Author: Layal Liverpool Edited by: Burcu Anil Kirmizitas
Gene editing with CRISPR
Our genes are made up of a string of chemical DNA “letters” joined together to make “words” called amino acids, which are the building blocks of the proteins that make us. Gene editing refers to altering the DNA, for example by changing, deleting or inserting one or more chemical letters.
The discovery that a bacterial defence system against virus infection can be repurposed for gene editing in human cells has been without doubt one of the greatest achievements in modern science. This system, called CRISPR-Cas9, allows bacteria to retain a memory of viruses that have infected them in the past so they can respond to them more efficiently during later encounters.
The bacteria essentially cut and paste the virus DNA into a part of their own DNA called CRISPR. When they subsequently re-encounter the same virus, this allows them to recognise the virus DNA and chop it up with a pair of molecular scissors called Cas9.
Scientists soon recognised that this CRISPR-Cas9 system could be harnessed to cut genes inside human cells too. The Cas9 enzyme can be guided to specific human genes and used to edit them in a controlled way. This has exciting applications for the treatment of diseases.
Gene therapy vs Gene editing
The idea of manipulating genes to cure disease is not new. Gene therapy was first used to treat human disease as far back as 1990. Since then it has mainly been used in rare genetic diseases such as severe combined immunodeficiency (SCID), to provide a healthy gene where there is a faulty one.
So why has there been so much excitement about CRISPR and gene editing? Well, instead of simply delivering a healthy gene, it is now possible to equip cells with the tools to fix the faulty gene directly. These tools include the Cas9 enzyme and special guide molecules, which target it to the gene in question.
This idea has been tested in an animal model of the human disease Duchenne muscular dystrophy (DMD). Patients with this disease suffer from muscle deterioration and premature death. The cause is a faulty dystrophin gene, which leads to problems with muscle contraction. By using CRISPR to remove the faulty part of the dystrophin gene, scientists were able to reverse the disease in mice.
Traditional gene therapy has been successful in treating rare diseases caused by defects in a single gene. The beauty of gene editing is that it has applications for the treatment of a wider range of diseases. CRISPR has been used to erase HIV DNA from latently infected cells in the lab and to engineer immune cells that are more effective at killing cancer cells.
CRISPR in the clinic
Killing cancer cells is the goal of the first human trial using CRISPR, which began in China in October 2016 and is ongoing. CRISPR is being used to switch off a gene called PD-1, which usually suppresses anti-tumour immunity and allows cancer cells to persist. Disabling this gene will release the brakes in immune cells, allowing them to become activated and destroy the cancer.
New Scientist recently reported that as many as 20 human trials involving CRISPR gene editing will be underway soon. In the PD-1 trial mentioned above, cells are edited outside of the patient and then inserted back in, however it is also possible to edit cells directly inside the body. Another on-going trial is for a HPV treatment gel, which when applied should be able to specifically delete HPV DNA from infected cells in the body. Left unchecked, HPV can cause cells to become cancerous.
Potential problems and possible solutions
A key limitation of CRISPR is the potential for so-called off-target effects. This is basically when the Cas9 enzyme cuts the wrong gene, for example a gene that happens to look similar to the target gene. This is not a trivial issue as editing the wrong gene in a patient could have serious consequences. It is for this reason that scientists have been working hard to find ways of limiting the off-target effects of CRISPR, for instance by modifying the Cas9 enzyme so that it becomes 'fussier' about where it cuts. As with any new treatment, the benefits of CRISPR will ultimately have to be weighed against the potential risk of off-target effects.
As the technology develops and improves, the ethical concerns around gene editing will also need to be addressed. In reference to CRISPR and gene editing, John Dupre, chair of the Nuffield Council for Bioethics and Professor of Philosophy of Science at the University of Exeter told the BBC that “It is highly desirable to involve the ethical and regulatory considerations as early as possible in the development of a possible transformational technique.”
It has been over a year since scientists in the UK were first granted permission by the UK Human Fertilisation and Embryology Authority (HFEA) to edit genes in human embryos for research purposes. Since then several studies have been conducted and early results have found that CRISPR is more effective in normal embryos as compared with non-viable embryos that could not develop into children. However, further research will be necessary to determine whether the success rate of this technique is high enough to justify its use in the clinic.
Looking to the future
It is also worth emphasising that in addition to medical applications, CRISPR provides an invaluable tool for biomedical researchers who investigate cellular processes in health and disease. The number of CRISPR trials is growing and early results are promising. As this technology develops, it will be important for scientists, clinicians and ethicists to continue working closely in deciding how best to implement novel treatments. Gene editing with CRISPR and similar technologies offers a bright future not only for the treatment of single gene disorders but for a wide range of diseases including infectious diseases and cancer.