A novel tissue-specific alternatively spliced form of the A-to-I RNA editing enzyme ADAR2

ADAR2, a member of the adenosine deaminase family of proteins, is the enzyme that edits the Q/R site in the GluR-B transcript, an important physiological A-to-I editing event. ADAR2 pre-mRNA undergoes a number of known alternative splicing events, affecting its function. Here we describe a novel alternatively spliced exon, located within intron 7 of the human gene, which we term "exon 7a". This alternatively spliced exon is highly conserved in the mammalian ADAR2 gene. It has stop codons in all three frames and is down regulated by NMD. We show that the level of exon 7a inclusion differs between different human tissues, with the highest levels of inclusion in skeletal muscle, heart and testis. In the brain, where the level of editing is known to be high, the level of exon 7a inclusion is low. The new alternative form was also found in supraspliceosomes, which constitute the nuclear pre-mRNA processing machine. The high conservation of the novel ADAR2 alternative exon in mammals indicates a physiological importance for this exon.