Sickle cell disease is a serious blood disorder that causes acute pain, severe anemia, infections, and vascular blockages that can lead to widespread organ damage and death. It is a genetic disease, the most common inherited blood disorder in the United States, where it occurs most often in African-Americans and Hispanics. Until the past few decades, most people with SCD did not live beyond young adulthood, but advances in treatment have improved and lengthened the lives of patients who have access to good medical care.
SCD affects millions of people worldwide, particularly those with African, Spanish, Mediterranean, and Indian ancestry. Some 120,000 infants are born with SCD every year worldwide. In the United States, approximately 1 in 500 African-Americans and 1 in 1,200 Hispanic Americans are born with SCD. Some 2 million Americans—including about 10 percent of the African-American population—carry one gene for SCD, the "sickle cell trait."
While bone marrow/stem cell transplantation appears to have successfully cured a small number of SCD patients, the vast majority of people with SCD are not candidates for this costly and high-risk procedure, meaning that there is not yet a common cure for the disease.
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The oxygen that we need to live is taken from the lungs to the rest of the body by red blood cells. A protein in the blood called hemoglobin carries the oxygen. In sickle cell disease, hemoglobin molecules stick to one another, distorting the shape of the blood cells.
Sickle cell disease is a group of inherited disorders caused by a defect in hemoglobin, the protein in red blood cells that transports oxygen from the lungs to the rest of the body and carries carbon dioxide back to the lungs.
Most people with SCD have a single defect on both copies of chromosome 11 that causes them to produce defective hemoglobin molecules, which bind together and form long, rodlike structures (polymers) when not carrying oxygen. This causes the red blood cells to become rigid and misshapen. While normal red blood cells are pliable, smooth, disk-shaped, and live for approximately 120 days in the bloodstream, red blood cells in SCD patients may have the shape of a crescent, or sickle, and typically survive for only about 10 to 20 days. Because of the abnormal cells' abbreviated life span, the body cannot replace them quickly enough, causing the blood to be chronically short of red blood cells, a condition called anemia.
These cells' distorted shape (and numerous other abnormalities) can prevent them from traveling through small blood vessels, causing them to accumulate and create blockages that deprive organs and tissues of oxygenated blood. Because this often painful process—known as vaso-occlusion—can damage tissue and vital organs, it can lead to life-threatening complications.
Some people with SCD have two different mutations, one on each copy of chromosome 11; in some of those cases, the disease can be slightly less severe than the most common form of SCD.
Sickle cell disease is inherited and can occur only when both parents carry the sickle cell trait. While children of any race can be born with the condition, in the United States it is most prevalent among African-Americans and second-most common among people of Hispanic origin. Worldwide, sickle cell disease occurs most frequently in people whose ancestry originated in regions that include:
- Sub-Saharan Africa. (In some parts of Africa, as many as 2 percent of babies are born with SCD, and up to 25 percent of the people are believed to carry the sickle cell trait.)
- Spanish-speaking regions of Central America, South America, and parts of the Caribbean
- Saudi Arabia
- Sri Lanka
- Mediterranean countries such as Turkey, Greece, and Italy
Scientists theorize that the defective gene that causes SCD evolved in places where the rates of fatal malaria epidemics were high. People with sickle cell trait (explained below), who had some abnormal hemoglobin in their red blood cells, were protected against malaria, because the unusual shape prevented the malaria organism from penetrating. So more of those people survived and reproduced, and they passed the sickle cell trait on to their children.
Understanding who gets sickle cell disease calls for a brief genetics lesson.
Children typically inherit one copy of the normal hemoglobin beta chain gene (hemoglobin A) from each parent. But some children instead inherit one or two mutated hemoglobin genes—usually genes known as hemoglobin S, C, D, E, or O.
Those who inherit a hemoglobin A gene from one parent and a hemoglobin S gene ("the sickle cell gene") from the other parent have what is known as sickle cell trait (AS), a condition in which red blood cells contain both hemoglobin A and hemoglobin S. The red blood cells of people with sickle cell trait usually contain slightly more hemoglobin A than S.
Sickle cell trait is a genetic state, not a disease, and it cannot develop into SCD. Having the sickle cell trait does, however, increase the odds of producing a child with two copies of the hemoglobin S gene, who therefore who has sickle cell disease.
• When both parents possess the sickle cell trait, each pregnancy has the following odds:
- 25 percent chance of producing a child with sickle cell disease (SS)
- 25 percent chance of producing a child with neither sickle cell trait nor sickle cell disease
- 50 percent chance of producing a child with sickle cell trait
• When one parent has SCD and the other has the sickle cell trait, each pregnancy has a 50 percent chance of producing a child who has either the sickle cell trait or sickle cell disease.
While most people with the sickle cell trait are unaffected carriers who don't experience SCD symptoms or complications, a very small number can develop problems when they are exposed to factors such as:
- High altitude
- Increased atmospheric pressure
- Low oxygen
- Severe dehydration
There are three common types of sickle cell disease:
• Hemoglobin SS disease. Also known as sickle cell anemia, hemoglobin SS disease is the most prevalent form of SCD in the United States, making up approximately 60 percent of cases. People with hemoglobin SS disease have two copies of the hemoglobin S gene.
• Hemoglobin SC disease. Also known as sickle-hemoglobin C disease, hemoglobin SC disease makes up about 25 percent of SCD cases. People with hemoglobin SC disease have one copy of the hemoglobin S gene and one copy of the hemoglobin C gene.
• Hemoglobin Sb (beta) thalassemia disease, including Sb
• The people representing the smallest number of SCD cases possess one hemoglobin S gene and one non-C mutated hemoglobin gene, such as Oarab. Such cases account for about 5 percent of SCD in the United States.
Although people with different varieties of SCD typically experience the same types of complications, the frequency and severity of those complications, as well as life expectancies, vary somewhat according to the types of hemoglobin gene mutations present.
Last reviewed on 1/28/10
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