CRISPR’s use as a gene editing tool has been exciting biologists for a decade. Now, the first CRISPR therapy to be endorsed in the UK faces scrutiny by the FDA, the USA’s drug regulation agency.
Exa-cel (dubbed Casgevy in Europe), a new therapy developed by Vertex Pharmaceuticals and CRISPR Therapeutics, manipulates gene coding to reintroduce foetal haemoglobin into adult patients to offset the effects of a mutation in one of the genes encoding adult haemoglobin. This aims to sickle cell disease and beta-thalassemia patients. With a combined 360,000 new cases per year, the proposed therapy could be life-changing for millions and extremely profitable for its developer. It is the first CRISPR-derived therapy that is seeking FDA approval.
Sickle cell disease is caused by an E6V (glutamate to valine at the 6th position) mutation in the HBB gene, which encodes the beta subunit of adult haemoglobin. Adult haemoglobin is assembled from 4 protein subunits – 2 alpha and 2 beta subunits. In sickle cell patients, the altered haemoglobin structure tends to polymerise when oxygen is not bound. This changes the shape of a red blood cell into the characteristic sickle shape. Symptoms are caused by either blockage of blood vessels or the resultant lack of oxygen delivery to respiring tissues. These can include vaso-occlusive crises (pain and tissue damage caused by blood vessel blockage), anaemia, and swelling of hands and feet.
Beta-thalassemia describes several diseases that result from decreased or absent beta-globin, again due to mutations in the HBB gene. The severity of symptoms varies with the extent of the mutation and are either caused by a lack of oxygen in tissues or by the body’s compensatory mechanisms. These symptoms include anaemia, enlarged liver and spleen and dark urine.
Exa-cel introduces foetal haemoglobin into patients to reverse symptoms. Foetal haemoglobin is assembled from 2 alpha and 2 gamma subunits. During pregnancy, the different composition of foetal haemoglobin gives it a higher affinity for oxygen, so oxygen can be passed from the mother to the foetus via the umbilical cord. Foetal haemoglobin is almost all lost 6 months after birth. The genes HGB1 and HGB2 that encode foetal haemoglobin are suppressed after birth due to BCL11A gene activity. This new therapy seeks to block the expression of BCL11A using CRISPR-Cas9 gene editing. This aims to enhance oxygen transport through the body.
Clustered regularly interspaced short palindromic repeat (CRISPR) sequences were first discovered in bacteria in 1987. Their purpose wasn’t fully identified until They are found in bacteria that have fought off viral infections. Phages (viruses that infect bacteria) hijack the bacteria’s genetic machinery to replicate themselves. If bacteria survive the infection, they create a CRISPR array from short repeating sections of the phage DNA. In case of a repeat infection, the long CRISPR array is transcribed and processed to individual RNA molecules. This RNA acts as a guide for Cas9 (CRISPR-associated protein 9) to cleave the complementary DNA sequences of the invading phage, preventing infection and phage replication. This mechanism acts as a bacterial immune system. Jennifer Doudna and Emmanuelle Charpentier developed this as a gene editing tool and published their work in 2012, leading to a Nobel Prize in Chemistry in 2020.
Exa-cel uses CRISPR-Cas9 gene editing of haematopoietic stem cells (HSCs). They are found in bone marrow and differentiate into blood cells (including red blood cells). The therapy will cause new red blood cells in patients to produce foetal haemoglobin by suppressing the BCL11A gene. The stand-out statistic of their data is that 9 months after treatment, 31 of 32 had not had a vaso-occlusive crisis, compared to an average of 4 per year before treatment.
CRISPR-Cas9 is a true game-changer as, although there have been previous gene editing methods, CRISPR is more precise, efficient, and cost-effective. Previous methods were hampered by off-target effects, high costs and difficulty applying the techniques in certain cells and tissues.
Off-target effects do remain a concern for this therapy. A similar CRISPR treatment, also for sickle cell disease, caused acute myeloid leukaemia in two trial patients. Although better than its predecessors, Cas9 is not a perfect set of ‘molecular scissors’. If it causes a mutation elsewhere, this could have terrible side effects. Other limitations are that only 40 patients have taken part in trials, which has been considered too few to test such off-target effects. A broader issue is the availability of HSCs for transplants. The NHS states ‘many people will eventually find a donor in the registry’, which isn’t particularly reassuring.
If Exa-cel does get approval, many other CRISPR gene therapies may follow. Such treatments have been researched for other inherited conditions, including cystic fibrosis, neurodegenerative diseases, cancers, and HIV. This won’t happen overnight – Vertex has proposed following its trial patients for 15 years to monitor long-term effects. The regulatory process is slow and considered but if it does prevail in this case, CRISPR technology could improve quality and length of life for many.
