Approximately 70,000 to 100,000 Americans have sickle cell disease. Though rare, it is the the most common form of inherited blood disorders. In Africa, the burden is much higher – Sub-Saharan Africa has an estimated 64% of the 300, 000 people born annually with sickle cell disease in the world, Sickle Cell Day each June 19th provides an opportunity to examine progress and persistent challenges in managing the disease.
Sickle Cell, present in affected individuals at birth, causes the production of abnormal hemoglobin. Normal hemoglobin protein, which resides inside red blood cells, attaches to oxygen in the lungs and carries it to all parts of the body. Healthy red blood cells are flexible enough to move through the smallest blood vessels.
In sickle cell disease, the hemoglobin is abnormal, causing red blood cells to be rigid and shaped like a sickle – the shape from which the disease takes its name. In order for a child to inherit sickle cell disease, both parents must have either sickle cell disease (two sickle cell genes) or sickle cell trait (one sickle cell gene). There are variations of sickle cell disease called sickle C or sickle thalassemia, which are serious conditions but sometimes less severe. If you have sickle cell disease, you will pass one sickle cell gene to your children.
Signs and symptoms of sickle cell disease usually begin in early childhood. Characteristic features of this disorder include a low number of red blood cells (anemia), repeated infections, and periodic episodes of pain. The severity of symptoms varies from person to person. Some people have mild symptoms, while others are frequently hospitalized for more serious complications.
Sickle cells can get stuck and block blood flow, causing pain and infections. Complications of sickle cell disease occur because the sickled cells block blood flow to specific organs. The worst complications include stroke, acute chest syndrome (a condition that lowers the level of oxygen in the blood), organ damage, other disabilities, and even premature death.
These signs and symptoms of sickle cell disease are caused by the “sickling” of red blood cells. When red blood cells sickle, they also break down prematurely, which can lead to anemia. Anemia can cause shortness of breath, fatigue, and delayed growth and development in children. The rapid breakdown of red blood cells may also cause yellowing of the eyes and skin, which are signs of jaundice.
Painful episodes can occur when sickled red blood cells- stiff and inflexible – get stuck in small blood vessels. These episodes deprive tissues and organs of oxygen-rich blood and can lead to organ damage, especially in the lungs, kidneys, spleen, and brain.
A particularly serious complication of sickle cell disease is high blood pressure in the blood vessels that supply the lungs (pulmonary hypertension). Pulmonary hypertension occurs in about one-third of adults with sickle cell disease and can lead to heart failure.
Mutations in the HBB gene cause sickle cell disease.
Hemoglobin consists of four protein subunits; typically, two alpha-globin subunits and two beta-globin subunits. The HBB gene provides instructions for making beta-globin. Variations of beta-globin result from different mutations in the HBB gene. One particular HBB gene mutation produces an abnormal version of beta-globin known as hemoglobin S (HbS). Other mutations in the HBB gene lead to additional abnormal versions of beta-globin, such as hemoglobin C (HbC) and hemoglobin E (HbE). HBB gene mutations can also result in an unusually low level of beta-globin; this abnormality is called beta thalassemia.
In people with sickle cell disease, at least one beta-globin subunit in hemoglobin is replaced with hemoglobin S. In sickle cell anemia, hemoglobin S replaces both beta-globin subunits in hemoglobin.
In other types of sickle cell disease, just one beta-globin subunit in hemoglobin is replaced with hemoglobin S. The other beta-globin subunit is replaced with a different abnormal variant, such as hemoglobin C. For example, people with sickle-hemoglobin C (HbSC) disease have hemoglobin molecules with hemoglobin S and hemoglobin C instead of beta-globin. If mutations that produce hemoglobin S and beta thalassemia occur together, individuals have hemoglobin S-beta thalassemia (HbSBetaThal) disease.
Sickle Cell Trait
Sickle cell trait is an inherited blood disorder that affects approximately 8 percent of African-Americans. Unlike sickle cell disease, in which patients have two genes that cause the production of abnormal hemoglobin, individuals with sickle cell trait carry only one defective gene and typically live normal lives without health problems related to sickle cell. Rarely, extreme conditions such as severe dehydration and high-intensity physical activity can lead to serious health issues, including sudden death, in individuals with sickle cell trait.
Sickle cell disease is more common in certain ethnic groups, including:
- People of African descent, including African-Americans (among whom 1 in 12 carries a sickle cell gene)
- Hispanic-Americans from Central and South America
- People of Middle Eastern, Asian, Indian, and Mediterranean descent
Because sickle cell disease symptoms can begin by four months of age, early diagnosis is critical. All newborns in the United States are now tested for the disease. Sickle cell disease can also be identified before birth by testing a sample of amniotic fluid or tissue from the placenta. People who carry the sickle cell gene can seek genetic counseling before pregnancy to discuss options.
Signs and Symptoms
Signs and symptoms of sickle cell disease can be mild or severe enough to require frequent hospitalizations. They may include:
- Anemia (looking pale)
- Dark urine
- Yellow eyes
- Painful swelling of hands and feet
- Frequent pain episodes
- Stunted growth
There are no standard treatments that cure sickle cell disease. However, there are regiments that help people manage and live with the disease. Treatment relieves pain, prevents infections, minimizes organ damage, and controls complications. At times, blood transfusions and other advanced options are needed.
Clinical trials provide access to experimental therapies for treating sickle cell disease. The American Society of Hematology (ASH) provides information on clinical trials for which you may be eligible. Researchers are looking at new drugs and also exploring the use of bone marrow transplants to treat sickle cell disease. Stem cell transplants, associated with significant risks, are appropriate only for some patients with severe forms of sickle cell disease and closely matched donors – typically family member.
Beyond this, the completion of the Human Genome Project and the use of CRISPR/Cas9 for gene editing have begun to transform the diagnosis and management of disease. Sickle cell disease has been considered a perfect model for genomic research because: 1) it is a monogenic disease and 2.) it has no cure despite the significant incidence of morbidity and mortality. Recent use of gene editing to minimize disease severity, and a single report of a patient who received successful treatment with gene therapy, highlight the potential for translating genome-based knowledge into health benefits for sickle cell patients.
It is important for you to talk with your doctor if you believe you may have sickle cell disease. If you carry the sickle cell trait, make sure you tell your doctor before getting pregnant as well. Depending on your condition, your doctor may refer you to a hematologist, a doctor who specializes in blood conditions.
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