By Easha Vigneswaran
Since the advent of CRISPR-Cas9 technology for gene editing, the medical world has been met with a new way of targeting many chronic genetic diseases. One of these includes sickle cell disease, a genetic blood disorder that results in defective haemoglobin. Whilst treatments exist to minimise the effects of the disease on people, none of them is a ‘cure’ for those carrying the genetic mutation. Following on from its discovery, CRISPR has been applied to many diseases including sickle cell disease (and its associated conditions), with the aim of improving the quality of life and ideally treating a disease that affects millions all over the world.
Sickle cell disease (SCD) is caused by a single mutation in the β globin gene causing a glutamate to valine amino acid substitution. This ultimately results in the haemoglobin being unable to polymerise correctly, therefore forming malformed red blood cells (sickle-shaped RBC) that are often deoxygenated. The disease has many symptoms such as haemolysis, pain and organ damage ultimately leading to lower life expectancy. As it is an inherited autosomal recessive condition, the gene is only inherited if both parents possess the mutated gene. Those that are carriers but unaffected (due to inheritance of only one copy) still do pose the risk of passing the trait onto their offspring.1
The current therapies and drugs that exist work to ameliorate the symptoms of SCD. The most common form of treatment is the use of blood transfusions and medications that are used to relieve and reduce those symptoms. Hydroxyurea is a cytotoxic drug used to increase foetal haemoglobin levels but this drug only has limited therapeutic effects.2 For those suffering from extreme cases of sickle cell diseases, they are eligible for haematopoietic stem cell transplants from sibling donors. However, with the limited availability of donors and the risk of rejection, this is not a viable long-term solution for sufferers of monogenic disease.3
This then brings us towards the modern era of research where scientists are now able to exploit gene editing technologies to treat genetically inherited disorders which are perfect candidates for CRISPR therapy. The primary target for using the CRISPR-Cas9 technology on SCD is to repair the mutation that exists in adult haemoglobin by correcting the gene. Using a mechanism called ‘homology-directed repair’, a donor template is introduced into the mutated area of the β globin gene so when the cell naturally repairs, it does so using the donor template DNA that has been engineered into the site via CRISPR-Cas9. This approach has been used carried out at Stanford University and has shown promising results with the aim of moving to clinical trials.4
Other gene targets that have been identified include the BCL11A gene which is responsible for the production of foetal haemoglobin. There is evidence that children post-birth who should suffer from SCD do not due to higher levels of foetal haemoglobin (HbF). Therefore, the second use of CRISPR uses a gene knockout technique as a way of increasing HbF persistence in adult erythrocytes by suppressing the gene that stops its production. This is done by the deletion of sections on the β globin gene or the introduction of SNPs in the γ globin gene. Another method is to target haematopoietic stem and progenitor cells, specifically the erythroid-specific enhancer region, to restore γ globin synthesis and foetal haemoglobin production increases. This is done by deleting the BCL11A gene in order to investigate if it is possible to increase HbF levels in SCD sufferers. Research suggests that targeting the foetal haemoglobin as opposed to adult haemoglobin may have more success.5
A current application of this method is currently being used in the CTX001 clinical trial conducted by CRISPR Therapeutic sand Vertex Pharmaceuticals. Bone marrow is harvested from the patients and gene-edited to inactivate the BC11A gene. The edited haematopoietic cells are then reintroduced into the patient for the proliferation of cells with increased foetal Hb levels. Since the first successful treatment on a patient, more people have participated in the trial. As far as the data suggests, the majority of these once SCD sufferers all appear to have minimal SCD symptoms indicating the potential sustained success of this therapy.5
Aside from CRISPR gene editing, other treatment methods are being investigated. One example of this is an anti-sickling gene therapy treatment that uses ‘LentiGlobin BB305’. This is a self-inactivating (SIN) lentiviral vector that contains the genetic code for a variant of the human β globin gene where SNPs have been introduced to introduce an ‘anti-sickling’ gene. Trials in one patient showed no sickle cell symptoms 15 months following the treatment.6 Another lentiviral gene therapy uses a zinc-finger nuclease that has been engineered against the γ globin causing it to be reactivated thereby increasing foetal haemoglobin production. The primary issue with a lot of these viral vector therapies is the efficacy and safety to be used in humans. The biggest danger is with immunogenicity and the potential for them to induce an inflammatory response in the donor which can lead to an attack on organ tissue.7
To conclude, it appears there is massive potential for treatments of sickle cell disease. Considering the clinical trial successes, it is highly probable that more and more of these treatments will be approved by regulating bodies and could soon be introduced as a treatment option for a larger group of people. There are inevitable limitations to using these treatments and the obvious ethical questions of gene editing remain. However, considering the lack of long term treatment solutions, it is evident why scientists want to explore the potential of CRISPR gene editing for SCD.
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