Gene editing, once a concept of science fiction alone, is now on the cusp of real-life implementation for the first time and 2023 will likely see the global approval of the first CRISPR-based therapy, exa-cel.
Vertex, who produce the therapy, has regulatory submissions underway in the USA and EU for sickle cell anaemia and β thalassaemia – the patient’s own cells are edited outside the body to produce high levels of foetal haemoglobin, which can perform the function that their own adult haemoglobin is unable to.
It’s a significant milestone as, although a number of gene therapies are already in use, these mostly focus on adding a functional copy of a faulty gene, whereas CRISPR-based systems have the potential to edit and correct faulty genes, preventing them from causing disease.
The application of this technology is clear in monogenic diseases and its potential is now being investigated for idiopathic diseases without a known single genetic cause, such as Parkinson’s disease (PD). Given that no disease-modifying therapies currently exist for this severely life-altering condition, the potential impact of this research is huge for healthcare professionals and patients.
Gene therapy today can be separated into four approaches. Gene inhibition involves silencing a faulty gene at the post-transcriptional level in order to prevent it from causing disease. Gene replacement, as discussed above, introduces a working copy of a gene into a patient whose own copy of that gene is faulty and does not produce an essential protein. The third approach is genetic modulation of immune cells, whereby new genes are introduced into immune components, such as T cells or T-cell receptors, to stimulate an immune response against a disease – examples include CAR T-cell therapies. These three gene therapy approaches have been used in various guises for some time, while the fourth approach, gene editing, is the newest kid on the block.
Gene editing describes the modification of a patient’s genome to treat a disease, using engineered or bacterial nucleases. CRISPR–Cas9, the system used by exa-cel, has revolutionised this process, making it faster and cheaper than ever before. Jennifer Doudna and Emmanuelle Charpentier won the Nobel Prize in 2020 for the discovery of CRISPR–Cas9, which is derived from the CRISPR–Cas adaptive immune system used by bacteria to degrade genetic material from invading viruses and plasmids. CRISPR– Cas9 therapies comprise two key components: a guide RNA strand, which can be engineered to target almost any DNA sequence in the genome and introduce site-specific changes; and the Cas9 endonuclease, a large multidomain protein that cleaves DNA. In addition to exa- cel, late-stage preclinical research is ongoing for CRISPR–Cas9 therapies for Duchenne muscular dystrophy, haemoglobinopathies and hereditary tyrosinaemia type 1.
Gene editing is a concept that has elicited discomfort and opposition from many who believe the potential for its misuse is too great. The case of the Chinese scientist jailed in 2019 for using CRISPR–Cas9 to genetically modify human embryos, resulting in three live births, has done little to assuage these fears. In this instance, a frameshift mutation was introduced to disable the CCR5 gene in an attempt to confer resistance against HIV infection; the ethical issues surrounding cases like this stray into the realm of eugenics, aside from the obvious safety issues.
However, there is staggering potential for this technology to be used for good. In PD, familial PD (for which causative genetic aberrations are known) represents only around 10% of cases, but has provided genetic targets for research into how gene editing might be used in the more common, idiopathic cases. Editing or up-/ down-regulating key genes known to become altered in PD is a promising avenue of research. Current strategies include improving dopamine bioavailability in the brain, inducing neuronal regeneration by targeting neurotrophic factors, editing neuromodulation-modifying genes in the subthalamic nucleus, or reducing the production of α-syn, the synuclein responsible for inducing dopaminergic neuron death.
Although idiopathic PD likely has multiple contributing factors not traceable to a single gene, it is still possible that patients may gain therapeutic benefit if key biological pathways are restored by gene editing. Animal model research is ongoing and greater knowledge is needed around the best genetic targets and safest delivery systems to avoid off-target effects. However, it is safe to say that CRISPR technology now offers an efficient, accurate and potentially safe toolkit for gene editing, and a unique opportunity to correct key biological systems in patients living with PD and other life-changing conditions.
References are available on request.
