Alpha Thalassemia Facts

What is alpha thalassemia?

Alpha thalassemia is a blood disorder that reduces the production of hemoglobin. Hemoglobin is the protein in red blood cells that carries oxygen to cells throughout the body.

In people with the characteristic features of alpha thalassemia, a reduction in the amount of hemoglobin prevents enough oxygen from reaching the body’s tissues. Affected individuals also have a shortage of red blood cells (anemia), which can cause pale skin, weakness, fatigue, and more serious complications.

Two types of alpha thalassemia can cause health problems. The more severe type is known as hemoglobin Bart hydrops fetalis syndrome or Hb Bart syndrome. The milder form is called HbH disease.

Hb Bart syndrome is characterized by hydrops fetalis, a condition in which excess fluid builds up in the body before birth. Additional signs and symptoms can include severe anemia, an enlarged liver and spleen (hepatosplenomegaly), heart defects, and abnormalities of the urinary system or genitalia. As a result of these serious health problems, most babies with this condition are stillborn or die soon after birth. Hb Bart syndrome can also cause serious complications for women during pregnancy, including dangerously high blood pressure with swelling (preeclampsia), premature delivery, and abnormal bleeding.

HbH disease causes mild to moderate anemia, hepatosplenomegaly, and yellowing of the eyes and skin (jaundice). Some affected individuals also have bone changes such as overgrowth of the upper jaw and an unusually prominent forehead. The features of HbH disease usually appear in early childhood, and affected individuals typically live into adulthood.

How common is alpha thalassemia?

Alpha thalassemia is a fairly common blood disorder worldwide. Thousands of infants with Hb Bart syndrome and HbH disease are born each year, particularly in Southeast Asia. Alpha thalassemia also occurs frequently in people from Mediterranean countries, North Africa, the Middle East, India, and Central Asia.

What genes are related to alpha thalassemia?

Alpha thalassemia typically results from deletions involving the HBA1 and HBA2 genes. Both of these genes provide instructions for making a protein called alpha-globin, which is a component (subunit) of hemoglobin.

People have two copies of the HBA1 gene and two copies of the HBA2 gene in each cell. Each copy is called an allele. For each gene, one allele is inherited from a person’s father, and the other is inherited from a person’s mother. As a result, there are four alleles that produce alpha-globin. The different types of alpha thalassemia result from the loss of some or all of these alleles.

Hb Bart syndrome, the most severe form of alpha thalassemia, results from the loss of all four alpha-globin alleles. HbH disease is caused by a loss of three of the four alpha-globin alleles. In these two conditions, a shortage of alpha-globin prevents cells from making normal hemoglobin. Instead, cells produce abnormal forms of hemoglobin called hemoglobin Bart (Hb Bart) or hemoglobin H (HbH). These abnormal hemoglobin molecules cannot effectively carry oxygen to the body’s tissues. The substitution of Hb Bart or HbH for normal hemoglobin causes anemia and the other serious health problems associated with alpha thalassemia.

Two additional variants of alpha thalassemia are related to a reduced amount of alpha-globin. Because cells still produce some normal hemoglobin, these variants tend to cause few or no health problems. A loss of two of the four alpha-globin alleles results in alpha thalassemia trait. People with alpha thalassemia trait may have unusually small, pale red blood cells and mild anemia. A loss of one alpha-globin allele is found in alpha thalassemia silent carriers. These individuals typically have no thalassemia-related signs or symptoms.

How do people inherit alpha thalassemia?

The inheritance of alpha thalassemia is complex. Each person inherits two alpha-globin alleles from each parent. If both parents are missing at least one alpha-globin allele, their children are at risk of having Hb Bart syndrome, HbH disease, or alpha thalassemia trait. The precise risk depends on how many alleles are missing and which combination of the HBA1 and HBA2 genes is affected.

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Alpha Thalassemia Trait

Alpha thalassemia is common in people of African, Southern Chinese, Southeast Asian, Middle Eastern and Mediterranean descent:

  • Alpha thalassemia affects the amount of hemoglobin in the red blood cells.
  • Adult hemoglobin (hemoglobin A) is made of alpha and beta globins.
  • Normally, people have four (4) genes for alpha globin with two (2) genes on each chromosome (αα/αα).
  • People with alpha thalassemia trait only have two (2) genes for alpha globin, so they make slightly lower amounts of hemoglobin.
  • All red blood cells contain hemoglobin, which carries oxygen from the lungs to all parts of the body.

What is Alpha Thalassemia trait?

There are two different types of alpha thalassemia trait.

  • The first type of alpha thalassemia trait has one alpha gene missing on each chromosome (α-/α-). This is called the trans form of alpha thalassemia trait.
  • The trans form of alpha thalassemia trait (α-/α-) is common in African-Americans (20-30%) and in people of African descent. It is rare for African-Americans to have the cis form of alpha thalassemia, but it can happen.
  • The second type of alpha thalassemia trait has two missing alpha genes on the same chromosome (αα/–). This is called the cis form of alpha thalassemia trait.
  • Both types of alpha thalassemia trait are common in people of Southeast Asian, Southern Chinese, Mediterranean and Middle Eastern descent, however, the cis type of trait is more common.

People with alpha thalassemia trait do not develop Hemoglobin H cell disease or Hydrops Fetalis later in life.

Source: St Jude’s Children’s Research Hospital



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