Alpha-Thalassemia

Introduction

Alpha-thalassemia is defined as an inherited hemolytic anemia mainly due to deletion found on the alpha globin gene(s) leading to reduced production or absence of alpha-globin chains needed for the formation of the hemoglobin tetramer.

Hemoglobinopathies, a group of genetic disorders, affect the structure or production of hemoglobin, the oxygen-carrying protein in red blood cells. These alterations in hemoglobin can lead to a range of conditions, including:

• Sickle cell disease (SCD): A severe hemoglobinopathy caused by a single point mutation in the beta-globin gene that results in the production of abnormal hemoglobin S. Hemoglobin S molecules tend to polymerize under low-oxygen conditions, causing red blood cells to sickle or adopt a crescent-like shape. This sickling can obstruct blood flow, leading to pain, tissue damage, and a range of complications.
• Thalassemia: A group of disorders characterized by reduced or absent production of either alpha-globin (alpha-thalassemia) or beta-globin chains (beta-thalassemia), the two components of normal hemoglobin. Thalassemia can present with mild to severe symptoms, depending on the type and severity of the genetic defect.
• Hemoglobin variants: These are less common hemoglobinopathies that involve single amino acid substitutions in the alpha-globin or beta-globin chains. Some variants may have no noticeable effects, while others can cause mild symptoms or even life-threatening complications.

Hemoglobinopathies are inherited disorders, meaning they are passed from parents to children through genes. The risk of developing hemoglobinopathy depends on the carrier status of the parents. If both parents carry a hemoglobinopathy gene, their offspring have a higher chance of inheriting the disorder.

What is Thalassemia? 

Thalassemia arises from mutations in the genes responsible for producing hemoglobin. These mutations lead to either reduced or absent production of alpha-globin or beta-globin chains, the two components of normal hemoglobin. Thus, thalassemia can be divided into alpha and beta thalassemia. 

The alpha-globin gene cluster is found on chromosome 16 while the beta-globin gene cluster is found on chromosome 11. The genes in the cluster are arranged according to the order of development and at different stages of development, different hemoglobin subtypes are formed to have different oxygen affinities. 

Global Prevalence and Significance of Alpha-Thalassemia

Alpha-thalassemia is one of the most prevalent single-gene blood disorders worldwide, affecting an estimated 270 million carriers, with the highest prevalence in Southeast Asia, the Mediterranean region, and parts of Africa and the Middle East. This widespread prevalence underscores the significant global burden of alpha-thalassemia and its impact on individuals and healthcare systems worldwide.

Genetic basis of Alpha-Thalassemia

There are two alpha-globin genes also known as HBA2 and HBA1 in a single allele while there is only one beta globin gene on a single allele. 

When a person has alpha-thalassemia, the production of the beta-globin chains is not affected while the production of the alpha-globin chain specifically is reduced or absent depending on the mutation involved. A hemoglobin needs 2 alpha- and 2 beta-globin chains to form a functional tetramer, thus when the alpha-globin is quantitatively reduced, there will be less Hb A available and free beta-globin chains will precipitate to form beta-globin tetramers which are also known as Hb H. Hemoglobin H occurs only with extreme limitation of alpha chain availability. 

Alpha-globin expression is very high even from very young fetal age compared to beta-globin which only rises postnatally. This indicates the importance of this gene and severe mutation of this gene can cause death in utero.

Genetic Inheritance of Alpha-Thalassemia

Alpha-thalassemia follows an autosomal recessive inheritance pattern. This means that the genes responsible for alpha-globin synthesis are located on non-sex chromosomes (autosomes), and both copies of the gene need to be mutated for the disorder to manifest.

To understand the inheritance pattern, let’s consider the two alleles (gene variants) of the alpha-globin gene: α and αs. The normal α allele produces functional alpha-globin chains, while the mutated αs allele produces either reduced or no alpha-globin chains.

An individual with two normal α alleles (αα) is considered a non-carrier of alpha-thalassemia and will not develop the disorder. An individual with one normal α allele and one mutated αs allele (αα^T) is considered a carrier of alpha-thalassemia. Carriers typically do not experience any symptoms or health complications, but they can pass the mutated α^T allele to their offspring.

The inheritance pattern becomes more apparent when considering couples with different carrier statuses.

Scenario 1: Both parents are αα^T carriers

In this scenario, each parent has one normal α allele and one mutated α^T allele. When they have a child, there are four possible outcomes:

1. 25% chance of the child having two normal α alleles (αα) and being a non-carrier
2. 50% chance of the child having one normal α allele and one mutated αs allele (αα^T) and being a carrier
3. 25% chance of the child having two mutated αs alleles (α^Tα^T) and developing alpha-thalassemia

Scenario 2: One parent is αα non-carrier and the other is αα^T carrier

In this scenario, one parent has two normal α alleles and is not a carrier, while the other parent has one normal α allele and one mutated αs allele. When they have a child, there are two possible outcomes:

1. 50% chance of the child having two normal α alleles (αα) and being a non-carrier
2. 50% chance of the child having one normal α allele and one mutated αs allele (αα^T) and being a carrier

Scenario 3: One parent is αα non-carrier and the other is α^Tα^T has alpha-thalassemia 

In this scenario, one parent has two normal α alleles and is not a carrier, while the other parent has two mutated α^T alleles and has alpha-thalassemia. When they have a child, there are two possible outcomes:

1. 50% chance of the child having one normal α allele and one mutated αs allele (αα^T) and being a carrier
2. 50% chance of the child having two mutated αs alleles (α^Tα^T) and developing alpha-thalassemia

Understanding the genetic inheritance of alpha-thalassemia is crucial for genetic counseling and carrier screening. Carrier screening allows individuals to determine their risk of passing the mutated α^T allele to their offspring, enabling informed decisions about family planning and preventing the transmission of alpha-thalassemia to future generations.

What are the types of mutations in alpha-thalassemia?

Alpha thalassemia mutations are mainly deletions and they usually delete one or both cis-alpha globin genes. 

• Complete abolishment or deactivation of both alpha globin genes in the same allele is known as alpha⁰-thalassemia. 
• Deletions affecting only a single gene deletion in the allele are known as alpha⁺-thalassemia. 
• There are also non-deletional alpha-plus thalassemia that are usually point mutations for example Hemoglobin Constant Spring and Hemoglobin Adana. Non-deletional alpha thalassemia has a more deleterious effect on the red blood cells as they are unstable and precipitate to cause membrane damage and release oxidative reactive species causing oxidative stress to the red cells. 

Pathogenesis of Alpha-Thalassemia

The anemia caused by alpha-thalassemia is usually dependent on how many genes are deleted and as there are 4 alpha globin genes in a person.

• Deletion on only one alpha globin gene is barely noticeable and sometimes could not even be picked up by a full blood count as the values are near normal.
• Deletion of 2 genes will give rise to alpha thalassemia trait or minor and has slight microcytic hypochromic anemia picture.
• Deletion of 3 alpha globin genes leads to Hb H disease. 
• Deletion of all four alpha globin genes can have a lethal effect. 

Clinical Picture of Alpha-Thalassemia

Alpha-thalassemia presents with a spectrum of clinical manifestations ranging from mild anemia to severe life-threatening complications. The severity of the condition depends on the number of affected alpha-globin genes.

Silent Carriers: No Noticeable Symptoms

Silent carriers of alpha thalassemia, individuals with one mutated alpha-globin gene, typically have no noticeable symptoms or signs of anemia. Their hemoglobin levels may be slightly lower than normal, but this typically doesn’t cause any health problems. Silent carriers can pass the mutated gene to their offspring, but they themselves do not experience any adverse effects from alpha-thalassemia.

Alpha-Thalassemia Minor: A Mild Form of the Disorder

Alpha thalassemia minor, also known as alpha thalassemia trait, is the most common form of the disorder, affecting approximately 5% of the world’s population. Individuals with alpha thalassemia minor have two mutated alpha-globin genes. They typically experience mild anemia, with hemoglobin levels slightly lower than normal. Other symptoms may include fatigue, weakness, and pale skin. However, these symptoms are usually mild and do not require any specific treatment.

Hemoglobin H Disease: A Moderate Form with Variable Symptoms

Hemoglobin (Hb) H disease, also known as alpha thalassemia intermedia, is an intermediate form of the disorder, characterized by the inheritance of three mutated alpha-globin genes. Hb H is the tetrameric formation of 4 beta-globin chains (β₄) due to the lack of alpha-globin chains and in turn lots of excess beta-globin chains. Individuals with hemoglobin H disease have moderate anemia and may experience symptoms such as fatigue, weakness, and pale skin. However, the severity of symptoms varies greatly from person to person. Some individuals with hemoglobin H disease may not experience any significant symptoms, while others may require occasional blood transfusions during periods of stress or illness.

Alpha Thalassemia Major: A Severe Form Requiring Regular Blood Transfusions

Alpha thalassemia major, the most severe form of the disorder, arises from the inheritance of four mutated alpha-globin genes. Individuals with alpha thalassemia major have significantly reduced or absent alpha-globin chain production, leading to Hb Bart’s hydrops fetalis which usually causes death in utero as it is incompatible with life outside the womb. Hydrops fetalis is a life-threatening condition characterized by excessive fluid accumulation in the fetus. Hydrops fetalis can lead to heart failure, respiratory failure, and death before or shortly after birth. Hb Bart is the tetrameric formation of 4 gamma-globin chains (ɣ₄). Hemoglobin Bart’s has an extremely high affinity for oxygen, meaning it binds oxygen very tightly. This makes it difficult for the hemoglobin to release oxygen to the tissues, leading to tissue hypoxia (inadequate oxygen supply).

Laboratory Investigations

Peripheral Blood Smear

Hypochromic microcytic anaemia with some target cells can be seen in the peripheral blood smear and golf ball inclusions can be seen in brilliant cresyl blue staining (BCB). 

The staining for Hb H inclusion bodies uses brilliant cresyl blue (BCB) or methylene blue (MB) as an oxidant to denature Hb H as intracellular inclusions. This method almost effectively confirms the presence of Hb H disease. 

HPLC

High performance liquid chromatography (HPLC) is used to quantitatively determine the different hemoglobin subtypes available in the blood as each hemoglobin subtype has a different molecular charge and will elute at different retention times which allows them to be separated. 

Hb H is a fast eluting subtype and thus will be seen as small peaks in the beginning of the run. 

Hb Bart’s is also a fast eluting band and is easily seen as peaks in the beginning of the HPLC run too. 

Capillary electrophoresis

Capillary electrophoresis is an upgraded technique to give a better resolution and separation of the hemoglobin subtypes. HPLC and capillary electrophoresis are gaining in popularity because these methods are more automated, the instruments are more user friendly, and they can be used to confirm hemoglobin variants observed with electrophoresis.

Hemoglobin electrophoresis

Hemoglobin electrophoresis is an old method which is based on the separation of hemoglobin molecules in an electric field primarily as a result of differences in total molecular charge. There are alkaline and acidic electrophoresis available. 

In alkaline electrophoresis, hemoglobin molecules assume a negative charge and migrate toward the anode (positive pole). Historically, alkaline haemoglobin electrophoresis was performed on cellulose acetate medium, but it is being replaced by agarose medium. Nonetheless, because some haemoglobins have the same charge and therefore the same electrophoretic mobility patterns, hemoglobins that exhibit an abnormal electrophoretic pattern at an alkaline pH may be subjected to electrophoresis at an acid pH for definitive separation. 

In an acid pH some haemoglobins assume a negative charge and migrate toward the anode, whereas others are positively charged and migrate toward the cathode (negative pole). 

Laboratory Interpretations

Silent carriers

Silent carriers of alpha-thalassemia only have one of the four genes deleted and there is a minimal decrease in the alpha-globin synthesis as it is well compensated by the other 3 functional alpha globin genes so the alpha/beta chain ratio is still quite balanced. They are usually asymptomatic and with near normal red cell indices. Through the blood count and blood film, silent carriers have minimal reduction in the red cell indices and are often undetectable in the peripheral blood film. 

Alpha-thalassemia trait

The alpha-thalassemia trait patients have mild anemia with MCV around 65-76 fL and MCH around 22 pg. Microcytic hypochromic red cells can be seen in the blood film. 

Patients with alpha-thalassemia trait have 2 genes deleted whether cis or trans-deletions. They have mild anemia and the microcytic and hypochromic cells are more prominent and easily detected through a full blood count and blood smear. 

HbH disease

Deletions of 3 out of the four genes will lead to Hb H disease. This compound heterozygosity leads to moderate anemia and an excess of free beta-globin chains that will precipitate to form Hb H. 

Hb H will be able to be detected using the HPLC or Hb electrophoresis and the Hb H golf ball inclusions can be seen in the peripheral blood smear using the brilliant cresyl blue staining technique. 

Non-deletional Hb H disease usually has a more severe picture. There is an increase in the reticulocyte count with microcytic hypochromic red cells, anisopoikilocytosis and target cells. Hb H inclusions can be seen in the blood film stained with brilliant cresyl blue stain. 

Alpha Thalassemia Major

Deletion of all four alpha-globin genes is incompatible with life outside of the uterus. Absence of alpha globin chain production will lead to free gamma globin chains that will precipitate to form Hb Bart’s. Hb Barts has an extremely high oxygen affinity which prevents the release of oxygen to be used by the tissue thus causing tissue hypoxia and major organ failures in utero. Hydrops fetalis babies have severe anemia and macrocytic red cells. There is severe anisopoikilocytosis with microcytic hypochromic red cells and nucleated red blood cells can be seen in the blood film. This will lead to hydrops fetalis babies, who die in utero or shortly after birth if no action is taken. The confirmation of alpha globin mutations can be done through GAP PCR for deletional mutations or ARMS PCR for non-deletional mutations. 

Treatment and Management of Alpha-Thalassemia

Silent carriers and alpha thalassaemia trait individuals do not require any treatment or management as they are asymptomatic. However, they should still be tested and their spouse if they plan to have a family and genetic counselling is required. 

Deletional hemoglobin H disease usually does not require any transfusion, however, iron overload does occur in adults and sometimes some form of renal damage. They should be monitored as they age. 

Non-deletional Hb H disease, however, may require frequent transfusion and iron chelation therapy due to iron overload as well as splenectomy if indicated. Patients who are planning to have a family will receive genetic counselling in order for them to understand their choices and the consequences of the disorder. 

Parents are usually given the option to terminate the pregnancy if alpha thalassemia major is found. Recent developments for hydrops foetalis include intrauterine blood transfusions which allow the baby to survive long enough for delivery and later for stem cell transplantation. There has also been a clinical trial on in utero hematopoietic stem cell transplantation. Clinical research on alpha thalassaemia is still ongoing.

Key Points for Alpha-Thalassemia

Definition:  An inherited hemolytic anemia due deletions or point mutation (uncommon) of the α-globin gene leading to reduced production or absence of α-chains needed for hemoglobin. It has an autosomal recessive inheritance pattern.

Genetics:

The normal α genotype is written as αα/αα. Mutations affecting α₂ causes more severe anemia compared to α₁ mutations as 75% of α chains are produced by α₂.

Terminologies: 

α⁰-thalassemia: deletion of both α genes on the same allele (–/αα)

α⁺-thalassemia: deletion of single α gene (-α/αα)

α^T-thalassemia: non-deletional mutations (α^Tα/αα)

Epidemiology: Highest prevalence in Southeast Asia followed by West Africa, Mediterranean, Middle East, India and Pacific Islands 

Classifications:

	Silent carrier state	α-Thalassemia minor	Hb H disease	Hb Barts hydrops fetalis syndrome
Pathogenesis
Signs and Symptoms	Asymptomatic	Asymptomatic	Chronic hemolytic anemia, splenomegaly, jaundice	Severely anemic, hydropic fetus
Pathophysiology	α/β chain ratio is almost balanced thus no hematologic abnormalities		Low α chain production → excess unpaired β chains → Hb H (β4) → prone to oxidation → precipitation forming inclusion bodies →  hemolytic anemia	No α chain production → excess unpaired γ chains → Hb Barts (γ4) → very high oxygen affinity →severely anoxic fetus
Laboratory investigations	Near normal with slight reduction in MCV & MCH	Hb normal or slightly reduced. Low MCV and MCH	FBC: Anemia (Hb 7 – 11 g/dL), variable reticulocytes count (5 – 10%). PBF: Microcytic hypochromic RBCS with marked poikilocytosis including target cells. Brilliant cresyl blue stain display golf ball-like inclusion bodies in RBCs. Bone marrow: erythroid hyperplasia	PBF: Severe microcytic hypochromic anemia (Hb 3 – 8 g/dL) with numerous nucleated RBCs.Bone marrow: marked erythroid hyperplasia
	None required		Intermittent transfusions	Abortion as hydropic pregnancies lead to toxemia and severe postpartum hemorrhage
Management	Genetic counselling and prenatal diagnosis recommended

Non-deletional α-thalassemia: Symptoms can be more severe than deletional α-thalassemia when inherited with α⁰-thalassemia

Rare forms of α-thalassemia: α-thalassemia retardation-16 (ATR-16) syndrome and α-thalassemia X-linked intellectual disability (ATRX) syndrome. Physical deformities, intellectual deformities are present with Hb H disease. 

Disclaimer: This article is intended for informational purposes only and is specifically targeted towards medical students. It is not intended to be a substitute for informed professional medical advice, diagnosis, or treatment. While the information presented here is derived from credible medical sources and is believed to be accurate and up-to-date, it is not guaranteed to be complete or error-free. See additional information.

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