On the 2nd March 2021, the Centre for Personalised medicine was honoured to host Professor Jennifer Doudna to deliver the annual Dr Stanley Ho memorial lecture. Professor Doudna is an outstanding biochemical scientist who was the recipient of the 2020 Nobel Prize in Chemistry in conjunction with Professor Emmanuelle Charpentier, for their discovery and development of CRISPR-Cas9 gene editing technology.
CRISPR has revolutionised our ability to edit the sequence of DNA, which has important applications for understanding genetics and huge promise for treating human genetic disease. In this fascinating and engaging talk, Professor Doudna introduced how CRISPR-Cas9 was discovered and harnessed, from its origin within a bacterial immune system to its translation to human medical research (along with many, many other fields).
CRISPR stands for ‘clusters of regularly interspaced short palindromic repeats’. In simpler terms, this means a CRISPR sequence is made of repeating sequences of nucleotides (the building blocks of DNA). These repeats are separated by ‘spacers’. Unlike the repeated blocks, these spacers are variable. In bacteria, the original home of CRISPR, these variable regions are made up of DNA sequences taken from a virus the bacteria has encountered before.
If the bacterium is attacked again by this virus, this CRISPR DNA is transcribed, and RNA is transcribed from the CRISPR region. The produced crRNA (CRISPR RNA) then guides a ‘cutting enzyme’, Cas9, to the foreign virus’ DNA, damaging it, and preventing the virus from causing harm. It’s a basic immune system, and one which is very effective.
One of the overwhelming advantages of CRISPR-Cas9 editing compared to other technologies is how easily specificity of the system (i.e. which specific DNA base it is targeted to) can be changed; simply by changing the provided guide RNA in the CRISPR system, the Cas9 enzyme’s target site can be easily switched. This high level of site specificity combined with the efficacy of CRISPR-Cas9 gene editing gives CRISPR technology a clear advantage compared to previous gene editing technologies.
However, being a novel technology, CRISPR is not without current limitation. Currently, while cleavage can occur with high specificity, there is no known biological agent capable of carrying out specific DNA single nucleotide substitutions which would be desirable to treat human diseases driven by single DNA base changes. Professor Doudna discussed exciting on-going work to investigate adaptation of mechanisms known to exist to edit RNA in bacteria.
Despite limitations, some very promising clinical translation has already taken place. For example, the New England Journal of Medicine  reported the findings from two patients with ÃŸ-thalassemia and sickle cell anaemia who both have been successfully treated through a CRISPR-Cas9 edited bone marrow transplant. However, further clinical use is hampered by significant limitations on cell-specific targeting. Currently, all CRISPR-Cas9 editing for human use must take place outside of the body, which can be invasive and result in significant side-effects when the cells are returned to the patient. Professor Doudna discussed some of the innovative approaches being taken in pre-clinical research to potentially target specific tissue systems within the body, including the extra challenge of targeting the central nervous system for potential treatment of neurodegenerative disease.
There is also the issue of cost of treatment. CRISPR-Cas9 therapy is not cheap, and treatment in the NEJM published trials costs around a million dollars a patient. There is naturally then, a financial as well as a scientific barrier to more widespread clinical implementation, and Professor Doudna discussed how part of her current work at the Innovative Genomics Institute (ICG) at the University of California, Berkeley and the University of California, San Francisco, is to increase both accessibility and affordability of CRISPR-Cas9 therapy.
It is fitting in the ongoing pandemic that Professor Doudna also discussed how the high sequence specificity of CRISPR is being developed for use in COVID-19 diagnostic testing, allowing for a much more rapid testing than PCR alternatives. The fluorescence emitted by a ‘positive’ test could potentially even be detected using a smartphone camera, according to research published in Cell using a CRISPR-Cas13a system (one of many alternative Cas enzymes being studied) . As the pandemic continues, this has huge potential for the future of commonplace and convenient SARS-CoV2 testing.
Professor Doudna rounded off the talk with a Q&A session, with audience questions including those on risk of immunogenicity of human harnessing of a bacterial system, Professor Doudna’s own opinions on the biggest barriers to therapeutic implementation, and discussion of some of the ethical concerns of human gene editing, to name a few.
We would like to extend a huge thank you to Professor Doudna for a fantastic and engaging talk and discussion. We hope everyone who was able to attend the webinar at this heavily subscribed event is excited (if you weren’t already!) about the potential CRISPR technology holds for the future. The talk is available on the CPM YouTube channel and is a fascinating watch which we highly recommend checking out!
Written by Holly Eggington, Secretary of the Oxford University Personalised Medicine Society (OUPM) and DPhil student at St Anne’s College
1. Frangoul, H. et al. CRISPR-Cas9 Gene Editing for Sickle Cell Disease and Î²-Thalassemia. N. Engl. J. Med. (2021). doi:10.1056/nejmoa2031054
2. Fozouni, P. et al. Amplification-free detection of SARS-CoV-2 with CRISPR-Cas13a and mobile phone microscopy. Cell (2021). doi:10.1016/j.cell.2020.12.001