Analysis of Potential Treatments for Sickle-Cell Anemia, Or Drepanocytosis, in Adults
Cameron D. Clarke, Cobb Research Laboratory, Howard University
This research paper discussed the clinical and evolutionary history of sickle-cell disease, also known as sickle-cell anemia, or drepanocytosis. It noted the genetic causes and physiological effects of sickle-cell disease, and the evolutionary and environmental factors involved in the disease’s emergence, finding that its proliferation was likely the result of a selective sweep of an adaptation of the red blood cells that had a secondary protective effect against malaria in heterozygous individuals. This also mentioned two of the possible treatment methods that could be used to eliminate or mitigate sickle-cell disease and its symptoms. The first treatment method analyzed was therapy of a patient with sickle-cell disease to increase the production of healthy red blood cells containing fetal hemoglobin. The second involved a bone marrow transplant to replace the defective bone marrow. It was found that while both treatments are effective in reducing the severity of sickle-cell disease, only a bone marrow transplant has been conclusively shown to be able to cure the condition, and even then, has only undergone trial testing among children. However, there is currently preliminary clinical research being conducted on gene therapy to promote the body’s continued production of fetal hemoglobin, preventing sickle-cell hemoglobin S from even developing, and curing sickle-cell disease completely.
Keywords: sickle-cell disease, anemia, drepanocytosis, hemoglobin S, red blood cells, hemoglobin A, erythrocytes, hemoglobin F, bone marrow, gene therapy, hydroxyurea, recombinant erythropoietin
Sickle cell disease, also known as sickle-cell anemia or drepanocytosis, is a hereditary blood disorder, characterized by a genetic mutation in the gene that codes for hemoglobin in the red blood cells of the body. Hemoglobin is an iron-based metalloprotein that binds to molecules of oxygen in the capillaries of the lungs, and then carries them through the bloodstream, releasing the oxygen molecules into the somatic cells, and binding to carbon dioxide, which is released back into the lungs with each exhalation (Maton). This paper will discuss three (3) distinct varieties of hemoglobin: Hemoglobin A (HbA), the normal adult form of hemoglobin; hemoglobin S (HbS), the diseased variety of hemoglobin; and hemoglobin F (HbF), fetal hemoglobin. Sickle cell disease occurs as a result of a mutation at a single nucleotide (A to T) of the β-globin gene, which results in glutamic acid being substituted by valine at position 7 (position 6 under the historic nomenclature. This causes the protein to form Hemoglobin S (HbS) in its final conformation, instead of Hemoglobin A (HbA), normal adult hemoglobin. Under normal conditions, the mutation is generally benign, causing no apparent effects on the secondary, tertiary, or quaternary structures of hemoglobin in conditions of normal oxygen concentration. Under conditions of low oxygen concentration, however, this mutation allows for the polymerization of the HbS itself. When HbS is in conditions of low oxygen saturation, the hydrophobic residues of the valine (formerly glutamic acid) at position 7 of the beta chain in hemoglobin are able to associate with the hydrophobic patch, causing hemoglobin S molecules to aggregate and form fibrous precipitates (Figure 1). These precipitates form long, interlocking strands within the blood cells, elongating and distorting its shape, and causing them to take on the distinctive malformed “sickle-like” shape that gives the condition its name. These “sickle” cells are far less efficient in carrying oxygen than the ordinary rounded cells, additionally, after a cell is sickled, it loses much of its elasticity, becoming much more inflexible. This rigidity makes the cells much more likely to become trapped in the small openings of the capillaries and narrow blood vessels. As cells accumulate in the blocked vessels, they can cause ischemia (oxygen deprivation) and cell death in the affected areas, a complication called a sickle-cell crisis. Sickle-cell crises are often extremely painful, and potentially fatal if the obstruction occurs around a vital organ and causes organ damage or failure. Additionally, the deformation of the cells by the hemoglobin fibers reduces the integrity of the cell membrane, making the cells much more likely to lyse. This damage is severe, to the extent that while healthy red blood cells may typically function for 90–120 days, sickled cells only last 10–20 days before lysis (Maakaron). This rapidly accelerated rate of hemolysis is from where the condition derives its designation as an anemia; although the bone marrow creates red blood cells at increased volume compared to healthy humans, it is simply unable to compensate for the rate of cell destruction. Patients are often left with lower-than-normal levels of erythrocytes, and can exhibit symptoms common to general anemia sufferers, including feeling tired, weakness, shortness of breath, or a poor ability to exercise (Stedman).
Sickle-cell disease is an allelic disorder, located on the chromosome 11, at 11p15. It has an autosomal recessive pattern of inheritance, in that the condition will only present itself when the allele for sickle-cell disease is passed from both parents during reproduction. In individuals that are heterozygous for sickle-cell disease (one copy of the diseased allele), also called “carriers,” the sickle cell load is greatly reduced, and symptoms generally only appear after prolonged oxygen deprivation, or severe dehydration. Evolutionarily, the emergence and proliferation of the sickle-cell allele are likely linked to the disease’s protective effect against malaria, a far more expansive and lethal disease, found in a similar range (Figures 2 & 3). This is due to the sickling of the cells interfering with the complex lifecycle of the parasite that causes malaria, Plasmodium malariae. Plasmodium reproduces within the erythrocytes. In a carrier for sickle-cell disease, the presence of the malaria parasite causes the red blood cells with defective hemoglobin to rupture prematurely, making the Plasmodium parasite unable to reproduce. Further, the polymerization of the hemoglobin affects the ability of the parasite to digest hemoglobin in the first place. This reduced efficacy of Plasmodium leads to shorter and less severe infections among sickle-cell carriers. Therefore, in areas with endemic malaria, chances of survival actually increase among heterozygotes. However, this protective effect does not extend to sickle cell homozygotes, or people with the disease, since the premature lysis of infected cells is a common cause of sickle-cell crises. The heterozygote advantage, however, is enough to account for the persistence of sickle-cell disease in areas with high endemic malaria. Analysis of the disease’s genetic history via restriction endonuclease analysis found that it most likely arose spontaneously in several different areas, with a different variant of the mutation emerging in each one. These variants are known as Cameroon, Senegal, Benin, Bantu, and Saudi-Asian. This evidence also supports the hypothesis that the mutation that causes sickle cell arose and proliferated as a result of the widespread malaria that is endemic to the regions.
Fetal Hemoglobin Treatment
In addition to corroborative evidence of evolutionary pressure for the emergence of sickle cell, the restriction endonuclease analysis of the disease also found that in some regions, including Senegal and Saudi-Asia, the sickle-cell mutation has variants that are correlated with increased and persistent production of hemoglobin F, also known as fetal hemoglobin (HbF) (Lanzkron). Fetal hemoglobin is the type of hemoglobin that is produced in the human fetus during the last seven months of development in the uterus. Functionally, fetal hemoglobin differs little from adult hemoglobin, apart from the fact that it has a slightly higher oxygen affinity, and bonds more tightly, but still temporarily, to the oxygen molecules in the capillaries of the lungs (Berg). This is due to the mixture of oxygenated and deoxygenated blood in the maternal blood that is delivered to the fetus via the umbilical vein. Generally, within six months of birth, humans stop producing fetal hemoglobin, and begin producing adult hemoglobin (HbA). In most cases, the switch from fetal hemoglobin to adult hemoglobin is relatively inconsequential. However, since the structure and genetics of fetal hemoglobin differ from those of adult hemoglobin, it is not affected by the same genetic mutations that govern the production of adult hemoglobin. When fetal hemoglobin production is switched off after birth, normal children begin producing adult hemoglobin (HbA). Children with sickle-cell disease instead begin producing a defective form of hemoglobin called hemoglobin S. However, among children with the sickle-cell trait, where fetal hemoglobin remains the predominant form of hemoglobin after birth, the frequency and severity of sickle-cell crises decrease relative to those where fetal hemoglobin production has ceased. This protective effect was observed in variants of the sickle-cell mutation in Senegal and Saudi-Asia, where it is believed to be a secondary adaptation, blunting some of the acute complications of sickle cell disease. Fortunately, much progress has been made in a clinical application for this phenomenon.
In a landmark study in the New England Journal of Medicine, it was found that not only did treatment of patients with hydroxycarbamide (hydroxyurea), an antineoplastic (tumor-inhibiting) drug, increase the quantity of fetal hemoglobin in erythrocytes, but that it also appeared to break down cells that were likely to sickle, further decreasing the risk of vaso-occlusive sickle-cell crises. Additionally, a second study discovered that the treatment of sickle-cell disease with a combination therapy of hydroxycarbamide and recombinant erythropoietin (a hormone involved in red blood cell synthesis) further increased levels of HbF, and further reduced the frequency of acute sickle-cell complications (Rodgers). Even more significant, this treatment method was found to have no major adverse side effects, and so would likely significantly improve patient outcomes and quality of life.
Important to note, however, is that the study was only conducted on adults, so it is not currently clear how much the success would translate to interventions in children. Additionally, hydroxyurea treatment has only been shown to reduce the frequency of sickle-cell crises, not prevent them entirely. As such, it is only a partially effective treatment, not a cure. It also has not been tested in an attempt to resolve a currently occurring sickle-cell crisis, and so cannot be recommended as an emergency intervention. Finally, the researchers involved with the study noted that “there is concern that long-term hydroxyurea therapy may be carcinogenic or leukemogenic, because some other antineoplastic agents have such effects” (Charache). As with any medication, additional research is necessary in order to more completely understand the mechanism of hydroxyurea therapy’s function in preventing crises, and its long-term effects on individuals and their children. However, this treatment is already in use in patients suffering from sickle-cell disease, and so far has seen widespread success.
On the horizon, researchers have begun testing whether gene therapy to stimulate permanent production of HbF in humans is feasible in order to more permanently treat, or even cure, sickle-cell disease. Preliminary studies have already found that it is possible to completely cure mice of a variant of sickle-cell disease by using gene therapy, and studies in humans have shown promising results, to the extent that gene therapy has moved on to the early stages of clinical trials as a permanent cure for sickle-cell disease (Pawliuk) (Wilson).
Hematopoietic Stem Cell Transplantation
For all of the promising research on the horizon, however, a cure for sickle cell disease already exists. Hematopoietic stem cell transplantation (HSCT), also known as bone marrow transplantation, has been shown to permanently cure children of sickle cell disease when it is successful. Allogenic bone marrow transplants, the type used in sickle-cell procedures, are a high-risk procedure, usually involving two people: the (healthy) donor and the recipient (patient). In the procedure, the donor and the recipient must both be tested to determine if they have similar or identical variants of the human leukocyte antigen (HLA) system. This network of genes is responsible for an individual’s immune response to foreign cells and molecules. If the two individuals are incompatible, or too dissimilar, the transplantation will fail, as the recipient’s immune system will either attack the donor’s stem cells, or the donor’s stem cells will cause an infection in the recipient’s tissues, a condition called graft-versus-host disease . Even if the HLA patterns of the individuals match, in order to minimize the possibility of a rejection, the patient must be subjected to immunosuppressant treatment to destroy their immune response system. This immunosuppression is part of what makes the procedure so high-risk; patients with compromised immune systems are extremely vulnerable to infection, such that even a relatively mundane disease such as the common cold can be fatal.
Once the recipient is immunosuppressed, the donor is placed under general anesthesia, and the hematopoietic stem cells are removed from a large bone of the donor, typically the pelvis, through a large needle that reaches the center of the bone. This technique is referred to as a bone marrow harvest (Figure 6). The cells are then implanted into the patient’s own bones, and after a few weeks to allow the cells to multiply, the patient is removed from immunosuppressants, and the procedure is completed.
Although HSCT, due to its relatively high risk, is generally only attempted in people with conditions that are extremely life-threatening, recent advances in the safety and standardization of the procedure have made it safe enough to attempt in individuals with less-threatening conditions, such as sickle-cell anemia. In such cases, however, studies have only been conducted among children. In a recent study by the New England Journal of Medicine found that of 22 patients upon whom bone-marrow transplants were performed, 20 survived the procedure itself, and 16 were found to be free of sickle-cell disease in the months following the procedure, survival and event-free survival at four years of 91 percent and 73 percent, respectively. Although these results are promising, a mortality rate of 9%, and a failure rate of 27% among children in a disease with only a 10% mortality likelihood by age 20 is clearly unacceptable, so more advancements must be made, both in the calculations of the likelihood of rejection, and in the safety of the procedure itself, before hematopoietic stem cell transplantation can be considered a viable and efficacious treatment for sickle-cell disease.
Of the methods described, as a practical solution for patients with mild to moderate sickle-cell disease, with only occasional, infrequent vaso-occlusive crises, the combination therapy of hydroxyurea and recombinant erythropoietin seems to be the ideal currently available treatment, both in terms of relative safety and likelihood of success, and in terms of effectiveness. Of course, as elaborated, all of the treatments require further research to conclusively determine their long-term health risks and benefits, but currently the risks of hydroxyurea are the most widely studied and well known. The most promising of the treatments, however, remains gene therapy, which, unlike hydroxyurea therapy, promises to completely cure sickle-cell disease, and without the complex surgery and high-risk immunosuppression of hematopoietic stem cell transplantations. Unfortunately, as this method is still in its earliest stages of clinical trial, it may be years before it becomes available to the average sickle-cell patient.
Sickle-cell disease is a stigmatized, misunderstood, and life-altering condition, but it is no longer the death sentence it once was. Hopefully, with additional study, evolutionary and clinical research, and public health outreach, we can make sickle-cell disease no more than a nuisance, instead of a potentially debilitating condition.***
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