Alpha-thalassemia

Alpha-thalassemia

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Alpha_thalassemia_chromosomes_-_Tamil

Alpha-thalassemia (α-thalassemia, α-thalassaemia) is a form of thalassemia involving the genes HBA1 and HBA2. Thalassemias are a group of inherited blood conditions which result in the impaired production of hemoglobin, the molecule that carries oxygen in the blood. Normal hemoglobin consists of two alpha chains and two beta chains; in alpha-thalassemia, there is a quantitative decrease in the amount of alpha chains, resulting in fewer normal hemoglobin molecules. Furthermore, alpha-thalassemia leads to the production of unstable beta globin molecules which cause increased red blood cell destruction. The degree of impairment is based on which clinical phenotype is present (how many genes are affected).

Signs and symptoms

The presentation of individuals with alpha-thalassemia consists of:

Signs and symptoms of alpha-thalassemia

Common	Uncommon
Anemia: People with alpha-thalassemia can experience both a decrease in the production of red blood cells (RBC) and an increase in the destruction of RBCs which can result in other symptoms such as fatigue and dizziness. Enlarged spleen Gallstones Delayed growth	Jaundice Pronounced forehead: This occurs due to the expansion of the bone marrow in the frontal bone in the skull.The bone marrow expands because more blood cells are being produced in order to compensate for the anemia. Extramedullary hematopoesis (blood formation outside of the bone marrow) Hypertension (in pregnancy)

Cause

Alpha-thalassemias are most commonly inherited in a Mendelian recessive manner. They are also associated with deletions of chromosome 16p.Alpha thalassemia can also be acquired under rare circumstances.

Pathophysiology

The mechanism sees that α thalassemias results in decreased alpha-globin production, therefore fewer alpha-globin chains are produced, resulting in an excess of β chains in adults and excess γ chains in newborns. The excess β chains form unstable tetramers called hemoglobin H or HbH of four beta chains. The excess γ chains form tetramers which are poor carriers of O₂ since their affinity for O₂ is too high, so it is not dissociated in the periphery. Homozygote α⁰ thalassaemias, where numerous γ₄ but no α-globins occur at all (referred to as Hb Barts), often result in death soon after birth.

Diagnosis

Diagnosis of alpha-thalassemia is primarily by laboratory evaluation and molecular diagnosis. Alpha-thalassemia can be mistaken for iron-deficiency anaemia on a full blood count or blood film, as both conditions have a microcytic anaemia. Serum iron and serum ferritin can be used to exclude iron-deficiency anaemia.

Types

Two genetic loci exist for α globin, thus four alleles are in diploid cells. Two alleles are maternal and two alleles are paternal in origin. The severity of the α-thalassemias is correlated with the number of affected α-globin alleles: the greater, the more severe will be the manifestations of the disease. When noting the genotype, an "α" indicates a functional alpha chain, and '-' a pathological one.

Alleles affected	Description	Genotype
One	This is known as alpha thalassemia silent and with this type, the effect on hemoglobin synthesis is minimal. Three α-globin genes are enough to permit normal hemoglobin production, and no clinical symptoms present. It occurs due to a deletion or non-deletion mutation.	- α/α α
Two	Haematopoiesis(production of blood cells) The condition is called alpha thalassemia trait; two α genes permit nearly normal production of red blood cells, but a mild microcytic hypochromic anemia is seen. The disease in this form can be mistaken for iron-deficiency anemia and treated inappropriately with iron. Alpha-thalassemia trait can exist in two forms: Alpha-thal-1 (- -/α α), involves cis deletion of both alpha genes on the same chromosome. Higher incidence in people of Asian descent when compared with the general population. Alpha-thal-2 (- α/- α), involves trans deletion of alpha genes; this occurs on different (homologous) chromosomes. Higher incidence in people of African descent when compared with the general population.	- -/α α or - α/- α
Three	This condition is called hemoglobin H disease; two unstable hemoglobins are present in the blood; hemoglobin Barts (tetrameric γ chains) and hemoglobin H (tetrameric β chains). Both of these unstable hemoglobins have a higher affinity for oxygen than normal hemoglobin.A microcytic hypochromic anemia with target cells and Heinz bodies (precipitated HbH) on the peripheral blood smear can occur, as well as hepatosplenomegaly. The disease is noticed in childhood or in early adult life; anemia and hepatosplenomegaly are noted.	- -/- α
Four	This is known as alpha thalassemia major; these fetuses are edematous, have little circulating hemoglobin, and the hemoglobin that is present is all tetrameric γ chains. When all four alleles are affected, the fetus likely will not survive gestation without in utero intervention; most infants with alpha-thalassemia major are stillborn with hydrops fetalis. Fetuses treated with intrauterine transfusions throughout pregnancy starting at an early gestational age can survive to birth with acceptable morbidity. After birth, the treatment options include bone marrow transplantation or continued chronic transfusions.	- -/- -
α α/α α = normal: 'α α' before the '/' represents one chromosome, and 'α α' after the '/', its homologous chromosome.

Laboratory Diagnosis

• Initial laboratory diagnosis should include a complete blood count and red blood cell indices. As well, a peripheral blood smear should be carefully reviewed.

• Hemoglobin analysis is important for the diagnosis of alpha-thalassemia as it determines the types and percentages of types of hemoglobin present. Several different methods of hemoglobin analysis exist, including hemoglobin electrophoresis, capillary electrophoresis and high-performance liquid chromatography.

• Molecular analysis of DNA sequences (DNA analysis) can be used for the confirmation of a diagnosis of alpha-thalassemia, particularly for the detection of alpha-thalassemia carriers (deletions or mutations in only one or two alpha-globin genes).

Treatment

Treatment for alpha-thalassemia may include blood transfusions to maintain hemoglobin at a level that reduces symptoms of anemia. The decision to initiate transfusions depends on the clinical severity of the disease. Splenectomy is a possible treatment option to increase total hemoglobin levels in cases of worsening anemia due to an overactive or enlarged spleen, or when transfusion therapy is not possible.However, splenectomy is avoided when other options are available due to an increased risk of serious infections and thrombosis.

Additionally, gallstones may be a problem that would require surgery. Secondary complications from febrile episode should be monitored, and most individuals live without any need for treatment.

Additionally, stem cell transplantation should be considered as a treatment (and cure), which is best done in early age. Other options, such as gene therapy, are still being developed.

A study by Kreger et al combining a retrospective review of three cases of alpha thalassemia major and a literature review of 17 cases found that in utero transfusion can lead to favorable outcomes. Successful hematopoietic cell transplantation was eventually carried out in four patients.

Epidemiology

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Malaria

Worldwide distribution of inherited alpha-thalassemia corresponds to areas of malaria exposure, suggesting a protective role. Thus, alpha-thalassemia is common in sub-Saharan Africa, the Mediterranean Basin, and generally tropical (and subtropical) regions. The epidemiology of alpha-thalassemia in the US reflects this global distribution pattern. More specifically, HbH disease is seen in Southeast Asia and the Middle East, while Hb Bart hydrops fetalis is acknowledged in Southeast Asia only.The data indicate that 15% of the Greek and Turkish Cypriots are carriers of beta-thalassaemia genes, while 10% of the population carry alpha-thalassaemia genes.

See also

• Beta-thalassemia
• Delta-thalassemia
• Hemoglobinopathy

NIH genetic and rare disease info

• NIH info on Alpha-thalassemia

Alpha-thalassemia is a rare disease.