ویرایش CRISPR در تحقیقات بیولوژیکی و زیست پزشکی CRISPR Editing in Biological and Biomedical Investigation

1. Introduction Few technological innovations may truly transform the landscape of genetic manipulations in biological and biomedical research, whereas the recently developed innovative genome editing tool- CRISPR/Cas9 has led to such a paradigm shift. CRISPR was conceived after a yogurt company identified it as an unexpected defense mechanism in bacteria to fight off viruses in 2007 (Travis, 2015). Further insight arrived in 2012, Cas protein was firstly demonstrated to cleave specific DNA sequences guided by short synthetic guide RNAs in vitro (Gasiunas et al., 2012). In vivo demonstrations followed in 2013, with the labs of Church, Doudna and Zhang quickly exhibiting the power of CRISPR-mediated genome modification in mammalian cells (Cong et al., 2013; Jinek et al., 2013; Mali et al., 2013b). In the short period since then, CRISPR/Cas revolution has ushered in an era of its own, by introducing a new scientific verb and giving rise to the now commonplace phrase: “CRISPR it!”(Dow, 2015). Owing to its capacity of scalable, affordable, and easy to engineer, CRISPR/Cas technologies fuel biological and biomedical investigations through efficient and versatile genetic modifications listed but not limited as deletion, knock-in, RNA regulation and chromatin modification of the targeted gene loci in various cell types and living organisms (Cong et al., 2013). All those promising applications made CRISPR/Cas systems to be selected by Science as “2015 Breakthrough of the Year” (Travis, 2015). Classical gene editing efforts most relied on the endogenous cellular homologous recombination (HR) pathway, that the insertion of exogenous DNA sequence must be mediated by long homologous sequences flanking the target genomic DNA site (Smithies et al., 1985). Spontaneous HR via transfecting exogenous donor DNA sequence is very inefficiently, at rates of 1 in every 103 to 109 cells, depending on the cell type and cell state (Thomas and Capecchi, 1986). Despite the low efficiency of editing by spontaneous HR, this approach could also induce undesired integration events, in which the exogenous DNA sequence is inserted into the genome at random sites more frequently than at the desired locus (Lin et al., 1985). So far, the most successful use of spontaneous HR was shown in mouse embryonic stem cells (ESCs), allowing researchers to generate geneticly modificated mice with a desired genotype efficiently (Capecchi, 1989). However, this ESCs plus HR system is still not available for genetic modifications in large animals because no well characterized ESCs have yet been isolated. Based on that the introduction of a double stranded break (DSBs) within genome would stimulate cellular DNA repairing (Rouet et al., 1994; Rudin et al., 1989), the programmable genome engineering nucleases are employed to introduce DSBs, which provide a key advance to overcome the above obstacles in large animals and non-human primates genome editing. After DSB, two different pathways mediate cellular DNA repairing,