What Is the Genetic Cause of Sickle Cell Anemia?

According to the Centers for Disease Control, sickle cell disease affects one in every 500 African-Americans born, and it is estimated that 2 million African-Americans (one in 12) carry the sickle cell trait. There is currently no cure for the disease, but treatment options are available.

The Facts

• Sickle cell disease occurs when the red blood cells contain mostly hemoglobin S, an abnormal type of hemoglobin. These red blood cells only live around 10-20 days; normal red blood cells live about 120 days. Hemoglobin S cells become stiff and sickle-, or crescent-, shaped; these stiff and abnormally shaped cells do not pass through small blood vessels as easily as normal blood cells and can cause blockages. The resulting reduction in blood flow can cause tissue damage.

Symptoms

• The most common symptom of sickle cell disease is anemia. The shape of sickle cells reduces their oxygen-carrying ability, which causes the anemia. Other possible symptoms include jaundice; pain crisis (also called sickle crisis), caused by tissue damage from blocked vessels; and acute chest syndrome, which is caused by blockage of the tiny blood vessels in the lungs and resembles pneumonia.

Inheritance

• Inheritance of sickle cell disease follows the same pattern as many other genetic traits. If one parent has sickle cell disease and the other parent is normal, all of their children will carry the sickle cell trait. If one parent has sickle cell disease and the other carries the trait (even if symptom-free), each of their children has a 50 percent chance of either having sickle cell disease or carrying the trait. If both parents are symptom-free but carry the trait, each of their children has a 25% chance of having sickle cell disease.

Common Forms

• There are several types of sickle cell disorder. A person with sickle cell trait has both normal hemoglobin (type A) and sickle cell hemoglobin (type S), but has more of type A and is usually symptom-free. In hemoglobin C disease, both sickle cells and type C cells are present; a person with this form of the disease will have some complications of sickle cell disease but to a lesser degree.

  Sickle cell anemia is the most common form of the disease and also the most severe; a person with sickle cell anemia has almost all type S hemoglobin. Hemoglobin S beta thalassemia combines a sickle cell gene and a gene for beta thalassemia, which is another kind of anemia. This version of the disease has two types: beta-zero thalassemia is more severe while beta-plus thalassemia is generally milder.

Rare Forms

• A person rarely can inherit one sickle cell gene and one of several other abnormal types of hemoglobin. These types are called hemoglobin SD disease (one sickle cell gene and one hemoglobin type D gene), hemoglobin SO (one sickle cell gene and one hemoglobin type O gene) and hemoglobin SE (one sickle cell gene and one hemoglobin type E gene). All of these can have symptoms similar to sickle cell anemia and each can range from mild to severe.