TALENs are Transcription Activator-Like Effector Nucleases. They are artificial restriction enzymes able to cut DNA only where they encounter a specific sequence of nucleotides.
The story begins with the discovery of a family of proteins secreted by plant pathogens in the genus Xanthomonas. These proteins — called TALEs (Transcription Activator-Like Effectors) act as transcription factors turning on genes in their plant host that promote their successful infection.
A TALE consists of a tandem array of repeated segments each consisting of 34 (usually) amino acids. The amino acid sequence of these repeats is mostly the same EXCEPT for the amino acids at positions #12 and #13. These pairs are quite variable.
Each of these pairs of amino acids binds to a specific nucleotide in DNA.Some frequent examples:
So a dozen or so tandem repeats can efficiently bind to a particular sequence of the same number of nucleotides in, for example, the promoter of a host gene. This usually enhances the transcription of that gene.
The one-to-one correspondence between pairs of amino acids and the particular nucleotide to which they can bind lays the groundwork for another tool with which a specific region of the genome can be targeted. If you know the nucleotide sequence of the genomic region you wish to alter, a TALE can be synthesized by assembling the corresponding sequence of repeats. Add to this construct an endonuclease able to cut through the DNA to which the TALE binds. The product: a TALEN.
Once within the nucleus of the cell whose genome you wish to alter, the TALEN cuts the DNA forming a double-stranded break (DSB). This can be repaired by nonhomologous end-joining (NHEJ) or homologous recombination (HR) [More].
Repair by NHEJ is error-prone and frequently introduces a mutation, e.g. a point mutation or an insertion or deletion (indel).
Up to now, TALENs have been used to alter genes in a number of species: plants (e.g., Arabidopsis), animals (e.g., C. elegans, Danio, Xenopus), fungi (e.g., budding yeast), and even human cells.
Still on the horizon, the hope that TALENs may become useful therapeutic tools for gene therapy in humans. But a word of caution: while they may be effective at altering the gene of interest, there may be other places in the human genome where the same target sequence occurs, and altering these might produce harmful side-effects.