Alpha Thalassemia 

The oxygen carrying capability of the red blood cells (RBC) relies on hemoglobin, a tetramer protein consisting of 2 pairs of globin chains bound to the heme molecule. There are 4 major types of globins labeled as alpha (α), beta (β), gamma (γ), and delta (δ). The dominant hemoglobin in adults (hemoglobin A) is composed of 2 alpha and 2 beta chains. This is achieved by the very tightly controlled globin chain production maintaining the ratio of alpha to non-alpha chains 1.00 (± 0.05). Thalassemia refers to a spectrum of diseases characterized by reduced or absent production of one or more globin chains, thus disrupting this ratio.

See the image below.

Minor forms of hemoglobins constitute a small percentage of the normal blood and are referred to as hemoglobin F (fetal), composed of 2 alpha chains and 2 gamma chains, and hemoglobin A2, composed of 2 alpha chains and 2 delta chains.

Relative excess of beta chains due to impaired production of alpha globin results in less stable chains. This leads to the clinical disease known as alpha thalassemia. Similarly, impaired production of beta globin chains manifests with a more severe disease known as beta thalassemia.^{[1, 2]}

The absence of normal production of a-chains results in a relative excess of γ-globin chains in the fetus and newborn, and β-globin chains in children and adults. Further, the β-globin chains are capable of forming soluble tetramers (beta-4, or HbH); yet this form of hemoglobin is unstable and tends to precipitate within the cell forming insoluble inclusions (Heinz bodies) that damage the red cell membrane. Furthermore, diminished hemoglobinization of individual red blood cells results in damage to erythrocyte precursors and ineffective erythropoiesis in the bone marrow, as well as hypochromia and microcytosis of circulating red blood cells.

Genes that regulate both synthesis and structure of different globins are organized into 2 separate clusters. The a-globin genes are encoded on chromosome 16 and the γ, δ, and β-globin genes are encoded on chromosome 11. Each individual normally carries a linked pair of a-globin genes, 2 from the paternal chromosome, and 2 from the maternal chromosome. Alpha thalassemia results when there is disturbance in production of α-globin from any or all four of the α-globin genes.

Normal hemoglobin biosynthesis requires an intact gene, silencers, enhancers, promoters, and locus control region (LCR) sequences. Several hundred mutations causing thalassemia have been described. These may affect any step in globin gene expression, transcription, pre-mRNA splicing, mRNA translation and stability, and post-translational assembly and stability of globin polypeptides.

The most common mechanism of aberrant a-globin production is due to deletions of either portions of the a-globin genes themselves or the genetic regulatory elements that control their expression. Regulatory elements may be located on the same chromosome (cis acting elements) or on separate chromosomes (trans acting elements).

Production of functional hemoglobin is also impaired in alpha thalassemia when point mutations, frame shift mutations, nonsense mutations, and chain termination mutations occur within or around the coding sequences of the a-globin gene cluster. These gene level mutations may in turn affect RNA splicing, initiation of mRNA translation, or result in the generation of unstable a-chain variants.

Mutations affecting transcription, pre-mRNA splicing, or canonical splice signals are rare causes of alpha thalassemia. Other forms of alpha thalassemia are caused by either premature or failed translation termination. More rare mutations have been found to cause thalassemia by interfering with the normal folding of otherwise normal globin peptide.

From a genetic standpoint, alpha thalassemias are extremely heterogeneous; however, phenotypic expression of alpha thalassemias may be described in simplified clinical terms related to the number of alpha globin genes affected:

• Alpha (0) thalassemia – More than 20 different genetic mutations that result in the functional deletion of both pair of a-globin genes have been identified. Individuals with this disorder are not able to produce any functional a-globin and thus are unable to make any functional hemoglobin A, F, or A2. This leads to the development of hydrops fetalis, also known as hemoglobin Bart, a condition that is incompatible with extra uterine life.
• Alpha (+) thalassemia – There are more than 15 different genetic mutations that result in decreased production of a-globin usually due to the functional deletion of 1 of the 4 alpha globin genes.^{[3, 4]} Based on the number of inherited alpha genes, alpha (+) thalassemia is subclassified into 3 general forms:

  • A- Thalassemia (-α/α α) is characterized by inheritance of 3 normal α-genes. These patients are referred to clinically as silent carrier of alpha thalassemia. Other names for this condition are alpha thalassemia minima, alpha thalassemia-2 trait, and heterozygosity for alpha (+) thalassemia minor. The affected individuals exhibit no abnormality clinically and may be hematologically normal or have mild reductions in red cell mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH).
  • B- Inheritance of 2 normal alpha genes due to either heterozygosity for alpha (0) thalassemia (α α/--) or homozygosity for alpha (+) thalassemia (-α/-α) results in the development of alpha thalassemia minor or alpha thalassemia-1 trait. The affected individuals are clinically normal but frequently have minimal anemia and reduced mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH). The red blood cell count is usually increased to over 5.5 x 1012/L.
  • C- Inheritance of one normal alpha gene (-α/--) results in abundant formation of hemoglobin H composed of tetramers of excess beta chains. This condition is known as HbH disease. The affected individuals have moderate to severe lifelong hemolytic anemia, modest degrees of ineffective erythropoiesis, splenomegaly, and variable bony changes.

Frequency

United States

Recent reports suggest an increasing incidence of all subtypes of alpha thalassemia in the United States secondary to immigration of individuals from endemic areas. It is estimated that about 15% of American blacks are silent carriers for α-thalassemia. In addition, α-thalassemia trait (minor) occurs in 3% of American blacks and in 1-15% of persons of Mediterranean origin.

According to the National Institutes of Health sponsored North American Thalassemia Clinical Research Network (TCRN) study of the epidemiology of thalassemia in North America, 59% of patients with alpha thalassemia have the (-α/--) genotype, 8% have 4 alpha gene deletion (--/--), and 33% have gene deletions with structural mutations.

International

It is estimated that there are 270 million carriers of mutant globin genes that can potentially cause severe forms of thalassemia. In addition, 300,000-400,000 severely affected infants are born every year, more than 95% of which occur in Asia, India, and the Middle East.

Before the introduction of DNA analysis, population surveys for alpha thalassemia were based entirely on the measurement of hemoglobin Bart levels in cord blood. However, single gene deletion heterozygotes do not always have detectable hemoglobin Bart in the neonatal period. As a result, reliable data on population frequencies for various types of alpha thalassemia are not always available.^[5]

Alpha thalassemia is common throughout parts of the world where malaria is endemic. Multiple studies have suggested that the presence of both single and double α -globin gene deletions confer a protective effect from malaria. Listed below are the approximate percentages of various populations with some forms of alpha thalassemia:

• Europe – 4-12%
• Middle East and western Asia - 12-55%^[6]
• Southeast Asia – 6-75%
• Africa – 11-50%
• South America and the Caribbean - 7%

Mortality/Morbidity

The morbidity and mortality of alpha thalassemia are related to the degree of imbalanced globin production and, therefore, correlate well with the number of affected α-globin genes. Individuals with milder alpha thalassemia phenotypes, including those with single and double gene deletions (-α/αα, --/ αα, -α/-α) have mild anemia as the only major morbidity associated with their disease^[7] . Patients with hemoglobin H (HbH) disease may develop hypersplenism, gallstones, leg ulcers, frequent infections, and various forms of venous thrombosis. The most severe form of alpha thalassemia, hemoglobin Bart is characteristic of individuals with no functional α-globin genes (--/--). Following a gestation of about 33 weeks, these infants develop hydrops fetalis syndrome and usually die in utero, during delivery, or within an hour or two of birth.

Race

Abnormalities affecting the α-globin genes have been documented in almost all ethnic groups yet are much more common in people of Asian, African, and Mediterranean heritage. The North American Thalassemia Clinical Research Network (TCRN) study showed that 85% of patients with alpha thalassemia are Asian, 4% are white, and 11% are of other ethnicities, including African, black, mixed ethnicity, and unknown.

Sex

Abnormalities of α-globin genes are equally distributed between males and females. A notable exception is the unusual alpha thalassemia associated with mental retardation, known as alpha thalassemia mental retardation-X syndrome (ATR-X), which affects exclusively males.^[8]

However, a recent report by Haas et al identified 2 females in a single center with alpha thalassaemia myelodysplastic syndrome (ATMDS) and mutations in the ATR-X gene (ATRX).^[8] The investigators observed that although it was possible females may be less likely to develop ATMDS if the inactivated copy of ATRX is reactivated throughout life, this hypothesis was ruled out in their study by the use of a cross-sectional analysis of healthy females aged newborn to 90 years to examine the pattern of ATRX inactivation.^[8]

Age

Alpha thalassemia is a genetic disorder, thus patients are born with the disorder, with the exception of patients with ATMDS, in which case patients are usually elderly with a mean age at diagnosis of 68 years.