RNA editing

RNA editing changes the information in an RNA molecule after transcription, through nucleotide substitution, insertion, or deletion. It differs from RNA splicing, which removes introns and joins exons, and from other maturation steps such as adding a cap or poly(A) tail. Editing modifies the RNA transcript; it does not, by itself, rewrite the DNA sequence from which that RNA was copied. The mechanisms and prevalence of editing differ across organisms and cellular compartments.

In animals, one prominent form is adenosine-to-inosine (A-to-I) editing, catalysed by adenosine deaminases acting on RNA (ADARs). These enzymes act on suitable double-stranded RNA structures and convert selected adenosines to inosines. Because cellular machinery commonly reads inosine as guanosine, an edit in a coding region can change how a codon is interpreted. Another substitution type is cytidine-to-uridine (C-to-U) editing, which in humans can be catalysed by APOBEC1 in particular RNA contexts. Some organisms also use insertion or deletion editing, including the addition or removal of uridines in certain mitochondrial transcripts of kinetoplastid protists. These are examples, not a single universal pathway.

The outcome depends on where an edit occurs. Editing in a protein-coding sequence may alter a codon and the resulting protein; editing in noncoding regions can affect RNA structure, stability, interactions, or processing. Some edited microRNA precursors are processed differently or can influence which RNA targets they recognize. In the human APOB transcript, C-to-U editing creates a stop codon and contributes to production of a shorter protein in the intestine, in contrast to the full-length form produced in the liver. Many editing sites are known, but the function of every observed edit is not established.

RNA editing expands the range of RNA and protein products that cells can produce from genomic instructions. Researchers compare DNA and RNA sequences, use sequencing, and study the enzymes and RNA structures involved to identify editing sites and test their effects. Editing is also distinct from RNA interference: RNA interference usually uses small RNAs and protein complexes to regulate or degrade target RNA, whereas editing chemically changes RNA sequence information. The two systems can interact, but they are not interchangeable. Studies of RNA editing are relevant to gene regulation, development, and disease biology; proposed therapeutic RNA editing remains an area of research.