Haemoglobin Variants

Common Haemoglobin Variants

• Haemoglobin S: This is the main haemoglobin in people with sickle cell disease. Approximately 8% of Britons of West 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 (“β^{S “}or “HbS”) 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 with infections or under a general anaesthetic). Sickled red blood cells lead to:
  • Haemolysis (premature breakdown of red blood cells) causing low haemoglobin levels (anaemia) and increase in the breakdown product bilirubin (causing yellow skin and eye-whites known as jaundice).
  • Blockage of small blood vessels, causing impaired circulation, resulting in strokes, bone pain, chest pain, shortness of breath from micro-clots in the lung, shortening of the fingers, prolonged and painful enlargement of the penis or in children of the spleen. In adults, repeated blockage of blood supply to the spleen usually results in loss of this organ.

A single HbS copy does not cause symptoms unless it is combined with another haemoglobin mutation, such as that causing Hb C (β^C).

• Haemoglobin C: About 2-3% of people of West African descent are heterozygotes for haemoglobin C (have one copy of β^C). Haemoglobin C disease (seen in homozygotes – those with two copies of β^C) 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 common in Southeast Asia, especially in Cambodia, Laos, and Thailand.. People who are homozygous for Hb E (have two copies of β^E) generally have a mild haemolytic anaemia (a condition where red blood cells break open), microcytic (small cells) 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 signs or symptoms – 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 main haemoglobin produced by the fetus, 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 increased in several congenital disorders. Levels can be normal or increased in beta thalassaemia and are frequently increased in individuals with sickle cell anaemia and in sickle cell plusbeta 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 fetal 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 fetuses with alpha thalassaemia. It is formed of four gamma (γ) protein chains when there is severe lack of alpha chains, in a manner similar to the formation of Haemoglobin H. Hb Barts 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, (typically bone pain or eye problems) 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 asymptomatic) to severe.

Haemoglobin S – beta thalassaemia. Sickle cell – beta thalassaemia varies in severity, depending on the beta thalassaemia mutation inherited. The more severe the beta-thalassaemia variant (so the less normal beta chain produced), the more the patient will resemble a homozygous sickle cell disease rather than trait individual. 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.

Laboratory testing for haemoglobin variants is an exploration of the microscopic appearance 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 them, and give the doctor an evaluation of the size 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 carrier or have one of the haemoglobin variants that cause microcytosis (for example, Hb E).

A blood film (also called a peripheral smear or a manual differential when different types of white cells are counted). In this test, a doctor or laboratory scientist looks at a thin layer of blood, treated with a special stain, on a slide, under a microscope. The number and types of white blood cells, and appearance of red blood cells and platelets can be assessed and evaluated to see if they are normal. A variety of disorders affects normal blood cell production.

The red blood cells may be abnormal such as:

• microcytic (small)
• Hypochromic (pale)
• Vary in size (anisocytosis) or shape (poikilocytosis)
• Have a nucleus (abnormal in a mature red blood cell)
• Have an abnormal shape (e.g. “target cells” that look like a bull’s-eye under the microscope, or “stomatocytes” with slit-like grooves across the middle).
• Have abnormal texture such as “basophilic stippling” (blue dots in the cell, around the nucleus).

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.

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. This has become a standard part of newborn screening in the UK. Infants with variants such as HbS 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, microcytosis, and/or hypochromasia. 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.