A novel mechanism suppressing the expression of truncated proteins from premature termination codon containing mRNAs

Abstract

[한글]

[영문]Frameshift and nonsense mutations are common in tumors with microsatellite instability, and mRNAs from these mutated genes have premature termination codons (PTCs). mRNAs containing PTCs are usually degraded by the nonsense-mediated mRNA decay (NMD) system. However, PTCs located within 50-55 nucleotides of the last exon-exon junction are not recognized by NMD (NMD-irrelevant), and some PTC-containing mRNAs can escape from the NMD system (NMD-escape). I investigated protein expression from NMD-irrelevant and NMD-escape PTC-containing mRNAs by Western blotting and transfection assays. The results showed that transfection of NMD-irrelevant PTC-containing genomic DNA generate truncated protein. In contrast, transfection of NMD-escape mutant TGFBR2 genomic DNA failed to generate expression of truncated proteins, while transfection of wild type TGFBR2 genomic DNA or mutant PTC-containing TGFBR2 cDNA generated expression of wild type protein and truncated protein, respectively. I found that NMD-escaped PTC-containing mRNAs were shifted into ribosomal subunits and monosome-containing fractions by polysome analysis. This nonsense-mediated translational repression (NMTR) appears to be post-initiation step since the CrPV IRES-mediated translation of PTC-containing mRNAs was repressed. These findings suggest a novel mechanism of gene expression regulation for PTC-containing mRNAs, in which the deleterious transcripts are regulated either by NMD or NMTR.