Introduction:
The field of sickle cell disease treatment has been advancing rapidly with the emergence of new therapies aimed at modifying the course of the disease and even the possibility of cures on the horizon. Until recently, the only established cure for sickle cell disease has been allogeneic stem cell transplantation. To maximize the long-term effectiveness of this therapy, it will be crucial to comprehend the various gene therapy in practice, distinguish their outcomes, and weigh the potential drawbacks.
What Is Sickle Cell Disease?
Sickle cell disease has been extensively studied for over a century, dating back to the first clinical report in 1910, which labeled it the first molecular disease. However, the progress in finding a cure for this SCD has been slow partly because SCD predominantly affects individuals in resource-limited areas and minority populations in wealthy nations. Additionally, the development of therapies that alter the disease has been sluggish, with only a single medication available until 2017.
SCD occurs as a result of the inheritance of at least a copy of sickle hemoglobin (HbS) and another copy of a gene encoding Hbs or another hemoglobin. HbS forms when a single E6V missense mutation in the beta-globin gene (HBB) replaces beta-6 glutamic acid with valine. On deoxygenation, HbS polymerizes, causing red blood cells to take on abnormal shapes and leading to various complications such as vaso-occlusion (a painful complication of sickle cell disease that occurs due to microvascular beds being occluded by sickle cells), irreversible organ damage, hemolysis, reduced quality of life, and premature death.
What Is the Cause of Sickle Cell Disease?
The fundamental cause of SCD pathology remains a single-point mutation that will produce an abnormal protein (HbS), which later undergoes hemoglobin polymerization within RBCs (red blood cells). The rate of HbS polymerization tends to vary widely and depends on multiple factors, including the amount of HbS per red blood cell, the amount of non-sickling hemoglobin per red blood cell, and the overall erythrocyte hemoglobin level. Therefore, having sufficient healthy normal adult hemoglobin (HbA) can prevent polymerization and lessen complications and symptoms, except in rare cases of severe psychological stress. Similarly, fetal hemoglobin (HbF) possesses an anti-sickling effect and can prevent or potentially eliminate hemoglobin polymerization when present in sufficient quantities within the erythrocyte.
What Is the Treatment of Sickle Cell Disease?
Until now, there have been four approved treatments for SCD. Hydroxyurea is the first approved medication used and has proven to reduce the frequency of vaso-occlusive crisis (VOC), lower stroke risk in a few patients, and improve the quality and length of life for many patients. However, Hydroxyurea (HU) is not universally accepted as a disease-modifying therapy because certain individuals still experience significant disease-related complications or struggle with drug adherence as a result of concerns regarding side effects, like intolerance and fertility issues. L-glutamine was approved as SCD treatment in 2017 and possesses an antioxidant property that can alleviate SCD and lessen the risk of VOC. However its long-term effectiveness is still debated. Latest medications like Voxelotor and Crizanlizumab have proved their efficiency by decreasing hemolysis and VOCs, respectively, but neither of them has a complete cure for SCD, and it requires lifelong treatment. These medications are still being researched for organ complications like pulmonary hypertension, avascular necrosis, etc.
What Are the Gene Therapy Techniques for Sickle Cell Disease?
1. Gene Addition Therapy: Gene addition therapy involves the introduction of a new gene, typically utilizing a viral vector for delivering to the stem cells a non-sickling globin. In this process, the original HbS gene remains unchanged, consequently ending up producing both the introduced hemoglobin and the original HbS. Numerous ongoing initiatives are employing lentiviral vectors (LVV) to host and transport new genes as part of therapy.
2. Gene Editing: Gene editing in the context of sickle cell disease is primarily associated with gene disruption. It involves using a guide molecule that precisely identifies and binds to a specific DNA segment and an enzyme to create double-stranded DNA breaks. This precise cut enables the creation of the DNA sequence with high accuracy, often resulting in deletion or insertions. This gene therapy typically focuses on a different region of DNA (distinct from HbS mutation) to enhance the production of HbF (fetal hemoglobin) while simultaneously reducing the HbS production. A common target is the BCL11A gene which negatively regulates HbF production, and gene therapy focuses on the deactivation of this HbF regulation, leading to an increase in its production.
3. Gene Silencing: Gene silencing is a method that regulates gene expression within a cell to prevent the synthesis of specific proteins. Like gene editing, this approach aims to inhibit the BC11A gene, leading to an increased HbF production while concurrently suppressing the production of HbS. Unlike gene editing, this gene therapy depends on viral vector delivery, like gene addition. It introduces an antisense molecule to mRNA (messenger RNA) to suppress the gene’s product without physically cutting the gene itself.
4. Gene Correction: Gene correction can be executed through various techniques, but typically, it involves the use of a guide RNA to pinpoint the mutation of interest, followed by editing with the concurrent introduction of template DNA containing the accurate sequence. This process guides homology-directed repair (HDR). Currently, this method is considered the least efficient, but ongoing research is done for gene correction. The efforts include direct base editing, DNA insertion, and prime editing. It is worth noting that this particular gene therapy is the sole approach currently seeking to eliminate HbS production while simultaneously introducing non-sickling hemoglobin.
Conclusion:
The upcoming frontier in gene therapy is anticipated to be gene correction therapy, which combines elements of both gene editing and gene addition. It is crucial to recognize that all forms of gene therapy, including gene addition, carry genuine and potential risks. In the case of gene addition, a primary concern revolves around the possibility of unintended cellular proliferation or the development of malignancies due to insertions at promoter sites. Recent findings from ongoing gene therapy research highlight the necessity for extended monitoring and systematic data collection using standardized data elements. This ensures that trial outcomes can be compared among different studies and against the natural progression of sickle cell disease.
