Development of Chimeric Cas9 Nucleases for Accurate and Flexible Genome Editing

Abstract

There has been tremendous amount of effort focused on the development and improvement of genome editing applications over the decades. Particularly, the development of programmable nucleases has revolutionized genome editing with regards to their improvements in mutagenesis efficacy and targeting feasibility. Programmable nucleases are competent for a variety of genome editing applications. There is growing interest in employing the programmable nucleases in therapeutic genome editing applications, such as correcting mutations in genetic disorders. Type II CRISPR-Cas9 bacterial adaptive immunity systems have recently been engineered as RNA-guided programmable nucleases. Native CRISPR-Cas9 nucleases have two stages of sequence-specific target DNA recognition prior to cleavage: the intrinsic binding of the Cas9 nuclease to a short DNA element (the PAM) followed by testing target site complementarity with the programmable guide RNA. The ease of reprogramming CRISPR-Cas9 nucleases for new target sequences makes them favorable genome editing platform for many applications including gene therapy. However, wild-type Cas9 nucleases have limitations: (i) The PAM element requirement restricts the targeting range of Cas9; (ii) despite the presence of two stages of target recognition, wild-type Cas9 can cleave DNA at unintended sites, which is not desired for therapeutic purposes; and (iii) there is a lack of control over the mutagenic editing product that is procuded. In this study, we developed and characterized chimeric Cas9 platforms to provide solutions to these limitations. In these platforms, the DNA-binding affinity of Cas9 protein from S. pyogenes is attenuated such that the target site binding is dependent on a fused programmable DNA-targeting-unit that recognizes a neighboring DNA-sequence. This modification extends the range of usable PAM elements and substantially improves the targeting specify of wild type Cas9. Furthermore, one of the featured chimeric Cas9 variants developed in this study has both robust nuclease activity and ability to generate predictable uniform editing products. These superior properties of the chimeric Cas9 platforms make them favorable for various genome editing applications and bring programmable nucleases one step closer to therapeutic applications.