TALENs: a widely applicable technology for targeted genome editing

Abstract

Engineered nucleases enable the targeted alteration of nearly any gene in a wide range of cell types and organisms. The newly-developed transcription activator-like effector nucleases (TALENs) comprise a nonspecific DNA-cleaving nuclease fused to a DNA-binding domain that can be easily engineered so that TALENs can target essentially any sequence. The capability to quickly and efficiently alter genes using TALENs promises to have profound impacts on biological research and to yield potential therapeutic strategies for genetic diseases.

Figure 1. Overview of TALENs and TALE repeat arrays

a | Schematic diagram of a TALEN. TALE repeats are shown as colored cylinders with a final carboxy-terminal truncated “half” repeat. Letters inside each repeat represent the two hypervariable residues. TALE-derived amino- and carboxy-terminal domains required for DNA-binding activity are shown as longer blue and grey cylinders, respectively. The non-specific nuclease domain from the FokI endonuclease is shown as a larger orange cylinder. b | TALENs bind and cleave as dimers on a target DNA site. Note that the TALE-derived amino- and carboxy-terminal domains flanking the repeats may make some contacts to the DNA. Cleavage by the FokI domains occurs in the “spacer” sequence that lies between the two regions of the DNA bound by the two TALEN monomers. c | Schematic diagram of a TALE-derived DNA-binding domain. The amino acid sequence of a single TALE repeat is expanded below with the two hypervariable residues highlighted in orange and bold text. d | TALE-derived DNA-binding domain aligned with its target DNA sequence. Note the matching of repeat domains to single bases in the target site according to the TALE code. Also note the presence of a 5’ thymine preceding the first base bound by a TALE repeat.

Figure 2. Nuclease-induced genome editing

a | Single nuclease-induced double-strand breaks (DSBs) in a gene locus can be repaired by either NHEJ (thin black arrow) or HDR (thick black arrows). NHEJ-mediated repair leads to introduction of variable length indel mutations. HDR with double-stranded DNA “donor templates” can lead to the introduction of precise nucleotide substitutions or insertions. b | Introduction of two nuclease-induced DSBs in cis on the same chromosome can lead to deletion of the intervening sequence or inversion (left panel) and introduction of two nuclease-induced DSBs on two different chromosomes can lead to the formation of translocations (right panel).