CRISPR-Cas9: a key player in gene therapy

The 2020 Nobel Prize in Chemistry was awarded to Emmanuelle Charpentier and Jennifer Doudna for their discovery and development of a novel method of genome editing. But what exactly is CRISPR-Cas9 and how does it work?

The main aim of gene therapy is to treat or prevent human disease through the manipulation of DNA or RNA. There are many ways scientists can utilise this method, such as replacing or deleting faulty genes or introducing disabling mutations into known promoters of disease. Usually, this sort of genetic manipulation either results in the elimination of a sinister gene or allows the expression of a mistakenly silenced gene.

There have been thousands of gene therapy clinical trials worldwide which have explored diverse genome manipulations resulting gene activation and inactivation. One advanced technique allows for targeted genome editing using ‘nucleases’ known as zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs). Whilst these names may be daunting, the main thing to understand is that such nucleases have the ability to cut nucleic acids such as DNA and RNA.

CRISPR-Cas9 is a technique which is becoming increasingly common in gene therapy. Similarly to the targeted genome editing techniques using ZFNs and TALENs, it uses a specific nuclease (Cas9) to cut DNA. It’s ease of design and ability to edit multiple genes simultaneously is desirable to researchers who wish to study the effects of genome editing in a cost-effective way.

The technique relies on a series of repeated RNA elements known as the ‘guide RNA’ attached to a pair of genetic scissors (the nuclease). These repeats are introduced into a cell and recognise a complementary piece of DNA, which is then cut out of the DNA by the scissors and replaced by a new arrangement of DNA repeats. Whilst targeted genome editing using ZFNs and TALENs uses the same process, CRISPR-Cas9 requires a shorter stretch of RNA bases to form the guide RNA, making it easier to design and more accurate.

Since its discovery in 2013, this technique has been used by researchers to edit the DNA of animals, plants and microorganisms. A previously time-consuming and difficult process can now be performed in a few weeks with high precision. Whilst the notion of genetically modified crops is still controversial for some, the ability of this technique to revolutionise disease therapy can only be seen as a good thing.

Cutting out a piece of undesirable or faulty DNA and replacing it with a healthy copy could theoretically cure a patient of a genetic disease. The guide RNA can be genetically engineered in the lab to recognise a specific fault in a patient’s DNA, offering hope of personalised treatment.

One field of science which is constantly looking out for advancements in gene therapy is cancer research. Given that cancer is primarily a genetic disease, gene therapy holds the promise of techniques such as CRISPR-Cas9 in fighting and preventing the disease before it emerges. One example is the use of CRISPR-Cas9 to inactivate oncogenic viruses which give rise to cancers such as nasopharyngeal carcinoma (Epstein-Barr virus) and liver cancer (HBV/ hepatitis C). Another option is to edit specific genes which are being over-expressed and causing havoc in cells e.g. in melanoma.

Whilst the vast majority of CRISPR-Cas9 experiments have been conducted on cells, within the last year there have been a few studies targeting the eye, the skin and the liver in humans, including for the treatment of hereditary blindness disorder. It is important to note that in these trials, only somatic (non-reproductive) cells are being targeted as there is a huge ethical concern surrounding the alteration a person’s DNA which could be passed down to their children.

There is no doubt that more studies will be carried out in the near future to test CRISPR-Cas9 in the treatment of human disease. Whilst currently all trials focus on easily accessible areas of the human body with a low risk of immune reaction, the coming years will bring with them exciting new developments which will allow the targeting of deeper tissues and organs.

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