Mechanisms and functions of RNA editing by adenosine deamination
(SFB projects 1706 /1716)

Intracellular modulators of A to I editing:
Adenosine deaminases that act on RNA convert adenosines to inosines in double-stranded and structured RNAs. Inosines are interpreted as guanosines by most cellular machineries. Consequently, this type of RNA-editing can lead to codon exchanges, introductio or deletion of splice sites, alteration of protein binding sites, and to different miRNA responses. Targets for ADAR-mediated editing comprise both coding- and non-coding RNAs, including miRNAs.
In vertebrates, three ADAR enzymes, ADAR1, ADAR2, and ADAR3 can be found. Lack of ADAR1 leads to embryonic lethality, while lack of ADAR2 shows a neurological phenotype caused by underediting of glutamate receptor B mRNA. ADAR3, finally, has no obvious enzymatic activity.
ADAR1 is almost ubiquitously expressed. Interestingly, however, editing levels differ considerably in various tissues. Moreover, editing activity seems misregulated in certain pathological conditions. It therefore seems likely that other cellular factors can modulate editing activity by either interfering with the substrate RNA or the ADAR enzyme. One goal of our ongoing project is aimed at identifying such cellular modulators of ADAR activity.

Impact of editing on coding targets:
Editing in coding regions of an mRNA can alter the amino acid composition of the encoded protein and thus alter protein function. Recently, we have identified a few editing events in coding regions of mRNAs leading to amino acid exchanges. We are currently testing the impact of these editing events in a mouse model. To do so, we replace the gene encoding the edited mRNA by a gene copy that is uneditable. Currently mice carrying such gene replacements are being crossed to homozygosity. Morphological, physiological, and behavioral analysis of these mice will give insight on the function of these highly conserved editing events in protein coding regions.

Editing in non-coding RNAs:
The vast majority of editing events identified so far, can be found in non-coding RNAs such as miRNAs, introns, and untranslated regions of protein coding mRNAs. It has been suggested that editing events in 3’ UTRs can affect expression of proteins encoded by these edited RNAs. We are currently testing this hypothesis using cells from mice lacking ADAR activity. Moreover, we are studying the stages and mechanisms by which putative editing sites can regulate gene expression.

Pri-miRNAs have been reported to be inefficiently processed upon editing. Following the introduction of inosines pri-miRNAs also become destabilized. Using a Xenopus oocyte injection system we are identifying the components involved in the destabilization of pri-miRNAs as well as their intracellular localization.