CRISPR/Cas9 Genome-editing Tool

Earlier this month the Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry to Emmanuelle Charpentier, Max Planck Unit for Science of Pathogens, Berlin, And Jennifer A Doudna, University of California, Berkeley, USA, for their work in developing the CRISPR/Cas9 genome-editing tool.

At the turn of the century, genome editing has become a prolific tool in both scientific research and the public eye. The ability to edit a person’s DNA and give them superhuman abilities has been an evocative plot device for many sci-fi stories, but such abilities have largely been beyond our capability. Whilst it is now relatively easy to sequence DNA to identify its code (and any mutations present), it is not possible to simply cut and paste DNA sequences together with scissors. DNA is tiny, and manipulating it requires knowledge of “molecular scissors”. This is very cumbersome and has a very low yield for the effort put in. And at such microscopic scales, it is easy to create mistakes and unwanted mutations alongside the genome editing. Producing new molecular scissors is no easy task and for this, we have to look towards nature.

CRISPR CAS9

The CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) Cas9 system was first discovered in bacteria, a possible immune system used to prevent infection by bacteria-invading viruses known as phages. The viruses inject this genetic sequence into the bacterial “DNA”, hijacking it to produce more viruses. At its core, the CRISPR system gives these bacteria the ability to recognise the precise genetic sequences that match these viruses and target them for destruction by the bacteria’s enzymes.

The Cas9 enzyme is guided by a guide RNA towards specific viral sequences in the DNA. Once the complex reaches a known segment the Cas9 enzyme cuts both strands of the DNA. In bacteria this removes the viral sequences entirely for destruction. But scientists can alter this system by choosing what the guide RNA recognises – this can be sections before a harmful mutated gene that the scientists want to excise. Alternatively they can create a cut in the DNA and introduce a mutation or a new gene, allowing the cell’s DNA repair mechanism to integrate it into the DNA.

BENEFITS

There are many advantages of the CRISP system. First of all, it is very specific, unlike other genome editing tools. The Cas9 enzyme will only cut the DNA where the guide RNA dictates, and because the guide RNA can be designed to recognise very unique sequences 20 characters long, it should in theory only cut the DNA at the single desired location.

Secondly, it is substantially less expensive. Previously scientists had to use an inefficient, cumbersome tool known as zinc finger nuclease that was difficult to engineer and cost $5,000 or more to order. In comparison scientists only need to order the RNA guide fragment – everything else can be put together from off the shelf components. The resulting costs can be as little as $30. This opens CRISPR to far more laboratories, allowing substantially more research to be done using this technique.   

The technique itself has in theory limitless possibilities. The ability to make edits at precise locations allows researchers to add or remove genes from a cell, introduce or remove mutations, and study how the presence or absence of these genes and mutations affects the workings of the cell. If these edits are made to the sperm or egg of an organism, the entire animal will have these genome edits, creating animal models of genetic disease. These models can be used to study how the disease functions, as well as test treatments for the disease.

Similarly the technique itself can be used as a treatment for known diseases. By introducing new, functioning genes to replace mutated diseased genes, it may become possible to treat certain genetic diseases. Cystic fibrosis, haemophilia and sickle cell are all resulting from mutations in genes coding for salt channels in cells and haemoglobin proteins in red blood cells respectively.  By replacing these defective genes, it may be possible for the body to produce functioning red blood cells that do not fragment or deform.

One such attempt has already been trialled in an animal model for tyrosinaemia, a human metabolic disease. Though the treatment required large volumes of liquid to be pumped into the body, and it only achieved a genome editing success rate of 0.4% in the liver, it highlights how this novel technique is already being used for new therapies.

DRAWBACKS

Of course, its not without limitations. We still need to study and refine the technique further to prevent off-target genome editing that could introduce new mutations and create new problems. If the CRISPR-CAS system introduces a mutation to a critical gene, it could seed the target animal (or person) with a new disease or cancer.

Furthermore we need to be careful with our laboratory work. One postdoc researcher mutated a virus to act as a transport vessel and carry CRISPR components into mice. By breathing in the virus, the CRISPR was able to engineer mutations in the mouse’s lungs and create a model for human lung cancer. However, if this virus were to mutate, and the restrictions placed on the CRISPR components to fail, the technique may have caused harm to nearby researchers accidentally breathing in the virus.

But overall? CRISPR is one of the most ingenious, incredible and innovative developments in genetic research, and one that will likely have the same impact on our healthcare as mapping the genome, or understanding DNA and genetics. It has allowed us to be unshackled by previously cumbersome techniques in gene editing, and expand our horizons on what is feasible in the world of genetics.