β-thalassemia

Beta-thalassemia (β-thalassemia) is an inherited blood disorder, a form of thalassemia resulting in variable outcomes ranging from clinically asymptomatic to severe anemia individuals. It is caused by reduced or absent synthesis of the beta chains of hemoglobin, the molecule that carries oxygen in the blood. Symptoms depend on the extent to which hemoglobin is deficient, and include anemia, pallor, tiredness, enlargement of the spleen, jaundice, and gallstones. In severe cases death ensues.

Beta thalassemia occurs due to a mutation of the HBB gene leading to deficient production of the hemoglobin subunit beta-globin; the severity of the disease depends on the nature of the mutation, and whether or not the mutation is homozygous. The body's inability to construct beta-globin leads to reduced or zero production of adult hemoglobin thus causing anemia. The other component of hemoglobin, alpha-globin, accumulates in excess leading to ineffective production of red blood cells, increased hemolysis, and iron overload. Diagnosis is by checking the medical history of near relatives, microscopic examination of blood smear, ferritin test, hemoglobin electrophoresis, and DNA sequencing.

As an inherited condition, beta thalassemia cannot be prevented although genetic counselling of potential parents prior to conception can propose the use of donor sperm or eggs. Patients may require repeated blood transfusions throughout life to maintain sufficient hemoglobin levels; this in turn may lead to severe problems associated with iron overload. Medication includes folate supplementation, iron chelation, bisphosphonates, and removal of the spleen. Beta thalassemia can also be treated by bone marrow transplant from a well matched donor, or by gene therapy.

Thalassemias were first identified in severely sick children in 1925, with identification of alpha and beta subtypes in 1965. Beta-thalassemia tends to be most common in populations originating from the Mediterranean, the Middle East, Central and Southeast Asia, the Indian subcontinent, and parts of Africa. This coincides with the historic distribution of ''Plasmodium falciparum'' malaria, and it is likely that a hereditary carrier of a gene for beta-thalassemia has some protection from severe malaria. However, because of population migration, β-thalassemia can be found around the world. In 2005, it was estimated that 1.5% of the world's population are carriers and 60,000 affected infants are born with the thalassemia major annually.

Signs and symptoms

Symptoms depend on the type and severity of thalassemia. Carriers of thalassemia genes may have no symptoms (thalassemia minor) or very mild symptoms with occasional crisis (thalassemia intermedia); individuals who are homozygous for the mutation have severe and life threatening symptoms (thalassemia major).

Individuals with beta-thalassemia major usually present within the first two years of life with symptomatic severe anemia, poor growth, and skeletal abnormalities. Untreated thalassemia major eventually leads to death, usually by heart failure.

Those with beta-thalassemia intermedia (those who are compound heterozygotes for the beta thalassemia mutation) usually present later in life with mild to moderate symptoms of anemia.

Beta thalassemia trait (beta thalassemia minor) involves heterozygous inheritance of a beta-thalassemia mutation. Individuals usually have microcytosis with mild anemia; they are usually asymptomatic or have mild symptoms. Beta thalassemia minor can also present as beta-thalassemia silent carriers; those who inherit a beta thalassemic mutation but have no hematologic abnormalities or symptoms.

Individuals with thalassemia thalassemia major and intermedia (to a lesser extent) are susceptible to health complications that involve the spleen (hypersplenism) and gallstones (due to hyperbilirubinemia from peripheral hemolysis). Additional symptoms of beta-thalassemia major or intermedia include the classic symptoms of anemia including fatigue, developmental delay in childhood, leg ulcers, and organ failure. Ineffective erythropoiesis (red blood cell production) can lead to expansion of the bone marrow in compensation; this can then lead to deformity, bone pain, and craniofacial abnormalities. Organs such as the liver and spleen that can also become enrolled in red blood cell production, leading to hepatosplenomegaly (enlargement of the liver and spleen).

People with thalassemia can get too much iron in their bodies, either from the disease itself as RBCs are destroyed, or as a consequence of frequent blood transfusions. Excess iron is not excreted, but forms toxic non-transferrin-bound iron. This can lead to organ damage, potentially affecting the heart, liver, endocrine system, bones and spleen. Symptoms include an irregular heartbeat, cardiomyopathy, cirrhosis of the liver, hypothyroidism, delayed puberty and fertility problems, brittle and deformed bones, and an enlarged spleen.

For clinical purposes, thalassemia is categorised as either transfusion-dependent thalassemia (TDT) or non-transfusion-dependent thalassemia (NTDT) are used. Patients are usually considered as having NTDT if they have received fewer than 6 red blood cell units in the past 6 months and none in the preceding 2 months.

Cause

Hemoglobin structural biology

Mutations

β-globin chains are encoded by the '' HBB gene'' on chromosome 11; in a healthy person with two copies on each chromosome, two loci encode the β chain.Robbins Basic Pathology, Page No:428 In beta thalassemia, a single faulty gene can be either asymptomatic or cause mild disease; if both genes are faulty this causes moderate to severe disease.

More than 350 mutations have been identified which can cause beta thalassemia; 20 of these account for 80% of beta-thalassemia cases.

Two major groups of mutations can be distinguished:
* Nondeletion forms: These defects, in general, involve a single base-pair substitution or small insertions near or upstream of the HBB gene.
* Deletion forms: base-pair deletions of different sizes involving the HBB gene produce syndromes such as hereditary persistence of fetal hemoglobin syndrome.
Mutations are characterized as (β^o) if they prevent any formation of β globin chains, and mutations are characterized as (β⁺) if they allow some β globin chain formation to occur.

Due to globin defects, beta thalassemia patients do not have normal levels of adult hemoglobin (HbA), and instead have elevated levels of HbA2 (α₂δ₂). Production of this form of hemoglobin may increase as a consequence of stress erythropoiesis.

Diagnosis

Prevention

Risk factors

Family history and ancestry are factors that increase the risk of beta-thalassemia. Depending on family history, if a person's parents or grandparents had beta thalassemia major or intermedia, there is a 75% (3 out of 4) probability (see inheritance chart at top of page) of the mutated gene being inherited by an offspring. Even if a child does not have symptomatic beta thalassemia they can still be a carrier, leading to an increased risk in future generations of their offspring having beta-thalassemia.

Beta thalassemia occurs most often in people of Mediterranean, Middle Eastern, Southern Asian, and African ancestry.

Counselling and screening

The American College of Obstetricians and Gynecologists recommends all people thinking of becoming pregnant should be offered testing to see if they have thalassemia trait. Genetic counseling and genetic testing are recommended for families who carry a thalassemia trait. Understanding the genetic risk, ideally before a family is started, would hopefully allow families to understand more about the condition and make an informed decision that is best for their family.

A number of countries have programs aimed at reducing the incidence of beta-thalassemia:-

* Cyprus has one of the highest carrier rates in the world. A program of premarital screening and counselling has, since the program's implementation in the 1970s, reduced the number of children born with thalassemia major from one of every 158 births to almost zero. Greece also has a screening program to identify people who are carriers.
* In Iran as a premarital screening, the man's red cell indices are checked first. If he has microcytosis ( mean cell hemoglobin < 27 pg or mean red cell volume < 80 fl), the woman is tested. When both are microcytic, their hemoglobin A2 concentrations are measured. If both have a concentration above 3.5% (diagnostic of thalassemia trait) they are referred to the local designated health post for genetic counseling.
* Large-scale awareness campaigns are being organized in India both by government and non-government organizations to promote voluntary premarital screening, with marriage between carriers strongly discouraged.

Treatment

Main article: Management of thalassemia

Combination hemoglobinopathies

A combination hemoglobinopathy occurs when someone inherits two different abnormal hemoglobin genes. If these are different versions of the same gene, one having been inherited from each parent it is an example of compound heterozygosity.

Some examples of clinically significant combinations involving beta thalassemia include:

* Hemoglobin C/ beta thalassemia: common in Mediterranean and African populations generally results in a moderate form of anemia with splenomegaly.
* Hemoglobin D/ beta thalassemia: common in the northwestern parts of India and Pakistan (Punjab region).

* Hemoglobin E/ beta thalassemia: common in Cambodia, Thailand, and parts of India, it is clinically similar to β thalassemia major or β thalassemia intermedia.
* Hemoglobin S/ beta thalassemia: common in African and Mediterranean populations, it is clinically similar to sickle-cell anemia.
* Delta-beta thalassemia is a rare form of thalassemia in which there is a reduced production of both the delta and beta globins. It is generally asymptomatic.

Epidemiology

Beta thalassemia is particularly prevalent among the Mediterranean peoples and this geographical association is responsible for its naming: ''thalassa'' (''θάλασσα'') is the Greek word for sea and ''haima'' (''αἷμα'') is the Greek word for blood. In Europe, the highest prevalence of beta-thalassemia trait is found in Greece, Turkey, and Mediterranean islands such as Sicily, Sardinia, Corsica, Cyprus, Malta and Crete.

Incidence

Beta thalassemia is most prevalent in the "thalassemia belt" which includes areas in Sub-Saharan Africa, and the Mediterranean extending into the Middle East and Southeast Asia. This geographical distribution is thought to be due to the beta-thalassemia carrier state (beta-thalassemia minor) conferring resistance to malaria. In 2005, it was estimated that 1.5% of the world's population are carriers and 60,000 affected infants are born with the thalassemia major annually.

Evolutionary adaptation

The thalassemia trait may confer a degree of protection against malaria, which is historically endemic in the regions where the trait is common. This is thought to confer a selective survival advantage on carriers (known as heterozygous advantage), thus perpetuating the mutation. In that respect, the various thalassemias resemble other genetic disorders affecting hemoglobin, such as sickle-cell disease or Hemoglobin C disease.

See also

* Alpha-thalassemia
* Anisopoikilocytosis
* Delta-thalassemia
* Hemoglobinopathy

References

Further reading

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External links

{{DEFAULTSORT:Beta-Thalassemia
Disorders of globin and globulin proteins
Hereditary hemolytic anemias
Rare diseases