Targeting DNA With Fingers and TALENs

Researchers have long recognized that the random introduction of new DNA into the genome could result in unpredictable genetic effects due to bilateral influences between transgenic and endogenous sequences. Hence, precise editing or replacement of mutant genes has been a major goal of gene therapy since its inception. That goal looked possible with the publication of studies demonstrating that targeted cleavage of chromosomal sequences and enhanced homologous recombination (HR) could be achieved using chimeric molecules composed of a nuclease domain and separate, designer DNA-recognition domains.(1,2) Zinc-finger nucleases (ZFNs) are artificial endonucleases that consist of a FokI cleavage domain tethered to engineered Cys(2)His(2) zinc-finger, DNA-binding polypeptides.(3-5) ZFNs have gained considerable momentum and are widely considered the most mature nuclease technology in the gene therapy field.(6) However, after 15 years in the spotlight, a new chimeric nuclease has emerged, Transcription Activator-Like (TAL) Effector Nucleases (TALENs).(7-10) Here, we describe functional and design characteristics of ZFNs and TALENs and discuss their expanding role as tools for research and gene therapy.