Hemoglobin Variants

Hemoglobin is made up of heme (the iron-containing portion of hemoglobin) and globin (amino acid chains that form a protein). Hemoglobin (Hgb) molecules are found in all red blood cells. They bind oxygen in the lungs, carry the oxygen throughout the body, and release it to the body's cells and tissues.

Normal hemoglobin types include:

  • Hemoglobin A (about 95% - 98%): Hgb A contains two alpha (α) chains and two beta (β) chains

  • Hgb A2 (2% - 3%): has two alpha (á) and two delta (ä) chains

  • Hgb F (up to 2%): the primary hemoglobin produced by the fetus during gestation; its production usually falls to a low level shortly after birth; Hgb F has two alpha (α) and two gamma (γ) chains

Hemoglobin variants are abnormal forms of hemoglobin that occur when changes (point mutations, deletions) in the globin genes cause changes in the amino acids that make up the globin protein. These changes may affect the structure of the hemoglobin, its behavior, its production rate, and/or its stability. Several hundred hemoglobin variants have been documented; however, only a few are common and clinically significant. The majority of these are beta chain variants.

These variants are inherited in an autosomal recessive fashion. A person inherits one copy of each beta globin gene from each parent. If one normal beta gene and one abnormal beta gene are inherited, the person is said to be a carrier or to be heterozygous for the abnormal hemoglobin. The abnormal gene can be passed on to any offspring but does not cause symptoms or health concerns in the carrier.

If two abnormal beta genes of the same type are inherited, the person is considered to have the disease and is homozygous for the abnormal hemoglobin. A copy of the abnormal beta gene will be passed on to any offspring.

If two abnormal beta genes of different types are inherited, the person is doubly or compound heterozygous. One of the abnormal beta genes will be passed on to each offspring.

COMMON HEMOGLOBIN VARIANTS

Hemoglobin S: This is the primary hemoglobin in people with sickle cell disease. Approximately 8% of Americans of African descent carry the sickle Hb mutation in one of their two beta genes (0.15% of African Americans have sickle cell disease). The mutation exists in the beta (β) chain gene, so those with Hgb S disease have two beta (βS) chains and two normal alpha (α) chains. The presence of hemoglobin S causes the red blood cell to deform and assume a sickle shape when exposed to decreased amounts of oxygen (such as might happen when someone exercises or in the peripheral circulation). Sickled red blood cells can block small blood vessels, causing pain and impaired circulation, decrease the oxygen-carrying capacity of the red blood cell, and decrease the cell's lifespan. A single beta (β) copy does not cause symptoms unless it is combined with another hemoglobin mutation, such as that causing Hgb C (βC).

Hemoglobin E: Hemoglobin E is one of the most common hemoglobin variants in the world. It is very prevalent in Southeast Asia, especially in Cambodia, Laos, and Thailand and in individuals of Southeast Asian descent. Hemoglobin E also is due to a mutation in the gene that creates the beta (β) chain. People who are homozygous for Hgb E (have two copies of βE) have a mild hemolytic anemia (caused by premature removal of red blood cells from the circulation), microcytosis (small red blood cells), and mild enlargement of the spleen. A single copy of hemoglobin E does not cause symptoms unless it is combined with another mutation, such as one for beta thalassemia trait.

Hemoglobin C: is another mutation in the gene for the beta (β) chain. About 2-3% of people of West African descent are heterozygotes for hemoglobin C (have one copy of βC). Hemoglobin C disease (seen in homozygotes - those with two copies of βC) is rare and relatively mild. It usually causes a minor amount of hemolytic anemia and a mild to moderate enlargement of the spleen.

Less Common Hemoglobin Variants

There are many other variants. Some are silent – causing no signs or symptoms – while others affect the functionality and/or stability of the hemoglobin molecule. Examples of other variants include: Hemoglobin D, Hemoglobin G, Hemoglobin J, Hemoglobin M, and Hemoglobin Constant Spring (a mutation in the alpha globin gene that results in an abnormally long alpha (α) chain and an unstable hemoglobin molecule). Additional examples are:

Hemoglobin F (Hgb F) is the primary hemoglobin produced by the fetus, and its role is to transport oxygen efficiently in a low oxygen environment. Production of Hgb F stops at birth and decreases to adult levels by 1-2 years of age. Hgb F may be elevated in several congenital disorders: levels can be normal to increased in beta thalassemia; often, levels are increased in individuals with sickle cell anemia and in sickle cell- beta thalassemia. Individuals with sickle cell disease and increased Hgb F often have a milder disease, as the F hemoglobin inhibits sickling of the red cells. Hgb F levels are also increased in a rare condition called hereditary persistence of fetal hemoglobin (HPFH). This is a group of inherited disorders in which Hgb F levels are increased with no hematological or clinical features of thalassemia. Different ethnic groups have different mutations causing HPFH. Hgb F can also be increased in some acquired conditions involving impaired red blood cell production. Leukemias and other myeloproliferative disorders often are also associated with elevated HgbF.

Hemoglobin H is an abnormal hemoglobin that occurs in some cases of alpha thalassemia. It is composed of four beta (β) globin chains and is created in response to a severe shortage of alpha (α) chains. Although each of the beta (β) globin chains is normal, the tetramer of 4 beta chains does not function normally. It has an increased affinity for oxygen, holding onto it instead of releasing it to the tissues and cells.

Hemoglobin Barts develops in fetuses with alpha thalassemia. Hgb Bart's is formed of four gamma (γ) chains when there is a shortage of alpha chains, in a manner analogous to the formation of Hemoglobin H. Hgb Bart's disappears shortly after birth (due to dwindling gamma chain production).

A person can also inherit two different abnormal genes, one from each parent. This is known as being compound heterozygous or doubly heterozygous. Several different clinically significant combinations are listed below.

Hemoglobin SC Disease. Inheritance of one beta S gene and one beta C gene results in Hemoglobin SC Disease. These individuals have a mild hemolytic anemia and moderate splenomegaly (enlargement of the spleen). Persons with Hgb SC disease may develop the same vaso-occulsive (blood vessel blocking) complications as seen in sickle cell anemia, but most cases are less severe.

Sickle Cell - Hemoglobin D Disease. Individuals with sickle cell - Hgb D disease have inherited one copy of hemoglobin S and one of hemoglobin D-Los Angeles (or D-Punjab). These patients may have occasional sickle crises and moderate hemolytic anemia.

Hemoglobin E - beta thalassemia. Individuals who are doubly heterozygous for hemoglobin E and beta thalassemia have an anemia that can vary in severity, from mild (or asymptomatic) to as severe as that seen in beta thalassemia intermedia.

Hemoglobin S - beta thalassemia. Sickle cell - beta thalassemia varies in severity, depending on the beta thalassemia mutation inherited. Some mutations result in decreased beta globin production (beta+) while others completely eliminate it (beta0). Sickle cell - beta+ thalassemia tends to be less severe than sickle cell - beta0 thalassemia. Patients with sickle cell - beta0 thalassemia tend to have more irreversibly sickled cells, more frequent vaso-occlusive problems, and more severe anemia than those with sickle cell - beta+ thalassemia. It is often difficult to distinguish between sickle cell disease and sickle cell - beta0 thalassemia.

LABORATORY TESTS

Laboratory testing for hemoglobin variants is an exploration of the "normalness" of the red blood cells (RBC's), an evaluation of the hemoglobin inside the RBC's, and an analysis of relevant gene mutations or deletions. Each test provides a piece of the puzzle, giving the clinician important information about which variants may be present. The tests that are ordered to search for hemoglobin variants are also used for thalassemia workups. Searching for both is important because thalassemia is sometimes inherited along with a hemoglobin variant.

CBC (complete blood count). The CBC is a snapshot of the cells circulating in your bloodstream. Among other things, the CBC will tell the doctor how many red blood cells are present, how much hemoglobin is in them, and give the doctor an evaluation of the size and shape of the red blood cells present. MCV (mean corpuscular volume) is a measurement of the size of the red blood cells. A low MCV is often the first indication of thalassemia. If the MCV is low and iron-deficiency has been ruled out, the person may be a thalassemia trait carrier or have one of the hemoglobin variants that cause microcytosis (for example, Hgb E).

Blood smear (also called peripheral smear and manual differential when white cells are examined). In this test, a trained laboratorian looks at a thin stained layer of blood, on a slide, under a microscope. The number and type of white blood cells, red blood cells, and platelets can be assessed and evaluated to see if they are normal and mature. A variety of disorders affect normal blood cell production. The red blood cells may be:

  • Microcytic (smaller than normal)

  • Hypochromic (pale - have less than the normal amount of reddish coloring - indicating less hemoglobin)

  • Varying in size (anisocytosis) and shape (poikilocytosis)

  • Nucleated (not normal in a mature RBC)

  • Have uneven hemoglobin distribution (producing "target cells" that look like a bull's-eye under the microscope).

The greater the percentage of abnormal-looking red blood cells, the greater the likelihood of an underlying disorder and of impaired oxygen-carrying capability.

Detection of hemoglobin variants. These tests identify the type and measure the relative amount of hemoglobins present in the red blood cells using either hemoglobin electrophoresis, isoelectric focusing, or high performance liquid chromatography. These techniques separate different hemoglobins based on their charge. Most of the common variants can be identified using one of these methods or a combination. The relative amounts of any variant hemoglobin detected can help to diagnose combinations of hemoglobin variants and thalassemia (compound heterozygotes).

DNA analysis. This test is used to investigate deletions and mutations in the alpha and beta globin producing genes. Family studies can be done to evaluate carrier status and the types of mutations present in other family members. DNA testing is not routinely done but can be used to help diagnose hemoglobin variants, thalassemia, and to determine carrier status.

TESTING FOR HEMOGLOBIN VARIANTS IS DONE TO:

  • Screen for common hemoglobin variants in newborns. In many states, this has become a standard part of newborn screening. Infants with variants, such as Hgb S, can benefit from early detection and treatment of sickle cell anemia.

  • Prenatal screening is also done in some areas on high-risk mothers: those with an ethnic background associated with a higher prevalence of hemoglobin variants (such as those of African descent), and those with affected family members. Screening may also be done in conjunction with genetic counseling prior to pregnancy to determine possible carrier status of parents.

  • Identify variants in asymptomatic parents with an affected child.

  • Identify hemoglobin variants in those with symptoms of unexplained anemia and/or microcytosis (small red blood cells) and hypochromasia (pale red blood cells). It may also be ordered as part of an anemia investigation.

Blood transfusions can interfere with hemoglobin variant testing. A patient should wait several months after a transfusion before having testing done. However, in patients with sickle cell disease, the test may be done after transfusion to determine if enough normal hemoglobin has been given to reduce the risk of damage from sickling of red blood cells.

Since newborn screening programs have started, including testing for hemoglobin variants, they have uncovered thousands of children who are carriers. This is due to new technology, not to increased prevalence of the gene mutations. The health of children is not affected by having single changed gene copies, but the availability of this new information has greatly increased the need for information about hemoglobin variants and their inheritance.