ENGINEERING HOMING ENDONUCLEASES TO MODIFY COMPLEX GENOMES

Gene targeting to selected chromosomal loci is greatly stimulated when free DNA ends are created that initiate double-strand break repair. Gene therapy reagents can be developed by engineering DNA endonucleases that cleave genomes at desired target sequences. Homing endonucleases are naturally occurring rare-cutting enzymes that have well understood DNA binding and DNA cleavage properties. Rational design methods as well as directed evolution strategies that involve genetic selections and screens using combinatorial libraries generate homing endonucleases with altered sequence specificities. Molecular switches are being introduced into these enzymes to regulate their activity. This article reviews the progress that has been made in constructing homing endonucleases for gene therapy and genome engineering, and discusses the challenges that remain.