Evolution of programmable zinc finger-recombinases with activity in human cells

Site-specific recombinases are important tools for genomic engineering in many living systems. Applications of recombinases are, however, constrained by the DNA targeting endemic of the recombinase used. A tremendous range of recombinase applications can be envisioned if the targeting of recombinase specificity can be made readily programmable. To address this problem we sought to generate zinc finger-recombinase fusion proteins (Rec(ZF)s) capable of site-specific function in a diversity of genetic contexts. Our first Rec(ZF), Tn3Ch15(X2), recombined substrates derived from the native Tn3 resolvase recombination site. Substrate Linked Protein Evolution (SLiPE) was used to optimize the catalytic domains of the enzymes Hin, Gin, and Tn3 for resolution between non-homologous sites. One of the evolved clones, GinL7C7, catalyzed efficient, site-specific recombination in a variety of sequence contexts. When introduced into human cells by retroviral transduction, GinL7C7 excised a 1.4 kb EGFP cassette out of the genome, diminishing fluorescence in approximately 17% of transduced cells. Following this template of rational design and directed evolution, Rec(ZF)s may eventually mediate gene therapies, facilitate the genetic manipulation of model organisms and cells, and mature into powerful new tools for molecular biology and medicine.