RNA truncation by premature polyadenylation attenuates human mobile element activity
Description
LINE-1 is the most prevalent human retroelement, and it contributes to genomic instability. LINE-1 elements comprise 17% of the human genome, which translates into about 5 x 105 LINE-1 copies, the majority of which are truncated at their 5' end. LINE-1 is the only member of the autonomous non-LTR retrotransposones family that is currently active. The full extent of LINE-1 mobility in somatic tissues and particularly in tumors is still not known LINE-1 expression is extremely low in all cell types of a mature organism except for testis. In contrast, significantly higher levels of LINE-1 expression were found in various cancer cells. The known factors involved in regulation of LINE-1 expression, such as hypomethylation often associated with malignant transformation and promoter activity, cannot fully explain the observed pattern of expression In addition, full-length LINE-1 is not detected by northern blotting in cell culture even when transiently expressed from the CMV promoter. Combined, these observations indicate a potential for posttranscriptional mechanisms that limit the amount of the full length LINE-1 mRNA. By using a polyadq program, we identified 20 putative polyadenylation (polyA) sites located only in the sense strand of the LINE-1.3 genome We hypothesize that the A-rich coding strand of the human LINE-1 contains multiple functional canonical and non-canonical polyadenylation signals resulting in truncation of full-length transcripts by premature polyadenylation To address this question we employed a biologically relevant assay that allows detection of the transiently transfected full-length human LINE-1 mRNA as well as any LINE-1 specific RNA species terminated at the functional internal polyA sites. We determined by sensitive northern blotting, 3' RACE, and EST analysis that both canonical and noncanonical internal polyadenylation sites are functional, and they contribute to a very limited amount of the full-length LINE-1 mRNAs present in cells. This attenuation lowers the rate of retrotransposition when assayed in HeLa cells Our observation of a novel application of the conventional mRNA 3 ' end formation extends the range of the known mechanisms for the control of mRNA levels in mammalian cells, and provides additional understanding of the selective pressures that act upon new retrotransposition events in reshaping the mammalian genome