and in vivo. RAI2 suppresses CRC by inhibiting AKT signaling.