CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. These are found in microbial and bacterial genomes. Scientists found these at the ends of genes where a sequence of DNA would be followed by the exact same sequence in reverse, then followed by a bunch of random “spacer DNA”, then repeat the same sequence followed by its reverse again then more of the “spacer DNA”. Weird right? When this was originally discovered, scientists thought nothing of it. But in 2005, scientists discovered that these palindromic repeats could be the microbial form of the immune system against bacteriophages! Why is that so exciting? Well, this discovery “led Eugene Koonin from the National Center for Biotechnology Information in Bethesda, Maryland, and his colleagues to propose that bacteria and archaea take up phage DNA, then preserve it as a template for molecules of RNA that can stop matching foreign DNA in its tracks, much the way eukaryotic cells use a system called RNA interference (RNAi) to destroy RNA (Elizabeth Pennisi, Science Magazine)”. So, this technique can be used to target human DNA and stop that in its tracks as well! Which would be extremely helpful in curing human genetic diseases.
So here’s how this process works: As discovered by Doudna and Emmanuelle Charpentier of the Helmholtz Centre for Infection Research and Hannover Medical School in Germany in 2011 reported in Nature, when CRISPR responds to an invading phage, the bacteria transcribe the spacers and the palindromic DNA into a long RNA molecule that the cell then cuts into short spacer-derived RNAs called crRNAs. An additional stretch of RNA, called tracrRNA, works with a protein called Cas9 which was found to be a nuclease. A nuclease is a specified enzyme that cuts DNA at two places that correspond to each strand of the DNA’s double helix. The tracrRNA works with Cas9 to produce crRNA, and together these all work together to attack foreign DNA that matches the crRNA.
The CRISPR systems are also much faster than other current methods of targeting DNA sequences such as TALENs, which require scientists to custom-make new proteins for each DNA target. CRISPR just uses RNA to target DNA, this means that in a few weeks, scientists can obtain tangible results that would take them months to replicate using other methods. But that’s not all! The CRISPR system is also much more efficient in human cells than TALENs are at cutting the target DNA, and CRISPR also works on more genes than TALENs do as well according to Church’s research group. This makes it possible to alter virtually ANY GENE using the CRISPR method, and it is also possible to fine-tune gene activity as well! A new method discovered by Doudna and Lei S. Qi from UC San Francisco and his colleagues called CRISPRi can be used to turn genes off in cells. The CRISPRi would work just like RNAi, reversibly turning off genes that match its sequence by binding to them and signaling them to be degraded.
All of these new advances in methods for gene regulation and genome editing are advancing incredibly quickly, which is good news for the future of clinical research and for people suffering from genetic diseases! Science like this makes me feel good about being a biology major!
-Posted by Ashley Condon (Group A)