Alpha-Thalassemia

Clinical Diagnosis

Alpha-thalassemia (α-thalassemia) has two clinically significant forms:

• Hemoglobin Bart hydrops fetalis (Hb Bart) syndrome, the most severe form of α-thalassemia, is characterized by fetal onset of generalized edema, ascites, pleural and pericardial effusions, and severe hypochromic anemia, in the absence of ABO or Rh blood group incompatibility. It is usually detected by ultrasonography at 22 to 28 weeks' gestation and can be suspected in an at-risk pregnancy at 13 to 14 weeks' gestation when increased nuchal thickness, possible placental thickness, and increased cardiothoracic ratio are present. Death in the neonatal period is almost inevitable. All four α-globin alleles are deleted or dysfunctional (inactivated).

• Hemoglobin H (HbH) disease should be suspected in an infant or child with a mild-to-moderate (rarely severe) microcytic hypochromic hemolytic anemia and hepatosplenomegaly. Mild thalassemia-like bone changes are present in approximately one-third of affected individuals. Unlike Hb Bart syndrome, HbH disease is compatible with survival into adulthood. HbH disease is a result of deletion or dysfunction of three of four α-globin alleles.

Alpha-thalassemia also has two carrier states:

• Alphaº-thalassemia generally results from deletion or dysfunction of two α-globin genes, in cis (--/αα) (See Molecular Genetic Testing)

• Alpha⁺-thalassemia usually results from deletion or dysfunction of one α-globin gene. Homozygosity for α⁺ thalassemia results in an α-thalassemia trait hematologic phenotype.

Testing

Molecular Genetic Testing

GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.

Genes. HBA1, the gene encoding α₁-globin, and HBA2, the gene encoding α₂-globin, are the two genes associated with α-thalassemia. They are localized to the telomeric region of chromosome 16p in a cluster containing the embryonically expressed HBZ gene encoding ζ-globin and a cis-acting regulatory element, HS-40, located 40 kb upstream of the HBZ gene. All regulatory-element and trans-acting mutations causing α-thalassemia also ultimately alter expression of all these genes:

• HBA1 is the α₁ gene.

• 

  

  Figure 1. Diagram of the α-globin gene cluster. (more...)

  

  Figure 1. Diagram of the α-globin gene cluster. In order from centromere to telomere, the genes include: α1 (HBA1, encoding α1-globin); α2 (HBA2, encoding α2-globin); the pseudogenes ψα1, ψα2, and ψζ1, which are not expressed; and ζ2 (HBZ, encoding ζ2-globin). HS-40 is a cis-acting regulatory element located 40 kb upstream of the HBZ gene. X, Y, and Z are homologous regions separated by non-homologous DNA regions; they constitute evolutionary duplication units. During meiosis, misalignment of chromosome homologs followed by reciprocal recombination at X, Y, or Z results in deletion-duplication events, i.e., the loss of a single α-globin gene on one chromosome and the triplication of α-globin genes on the other chromosome.

  HBA2 is the α₂ gene; it is 20 kb away from the embryonic ζ gene (Figure 1).

Clinical testing

• Targeted mutation analysis

  • Deletions. (1) Polymerase chain reaction (PCR)-based methods that use specific primers flanking the deletion breakpoints detect deletion of a single α-globin gene (α⁺-thalassemia mutations) and deletion of both α-globin genes on one chromosome 16 (αº-thalassemia mutations). Primer panels targeted to the most common mutations found in the area of geographic origin of the proband can be used. [Galanello et al 1998, Chong et al 2000, Old 2001]. (2) Southern blot analysis or multiple ligation-dependent probe amplification (MLPA) [Harteveld et al 2005] may be used to detect less common or novel deletions.

    Deletion of a single α-globin gene (α⁺-thalassemia mutations). Reciprocal recombination between either the Z boxes (Figure 1), which are 3.7 kb apart, or the X boxes, which are 4.2 kb apart, deletes one α-globin gene. The two resulting deletions are referred to respectively as the 3.7-kb rightward deletion (-α^3.7) and the 4.2-kb leftward deletion (-α^4.2).

    In addition to these two common deletions, three rare deletions involving a single α-globin gene have been reported.

    Deletion of both α-globin genes on one chromosome (αº-thalassemia mutations). More than 20 different deletions ranging from approximately 6 kb to more than 300 kb and removing both α-globin genes (and sometimes the embryonic HBZ gene) have been reported. The most common are −−^SEA, −−^FIL, −−^MED, which in the homozygous state result in Hb Bart syndrome. When any of these alleles occur in combination with another allele carrying a single α-globin gene deletion (e.g., -α^3.7), the result is HbH disease.

  • Sequence variants such as Hb^{Constant Spring} (Hb^CS), a common missense mutation of the termination codon of HBA2, lead to an elongated protein chain and can be detected indirectly by digestion of the amplified gene by MseI restriction endonuclease by ARMS or directly by sequence analysis [Tangvarasittichai et al 2005]. Several of the non-deletion mutations, which downregulate α-globin expression or result in a dysfunctional protein α chain, create or destroy a restriction enzyme site and may be detected by restriction enzyme digestion of the amplified product (i.e., NcoI digestion for detection of initiation codon mutation of HBA2 [HBA2:c.2T>C] and HphI digestion for the pentanucleotide HBA2 IVS-1 deletion [HBA2:c.95+2_95+6delTGAGG]).

• Sequence analysis can be used to identify point mutations (including rare termination codon mutations and hyperunstable α-globin variants) in the coding regions of HBA1 and HBA2 when an α-globin deletion is not identified and suspicion for α-thalassemia is high [Traeger-Synodinos et al 2000].

  Note: A mutation in the HS-40 regulatory region located 40 kb upstream from the α-globin cluster, described in several pedigrees, would not be detected by DNA analysis of the alpha cluster region alone.

• Deletion/duplication analysis can be used to detect common, rare, and/or novel deletions and duplications involving HBA1 and HBA2.

Table 3 summarizes molecular genetic testing for this disorder.

Table 3. Molecular Genetic Testing Used in Alpha-Thalassemia

Gene Name	Test Method	Mutations Detected	Percent of Alleles	Mutation Detection Frequency by Test Method	Test Availability
HBA1 and HBA2	Targeted mutation analysis	Deletions 1	~90% 2	Variable	Clinical
		HBA2 sequence variants 3	Variable 4	Variable 4
	Sequence analysis	HBA1, HBA2 sequence variants	~ 9%–10%3	Theoretically 100%
	Deletion/ duplication analysis	Deletions and duplications	Unknown	Variable 4

1. May detect both deletions of a single α-globin gene and two α-globin gene deletions (either --/αα or -α/-α) on one chromosome. Deletions detected may vary among laboratories.

2. Varies by population

3. Targeted analysis of known sequence variants by restriction endonuclease digestion or other direct DNA methods

4. Varies by population, testing laboratory, and test method

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Testing Strategy

The following screening tests can be used if α-thalassemia is suspected:

• Red blood cell indices

• Peripheral blood smear

• Red blood cell supravital stain of peripheral blood

• Qualitative and quantitative hemoglobin analysis

To confirm the diagnosis in a proband. Molecular genetic testing

Carrier testing for at-risk relatives. Molecular genetic testing is requested in the parents of individuals with Hb Bart syndrome and HbH disease.

Prenatal diagnosis is requested in pregnancies at risk for Hb Bart syndrome.

Genetically Related (Allelic) Disorders

Alpha-thalassemia retardation-16 (ATR-16) syndrome, a contiguous gene deletion syndrome, results from a large deletion on the short arm of chromosome 16 from band 16p13.3 to the terminus, which removes the HBA1 and HBA2 genes together with other flanking genes. Among the few reported individuals with deletion of 16p (without deletion or duplication of other genomic material), microcephaly and short stature were variable; IQ ranged from 53 to 76 [Lindor et al 1997, Gibson et al 2008]. Facial features are distinctive; talipes equinovarus (club foot) is common, as are hypospadias and cryptorchidism in males [Lindor et al 1997]. Typically, hematologic features are those of the α-thalassemia trait reflecting deletion of HBA1 and HBA2 in cis configuration (i.e., --/ αα ). While routine cytogenetic studies may be sufficient to identify the deletion, in some instances, cryptic subtelomeric deletions are detected only with subtelomeric FISH studies, MLPA, or array CGH [Harteveld et al 2007, Gibson et al 2008]. The deletion may be de novo or inherited from a parent who carries a balanced chromosome rearrangement.

Acquired α-thalassemia (α-thalassemia-myelodysplastic syndrome [ATMDS]). In the context of a clonal myeloid disorder such as myelodysplastic syndrome, somatic mutations causing an acquired form of α-thalassemia-HbH disease in individuals who were previously hematologically normal may arise [Steensma et al 2005]. Red cell indices are usually hypochromic and microcytic, in contrast to the normocytic or macrocytic indices typical of myelodysplastic syndrome. Although most cases of ATMDS have been linked to mutations in the ATRX gene on the X chromosome [Gibbons et al 2003, Steensma et al 2004a], acquired deletions of chromosome 16p may be causative [Steensma et al 2004b]. For unknown reasons, some individuals with myeloid disorders have small amounts (<1%) of HbH.

Alpha-thalassemia X-linked mental retardation syndrome is NOT an allelic disorder. (See Differential Diagnosis)

Natural History

The clinically significant phenotypes of alpha-thalassemia (α-thalassemia) are hemoglobin Bart hydrops fetalis (Hb Bart) syndrome and hemoglobin H (HbH) disease. The severity of the α-thalassemia syndromes depends on the extent of α-globin chain defect (See Genotype-Phenotype Correlations).

Hb Bart syndrome is the most severe clinical condition related to α-thalassemia. It is usually not compatible with postnatal life. Affected fetuses are either stillborn or die soon after birth. Red cells with Hb Bart have an extremely high oxygen affinity and are incapable of effective tissue oxygen delivery.

The clinical features are severe anemia, marked hepatosplenomegaly, diffuse edema, heart failure, and extramedullary erythropoiesis.

Developmental abnormalities, including hydrocephaly and cardiac and urogenital defects, have been reported.

Maternal complications during pregnancy commonly include: preeclampsia (hypertension, edema, and proteinuria), polyhydramnios (excessive amniotic fluid) or oligohydramnios (reduced amniotic fluid), antepartum hemorrhage, and premature delivery.

HbH disease. The phenotype of HbH disease varies [Chui et al 2003, Origa et al 2007]. Although clinical features usually develop in the first years of life, it may not present until adulthood or may be diagnosed only during routine hematologic analyses in asymptomatic individuals.

Individuals with HbH disease may develop hypersplenism and gallstones and experience acute episodes of hemolysis in response to oxidant drugs and infections.

While the majority of individuals with HbH disease have minor disability, some are severely affected, requiring regular blood transfusions; very rarely, some have hydrops fetalis [Lorey et al 2001, Chui et al 2003].

Iron overload is uncommon but has been reported in older individuals, usually as a result of repeated blood transfusions or increased iron absorption.

Pregnancy is possible in women with HbH disease; however, worsening of anemia requiring blood transfusion has been reported [Origa et al 2007].

Genotype-Phenotype Correlations

The phenotype of the α-thalassemia syndromes depends on the degree of α-globin chain deficiency relative to β-globin production. The correlation between different α-thalassemia mutations, α-globin mRNA levels, α-globin synthesis, and clinical manifestations of α-thalassemia is well documented. The wide spectrum of hematologic and clinical phenotypes results from the presence and interaction of many α-thalassemia mutations.

The different α-thalassemia mutations vary widely in severity. From most to least severe, they are: non-deletion HBA2, -α^3.7 (because of compensatory increase of the α-globin gene output from the remaining HBA1 gene), and non-deletion HBA1. For the -α^4.2 deletion, evidence is inconclusive for a compensatory increase in the expression of the remaining α gene. The phenotype may be modified by triplication or quadruplication of the α-globin genes on one chromosome.

Alpha-thalassemia silent carrier results from a deletion or "non-deletion" mutation that inactivates one of the two α-globin genes (i.e., HBA1 or HBA2) on one chromosome (α⁺-thalassemia).

Non-deletion α⁺-thalassemia defects include the following:

• Inactivating point mutations, including those important for gene expression (initiation codon mutation [HBA2:c.2T>C]); splicing sites (HBA2:c.95+2_95+6delTGAGG); termination codon Hb^{Constant Spring}

• A frameshift caused by a deletion/insertion in the coding regions (i.e., (HBA2:c.94_95delAG), HBA2:c.[339C>G; 340_351delCTCCCCGCCGAG])

• Rarely, very rapid post-synthetic degradation of a hyper-unstable α-globin variant. Non-deletion forms of α-thalassemia mainly occur in HBA2.

Carriers of α⁺ thalassemia may have a completely silent hematologic phenotype or may present with a moderate, thalassemia-like hematologic picture (i.e., reduced MCV and MCH, but normal HbA2 and HbF), similar to carriers of αº-thalassemia (See Alpha-thalassemia trait).

Alpha-thalassemia trait results from deletion or inactivation of two α-globin genes (--/αα in cis configuration or -α/-α in trans configuration). Carriers of αº-thalassemia show microcytosis (low MCV), hypochromia (low MCH), normal percentages of HbA2 and HbF, and RBC inclusion bodies.

Note: While the phenotype between cis configuration and trans configuration may not vary significantly, the genetic counseling implications are significant. See Genetic Counseling.

HbH disease results from deletion or inactivation of three α-globin genes, usually as a result of the compound heterozygous state for αº-thalassemia and α⁺-thalassemia. The phenotype of HbH disease (chronic microcytic, hypochromic hemolytic anemia of variable severity) mainly correlates with the severity of the α⁺-thalassemia defect:

• Individuals with non-deletion HbH disease have a more severe phenotype with earlier presentation, more severe anemia, jaundice, bone changes, and greater hepatosplenomegaly. As a consequence of the more severe hematologic phenotype, they may need red cell transfusions more frequently than individuals with deletion HbH.

• Individuals who are homozygous for non-deletion α-thalassemia defect (i.e., 2 of 4 α genes affected, but both with non-deletion mutations) may have HbH disease. For example, homozygotes for Hb^{Constant Spring} show a mild hemolytic anemia. Red blood cell indices are characterized by low red blood cell count, normal MCV, and slightly decreased MCV. Hb electrophoresis shows HbA - HbA2, Hb^{Constant Spring} (2.6%-11.6%), and Hb Bart.

Figure 1

Hb Bart syndrome results from deletion of four α-globin chains and rarely may involve non-deletion defects.

Nomenclature

The α-thalassemias are classified on the basis of the total globin production from each of the two α-globin genes present on each chromosome 16:

• When both α-genes on a chromosome are deleted (--/αα) or inactivated, the condition is called α°-thalassemia (no output of α-globin from the chromosome). Previously, α° thalassemia was called α-thalassemia 1.

• When one α-gene on a chromosome is deleted or inactivated by a mutation (i.e., non-deletion mutation) the condition is called α⁺ thalassemia (previously known as α-thalassemia 2). In this case, some α globin is produced. Non-deletion mutants are indicated as (α^NDα/) or (α^Tα/).

Prevalence

Since the early 1960s, prevalence of α-thalassemia has been determined in several populations using the percent Hb Bart in cord blood. However, because not all newborns with α-thalassemia (mainly those with α⁺-thalassemia) have increased Hb Bart, the prevalence of α-thalassemia derived from this measure may be underestimated.

More precise data have been obtained using restriction endonuclease analysis. For detailed references for the frequency of α-thalassemia in each population, see Bernini [2001].

Africa

• The highest gene frequency (0.30-0.40) of the -α^3.7allele (causing α⁺-thalassemia) has been observed in the equatorial belt including Nigeria, Ivory Coast, and Kenya.

• Alphaº-thalassemia has been reported very rarely in North Africa and in the African-American population.

The Mediterranean

• Alpha⁺-thalassemia (caused by the -α^3.7allele) is common, with the highest frequency reported in Sardinia (0.18) and the lowest in Spain.

• Alphaº-thalassemia is very rare (0.002); thus, Hb Bart hydrops fetalis is only rarely reported.

• A remarkable aspect of α-thalassemia (mainly α⁺) in the Mediterranean population is the heterogeneity of mutations, particularly the non-deletion mutations.

The Arabian Peninsula

• Gene frequencies of the -α^3.7 allele (causing α⁺-thalassemia) vary from 0.01 to 0.67, with the highest values being observed in Oman.

• Alphaº-thalassemia determinants are extremely rare.

India

• Deletion α⁺-thalassemia reaches very high frequency (0.35-0.92) in the Indian tribal population of Andra Pradesh; in other tribes, the frequency is much lower (0.03-0.12). Both the -α^3.7 allele and the -α^4.2 allele variably contribute to the deletion α⁺-thalassemia.

• Alphaº-thalassemia is very rare.

Southeast Asia

• Alphaº-thalassemia (−−^SEA, −−^THAI, −−^FIL) and α⁺-thalassemia are very common, causing a major public health burden.

• The non-deletion form of α⁺-thalassemia, caused by the Hb^{Constant Spring} allele, is common.

• The incidence of Hb Bart hydrops fetalis is expected to be between 0.5 and five per 1000 births and HbH disease between four and 20 per 1000 births.

Oceania

• The distribution of α⁺-thalassemia, extensively studied by DNA-based methods, follows a pattern consistent with the degree of malaria endemicity. The prevalence of α⁺-thalassemia is low in the highlands and high in the coastal areas and the lowlands where malaria is hyperendemic.

• The specific mutations causing α-thalassemia have not been identified in all areas; for example, some affected individuals on the island of Vanuatu have normal α-globin genes without deletions or mutations. Mutations in a regulatory element are suspected.

• Alphaº-thalassemia is very rare.

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with alpha-thalassemia (α-thalassemia), the following phenotype-based evaluations are recommended:

• Hemoglobin Bart hydrops fetalis (Hb Bart) syndrome. See Prenatal Testing.

• Hemoglobin H (HbH) disease. Differentiation of deletion (mild) from non-deletion (moderate-to-severe) forms of HbH disease by appropriate molecular genetic testing of HBA1 and HBA2 is important at presentation because of varying severity.

Treatment of Manifestations

Hb Bart syndrome currently has no effective treatment.

HbH disease

• Most individuals with HbH disease are clinically well and survive without any treatment.

• Occasional red blood cell transfusions may be needed if the hemoglobin level suddenly drops because of hemolytic or aplastic crises.

• Chronic red blood cell transfusions should be considered in selected individuals only. Clear indications for red blood cell transfusions are severe anemia affecting cardiac function and massive erythroid expansion, resulting in severe bone changes and extramedullary erythropoiesis. (Note: These events are quite rare in HbH disease.)

• Splenectomy should be performed only in case of massive splenomegaly or hypersplenism; but the risk of severe, life-threatening venous thrombosis should be considered.

• Other complications, such as gallstones and leg ulcers, require appropriate medical or surgical treatment.

Prevention of Primary Manifestations

Hb Bart hydrops fetalis syndrome

• Early treatment with intrauterine transfusions or in utero hematopoietic stem cell transplantation has been unsuccessful and also may be not be justified in view of the unknown future risks for normal development. In fact, these neonates have marked cardiopulmonary problems and a high frequency of congenital malformations (patent ductus arteriosus, limb and genital abnormalities) in addition to the hematopoietic failure. In those infants surviving the immediate post-natal period, subsequent development has been abnormal. All these infants obviously require regular blood transfusions and iron chelation therapy. Given these results, further human experimentation should be discouraged until more effective therapies (e.g., somatic gene therapy) are available.

• Because of the severity of Hb Bart hydrops fetalis syndrome and the risk of maternal complications during the pregnancy with a fetus with this disorder, prenatal diagnosis and early termination of at-risk pregnancies are usually recommended.

Prevention of Secondary Complications

HbH disease

• During febrile episodes, a clinical evaluation is recommended because of the increased risk of hemolytic crisis (similar to G6PD deficiency, hemolysis in HbH disease can be triggered by infection or oxidative stresses).

• When chronic blood transfusions are instituted for individuals with HbH disease, the management should be the same as for all individuals who have been polytransfused, including use of iron chelation therapy (see Beta-Thalassemia).

• Some clinicians recommend folic acid supplementation, as for other hemolytic anemias.

• If splenectomy is required, antimicrobial prophylaxis is usually provided, at least until age five years, to decrease the risk of overwhelming sepsis caused by encapsulated organisms. Despite the use of antimicrobial prophylaxis, a careful clinical evaluation of splenectomized individuals with fever is recommended.

Surveillance

HbH disease

• Hematologic evaluation every six to 12 months to determine the steady state levels of hemoglobin

• In children, assessment of growth and development every six to 12 months

• Monitoring of iron load with annual determination of serum ferritin concentration in individuals who have been transfused, in older individuals, and in those given inappropriate iron supplementation

Agents/Circumstances to Avoid

HbH disease

• Inappropriate iron therapy

• Oxidant drugs including sulphonamides; some antimalarials, because of the risk of hemolytic crisis

Testing of Relatives at Risk

The sibs of a proband should be tested as soon as possible after birth to determine if they have HbH disease so that appropriate monitoring can be instituted.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions. Note: Therapy studies for this disorder may not currently be ongoing.

Other

Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

See Consumer Resources for disease-specific and/or umbrella support organizations for this disorder. These organizations have been established for individuals and families to provide information, support, and contact with other affected individuals.

Mode of Inheritance

Alpha-thalassemia (α-thalassemia) is usually inherited in an autosomal recessive manner.

Risk to Family Members — Hemoglobin Bart Hydrops Fetalis (Hb Bart) Syndrome

Parents of a proband

• The parents of a fetus with Hb Bart hydrops fetalis are obligate heterozygotes for two α-globin gene deletions (αº-thalassemia or α-thalassemia trait).

• Individuals with α-thalassemia trait typically have mild hypochromia (low MCH) microcytosis (low MCV) anemia and normal HbA2 and HbF.

Sibs of a proband

• At conception, each sib of a proband with Hb Bart hydrops fetalis has a 25% chance of having Hb Bart hydrops fetalis syndrome, a 50% chance of having αº-thalassemia (α-thalassemia trait), and a 25% chance of being unaffected and not a carrier.

• Once an at-risk sib is known to be unaffected, the risk of his/her having αº-thalassemia (α-thalassemia trait) is 2/3.

Offspring of a proband. The Hb Bart hydrops fetalis syndrome is usually not compatible with postnatal life.

Other family members of a proband. Each sib of the proband’s parents is at 50% risk of being a carrier of two α-globin gene deletions (αº-thalassemia or α-thalassemia trait).

Individuals with α-thalassemia trait typically have mild hypochromia (low MCH), microcytosis (low MCV), anemia, and normal HbA2 and HbF.

Risk to Family Members — Hemoglobin H (HbH) Disease

Parents of a proband

• The parents of a child with HbH disease usually have different types of αº-thalassemia (α-thalassemia trait) and α⁺-thalassemia (α-thalassemia silent carrier) mutations:

  • Most commonly, one parent has αº-thalassemia and the other parent has α⁺-thalassemia. Rarely, one parent has αº-thalassemia and the other parent has α⁺-thalassemia resulting from a non-deletion mutation.

  • Uncommonly, both parents carry a specific HBA2 non-deletion α-thalassemia mutation (α^TSaudi, -α^{IN: 2 bp del}) or an initiation codon mutation (HBA2:c.2T>C).

• Individuals with αº-thalassemia typically have mild microcytosis and normal HbA2 and HbF.

• Individuals with α⁺-thalassemia are either hematologically normal or have a mild reduction of MCV and MCH.

Sibs of a proband

• At conception, each sib of an individual with HbH disease has a 25% chance of having HbH disease, a 25% chance of having αº-thalassemia (α-thalassemia trait), a 25% chance of having α⁺-thalassemia (α-thalassemia silent carrier), and a 25% chance of being unaffected and not a carrier.

• Once an at-risk sib is known to be unaffected, the risk of his/her having either αº-thalassemia or α⁺-thalassemia is 2/3.

Offspring of a proband

• Each child of an individual with HbH disease inherits the mutation for either αº-thalassemia (α-thalassemia trait) or α⁺-thalassemia (α-thalassemia silent carrier) and is thus an obligate heterozygote.

• Given the high carrier rate of αº-thalassemia in certain populations, it is appropriate to offer carrier testing to the reproductive partner of an individual with αº-thalassemia.

Other family members of a proband with either Hb Bart hydrops fetalis or HbH disease. Each sib of the proband's parents is at risk of having α⁺-thalassemia (α-thalassemia silent carrier) and/or αº-thalassemia (α-thalassemia trait), depending on the genetic status of the relative.

Carrier Detection

Individuals who should be considered for carrier testing:

• Family members

• Members of ethnic groups at risk (see Prevalence)

• Gamete donors for assisted reproductive technologies (ART)

Alpha⁺-thalassemia carriers (α-thalassemia silent carriers) may have normal hematologic findings or may have a moderate, thalassemia-like hematologic picture similar to that of a carrier of αº-thalassemia. Identification relies on molecular genetic testing of the α-globin genes, HBA1 and HBA2.

Population Screening

Alphaº-thalassemia. Because of the high carrier rate for αº-thalassemia in certain populations and the availability of genetic counseling and prenatal diagnosis, it is ideal to screen couples who are members of at-risk populations for αº-thalassemia carrier status to identify those at risk of conceiving a fetus with Hb Bart hydrops fetalis syndrome prior to or in early pregnancy:

• If both members of a couple are carriers of an αº-thalassemia deletion mutation (e.g., genotype αα/--^SEA), each of their offspring has a 1/4 risk of having Hb Bart hydrops fetalis syndrome.

• If both members of the couple are carriers of the αº-thalassemia deletion mutation in which both HBA1 and HBA2 genes along with the HBZ are deleted (i.e., genotype αα/--^FIL or genotype of αα/--^THAI), they are not at risk of having offspring with Hb Bart hydrops fetalis syndrome, because homozygotes for such mutations are lost shortly after conception.

• If both members of the couple are carriers of a deletion involving both the HBA1 and HBA2 genes, but only one of them has a deletion that extends into the HBZ gene (e.g., a couple with genotypes αα/--^SEA and αα/--^FIL), the couple is at risk of having offspring with Hb Bart hydrops fetalis syndrome because the single HBZ gene in the fetus produces sufficient ζ-globin for fetal development [Chui & Waye 1998].

Alpha⁺-thalassemia. Prospective identification of carriers of α⁺-thalassemia is not strongly indicated, as the offspring of these carriers are not at risk for Hb Bart hydrops fetalis syndrome.

Related Genetic Counseling Issues

See Management for information on testing at-risk relatives for the purpose of early diagnosis and treatment.

Family planning

• The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.

• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who have HbH disease, are carriers, or are at risk of being carriers.

DNA banking. DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. DNA banking is particularly relevant when the sensitivity of currently available testing is less than 100%. See for a list of laboratories offering DNA banking.

Prenatal Testing

High-risk pregnancies. Prenatal testing is available for couples confirmed by DNA analysis to be at risk of having a fetus with Hb Bart hydrops fetalis syndrome because both parents are carriers of deletion αº-thalassemia. Molecular genetic testing can be performed either on fetal DNA extracted from cells obtained by chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation or by amniocentesis usually performed at about 15 to 18 weeks' gestation.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Ultrasound examination. Ultrasonography can also be useful in the management of pregnancies at risk for Hb Bart hydrops fetalis syndrome. In the first trimester, increased nuchal thickness, particularly in an at-risk pregnancy, should prompt appropriate evaluation.

Indeterminate-risk pregnancies. An indeterminate-risk pregnancy is a pregnancy in which:

• One parent is an αº-thalassemia carrier and the other has an α-thalassemia-like hematologic picture but no αº-thalassemia mutation identified by molecular genetic testing.

• The mother is a known αº-thalassemia carrier and the father is unknown or unavailable for testing. This is of concern if the father belongs to a population with a high carrier rate for αº-thalassemia.

In both cases, the options for prenatal testing should be discussed in the context of formal genetic counseling. In such cases, the strategy is fetal DNA analysis for the known αº-thalassemia mutation; if the known αº-thalassemia mutation is present, globin chain synthesis analysis is performed using a fetal blood sample obtained by percutaneous umbilical blood sampling (PUBS) at about 18 to 21 weeks' gestation.

Preimplantation genetic diagnosis (PGD). PGD may be available for families in which the disease-causing mutations have been identified. For laboratories offering PGD, see .

Molecular Genetic Pathogenesis

HBA1, HBA2, and HBZ

Figure 1

Figure 1

As a result of unequal genetic exchange, individuals who are phenotypically normal may have four, five, or six α-globin genes and two to six HBZ-like genes. HBA-like globin genes are arranged in the cluster in the order in which they are expressed during development. The genes encoding the α₁-globin chain (HBA1) and the α₂-globin chain (HBA2) display a marked homology resulting from repeated rounds of gene conversion.

The level of transcription of the two genes differs, as HBA2 produces two to three times more α-globin than HBA1. Regarding the translation profile of HBA1 mRNA and HBA2 mRNA, contrasting results in which percentages of HBA2 mRNA are higher or only slightly higher than percentages of HBA1 mRNA have been reported. The different expression of the two α-globin genes has important clinical implications for the amount of structural α-variant resulting from mutation of HBA1 or HBA2, and for the pathophysiology of the deletion and non-deletion pathologic variants of the HBA1 and HBA2 genes.

Figure 1

Normal allelic variants. Both HBA1 and HBA2 genes have three coding exons. The mRNAs produced by the HBA1 and HBA2 genes have identical coding regions and can be distinguished only by their 3' UTR.

• Alpha⁺-thalassemia. Reciprocal recombination between the Z boxes, which are 3.7 kb apart, or between the X boxes, 4.2 kb apart, gives rise to chromosomes with a single α-globin gene. The two resulting α-thalassemia mutations are referred to respectively as the 3.7-kb rightward deletion (-α^3.7) and the 4.2-kb leftward deletion (-α^4.2) (Figure 1):

  • In relation to the location of the crossover within the Z box, the -α^3.7 deletion is subdivided into three varieties named I, II, and III.

  • In addition to the -α^3.7 and the -α^4.2 common alleles, other rare deletions involving a single α-globin gene have been reported.

  • These recombinational events also result in the production of chromosomes containing three α-globin genes. A triplicated α-globin gene inherited with heterozygous β-thalassemia results in a mild thalassemia intermedia phenotype.

• Alphaº-thalassemia. Extended deletions varying from 100 kb to more than 250 kb and removing both α-globin genes (HBA1 and HBA2) (and sometimes the embryonic HBZ gene) result in the complete absence of α-chain production from that allele. Most such deletions are founder mutations that arose by one of several molecular mechanisms, including illegitimate recombination, reciprocal translocation, and truncation of chromosome 16. More than 20 different αº-thalassemia deletions have been reported to date:

  • The most common alleles are the Southeast Asian (--^SEA) and the Filipino (--^FIL) types.

  • Two deletion alleles, -α^5.2 and -α^20.5, which remove HBA2 and part of HBA1, produce αº-thalassemia [Higgs 2001].

  • A deletion removing HBA1 and the theta gene (HBQ1) and extending downstream centromeric from the α-globin gene cluster results in αº-thalassemia. The silencing of intact HBA2 in this chromosome is related to an antisense RNA transcribed from the widely expressed LUC7L gene, becoming juxtaposed to the normal HBA2 by the deletion, and running through the HBA2 sequences [Tufarelli et al 2003].

  • Nine deletions of the HS-40 region also result in the silencing of the intact α-globin genes, thereby producing αº-thalassemia [Higgs 2001].

Non-deletion α-thalassemia. Less frequently, α-thalassemia results from single point mutations or oligonucleotide insertion/deletion in regions critical for α-globin gene expression. In non-deletion α-thalassemia, the affected gene is denoted T (e.g., α^TSaudi). Considered as a group, the non-deletion α-thalassemia mutations appear to have a more severe effect on α-globin gene expression and hematologic phenotype than single α-globin gene deletions. This phenomenon may be explained by the majority of the mutations affecting HBA2, whose expression may predominate over HBA1 [Higgs 2001]. No compensatory increase in expression in the remaining functional α gene occurs when the other is inactivated by a point mutation, in contrast to the compensatory increase in expression in the remaining functional α gene when a single α-globin gene is deleted (e.g., the -α^3.7 deletion).

At present, more than 45 well-defined causes of non-deletion α-thalassemia are known.

The most common non-deletion mutation, which is frequently seen in Southeast Asia, is Hb^{Constant Spring} (Hb^CS), resulting from a mutation of the stop codon of HBA2. This mutation leads to the production of an α-globin chain elongated by 31 amino acids. Hb^CS is produced in very small amounts because its mRNA is unstable. Heterozygotes for Hb^CS and other rare elongated variants, along with the presence of the Hb variant, produce the αº-thalassemia phenotype.

Some of the mutations causing α-chain structural variants may occur in a chromosome with only one α-globin gene (e.g., HbQ^Thailand, HbG^Philadelphia). (For more information, see Table A: locus-specific databases and HGMD.)

Normal gene product: The α-globin chains produced by HBA1 and HBA2 mRNAs have identical amino acid sequences. The heterodimer protein hemoglobin A is made up of two α chains and two β chains.

Figure 1

Literature Cited

Published Statements and Policies Regarding Genetic Testing

Revision History

• 14 July 2008 (me) Comprehensive update posted live

• 1 November 2005 (me) Review posted to live Web site

• 3 January 2005 (rg) Original submission