Zinc-finger nucleases (ZFNs) have been successfully used for rational genome engineering in a variety of cell types and organisms. ZFNs consist of a non-specific FokI endonuclease domain and a specific zinc-finger DNA-binding domain. Because the catalytic domain must dimerize to become active, two ZFN subunits are typically assembled at the cleavage site. The generation of obligate heterodimeric ZFNs was shown to significantly reduce ZFN-associated cytotoxicity in single-site genome editing strategies. To further expand the application range of ZFNs, we employed a combination of in silico protein modeling, in vitro cleavage assays, and in vivo recombination assays to identify autonomous ZFN pairs that lack cross-reactivity between each other. In the context of ZFNs designed to recognize two adjacent sites in the human HOXB13 locus, we demonstrate that two autonomous ZFN pairs can be directed simultaneously to two different sites to induce a chromosomal deletion in ∼10% of alleles. Notably, the autonomous ZFN pair induced a targeted chromosomal deletion with the same efficacy as previously published obligate heterodimeric ZFNs but with significantly less toxicity. These results demonstrate that autonomous ZFNs will prove useful in targeted genome engineering approaches wherever an application requires the expression of two distinct ZFN pairs.

Zinc finger nucleases (ZFN) have been used to direct precise modifications to the genetic information in living cells at high efficiency. An important consideration in the design of ZFNs is the number of zinc fingers that are required for efficient and specific cleavage. We examined dimeric ZFNs composed of 1+1, 2+2, 3+3, 4+4, 5+5 and 6+6 zinc fingers, targeting 6, 12, 18, 24, 30 and 36 bp, respectively. We found that 1+1 and 2+2 fingers supported neither in vitro cleavage nor single-strand annealing in a cell-based recombination assay. An optimum of ZFN activity was observed for 3+3 and 4+4 fingers. Surprisingly, 5+5 and 6+6 showed significantly reduced activity. While the extra fingers were not found to dramatically increase toxicity, directly inhibit recombination, or perturb the ZFN target site, we demonstrate the ability of subsets of three fingers in six-finger arrays to bind independently to regions of the target site, possibly explaining the decrease in activity. These results have important implications for the design of new ZFNs, as they show that in some cases an excess of fingers may actually decrease the performance of engineered multi-finger proteins. Maximum ZFN activity will require an optimization of both DNA binding affinity and specificity.

The technical advances in developing artificial endonucleases, such as zinc finger nucleases (ZFNs), have opened a wide field of applications in the genome engineering arena, including the therapeutic correction of mutated genes in the human genome. Gene editing frequencies of up to 50% in human cells under non-selective conditions reveal the power of the ZFN technology. Activity and toxicity of ZFNs are determined by a number of parameters, including the specificity of DNA binding, the kinetics of dimerization of the two ZFN subunits, and the catalytic activity. In order to investigate these parameters individually, a cell-free system that models these reactions is essential. Here, we present a simple and fast method for the functional testing of ZFNs in vitro.