Increased RNA editing in EAAT2 pre-mRNA from amyotrophic lateral sclerosis patients: involvement of a cryptic polyadenylation site.

The astroglial EAAT2 glutamate transporter is essential for clearing glutamate in the central nervous system and protecting against excitotoxicity. It is implicated in amyotrophic lateral sclerosis (ALS, the most common type of motor neurone disease) where less EAAT2 is found, possibly involving aberrant intron 7 retention transcripts. We report adenine/inosine RNA editing at a novel site in intron 7 of EAAT2 pre-mRNA that appears to activate a cryptic alternative polyadenylation site, generating intron 7 retention transcripts. This polyadenylation site includes two overlapping polyadenylation signals opposite the editing site in a strong stem-loop, which is highly conserved in primates. In pre-mRNA, we observed variable editing levels at this site, which were significantly higher in spinal cord (p=0.001) and motor cortex (p=0.005) from ALS patients, but not in cerebellum, demonstrating specificity for clinically relevant regions. By contrast, incomplete mRNA molecules polyadenylated in intron 7 are always completely edited. Cell culture experiments confirm this strong correlation between editing and polyadenylation in intron 7, strongly suggesting activation of the alternative polyadenylation site by editing. Prediction of inosine base-pairing from published data suggests that RNA editing releases the polyadenylation signals from the stem-loop, providing a plausible mechanism. To the best of our knowledge, this is the first report of RNA editing activating an alternative polyadenylation signal.