I wanted to write a quick note about the use of CRISPR/Cas system in gene editing. Several labs in parallel have developed a CRISPR system (Clustered Regularly Interspaced Short Palindromic Repeats) for gene editing purposes in a variety of organisms from zebra fish to humans. The CRISPR/Cas systems and their function as an immunity-type response in bacteria is on its own very exciting and I encourage you to read about it, if you haven’t (e.g. see this paper in Science from 2010 or simply visit wikipedia).
In short, this system records foreign DNA and uses an RNA intermediate (crRNA) to target other encounters of the same invasive DNA species via specific nucleases (i.e. Cas). But how unbelievably “cool” this system is aside, recently it has been adopted for gene editing. In this context, the crRNA is replaced with a target sequence (a sequence that we want to alter in the genome). The activity of the CAS/crRNA complex, if properly expressed, then results in double stranded breaks at the site of interest. The cell then uses end-joining repair system to correct the break; however, the error-prone nature of this mechanism results in deletions at the site of action. Now if the target sequence was selected from an active gene, this mechanism would effectively mutate the gene into an inactive copy.
CAS system structure
Even better, by modifying the CAS enzymes, we can limit the nuclease activity to a single nick in the DNA, as opposed to double-stranded breaks. In this case, the cell employs homologous recombination to repair the nick and if we provide a mutated homologous sequence in trans, the system may use it as a template to correct the nicked site and in effect transfer the mutation to the genome with surgical precision.
Not only these are all very exciting, this is a prime example of how important basic research is. Just imagine the first grant written for studying the CRISPRs, and I’m paraphrasing here, “ahem…, there are these repetitive sequences in bacteria and some obscure archaea… we have no idea what they do, but we kind of wanna know… so fund us may be?”. Pursuing this simple curiosity however, has resulted in a promising method for genome editing in humans and is poised to transform how we do genetics (mainly due to its low cost of implementation). This is a very good example of where targeted funding of “translational research” fails. Ultimately, leaps in life sciences (and science in general) come from systems that we don’t even know exist. The same goes for other amazing tools that have become mainstays of molecular biology. Similarly, I assume the proposal for studying fluorescent proteins went something like this: “well… we have this cool organism that glows in the dark. We kind of wanna know why. Will it cure cancer? probably not…”. But all kidding aside, these are all very good reminders of how important basic research is to our collective knowledge.
Sources: “Cong et al, 2013, Multiplex Genome Engineering Using CRISPR/Cas Systems, Science DOI: 10.1126/science.1231143″ among others.