CRISPR systems Essay

CRISPR systems are adaptable immune mechanisms used by many bacteria to protect themselves from foreign nucleic acids, such as viruses or plasmids (30,45-47). Type II CRISPR systems incorporate sequences from invading DNA between CRISPR repeat sequences encoded as arrays within the bacterial host genome. Transcripts from the CRISPR repeat arrays are processed into CRISPR RNAs (crRNAs), each harboring a variable sequence transcribed from the invading DNA, known as the "protospacer" sequence, and part of the CRISPR repeat. Each crRNA hybridizes with a second RNA, known as the transactivating CRISPR RNA (tracrRNA) (48), and these two RNAs complex with the Cas9 nuclease (43). The protospacer-encoded portion of the crRNA directs Cas9 to cleave complementary target-DNA sequences, if they are adjacent to short sequences known as protospacer adjacent motifs (PAMs). Protospacer sequences incorporated into the CRISPR locus are not cleaved presumably because they are not next to a PAM sequence. The type II CRISPR system from S. pyogenes has been adapted for inducing sequence-specific DSBs and targeted genome editing (43). In the simplest and most widely used form of this system, two components must be introduced into and/or expressed in cells or an organism to perform genome editing the Cas9 nuclease and a guide RNA (gRNA), consisting of a fusion of a crRNA and a fixed tracrRNA.(49)
Targeted genome editing using engineered nucleases has rapidly gone from being a niche technology to a mainstream method used by many biological researchers. This widespread adoption has been largely fueled by the emergence of the clustered, regularly interspaced, short palindromic repeat (CRISPR) technology, an important new approach for generating RNA-guided nucleases, such as Cas9, with customizable specificities. Genome editing mediated by these nucleases has been used to rapidly, easily and efficiently modify endogenous genes in a wide variety of biomedically important cell types and in organisms that have traditionally been challenging to manipulate genetically. The power of these systems to perform targeted, highly efficient alterations of genome sequence and gene expression will undoubtedly transform biological research and spur the development of novel molecular therapeutics for human disease.(49)
Complex and dynamic transcription regulation of multiple genes and their pathways drives many essential cellular activities, including genome replication and repair, cell division and differentiation, and disease progression and inheritance. Understanding the complex functions of a gene network requires the ability to precisely manipulate and perturb expression of the desired genes by repression or activation.(50) The science of genetics relies heavily on the analysis of mutations and the phenotypes they cause. Many geneticists seek to direct mutations and precise sequence changes to particular genes of interest. Targetable nucleases provide that capability. Targeted genome-editing technology continues to create intense excitement with each new technological advance.(51-53) The development of tools to generate DNA breaks, activate,(54) repress or label genomic loci,(55,56) and remodel chromatin (57) in a controlled, targeted manner will greatly aid the studies of a wide range of biological issues, including gene and genomic functions. The ability to specifically modify the genome also holds great promise for targeted gene therapies.(58)