Gene Editing Approaches against Viral Infections and Strategy to Prevent Occurrence of Viral Escape

Powerful new gene editing techniques promise groundbreaking opportunities for novel therapeutic options to important illnesses, including cancer, genetic disorders [1], and viral infections [2]. These techniques include zinc finger nucleases (ZFN) [3], transcription activator-like effector nucleases (TALEN) [4], and clustered regulatory interspaced short palindromic repeat (CRISPR)-associated 9 (Cas9) [5, 6]. In particular, CRISPR/Cas9 provides an effective, highly specific, and versatile tool applicable to important human viruses, including HIV-1 [7]. CRISPR/Cas9 is elegant and simple compared to ZFN and TALEN because it uses one or more guide RNAs (gRNA), which are simple to produce and specifically target any sequence in an adaptable and flexible way for different targets, such as viral genes, by changing the gRNA sequence [8]. Cas9 cuts both strands of the DNA target, resulting in a double-strand break (DSB), usually repaired by nonhomologous end-joining (NHEJ), an error-prone arm of the DNA repair pathway that introduces insertions and deletions (InDels) at the break site. While InDel introduction is quite rare, the products of accurate repair are targets for recleavage by CRISPR/Cas9, whereas InDel products are not, and, hence, InDel products accumulate with repeated cleavage cycles [9, 10]. In this short review, we discuss CRISPR/Cas9 in combating human viruses by specifically targeting and disrupting essential viral genes. We will also discuss generation of viral escape mutants resistant to Cas9/gRNA and how this may be overcome.