Hemoglobin From Patients With Sickle Cell Disease example essay topic

We feel that this report looks a lot better single-spaced. A Brief History of Sickle Cell Disease Sickle Cell Disease in African Tradition Sickle cell disease has been known to the peoples of Africa for hundreds, and perhaps thousands, of years. In West Africa various ethnic groups gave the condition different names: Ga tribe: Chwechweechwe Faut e tribe: Nwiiwii Ewe tribe: Nuidudui Twi tribe: Ahotutuo Sickle Cell Disease in the Western Literature Description of Sickle Cell Disease In the western literature, the first description of sickle cell disease was by a Chicago physician, James B. Herrick, who noted in 1910 that a patient of his from the West Indies had an anemia characterized by unusual red cells that were "sickle shaped". Relationship of Red Cell Sickling to Oxygen In 1927, Hahn and Gillespie showed that sickling of the red cells was related to low oxygen.

Deoxygenation and Hemoglobin In 1940, Sherman (a medical student at Johns Hopkins) noted a birefringence of deoxygenate d red cells, suggesting that low oxygen altered the structure of the hemoglobin in the molecule. Protective Role of Fetal Hemoglobin in Sickle Cell Disease Janet Watson, a pediatric hematol ist in New York, suggested in 1948 that the paucity of sickle cells in the peripheral blood of newborns was due to the presence of fetal hemoglobin in the red cells, which consequently did not have the abnormal sickle hemoglobin seen in adults. Abnormal Hemoglobin in Sickle Cell Disease Using the new technique of protein electrophoresis, Linus Pauling and colleagues showed in 1949 that the hemoglobin from patients with sickle cell disease is different than that of normals. This made sickle cell disease the first disorder in which an abnormality in a protein was known to be at fault. Amino Acid Substitution in Sickle Hemoglobin In 1956, Vernon Ingram, then at the MRC in England, and J.A. Hunt sequenced sickle hemoglobin and showed that a glutamic acid at position 6 was replaced by a valine in sickle cell disease. Using the known information about amino acids and the codons that coded for them, he was able to predict the mutation in sickle cell disease.

This made sickle cell disease the first known genetic disorder. Preventive Treatment for Sickle Cell Disease Hydroxyurea became the first (and only) drug proven to prevent complications of sickle cell disease in the Multicenter Study of Hydroxyurea in Sickle Cell Anemia, which was completed in 1995. How Does Sickle Cell Cause Disease The Mutation in Hemoglobin Sickle cell disease is a blood condition primarily affecting people of African ancestry. The disorder is caused by a single change in the amino acid building blocks of the oxygen-transport protein, hemoglobin. This protein, which is the component that makes red cells "red", has two subunits.

The alpha subunit is normal in people with sickle cell disease. The -subunit has the amino acid valine at position 6 instead of the glutamic acid that is there normally. The alteration is the basis of all the problems that occur in people with sickle cell disease. The schematic diagram shows the first eight-of the 146 amino acids in the -globin subunit of the hemoglobin molecule. The amino acids of the hemoglobin protein are represented as a series of linked, colored boxes.

The lavender box represents the normal glutamic acid at position 6. The dark green box represents the valine in sickle cell hemoglobin. The other amino acids in sickle and normal hemoglobin are identical. The molecule, DNA (deoxyribonucleic acid), is the fundamental genetic material that determines the arrangement of the amino acid building blocks in all proteins. Segments of DNA that code for particular proteins are called genes. The gene that controls the production of the -subunit of hemoglobin is located on one of the 46 human chromosomes (chromosome #11).

People have twenty-two identical chromosome pairs (the twenty-third pair is the unlike X and Y-chromosomes that determine a person's sex). One of each pair is inherited from the father, and one from the mother. Occasionally, a gene is altered in the exchange between parent and offspring. This event, called mutation, occurs extremely infrequently. Therefore, the inheritance of sickle cell disease depends totally on the genes of the parents. If only one of the -globin genes is the "sickle" gene and the other is normal, the person is a carrier.

The condition is called sickle cell trait. With a few rare exceptions, people with sickle cell trait are completely normal. If both -globin genes code for the sickle protein, the person has sickle cell disease. Sickle cell disease is determined at conception, when a person acquires his / her genes from the parents. Sickle cell disease cannot be caught, acquired, or otherwise transmitted. The hemoglobin molecule (made of alpha and -globin subunits) picks up oxygen in the lungs and releases it when the red cells reach peripheral tissues, such as the muscles.

Ordinarily, the hemoglobin molecules exist as single, isolated units in the red cell, whether they have oxygen bound or not. Normal red cells maintain a basic disc shape, whether they are transporting oxygen or not. The picture is different with sickle hemoglobin. Sickle hemoglobin exists as isolated units in the red cells when they have oxygen bound. When sickle hemoglobin releases oxygen in the peripheral tissues, however, the molecules tend to stick together and form long chains or polymers. These polymers distort the cell and cause it to bend out of shape.

When the red cells return to the lungs and pick up oxygen again, the hemoglobin molecules resume their solitary existence (the left of the diagram). A single red cell may traverse the circulation four times in one minute. Sickle hemoglobin undergoes repeated episodes of polymerization and depolymerization. This "Ping-Pong" alteration in the state of the molecules damages the hemoglobin and ultimately the red cell itself. Polymerized sickle hemoglobin does not form single strands. Instead, the molecules group in long bundles of 14 strands each that twist in a regular fashion, much like a braid.

These bundles self-associate into even larger structures that stretch and distort the cell. An analogy would be a water ballon that formed ice sickles that extended and stretched the ballon. The stretching of the rubber of the ballon is similar to what happens to the membrane of the red cell. Despite their imposing appearance, the forces that hold these sickle hemoglobin polymers together are very weak. The abnormal valine amino acid at position 6 in the -globin chain interacts weakly with the globin chain in an adjacent sickle hemoglobin molecule. The complex twisting, 14-strand structure of the bundles produces multiple interactions and cross-interactions between molecules.

On the other hand, the weak nature of the interaction opens one strategy to treat sickle cell disease. Some types of hemoglobin molecules, such as that found before birth (fetal hemoglobin), block the interactions between the hemoglobin S molecules. All people have fetal hemoglobin in their circulation before birth. Fetal hemoglobin protects the unborn and newborns from the effects of sickle cell hemoglobin. Unfortunately, this hemoglobin disappears within the first year after birth. One approach to treating sickle cell disease is to rekindle production of fetal hemoglobin.

The drug, Hydroxyurea induces fetal hemoglobin production in some patients with sickle cell disease and improves the clinical condition of some patients. The Sickle Red Cell The schematic diagram shows the changes that occur as sickle or normal red cells release oxygen in the microcirculation. The upper panel shows that normal red cells retain their biconcave shape and move through the microcirculation (capillaries) without problem. In contrast, the hemoglobin polymerizes in sickle red cells when they release oxygen, as shown in the lower panel. The polymerization of hemoglobin deforms the red cells.

The problem, however, is not simply one of abnormal shape. The membranes of the cells are rigid due in part to repeated episodes of hemoglobin polymerization / depolymerization as the cells pick up and release oxygen in the circulation. These rigid cells fail to move through the microcirculation, blocking local blood flow to a microscopic region of tissue. Amplified many times, these episodes produce tissue hypoxia (low oxygen supply). The result is pain, and often damage to organs. The damage to red cell membranes plays an important role in the development of complications in sickle cell disease.

Robert Hebbel at the University of Minnesota and colleagues were among the first workers to show that the heme component of hemoglobin tends to be released from the protein with repeated episodes of sickle hemoglobin polymerization. Some of this free heme lodges in the red cell membrane. The iron in the center of the heme molecule promotes formation of very dangerous compounds, called oxygen radicals. These molecules damage both the lipid and protein components of the red cell membrane. As a consequence, the membranes become stiff. Also, the damaged proteins tend to clump together to form abnormal clusters in the red cell membrane.

Antibodies develop to these protein clusters, leading to even more red cell destruction (hemolysis). Red cell destruction or hemolysis causes the anemia in sickle cell disease. The production of red cells by the bone marrow increases dramatically, but is unable to keep pace with the destruction. Red cell production increases by five to ten-fold in most patients with sickle cell disease. The average half-life of normal red cells is about 40 days. In-patients with sickle cell disease, this value can fall to as low as four days.

The volume of "active" bone marrow is much expanded in-patients with sickle cell disease relative to normal in response to demands for higher red cell production. The degree of anemia varies widely between patients. In general, patients with sickle cell disease have hematocrits that are roughly half the normal value (e. g., about 25% compared to about 40-45% normally). Patients with hemoglobin SC disease (where one of the -globin genes codes for hemoglobin S and the other for the variant, hemoglobin C) have higher hematocrits than do those with homozygous Hb SS disease.

The hematocrits of patients with Hb SC disease run in low- to mid-thirties. The hematocrit is normal for people with sickle cell trait. How Do People Get Sickle Cell Disease Sickle cell disease is an inherited condition. The genes received from one's parents before birth determine whether a person will have sickle cell disease. Sickle cell disease cannot be caught or passed on to another person. The severity of sickle cell disease varies tremendously.

Some people with sickle cell disease lead lives that are nearly normal. Others are less fortunate, and can suffer from a variety of complications. How Are Genes Inherited At the time of conception, a person receives one set of genes from the mother (egg) and a corresponding set of genes from the father (sperm). The combined effects of many genes determine some traits (hair color and height, for instance). One gene pair determines other characteristics. Sickle cell disease is a condition that is determined by a single pair of genes (one from each parent).

Inheritance of Sickle Cell Disease The genes are those which control the production of a protein in red cells called hemoglobin. Hemoglobin binds oxygen in the lungs and delivers it to the peripheral tissues, such as the liver. Most people have two normal genes for hemoglobin. Some people carry one normal gene and one gene for sickle hemoglobin. This is called "sickle cell trait". These people are normal in almost all respects.

Problems from the single sickle cell gene develop only under very unusual conditions. People who inherit two genes for sickle hemoglobin (one from each parent) have sickle cell disease. With a few exceptions, a child can inherit sickle cell disease only if both parents have one gene for sickle cell hemoglobin. The most common situation in which this occurs is when each parent has one sickle cell gene.

In other words, each parent has sickle cell trait. Figure 1 shows the possible combination of genes that can occur for parents each of whom has sickle cell trait. Figure 1. (ABOVE) Inheritance of sickle genes from parents with sickle cell trait. As shown in the graphic, the couple has one chance in four that the child will be normal, one chance in four that the child will have sickle cell disease, and one chance in two that the child will have sickle cell trait. A one-in-four chance exists that a child will inherit two normal genes from the parents.

A one-in-four chance also exists that a child will inherit two sickle cell genes, and have sickle cell disease. A one-in-two chance exists that the child will inherit a normal gene from one parent and a sickle gene from the other. This would produce sickle trait. These probabilities exist for each child independently of what happened with prior children the couple may have had. In other words, each new child has a one-in-four chance of having sickle cell disease. A couple with sickle cell trait can have eight children, none of whom have two sickle genes.

Another couple with sickle trait can have two children each with sickle cell disease. The inheritance of sickle cell genes is purely a matter of chance and cannot be altered. Do Factors Other Than Genes Influence Sickle Cell Disease Sickle cell disease is quite variable in itself. Other blood conditions can influence sickle cell disease, however. One of the most important is thalassemia. One form of thalassemia, called -thalassemia, reduces the production of normal hemoglobin.

A person with one normal hemoglobin gene and one thalassemia gene has thalassemia trait (also called thalassemia minor). Parents who have sickle cell trait and thalassemia trait have one chance in four of having a child with one gene for sickle cell disease and one gene for -thalassemia (Figure 2). This condition is sickle -thalassemia. The severity varies. Some patients with sickle -thalassemia have a condition as severe as sickle cell disease itself. People of Mediterranean origin who have a sickle condition most often have sickle -thalassemia.

Figure 2. (BELOW ON LAST PAGE) Inheritance of hemoglobin genes from parents with sickle cell trait and thalassemia trait. As illustrated, the couple has one chance in four that the child will have the genes both for sickle hemoglobin and for thalassemia. The child would have sickle -thalassemia.

The severity of this condition is quite variable. The nature of the thalassemia gene (o or +) greatly influences the clinical course of the disorder. Another disorder that interacts with sickle cell disease is "hemoglobin SC disease". The abnormal hemoglobin C gene is relatively harmless.

Even people with two hemoglobin C genes have a relatively mild clinical condition. When hemoglobin C combines with hemoglobin S, the result is "hemoglobin SC disease". On average, hemoglobin SC disease is milder than sickle cell disease. However, some patients with hemoglobin SC disease have a clinical condition as severe as any with sickle cell disease.

The reason for the marked variability in the clinical course of hemoglobin SC disease is unknown. We do know that the tendency of hemoglobin C to produce red cell dehydration is a major reason that the combination of hemoglobins S and C is so problematic. Figure 3. (ABOVE) Inheritance of hemoglobin genes from parents with sickle cell trait and hemoglobin C trait. As illustrated, the couple has one chance in four that the child will have the genes both for sickle hemoglobin and for hemoglobin C. The child would have hemoglobin SC disease. Most patients with hemoglobin SC disease have a milder condition than occurs with sickle cell disease (two sickle genes).

Unfortunately, some patients run a clinical course that is undistinguishable from sickle cell disease. Are There Tests That Can Tell Me Whether I Have Sickle Cell Trait The answer is yes. Routine "blood counts" commonly performed in doctors' offices do not give hints about the presence of sickle cell trait. The blood counts of most people with sickle cell trait are normal. Only a special test, called a "hemoglobin electrophoresis" indicates reliably whether a person has sickle trait. In addition, the hemoglobin electrophoresis will detect hemoglobin C and -thalassemia.

How Can I Be Tested for Sickle Cell Trait Most large hospitals and clinics can perform routine hemoglobin electrophoresis. Smaller laboratories send the test to commercial firms for testing. If you are concerned about the possibility of having sickle cell trait, you should speak with your doctor. Overview Everyone with sickle cell disease shares the same gene mutation. A thymine replaces an adenine in the DNA encoding the -globin gene. Consequently, the amino acid valine replaces glutamic acid at the sixth position in the -globin protein product.

The change produces a phenotypically recessive characteristic. Most commonly sickle cell disease reflects the inheritance of two mutant alleles, one from each parent. The final product of this mutation, hemoglobin S is a protein whose quaternary structure is a tetramer made up of two normal alpha-polypeptide chains and two aberrant's-polypeptide chains. The primary pathological process leading ultimately to sickle shaped red blood cells involves this molecule. After deoxygenation of hemoglobin S molecules, long helical polymers of HbS form through hydrophobic interactions between the's-6 valine of one tetramer and the -85 phenylalanine and -88 leucine of an adjacent tetramer.

Deformed, sickled red cells can occlude the microvascular circulation, producing vascular damage, organ infarcts, painful crises and other such symptoms associated with sickle cell disease. Although everyone with sickle cell disease shares a specific, invariant genotypic mutation, the clinical variability in the pattern and severity of disease manifestations is astounding. In other genetic disorders such as cystic fibrosis, phenotypic variability between patients can be traced genotypic variability. Such is not the case, however, with sickle cell disease. Physicians and researchers have sought explanations of the variability associated with the clinical expression of this disease.

The most likely causes of this inconstancy are disease-modifying factors. I have reviewed the role of some of these factors, and tried to ascertain the clinical importance of each. Fetal Hemoglobin Augmented post-natal expression of fetal hemoglobin is perhaps the most widely recognized modulator of sickle cell disease severity. Fetal hemoglobin, as its name implies is the primary hemoglobin present in the fetus from mid to late gestation.

The protein is composed of two alpha-subunits and two gamma-subunits. The gamma-subunit is a protein product of the -gene cluster. Duplicate genes duplicate upstream of the -globin gene encodes fetal globin. Fetal hemoglobin binds oxygen more tightly than does adult hemoglobin A. The characteristic allows the developing fetus to extract oxygen from the mother's blood supply.

After birth, this trait is no longer necessary and the production of the gamma-subunit decreases as the production of the -globin subunit increases. The -globin subunit replaces the gamma-globin subunit in the hemoglobin tetramer so that eventually adult hemoglobin replaces fetal hemoglobin as the primary component red cells. HbF levels stabilize during the first year of life, at less than 1% of the total hemoglobin. In cases of hereditary persistence of fetal hemoglobin, that percentage is much higher. This persistence substantially ameliorates sickle cell disease severity. Mechanism of Protection Two properties of fetal hemoglobin help moderate the severity of sickle cell disease.

First, HbF molecules do not participate in the polymerization that occurs between molecules of deoxyHbS. The gamma-chain lacks the valine at the sixth residue to interact hydrophobic ally with HbS molecules. HbF has other sequence differences from HbS that impede polymerization of deoxyHbS. Second, higher concentrations of HbF in a cell infer lower concentrations of HbS.

Polymer formation depends exponentially on the concentration of deoxyHbS. Each of these effects reduces the number of irreversibly sickle cells (ISC). Hemoglobin F Levels and Amelioration of Sickle Cell Disease The level of HbF needed to benefit people with sickle cell disease is a key question to which different studies supply varying answers. Bailey examined the correlation between early manifestation of sickle cell disease and fetal hemoglobin level in Jamaicans. They concluded that moderate to high levels of fetal hemoglobin (5.4-9.7% to 39.8%) reduced the risk for early onset of dactylics, painful crises, acute chest syndrome, and acute splenic sequestration. Platt examined predictive factors for life expectancy and risk factors for early death (among Black Americans).

In their study, a high level of fetal hemoglobin ( 8.6%) augured improved survival. Kosh y et al. reported that fetal hemoglobin levels above 10% were associated with fewer chronic leg ulcers in American children with sickle cell disease. Other studies, however, suggest that protection from the ravages of sickle cell disease occur only with higher levels of HbF. In a comparison of data from Saudi Arabs and information from Jamaicans and Black Americans, Perrine et al. found that serious complications occurred only 6% to 25% as frequently in Saudi Arabs as North American Blacks. In addition mortality under the age of 15 was 10% as great among Saudi Arabs. Further, these patients experienced no leg ulcers, reticulocyte counts were lower and hemoglobin levels were higher on average.

The average a fetal hemoglobin level in the Saudi patients ranged between 22-26.8%. This is more than twice that reported in studies mentioned above. Kar et al. compared patients from Orissa State, India to Jamaican patients with sickle cell. These patients also had a more benign course when compared with Jamaican patients. The reported protective level of fetal hemoglobin in this study was on average 16.64%, with a range of 4.6% to 31.5%. Interestingly, -globin locus haplotype analysis shows that the Saudi HbS gene and that in India have a common origin (see below).

These studies suggest that the level of fetal hemoglobin that protects against the complications of sickle cell disease depend strongly on the population group in question. Among North American blacks, fetal hemoglobin levels in the 10% range ameliorate disease severity. The higher average level of fetal hemoglobin could contribute to the generally less severe disease in Indians and Arabs. Another study that suggests only a small role at best for fetal hemoglobin as a modifier of sickle cell disease severity was reported by El-Hazmi. The subjects were Saudi Arabs in whom a variety of symptoms associated with sickle cell disease were assessed to form a "severity" index.

The author concluded that among his patients, no correlation existed between HbF and the severity index. However, his analysis has a fundamental flaw. El-Hazmi failed to examine the effect of HbF on each of these symptoms individually. Their important information and an association between fetal hemoglobin levels specific disease manifestations could be concealed in his data. However, the study reinforces the conclusion that fetal hemoglobin levels most likely work in conjunction with other moderating factors to determine clinical severity in-patients with sickle cell disease. Alpha-Thalassemia Concurrent alpha-thalassemia has also been examined as a modifier of sickle cell disease severity.

Alpha-thalassemia, like sickle cell disease, is a genetically inherited condition. The loss of one or more of the four genes encoding the alpha globin chain (two each on chromosome 16) produces alpha-thalassemia. A gene deletion most commonly is at fault. The deletion results from unequal crossover between adjacent alpha-globin genes during the prophase I of meiosis I. Such a crossover leaves one gamete with one alpha-gene and the other gamete with three alpha genes. Upon fertilization the zygote can have 2, 3, 4, or 5 alpha genes depending on the make up of the other parental gamete.

In people of African descent, the most common haploid gamete of this type is alpha-thal-2 in which there is one deletion on each of the number 16 chromosomes in the patient. Heterozygote's for this allele, therefore, have three alpha genes (one alpha gene on one of the number 16 chromosomes, two alpha genes on the other). Embury et al. (1984) examined the effect of concurrent alpha-thalassemia and sickle cell disease. Based on prior studies, they proposed that alpha-thalassemia reduces intra erythrocyte HbS concentration, with a consequent reduction in polymerization of deoxyHbS and hemolysis.

They investigated the effect of alpha gene number on properties of sickle erythrocytes important to the hemolytic and rheological consequences of sickle cell disease. Specifically they looked for correlations between the alpha gene number and irreversibly sickled cells, the fraction of red cells with a high hemoglobin concentration (dense cells), and red cells with reduced deformability. The investigators found a direct correlation between the number of alpha-globin genes and each of these indices. A primary effect of alpha-thalassemia was reduction in the fraction of red blood cells that attained a high hemoglobin concentration. These dense cells result from potassium loss due to acquired membrane leaks.

The overall deformability of dense R BCs is substantially lower than normal. This property of alpha-thalassemia was confirmed by comparison of red cells in people with or without 2-gene deletion alpha-thalassemia (and no sickle cell genes). The cells in the nonthalassemic individuals were denser than those from people with 2-gene deletion alpha-thalassemia. The difference in median red cell density produced by alpha-thalassemia was much greater in-patients sickle cell disease.

Reduction in overall hemoglobin concentration due to absent alpha genes is not the only mechanism by which alpha-thalassemia reduces the formation of dense and irreversibly sickled cells. In reviewing the available literature, Embry and Steinberg suggested that alpha-thalassemia moderate's red cell damage by increasing cell membrane redundancy. This protects against sickling-induced stretching of the cell membrane. Potassium leakage and cell dehydration would be minimized. These two papers by Embury et al. give some insight into the moderation of sickle cell disease severity by alpha thalassemia. Some deficiencies exist, nonetheless.

The first paper makes no mention of the patient pool. Unspecified are the number of patients used, their ethnicity, or their state of health when blood samples were taken. This information would help establish the statistical reliability of the data, and its applicability across patient groups. Despite these limitation, the work provides important insight into the mechanisms by which alpha-thalassemia ameliorates sickle cell disease severity. Ballas et al reached different conclusions regarding alpha thalassemia and sickle cell disease than did Embury et al.

They reported that decreased red blood cell deformability was associated with reduced clinical severity of sickle cell disease. Patients with more highly deformable red cells had more frequent crises. They also found that fewer dense cells and irreversible sickle cells correlated inversely with the severity of painful crises. Like Embury et al., Ballas and colleagues found alpha thalassemia was associated with fewer dense red cells.

In addition, Ballas' group found that alpha thalassemia was associated with less severe hemolysis. However they reached no clear conclusion concerning alpha gene number and deformability of RBC except to note that the alpha thalassemia was associated with less red cell dehydration. The two studies are not completely at odds. Both state that concurrent alpha-thalassemia reduces hemolytic anemia. They agree that this occurs through reduction in the number of dense cells, a number directly related to the fraction of irreversibly sickled cells.

Embury et al. concludes that through this mechanism red blood cell deformability is increased. The investigators diverge, however, on the relationship to clinical severity of dense cells and rigid cells. Ballas et al. asserts that both the reduction of dense cells and rigid cells contribute to disease severity. 32 c.