Characterization of DNA polymerase beta splicing variants in gastric cancer: the most frequent exon 2-deleted isoform is a non-coding RNA.

DNA repair polymerase beta (Pol beta) gene variants are frequently associated with tumor tissues. In this study a search for Pol beta mutants and splice variants was conducted in matched normal and tumor gastric tissues and blood samples from healthy donors. No tumor associated mutations were found while a variety of alternative Pol beta splicing variants were detected with high frequency in all the specimens analysed. Quantitative PCR of the Pol beta variant lacking exon 2 (Ex2Delta) and the isoforms with exon 11 skipping allowed to clarify that these variants are not tumor- neither tissue-specific and their levels vary greatly among different individuals. The most frequent Ex2Delta variant was further characterized. We clearly demonstrated that this variant does not encode protein, as detected by both western blotting and immunofluorescence analysis of human AGS cells expressing HA-tagged Ex2Delta. The lack of translation was confirmed by comparing the DNA gap-filling capacity and alkylation sensitivity of wild type and Pol beta null murine fibroblasts expressing the human Ex2Delta variant. We showed that the Ex2Delta transcript is polyadenylated and its half-life is significantly longer than that of the wild type mRNA as inferred by treating AGS cells with actinomycin D. Moreover, we found that it localizes to polyribosomes suggesting a role as post-transcriptional regulator. This study identifies a new type of DNA repair variants that do not give rise to functional proteins but to non-coding RNAs that could either modulate target mRNAs or represent unproductive splicing events.