Clustered Regularly Interspaced Short Palindromic Repeats/ CRISPR-associated protein 9 (CRISPR/Cas9), a revolutionary gene-editing technology has been used to correct a single mutation in the beta subunit of haemoglobin (an oxygen-carrying protein) that causes sickle cell disease, a painful inherited blood disorder. In a different approach, the foetal form of haemoglobin (HbF) – which has the correct haemoglobin sequence – is reactivated in adults using CRISPR/Cas9 (brand name Casgevy). A single treatment with Casgevy alleviated the frequent pain crises in 96% of patients for over 18 months, who went for several months without blood transfusions. Almost 98% of patients who completed Casgevy treatment experienced significant relief from the painful symptoms and avoided hospital admissions for 3.5 years. Casgevy has been approved for NHS patients and may cure sickle cell disease.
CRISPR/Casp9, a breakthrough in gene-editing
In 2020, Emmanuelle Charpentier and Jennifer Doudna were awarded the Nobel Prize for chemistry for developing a powerful gene-editing tool which they called CRISPR/Cas9 for altering the DNA of many organisms with high precision and accuracy. Their outstanding work has revolutionised research in biomedicine and the life sciences and promises to lead to the treatment and cure of many human diseases, including inherited genetic disorders.
CRISPR/Cas9 (CRISPR for short) acts as a molecular ‘scissors’ to cut the DNA at the precise location where the changed DNA sequence is to be introduced. Changes may be additions, deletions, or substitutions of DNA. CRISPR has many applications in biology and medicine including structure and function analysis of proteins, screening for novel drug targets, diagnostics, and identifying pathogenic genes, in addition to correcting inherited genetic defects.
Sickle cell disease and current treatment options
Inherited genetic disorders and many other diseases are frequently associated with one or a small number of mutations in key proteins leading to the loss of their function. For example, in healthy people, position 6 of the amino acid sequence in the beta-globin subunit of haemoglobin is glutamic acid (coded by GAG), whereas in sickle cell disease, GAG has undergone mutation to GTG which codes for valine. This mutation alone is sufficient to generate haemoglobin S (HbS), the deoxygenated sickle cell disease-associated form of haemoglobin. The affected cells have a reduced lifespan and are compromised in their ability to transport oxygen. They display a rigid, distorted sickle shape that clogs blood vessels, disrupts circulation, causes pain and anaemia, and results in organ damage over time.
Before the advent of CRISPR, the only realistic treatment options for sickle cell disease were frequent blood transfusions every 3-5 weeks or bone marrow transplantation, which carries the risk of rejection and is only available for about 15% of patients. Blood transfusions give only temporary relief, while bone marrow transplants have a low success rate, risk of infections, and require a matching donor. On the other hand, CRISPR technology does not require a bone marrow donor and has a higher success rate.
CRISPR corrects genetic defects that cause sickle cell disease
There are two approaches to CRISPR-mediated re-expression of the correct haemoglobin gene (HbB). In one approach, haemopoietic stem cells are withdrawn from the patient’s bone marrow. These cells are CRISPR-edited to restore glutamic acid (coded by GAG) at position 6, then infused back into the patient. Because the pre-existing HbS sickle cells have a reduced lifespan, they are quickly outgrown by the CRISPR-engineered cells. Clinical trials of this method have shown considerable promise.
The second method relies on the unexpected observation that some individuals with the GTG (valine) mutation in HbS have no sickle cell symptoms. This was found to be due to a mutation which inactivates the BCLIIA gene whose normal function is to keep the foetal form of haemoglobin (HbF) switched off in adulthood. By employing CRISPR to knock out the BCLIIA gene, the re-expression of HbF with the correct sequence at position 6 occurs.
In late 2023, the American Food and Drug Administration (FDA) approved the world’s first CRISPR-mediated gene therapy for the alleviation of a human disease – the reactivation of foetal HbF under the brand name Casgevy. This single treatment alleviated the frequent pain crises in 96% of subjects for over 18 months, who also went several months without blood transfusions in the hospital.
There are around 17,000 people suffering from sickle cell disease in the UK alone, and it is particularly common among people of African and Caribbean descent. In late 2024, Casgevy was approved for use on the NHS in the UK for an initial cohort of 50 sickle cell patients, giving them the prospect of a cure for this devastating disease. Clinical trials revealed the remarkable power of a single treatment with Casgevy. Almost 98% of subjects avoided being admitted to hospital for over 3 years after starting Casgevy therapy, and they experienced significant relief from the unbearable symptoms, restoring their freedom and independence.
Challenges, future directions, and downsides of CRISPR
There are ongoing CRISPR-based gene-editing experiments and clinical trials for a variety of genes other than those coding for sickle cell disease. Funding for Casgevy has recently been approved for the NHS to treat beta-thalassemia, a blood disorder related to sickle cell disease that is caused by mutations in beta-globin that lead to the underproduction of haemoglobin. Unlike sickle cell disease, the red blood cells in beta-thalassemia do not form sickle shapes.
CRISPR therapy is a gruelling process involving bone marrow replacement with side effects that include low levels of platelets and white blood cells, abdominal pain, headache, and vomiting. But it is a single treatment and the side effects are nowhere near as serious compared with sickle cell disease. Low levels of off-target mutations have been reported with Casgevy, raising concerns about possible long-term health complications of these unforeseen errors, but their consequences are so far unclear.
In the UK, the current cost to the NHS for sickle cell disease therapy is around £1.65mn for each patient, which is partially offset by not having to use powerful drugs to manage symptoms or make frequent hospital visits for blood transfusions. The NHS is in negotiations with the supplier of Casgevy, so the price will come down and more people will benefit. However, there are at present insufficient health centres and hospitals around the world with the equipment and expertise to perform CRISPR gene therapy, but this is starting to change. One patient who successfully underwent CRISPR treatment for severe sickle cell disease said “It’s like being born again” and “wow, I can’t believe I lived with that.” He and others are being closely monitored to determine whether in the long term their treatment can be considered a cure for sickle cell disease.
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