What are they?
Haemoglobin variants are abnormal forms of haemoglobin. Made up of haem, an iron-containing portion, and globin, amino acid chains that form a , haemoglobin (Hb or 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 haemoglobin types include:
- Hb A - makes up about 95%-98% of Hb found in adults); contains two alpha (α) protein chains and two beta (β) protein chains
- Hb A2 - makes up about 2%-3% of Hb; has two alpha (α) and two delta (δ) protein chains
- Hb F - makes up to 2% of Hb found in adults; has two alpha (α) and two gamma (γ) protein chains; the primary haemoglobin produced by the foetus during pregnancy; its production usually falls to a low level shortly after birth
Haemoglobin variants occur when genetic changes in the globin genes cause alterations in the that make up the globin protein. These changes may affect the structure of the haemoglobin, its behavior, its production rate, and/or its stability. There are four that code for alpha globin chains and two genes that code for the beta globin chains. (For general information on genetic testing, see Genetic Testing) The most common alpha-chain-related condition is alpha thalassaemia. Its severity is governed by the number of genes affected. (See Thalassaemia for more information.)
Beta chain haemoglobin variants are inherited in an autosomal fashion. This means that the person must have two altered gene copies, one from each parent, to have a haemoglobin variant-related disease. If one normal beta gene and one abnormal beta gene are inherited, the person is for the abnormal haemoglobin, a . The abnormal gene can be passed on to any offspring, but it does not cause symptoms or health concerns in the carrier.
If two abnormal beta genes of the same type are inherited, the person is . The person would produce the associated haemoglobin variant and may have some associated symptoms and potential for complications. The severity of the condition depends on the genetic and varies from person to person. A copy of the abnormal beta gene would be passed on to any offspring.
If two abnormal beta genes of different types are inherited, the person is doubly or compound heterozygous. The affected patient would typically have symptoms related to one or both of the haemoglobin variants that he or she produces. One of the abnormal beta genes would be passed on to each offspring.
Several hundred beta chain haemoglobin variants have been documented; however, only a few are common and clinically significant. They are discussed on the next page.
Common Haemoglobin Variants
- Haemoglobin S: This is the primary haemoglobin 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). Those with Hb S disease have two abnormal beta (βS) chains and two normal alpha (α) chains. The presence of haemoglobin 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). 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 (βS) copy does not cause symptoms unless it is combined with another haemoglobin mutation, such as that causing Hb C (βC).
- Haemoglobin C: About 2-3% of people of West African descent are heterozygotes for haemoglobin C (have one copy of βC). Haemoglobin C disease (seen in homozygotes – those with two copies of βC) is rare and relatively mild. It usually causes a minor amount of haemolytic anaemia and a mild to moderate enlargement of the spleen.
- Haemoglobin E: Haemoglobin E is one of the most common beta chain haemoglobin variants in the world. It is very prevalent in Southeast Asia, especially in Cambodia, Laos, and Thailand and in individuals of Southeast Asian descent. People who are for Hb E (have two copies of βE) generally have a mild haemolytic anaemia, red blood cells, and a mild enlargement of the spleen. A single copy of the haemoglobin E gene does not cause symptoms unless it is combined with another mutation, such as the one for beta thalassaemia trait.
Less Common Haemoglobin Variants
There are many other variants. Some are silent – causing no or – while others affect the functionality and/or stability of the haemoglobin molecule. Examples of other variants include: Haemoglobin D, Haemoglobin G, Haemoglobin J, Haemoglobin M, and Haemoglobin Constant Spring, a mutation in the alpha globin gene that results in an abnormally long alpha (α) chain and an unstable haemoglobin molecule. Additional beta chain variant examples are:
- Haemoglobin F: Hb F is the primary haemoglobin produced by the foetus, and its role is to transport oxygen efficiently in a low oxygen environment. Production of Hb F stops at birth and decreases to adult levels by 1-2 years of age. Hb F may be elevated in several disorders. Levels can be normal to increased in beta thalassaemia and are frequently increased in individuals with sickle cell anaemia and in sickle cell-beta thalassaemia. Individuals with sickle cell disease and increased Hb F often have a milder disease, as the F haemoglobin inhibits sickling of the red cells. Hb F levels are also increased in a rare condition called hereditary persistence of foetal haemoglobin (HPFH). This is a group of inherited disorders in which Hb F levels are increased without the signs or clinical features of thalassaemia. Different ethnic groups have different mutations causing HPFH. Hb F can also be increased in some acquired conditions involving impaired red blood cell production. Leukaemias and other myeloproliferative disorders often are also associated with elevated Hb F.
- Haemoglobin H: Hb H is an abnormal haemoglobin that occurs in some cases of alpha thalassaemia. It is composed of four beta (β) globin chains and is produced 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.
- Haemoglobin Barts: Hb Barts develops in foetuses with alpha thalassaemia. It is formed of four gamma (γ) protein chains when there is a shortage of alpha chains, in a manner similar to the formation of Haemoglobin H. Hb 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.
Haemoglobin SC Disease. Inheritance of one beta S gene and one beta C gene results in Haemoglobin SC Disease. These individuals have a mild haemolytic anaemia and moderate enlargement of the spleen. Persons with Hb SC disease may develop the same vaso-occulsive (blood vessel blocking) complications as seen in sickle cell anaemia, but most cases are less severe.
Sickle Cell – Haemoglobin D Disease. Individuals with sickle cell – Hb D disease have inherited one copy of haemoglobin S and one of haemoglobin D-Los Angeles (or D-Punjab). These patients may have occasional sickle crises and moderate haemolytic anaemia.
Haemoglobin E – beta thalassaemia. Individuals who are doubly heterozygous for haemoglobin E and beta thalassaemia have an anaemia that can vary in severity, from mild (or ) to severe.
Haemoglobin S – beta thalassaemia. Sickle cell – beta thalassaemia varies in severity, depending on the beta thalassaemia mutation inherited. Some mutations result in decreased beta globin production (beta+) while others completely eliminate it (beta0). Sickle cell – beta+ thalassaemia tends to be less severe than sickle cell – beta0 thalassaemia. Patients with sickle cell – beta0 thalassaemia tend to have more irreversibly sickled cells, more frequent vaso-occlusive problems, and more severe anaemia than those with sickle cell – beta+ thalassaemia. It is often difficult to distinguish between sickle cell disease and sickle cell – beta0 thalassaemia.
How are they tested?
Laboratory testing for haemoglobin variants is an exploration of the “normalness” of the red blood cells (RBCs), an evaluation of the haemoglobin inside the RBCs, 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 haemoglobin variants are also used for thalassaemia workups. Searching for both is important because thalassaemia is sometimes inherited along with a haemoglobin variant.
FBC (full blood count)
. The FBC is a snapshot of the cells circulating in your bloodstream. Among other things, the FBC will tell the doctor how many red blood cells are present, how much haemoglobin is in thaem, and give the doctor an evaluation of the size and shape of the red blood cells present. Mean corpuscular volume (MCV) is a measurement of the size of the red blood cells. A low MCV is often the first indication of thalassaemia. If the MCV is low and iron-deficiency has been ruled out, the person may be a thalassaemia trait or have one of the haemoglobin variants that cause (for example, Hb E).
Blood film (also called a peripheral blood smear and a manual differential when white cells are examined). In this test, a trained medical scientist or pathologist looks at a thin layer of blood, treated with a special stain, 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:
- Varying in size (anisocytosis) and shape (poikilocytosis)
- Having a nucleus (not normal in a mature RBC)
- Having uneven haemoglobin 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.
Haemoglobin variant testing. These tests identify the type, and measure the relative amount, of haemoglobins present in the red blood cells. Most of the common variants can be identified using one of these tests or a combination. The relative amounts of any variant haemoglobin detected can help diagnose combinations of haemoglobin variants and thalassaemia (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 haemoglobin variants, thalassaemia, and to determine carrier status. DNA sequencing can be used to identify rare and new haemoglobin variants but this is not a routine blood test.
Is any test preparation needed?
No test preparation is needed.
Why are they done?
Testing for haemoglobin variants is done to:
- Screen for common haemoglobin variants in newborns. In all states, this has become a standard part of newborn screening. Infants with variants such as Hb S can benefit from early detection and treatment.
- Prenatal screening is also done in some areas on high-risk mothers: those with an ethnic background associated with a higher prevalence of haemoglobin variants (such as those of African descent) and those with affected family members. Screening may also be done in conjunction with genetic counselling prior to pregnancy to determine possible carrier status of parents.
- Identify variants in asymptomatic parents with an affected child.
- Identify haemoglobin variants in those with symptoms of unexplained anaemia, , and/or . It may also be ordered as part of an anaemia investigation.
Is there anything else I should know?
Blood transfusions can interfere with haemoglobin 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 a transfusion to determine if enough normal haemoglobin has been given to reduce the risk of damage from sickling of red blood cells.
Since newborn screening programs have started including testing for haemoglobin variants, they have uncovered thousands of children who are carriers. This is due to new technology, not to an 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 haemoglobin variants and their inheritance.
Diabetic patients may have an increase in a form of haemoglobin known as haemoglbn A1c. (see HbA1c).
Elsewhere On The Web
The Sickle Cell Disease Association of America (USA)
Sickle Cell Information Center (USA)
National Institutes of Health: What is Sickle Cell Anemia? (USA)
The Children's Hospital of Oakland, Northern California Comprehensive Thalassemia Center (USA)
University of Rochester Medical Center, Division of Genetics Hemoglobinopathies (USA)
National Newborn Screening & Genetics Resource Center, Newborn Screening (USA)
Cooley’s Anemia Foundation (USA)
National Human Genome Research Institute: Sickle Cell Disease (USA)
National Human Genome Research Institute: Thalassemia (USA)
Last Review Date: December 5, 2011