A sizeable fraction of the risk for psychiatric illness can be attributed to the genes that we inherit (Polderman et al, 2015). The challenge of identifying gene variants that influence vulnerability to psychiatric illness, which often operate only after interacting with environmental risk factors, is daunting. Nevertheless, advances in genomics are yielding new insights into these risk-modifying gene variants.
A major breakthrough that will facilitate our understanding in this area is the development of CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats) (Cong et al, 2013; Jinek et al, 2013); Heidenreich and Zhang, 2016). CRISPR technology utilizes a bacterial borne RNA-guided DNA nuclease defense mechanism to elicit specific gene mutations. Scientists have exploited the mechanism by which bacteriophages transcribe part of a pathogenic genome into guide RNAs (gRNAs) that, as their name implies, serve to guide the endonuclease, Cas9, to cut the DNA of the invading pathogen. The resulting DNA breaks are repaired by either non-homologous end joining (NHEJ) or homology-directed repair (HDR). The NHEJ pathway efficiently ligates broken ends of DNA strands, but often introduces nucleotide insertions or deletions resulting in frameshift mutations that can disrupt expression of the repaired gene. The less efficient but higher fidelity HDR pathway includes a specific DNA sequence to be inserted at the break site.
Using CRISPR-mediated NHEJ, mutations in genes of interest can be generated efficiently, thus reducing the time between gene discovery and investigation of mechanisms of action. Delivery of the CRISPR components (gRNA, tracrRNA, and Cas9) to cells or model organisms is efficient and can, for example, be accomplished through the transfection of mRNA or delivery via adeno-associated viruses (AAVs). The recent generation of the Cre-inducible Cas9 mouse allowed for the delivery of a single AAV containing the gRNA, tracrRNA, and Cre. This model can be used to delete genes of interest in discrete populations of neurons in the adult brain of laboratory animals with relative ease. This model illustrated efficient knockout in their gene of interest by 80% in the prefrontal cortex (Platt et al, 2014). The ability of CRISPR to target more than one gene simultaneously is particularly useful for investigating the role for gene × gene interactions in psychiatric illness. In addition, CRISPR-mediated HDR, when used in combination with specifically designed DNA templates, can replicate alleles that influence the risk of psychiatric illness or, ultimately, to correct risk-modifying alleles.
CRISPR is an efficient, precise, and versatile genome editing tool. Nevertheless, the technology is still in its infancy and further maturation is necessary before its full potential is realized. For example, CRISPR-mediated HDR is inefficient in post-mitotic cells, making it difficult to insert disease-relevant mutations into the neurons of laboratory animals. Although CRISPR is highly specific, it can suffer from off-target actions, a liability that is being advanced in new iterations of the technology (Slaymaker et al, 2016). Nevertheless, CRISPR and other genome editing technologies are poised to markedly increase our understanding of the biological underpinnings of psychiatric illnesses and to ultimately advance therapeutic discovery.
Funding and disclosure
This work was supported by grants DA025983, AA024292, MH112168, NS083614 from the NIH (PJK), a NARSAD-distinguished investigator grant (PJK), and a post-doctoral fellowship from the Canadian Institutes of Health Research (SC). PJK is a shareholder in Eolas Therapeutics and is a consultant for Florida House Experience. The remaining author declares no conflict of interest.
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