Introduction
Human health involves a multitude of pathological conditions that tend to develop either throughout life or have innate adverse health effects. For example, inherited diseases pose the greatest danger to the population because they can suppress the health of related individuals over time. At the same time, hereditary diseases are more difficult to control because their development is closely related to the disruption of the genetic functions of the body. On the other hand, such a set of pathological conditions has an increased academic interest because their study covers problems of both physiological and molecular-genetic spectrums. Among many hereditary diseases, special attention should be paid to sickle cell anemia, which manifests itself as a change in the shape of globular proteins. More specifically, during sickle cell anemia, the patient’s standard hemoglobin form undergoes abnormal mutational changes, causing the protein to take on an unnatural crystalline structure. This report aims to summarize and critically discuss the available scientific information regarding the biochemical side of sickle cell anemia, namely the implementation of mutagenic processes, metabolic pathways, and innovative ways of prevention and treatment.
A Short Introduction to Sickle Cell Anemia
Human blood is a valuable biological fluid, rich in many vital substances, among which the presence of hemoglobin. Typically, hemoglobin is a complex iron-containing tetramer with the ability to actively bind to oxygen (AMBOSS, 2021). In this way, hemoglobin actively transports oxygen through the tissues and cells of the human body because the nature of the chemical bond between the protein and oxygen is reversible. Hemoglobin itself does not migrate freely in the bloodstream in molecules but is instead included in the composition of red blood cells. In addition, as its name implies, hemoglobin contains an iron ion, which allows the four subunits of the protein to be combined into a single complex.
The prosthetic center of hemoglobin molecules, represented by divalent iron, realizes the strong binding of four polypeptide chains, among which α1, α2, β1, and β2 are distinguished (AMBOSS, 2021). Together, the subunits and the iron-containing core combine to form a tetrahedral form, with each of the four monomolecules having its active oxygen-binding center. Thus, one hemoglobin can transport up to eight oxygen atoms, or 1.34 mL of dissolved gas (Rizvi et al., 2017). Since the hemoglobin molecule is inherently complex, its biosynthesis in cells should also not be considered an exact and straightforward process.
The alpha- and beta-chains of primary globin are even produced in different chromosomes. For example, the alpha-chain genes of the future hemoglobin are localized in the short arm of the 16th chromosome, with a total cluster of up to 30,000 base pairs. In contrast, the beta-chain genes are encoded by a sequence of 438 base pairs and are localized in the short arm of the 11th chromosome (NCBI, 2021). Consequently, the extended site of total hemoglobin synthesis increases the likelihood of deleterious mutations that can lead to abnormal protein structures.
Sickle cell anemia is commonly referred to as mutagenic changes occurring directly in the beta-chain of globin: consequently, the location of such changes is due to chromosome 11. In short, in sickle cell anemia, the patient’s red blood cells contain an abnormal form of hemoglobin, HbS, and as a result, change their shape to sickle cell: Figure 1. This cell geometry prevents active oxygen binding, and, as a consequence, the protein is no longer able to transport vital molecules of the element through the blood. The patient’s tissues are severely hypoxic, and since the disease is congenital, delays in mental and physical development due to oxygen deficiency are most likely.
Mutagenesis
As it is known, DNA is a macromolecular polynucleotide sequence that is based on four nitrogenous bases: thymine, adenine, guanine, and cytosine. These bases are divided into two groups according to their chemical structure. Adenine and guanine make up the pyrimidine components of DNA, whereas thymine and cytosine are part of the purine bases (Ferry, 2019). Accordingly, complementary bonds of molecules from different groups are realized in forming hydrogen bonds that form the three-dimensional helical structure of DNA: thymine binds only to adenine, while cytosine can bind to guanine. In sickle cell anemia, the study of DNA structure is critical because this mutation affects the change in the nucleotide sequence in the eleventh chromosome. More specifically, a CTT triplet is localized in the short arm of this chromosome, which creates a copy, GAA, on the informational RNA during translation. This triplet can then be said to encode the glutamine amino acid, which forms the standard structure of hemoglobin (NIH, 2020). In a point mutation, thymine is replaced with adenine, and as a result, the triplet takes the form of CAT. As a result, the 6th position of the beta chain of normal hemoglobin has a valine instead of glutamic acid, making the total protein abnormal.
The Nature of Inheritance of the Mutation
Hereditary diseases can be transmitted either recessively or dominantly if the sex chromosomes are not involved. Sickle cell anemia belongs to the autosomal recessive type of inheritance, which means that the embryo must receive the recessive gene from both parents at once to manifest the active form of the pathology. However, it also means that the family of two carriers must have heterozygous offspring that are not sick themselves but can pass on the abnormal gene. For such children, it is fair to clarify that the development of sickle cell anemia is still possible in rare situations (Sickle cell anemia, 2019). Consequently, the patient’s history should include an accurate categorization of the disease, whether heterozygous or homozygous.
Biochemical Features of the Disease
A point change in a nucleotide in a triplet entails much more mutagenic transformations than coding for a new amino acid. Abnormal hemoglobin produced due to a point single-nucleotide mutation has significant biochemical differences from the standard protein structure. Primarily, it should be emphasized that there are two forms of hemoglobin in general, depending on the total oxygenation of the blood. If the erythrocyte does not transport oxygen, such hemoglobin is called deoxygenated. In this state, HbA does not bind oxygen molecules and therefore moves toward the heart. Since the structure of the beta-chain has changed, the properties of the entire hemoglobin macromolecule also change; namely, a sticky area is generated on its surface. The formation of such a site initiates the mechanism of polymerization of abnormal HbS due to which protein conglomerates are formed in the erythrocyte. Such clots have a gel-like consistency and are substantially less soluble (Maakaron, 2021). More specifically, if the concentration of abnormal protein exceeds 30 g/dL, HbS tend to aggregate into polymerase conglomerates. Such abnormal conglomerates can not only deform red blood cells, giving them a sickle shape but also provoke autohemolysis and tissue ischemia.
Although it happens that abnormal hemoglobins return to their former, non-sickle shape during oxygenation, this possibility is lost with the accumulation of the sickle effect. It is accurately known that between 5% and 50% of all patient’s red blood cells have a rigid sickle shape fixation and cannot be rehabilitated over time (Maakaron, 2021). In other words, erythrocytes are rendered incapable of assuming a biconvex shape, an(d the patient enters a state of severe sickle cell anemia. In addition, changes at the ionic level are also observed. Sodium-potassium adenosine triphosphatase is known to balance sodium and potassium ions through the release of the former into the external environment and the accumulation of the latter inside the cell (Waugh, 2019). This maintains the resting potential of the cell and controls cellular volume. During sickle cell anemia, the situation changes: sodium ions are actively accumulated while intracellular potassium concentration rapidly decreases. Disruption of adenosine triphosphatase also affects an active increase in the presence of calcium cations in the cytoplasm. This amount in sickle cell anemia increases about fourfold compared to normal, resulting in a stiffer erythrocyte membrane (Maakaron, 2021). In turn, this allows abnormal erythrocytes to retain their abnormal shape. It has been shown that a possible cause initiating the appearance of sickling in erythrocytes and the appearance of abnormal protein, in general, is the enhancement of adenosine signaling (Adebiyi et al., 2019). More specifically, there is an amplification of adenosine transmission through erythrocyte ADORA2B receptors, which leads to activation of the cellular AMPK protein kinase that controls cellular energy balance (Figure 2). In turn, this imbalance leads to increased production of 2,3-BPG and, consequently, increased polymerization processes of abnormal hemoglobins.
Another negative biochemical manifestation of sickle cell anemia is the increased adhesive capacity of abnormal red blood cells. In addition to the fact that HbS can stick together into conglomerates, abnormal red blood cells also have increased adhesiveness, causing them to attach to the surface of the endothelium (Zhang, 2019). In turn, endothelial adhesion leads to endothelial dysfunction, which becomes the cause of crises. Adhesion is further initiated by hypoxia, as dissolved oxygen deficiency rapidly reduces nitric oxide production. As a result, the lack of nitric oxides in the blood leads to vasoconstriction, which only accelerates the initiation of thrombosis and tissue ischemia. Finally, bound sickle-shaped erythrocytes on the endothelium produce large amounts of VLA-4 antigen, which is responsible for increasing the adhesive properties of leukocytes (Maakaron, 2021). Thus, blood vessels from within tend to a state of complete dysfunction.
In addition to the attachment of sickle cell erythrocytes to the endothelial surface, a severely adverse effect of sickle cell anemia is the intensification of hemolytic processes. More specifically, the driving forces behind erythrocyte hemolysis are not unequivocal. For example, sickle cells attach to the surfaces of macrophages, which in turn causes erythrophagocytosis (Hod, 2019). At the same time, it has been studied that when abnormal cells are oxygenated, their membrane fibers can break down, which also causes intravascular hemolysis. In other words, the biochemical features of erythrocyte adhesion in sickle cell anemia are a life-shortening severe factor for the patient.
Immune Features of the Disease
Noteworthy is the fact that the leading share of those exposed to the disease is the dark-skinned population, especially in regions endemic for malaria. It should be recalled that malaria is an acute infectious disease characteristic of southern countries in which a patient’s blood is infected with malarial plasmodium through an intermediate host in the form of a mosquito (White, 2018). In this case, the patient’s blood becomes infected with the pathogen, which gradually depresses it, which is manifested by fever, splenomegaly, hepatomegaly, and anemia. However, people suffering from sickle cell anemia have a peculiar resistance to infection by malarial plasmodium. In particular, the abnormal shape of the protein prevents free fixation and penetration of the unicellular inside the erythrocyte, resulting in the sickle-shaped red cells being an escape from malaria (Uyoga et al., 2019). Such claims are valid not only for homozygous patients but also for heterozygotes, in whom HbS is produced, but in markedly reduced amounts. Thus, carriers show partial resistance to malaria but may be affected by it. In general, this character of resistance is a sufficient explanation of the reason for the wide prevalence of abnormal gene mutation in such regions.
Pathophysiological Manifestations
Sickle cell anemia is an inherited disease that affects the human cardiovascular system. As has become apparent from the previous sections, in a carrier family, the probability of getting the disease does not exceed 25%, but if received, the individual’s life is severely threatened. It should be noted that the clinical manifestation of this disease becomes evident by the first months of the life of the newborn when the number of hemoglobin increases in their blood. In sickle cell anemia, the child’s sickle cell count reaches the 90% mark by the fourth month, indicating the potential for dangerous complications associated with hypoxia. In more detail, oxygen deficit in the central nerve fiber tissues leads to inhibition of cell differentiation and, consequently, developmental delays. The infant’s bone and cartilage formations also fail to develop since the lack of oxygen does not allow for a strong bone skeleton. Thus, a sick child often has acrocephaly, thickening of the frontal bones and lordosis, and kyphosis in uncharacteristic areas of the spine. Furthermore, it is evident that the degree of the disease closely correlates with the concentration of sickle cell erythrocytes in the blood: the higher their value, the more likely severe manifestations of sickle cell anemia.
In this regard, it is evident that this type of anemia requires careful medical care. Even seemingly minor factors may initiate complications. Stress conditions and even dehydration can lead to a hemolytic crisis in which the patient’s red blood cells are destroyed. At the same time, about thirty percent of all patients notice the development of autosplenectomy, during which spleen dysfunction is observed. In turn, this condition leads to a sharp decrease in the immune status of the patient and, as a result, more frequent cases of infection.
Diagnosis
Since the disease is hereditary, the first signals of its manifestation are weakness and swelling in the limbs, caused by pathological conditions in the peripheral bones. An accurate diagnosis can also be made by taking blood tests and a detailed morphological study of the hemoglobin structures later. In addition, because of the heredity of the disease, the therapist will need a family history of the disease since the recessive nature of sickle cell anemia implies the ancestral manifestation of the disease.
However, the best and earliest result can be obtained with a prenatal diagnosis of the disease. Specifically, DNA sampling from the chorion can be performed as early as 8-12 weeks of pregnancy, so the physician will have time to prepare the family (Short et al., 2018). The primary treatment aims to prevent infections from developing in the patient, so weekly doses of penicillin may be used (Fischer, 2018). Another therapy vector is to stimulate hematopoiesis processes to allow more red blood cells to be produced in the blood. For this purpose, hematologists often recommend folic acid, which is a natural enzyme (Williams et al., 2020). Thus, treatment of sickle cell anemia is possible and actively practiced even before the infant is born if diagnosed promptly.
Conclusion
In conclusion, special attention should be paid to sickle cell anemia among the severe hereditary diseases, which negatively affects the patient’s blood system. During mutations in the eleventh chromosome, a point nucleotide substitution occurs, replacing glutamic acid in the protein with valine. The change in the beta chain entails abnormalization of the total hemoglobin: it has increased adhesive properties. HbS tend to combine into polymers and attach to the endothelium, resulting in erythrocyte dysfunction. Treatment exists, but the potential for disease development must be detected as early as possible to be as effective as possible.
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