RNA-binding motif protein 35A is a novel tumor suppressor for colorectal cancer.

The frequent occurrence of inactivating gene mutations in tumors suggests a tumor suppressor function of the mutated gene. The RNA binding motif protein 35A (RBM35A) is mutated in approximately 50% of analyzed primary colon tumors with microsatellite instability. The Tet-off regulated ectopic expression of RBM35A gene in RBM35A-null LS180 colon carcinoma cells inhibited anchorage-independent growth in vitro, suppressed tumorigenic potential in vivo and enhanced adhesiveness of these cancer cells. Using microarray hybridization we found that in response to RBM35A expression a small fraction of genes showed a decrease in polysome-associated mRNA. Experiments using cell-free in vitro translation system demonstrated that RBM35A differentially affects translation of luciferase reporter mediated by various 5'untranslated regions (UTR). We found that Gibbs energy value (DeltaG) of secondary structure formed by 5'UTRs of mRNAs can account for differential effect of RBM35A on reporter translation efficiency. Targeted mutation in the FOS 5'UTR sequence, which increased the DeltaG value of hairpin stem formation, resulted in a stronger inhibitory effect of RBM35A on reporter translation efficiency mediated by this UTR. Immunoblotting revealed that ectopic expression of RBM35A in LS180 cells caused alterations in protein levels for several cancer related genes. Our results demonstrate for the first time that RBM35A functions as a tumor suppressor in colon cancer cells. We propose that RBM35A is involved in posttranscriptional regulation of a number of genes by exerting a differential effect on protein translation via 5'UTRs of mRNAs.