what makes only black people prone to sickle cell?

Question

I often wonder; why is it that only black people suffer from sickle cell? can it be completely treated? is it because whites don't suffer from this malaise that cure has not been found?

Answer 1

Sickle cell disease is a general term for a group of genetic disorders caused by sickle hemoglobin (Hgb S or Hb S). In many forms of the disease, the red blood cells change shape upon deoxygenation because of polymerization of the abnormal sickle hemoglobin. This process damages the red blood cell membrane, and can cause the cells to become stuck in blood vessels. This deprives the downstream tissues of oxygen and causes ischemia and infarction. The disease is chronic and lifelong. Individuals are most often well, but their lives are punctuated by periodic painful attacks. In addition to periodic pain, there may be damage of internal organs, such as stroke. Lifespan is often shortened. It is common in people from parts of the world where malaria is or was common, especially in West Africa or in descendents of those peoples. Sickle cell disease can occur in any individual of any color or ethnicity, however.

"Sickle cell anemia" is the name of a specific form of sickle cell disease in which there is homozygosity for the mutation that causes Hgb S. Other forms of sickle cell disease include sickle-hemoglobin C disease, sickle beta-plus-thalassemia, and sickle beta-zero-thalassemia. Unlike sickle cell anemia, these other forms of sickle cell disease are compound heterozygous states in which the person has only one copy of the mutation that causes Hgb S and one copy of another abnormal hemoglobin gene. It is important to know that "sickle cell anemia" is the proper name of a specific type of "sickle cell disease", and that "sickle cell disease" is a non-specific term. Because the different forms of sickle cell disease are quite different, one must be sure to specify the exact form of the disease in question. The disease was originally called sickle-cell anemia but this name has fallen out of favor in medical contexts because anemia is not the only symptom of the disease.

Signs and symptoms

Patients with sickle cell anemia have steady-state or baseline anemia that varies in severity, with hemoglobin levels of 6-9 g/dl typical. Reticulocyte counts are elevated, reflecting new red blood cells replacing the rapidly destroyed older cells - red blood cell life span is markedly reduced in this disease. Often, the white blood cell and platelet counts are elevated, and these cells may contribute to vaso-occlusion.

Vaso-occlusive crisis

Vaso-occlusive crisis is caused by sickle shaped red blood cells that obstruct capillaries and restrict bloodflow to an organ, resulting in ischemia, pain, and organ damage.

Because of its narrow vessels and function in clearing defective red blood cells, the spleen is frequently affected. It is usually infarcted before the end of childhood in individuals suffering from sickle cell anemia. This autosplenectomy increases the risk of infection from encapsulated organisms; preventive antibiotics and vaccinations are recommended for those with such asplenia. Liver failure may also occur with time.

Bone is also a common target of vasoocclusive damage, especially when the bone is particularly weight-bearing. Such damage may result in avascular necrosis (especially of the femur) and bone deterioration. The pain experienced by sickle-cell patients is also due to the bone ischemia.

A recognized type of sickle crisis is the acute chest crisis, a condition characterized by fever, chest pain, hard breathing, and pulmonary infiltrate on chest x-ray. Given that pneumonia and intra-pulmonary sickling can both produce these symptoms, the patient is treated for both conditions. Treatment consists of admission, oxygen, close monitoring, and intravenous antibiotics.

Other sickle-cell crisis

Aplastic crisis. An acute worsening of the patient's baseline anemia producing pallor, tachycardia, and fatigue. This crisis is triggered by parvovirus B19, which directly affects erythropoiesis (production of red blood cells). Reticulocyte counts drop dramatically during the illness and the rapid turnover of red cells leads to the drop in hemoglobin. Most patients can be managed supportively; some need blood transfusion.
Splenic sequestration crisis. An acute, painful enlargement of the spleen. Management is supportive, sometimes with blood transfusion.

Complications

Sickle cell anemia can lead to various complications, including:

Overwhelming post (auto)splenectomy infection - due to functional asplenia, caused by encapsulated organisms such as Streptococcus pneumoniae and Haemophilus influenzae. Daily penicillin prophylaxis is the most commonly used treatment during childhood with some hematologists continuing treatment indefinitely. Patients benefit today from routine vaccination for
Haemophilus influenzae, Pneumococcus and Meningococcus.
Stroke - progressive vascular narrowing (occlusion) can prevent oxygen from reaching the brain, leading to stroke; cerebral infarction occurs in children, and cerebral hemorrhage in adults.
Cholelithiasis and cholecystitis (gallstones) - prolonged hemolysis may lead to excessive bilirubin production and precipitation, leading to gallstones
Avascular necrosis (aseptic bone necrosis) of the hip
Decreased immune reactions due to hyposplenism (malfunctioning of the spleen)
Priapism and infarction of the penis (in men)
Osteomyelitis (bacterial bone infection) - salmonella is noted much more commonly than in the general population, but staphylococcus is still the most common pathogen.
Opioid addiction (see below)

Diagnosis

Attacks are diagnosed clinically, i.e. there is no gold standard diagnostic test. Hemolysis (anemia and jaundice) is often present, although for painful crises the diagnosis depends essentially on how the patient describes the pain.

Abnormal hemoglobin forms are detected on hemoglobin electrophoresis, a form of gel electrophoresis on which the various types of hemoglobin move at varying speed. Sickle cell hemoglobin (HbSS) and Hemoglobin C with sickling (HbSC)--the two most common forms--can be identified from there. Genetic testing is rarely performed.

Pathophysiology

Sickle cell anemia is caused by a mutation in the β-globin chain of hemoglobin, replacing glutamic acid with less polar valine at the sixth position of the β chain . The association of two wild type α-globin subunits with two mutant β-globin subunits forms hemoglobin S, which polymerises under low oxygen conditions causing distortion of red blood cells and a tendency for them to lose their elasticity.

New erythrocytes are quite elastic, which allows the cells to deform to pass through capillaries. Often a cycle occurs because as the cells sickle, they cause a region of low oxygen concentration which causes more red blood cells to sickle. Repeated episodes of sickling causes loss of this elasticity and the cells fail to return to normal shape when oxygen concentration increases. These rigid red blood cells are unable to flow through narrow capillaries, causing vessel occlusion and ischemia.

Genetics

Sickle-cell disease is inherited in the autosomal recessive pattern, depicted above.The allele responsible for sickle cell anemia is autosomal recessive. A person who receives the defective gene from both father and mother develops the disease; a person who receives one defective and one healthy allele remains healthy, but can pass on the disease and is known as a carrier. If two parents who are carriers have a child, there is a 1-in-4 chance of their child developing the illness and a 1-in-2 chance of their child just being a carrier.

The gene defect is a known mutation of a single nucleotide (A to T) of the β-globin gene, which results in glutamic acid to be substituted by valine at position 6. Hemoglobin S with this mutation are referred to as HbS, as opposed to the more normal adult HbA. The genetic disorder is due to the mutation of a single nucleotide, from a GAG to GTG codon mutation. This is normally a benign mutation, causing no apparent effects on the secondary, tertiary, or quaternary structure of hemoglobin. What it does allow for, under conditions of low oxygen concentration, is the polymerization of the HbS itself. In people heterozygous for HbS (carriers of sickling hemoglobin), the polymerization problems are minor. In people homozygous for HbS, the presence of long chain polymers of HbS distort the shape of the red blood cell, from a smooth donut-like shape to ragged and full of spikes, making it fragile and susceptible to breaking within capillaries. Carriers only have symptoms if they are deprived of oxygen (for example, while climbing a mountain) or while severely dehydrated. For those afflicted with the disease, however, vasocclusive events can be a painful part of life (normally they occur 0.8 times per year per patient). This occurs when HbS becomes deoxygenated it undergoes an abnormal change in shape and consistency which can lead to a change in the shape and plasticity in the cell wall of the RBC, an occurrence called sickling.

The sufferers of the illness have a reduced life span. It is believed that carriers (sickle cell trait) are relatively resistant to malaria. Since the gene is incompletely recessive, carriers have a few sickle red blood cells at all times, not enough to cause symptoms, but enough to give resistance to malaria. Because of this, heterozygotes have a higher fitness than either of the homozygotes. This is known as heterozygote advantage.

The malaria parasite has a complex life cycle and spends part of it in red blood cells. In a carrier, the presence of the malaria parasite causes the red blood cell to rupture, making the plasmodium unable to reproduce. Further, the polymerization of Hb affects the ability of the parasite to digest Hb in the first place. Therefore, in areas where malaria is a problem, people's chances of survival actually increase if they carry sickle cell anemia.

Due to the above phenomenon, the illness is still prevalent, especially among people with recent ancestry in malaria-striken areas, such as Africa, the Mediterranean, India and the Middle East. In fact, sickle-cell anemia is the most common genetic disorder among African Americans; about 1 in every 12 is a carrier.

The evolution of sickle-cell anaemia is probably an example of Baldwinian evolution, whereby humans modify their environment and thus change the selective pressures. As humans in tropical areas in Africa and elsewhere developed agriculture and animal husbandry, they expanded the niche for Anopheles mosquitoes that could transmit the malaria parasite.

It is interesting that in the USA, where there is no endemic malaria, the incidence of sickle cell anaemia amongst people of African descent is much lower than in West Africa and falling. Without endemic malaria from Africa, the condition is purely disadvantageous, and will tend to be bred out of the affected population. See the Price equation article for a simplified mathematical model of the genetic evolution of sickle cell anemia.

Treatment

Febrile illness

Children with fever are screened for bacteremia i.e. complete blood count, reticulocyte count and blood culture taken. Younger children (varies from center to center) are admitted for intravenous antibiotics while older children with reassuring white cell counts are managed at home with oral antibiotics. Children with previous bacteremic episodes should be admitted.

Painful (vaso-occlusive) crises

Most patients with sickle cell disease have intensely painful episodes called vaso-occlusive crises. The frequency, severity, and duration of these crises vary tremendously, however. Painful crises are treated symptomatically with analgesics; pain management requires opioid administration at regular intervals until the crisis has settled. For milder crises a subgroup of patients manage on NSAIDs (such as diclofenac or naproxen). For more severe crises most patients require inpatient management for intravenous opioids; patient-controlled analgesia (PCA) devices are commonly used in this setting. Diphenhydramine is effective for the itching associated with the opioid use.

Acute chest crisis

Management is similar to vaso-occlusive crises with the addition of antibiotics (usually a third generation cephalosporin), oxygen supplementation for hypoxia, and close observation. Should the pulmonary infiltrate worsen or the oxygen requirements increase, simple blood transfusion or exchange transfusion is indicated. The latter involves the exchange of a significant portion of the patients red cell mass for normal red cells, which decreases the percent hemoglobin S in the patient's blood.

Hydroxyurea

The first approved drug for the causative treatment of sickle cell anemia, hydroxyurea, was shown to decrease the number and severity of attacks in a study in 1995 (Charache et al) and shown to increase survival time in a study in 2003. This is achieved by reactivating fetal hemoglobin production in place of the hemoglobin S that causes sickle cell anaemia. Hydroxyurea had previously been used as a chemotherapy agent, and there is some concern that long-term use may be harmful, but it is likely that the benefits outweigh the risks.

Future treatments

Various approaches are being sought for preventing sickling episodes as well as for the complications of sickle-cell disease. Other ways to modify Hb switching are being investigated.

Nipoxan (or Hemoxin), previously Niprisan or Nix-0699, is a phytochemical (ethanol/water extract of Piper guineenses seeds, Pterocapus osum stem, Eugenia caryophyllum fruit, and Sorghum bicolor leaves) has received orphan drug status in the USA and Europe.

Gene therapy is under investigation.

Situation of carriers

People who are known carriers of the disease often undergo genetic counseling before they have a child. A test to see if an unborn child has the disease takes either a blood sample from the unborn or a sample of amniotic fluid. Since taking a blood sample from a fetus has risks, the latter test is usually used.

After the mutation responsible for this disease was discovered in 1979, the U.S. Air Force required African American applicants to test for the mutation. It dismissed 143 applicants because they were carriers, even though none of them had the condition. It eventually withdrew the requirement, but only after a trainee filed a lawsuit.

History

The cause of this collection of clinical findings was unknown until the description of the sickle cells in 1910 by the Chicago cardiologist and professor of medicine James B. Herrick (1861-1954) whose intern Ernest Edward Irons (1877-1959) found "peculiar elongated and sickle shaped" cells in the blood of Walter Clement Noel, a 20 year old first year dental student from Grenada after Noel was admitted to the Presbyterian Hospital in December 1904 suffering from anemia. Noel was readmitted several times over the next three years for "muscular rheumatism" and "bilious attacks" while an undergraduate. Noel completed his studies and returned to capital of Grenada (St. George's) to practice dentistry. He died of pneumonia in 1916 and is buried in the Catholic cemetery at Sauteurs in the north of Grenada [1].

The disease is very occasionally called "Herrick's syndrome" for this reason.

The disease was named "sickle cell anemia" by Vernon Mason in 1922. In retrospect some elements of the disease had been recognized earlier: a paper in the Southern Journal of Medical Pharmacology in 1846 described the absence of a spleen in the autopsy of a runaway slave. The African medical literature reported this condition in the 1870's where it was known locally as ogbanjes ('children who come and go') because of the very high infant mortality in this condition. And a history of the condition tracked reports back to 1670 in one Ghanian family [2]. Also, the practice of using tar soap to cover blemishes caused by sickle cell sores was prevalent in the African American community.

The origin of the mutation that led to the sickle cell gene was initially thought to be in the Arabian peninsula, spreading to Asia and Africa. It is now known, from evaluaton of chromosome structures, that there have been at least four independent mutational events, three in Africa and a fourth in either Saudi Arabia or central India [3]. These independent events occurred between 3000 and 6000 generations ago, approximately 70000-150000 years.

Answer 2

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Answer 3

Sickle cell is a recessive trait that is found in the black population. The mutation first happened and began spreading amongst black people. In order for a baby to get the sickle cell disease, it would have to get the gene from it from both parents. This makes it really unlikely that a white baby could get it--maybe in a hundred years or so you will start seeing sickle cell in white people (or, if you consider anyone with a drop of black blood in their ancestry black, you will never see it), because you won't see this until interracial babies have interracial babies for a few generations, so that a baby with a black ancestors on both sides of the family could be born with white skin.

Another reason sickle cell is so prevelant, and hasn't been weeded out through selection like other undesireable mutations, is that there is one benefit of sickle cell that has helped to keep it in the population. People with sickle cell can't catch malaria, so at times in history when malaria would sweep through an area, it would sicken or kill normal-blooded people while not affecting people with sickle-cell, making it so a larger percentage of the population would have the sickle-cell disease, and pass it to their children. Also, since it is recessive, both parents could be normal (even though they both carry the gene), and pass this gene to their child who would get sickle cell. A person would have no idea whether they or their mate is a carrier of the disease, unless they understood basic genetics and had family histories of the disease.

Why don't they cure it? The answer is partly technology, but mostly money. The technology to fix genetic disorders is really in its infancy. The human genome project was started in the late nineties, and has given answers that will lead us to be able to treat genetic disorders, but we are many, many years from finding cures for them. Treatments, however, have taken a giant leap forward, but still require extensive research to figure out a drug that might work, and then the drug has to go through extensive testing before it becomes available.

This new knowledge about treating genetic disorders is being used now and is bringing exciting new cures, but of course the money for the research comes from corporations that are trying to make money. Since sickle cell affects such a small part of the entire population, the drug companies find that it doesn't make financial sense spend millions to develop a drug that only a few will use. They can make more money curing more 'popular' diseases. Its sad that no one will help sick people even if there isn't money to be made, but it's just the reality of the situation.

Answer 4

It's not only black people, but mostly, and it's a good case of seeing how evolution works, on a human timeframe.

In Europe, say, sickle cell is detrimental to health, so Europeans that have it tend to have less children than the average (because they die younger, or they don't want to have sick children, etc). Therefore, the gene doesn't spread much.

In Eastern Africa, though, sickle cell is *good* for you, because it protects you against malaria. So, even if you still die younger because of the sickle cell, your neighbours die even more young because of malaria. So people with sickle cell tend to have more children than the rest, and the gene spreads.

Answer 5

Sickle cell disease is a general term for a group of genetic disorders caused by sickle hemoglobin (Hgb S or Hb S). In many forms of the disease, the red blood cells change shape upon deoxygenation because of polymerization of the abnormal sickle hemoglobin. This process damages the red blood cell membrane, and can cause the cells to become stuck in blood vessels. This deprives the downstream tissues of oxygen and causes ischemia and infarction. The disease is chronic and lifelong. Individuals are most often well, but their lives are punctuated by periodic painful attacks. In addition to periodic pain, there may be damage of internal organs, such as stroke. Lifespan is often shortened. It is common in people from parts of the world where malaria is or was common, especially in West Africa or in descendents of those peoples. Sickle cell disease can occur in any individual of any color or ethnicity, however. ...

Answer 6

It's one of those diseases which is caused by a recessive gene, so you have to have both a father and a mother with the gene (both of whom could have one copy of the gene and be healthy) in order to get the disease.
Having just one copy of the gene gives you some protection against malaria, so it makes sense for people who live in areas where malaria is common to be more likely to have the gene. This is mostly in hot climates, where black skin is also more common - thus the two things tend to go together.
Because it's a genetic disease, it's difficult to find a cure for it - I believe most treatments are palliative (they treat the symptoms, not the disease).
Whether more attention would be paid to curing it if it were more common amongst whites is a difficult question to answer - the drug companies only research drugs which will make them money, so maybe they think there's too little profit in it for them?

Answer 7

South Asians (Indians),Italians,Greeks and other non-Black peoples can be highly prone to sickle cell anemia

Answer 8

Sugarless chewing gum can suppress your appetite in a very pinch.

Answer 9

both are roughly the same except when it comes to protein. MOST veggies include a respectable amount of protein; fruits does not.

Answer 10

Follow an increasingly low fat protein/green vegetable diet

Answer 11

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