DiagnosisClinical DiagnosisAlpha-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.TestingHematologic TestingRed blood cell indices show microcytic anemia in HbH disease or α-thalassemia trait; indices are usually normal in silent carriers and macrocytic in Hb Bart syndrome as a result of extreme reticulocytosis and megaloblastoid erythropoiesis (Table 1).Table 1. Red Blood Cell Indices in Adults with Alpha-ThalassemiaView in own windowRed Blood Cell IndicesNormalAffectedCarrier ^{1MaleFemaleHemoglobin Bart hydrops fetalis 2Hemoglobin H disease 3Alpha-thalassemia trait 4(--/αα or -α/-α)Alpha-thalassemia silent carrierMean corpuscular volume (MCV, fl)89.1±5.0187.6±5.5136±5.1Children: 56±5
Adults: 61±471.6±4.181.2±6.9Mean corpuscular hemoglobin (MCH, pg)30.9±1.930.2±2.131.9±918.4±1.222.9±1.326.2±2.3Hemoglobin (Hb, g/dL)15.9±1.014.0±0.93-8Male: 10.9±1.0
Female: 9.5±0.8Male: 13.9±1.7
Female: 12.0±1.0Male: 14.3±1.4
Female: 12.6±1.21. Higgs & Bowden [2001]2. Vaeusorn et al [1985]3. Galanello et al [1992]4. Alpha-thalassemia carriers with the two gene in cis (--/αα) genotype have slightly lower RBC indices.ReticulocytosisHb Bart syndrome. Variable, may be more than 60%HbH disease. Moderate, between 3% and 6%Peripheral blood smearHb Bart syndrome. Large, hypochromic red cells and severe anisopoikilocytosisHbH disease. Microcytosis, hypochromia, anisocytosis, poikilocytosis (spiculated tear-drop and elongated cells), and very rare nucleated red blood cells (i.e., erythroblasts)Carriers. Reduced MCV, MCH, and RBC morphologic changes that are less severe than those in affected individuals; erythroblasts are not seen.Supravital stain to detect RBC inclusion bodies. HbH inclusions (β₄ tetramers) can be demonstrated in 5% to 80% of the erythrocytes of individuals with HbH disease following incubation of fresh blood smears with 1% brilliant cresyl blue (BCB) for four to 24 hours. Small amounts of inclusions can also be detected in subjects with α-thalassemia trait and the silent carrier state as well.Qualitative and quantitative hemoglobin analysis (by cellulose acetate electrophoresis, weak-cation high-performance liquid chromatography [HPLC], and supplemental techniques such as isoelectric focusing and citrate agar electrophoresis) identifies the amount and type of Hb present. Hb types most relevant to α-thalassemia:Hemoglobin A (HbA). Two α-globin chains and two β-globin chains (α₂β₂)Hemoglobin H (HbH). Four β-globin chains (β₄)Hemoglobin Bart (Hb Bart). Four γ-globin chains (γ₄)Hemoglobin Portland. Two ζ-globin chains and two γ-globin chains (ζ₂γ₂)The Hb pattern in α-thalassemia varies by α-thalassemia type (Table 2).Table 2. Hemoglobin Patterns in Alpha-Thalassemia (Age >12 Months)View in own windowHemoglobin TypeNormalAffectedCarrierHb Bart hydrops fetalis syndrome 1HbH disease 2Alpha-thalassemia trait 3Alpha-thalassemia silent carrier 4HbA96%-98%060%-90%96%-98%96%-98%HbF<1%0<1.0%<1.0%<1.0%Hb Bart085%-90%2%-5%00HbH000.8%-40%00HbA2 2%-3%0<2.0%1.5%-3.0%2%-3%1. Deletion or inactivation of all four α-globin chains makes it impossible to assemble HbF and HbA. Fetal blood contains mainly Hb Bart (γ4) and 10%-15% of the embryonic hemoglobin Portland (ζ2γ2).2. Deletion or inactivation of three α-globin chains3. Deletion or inactivation of two α-globin chains either in cis configuration (--/αα) or in trans configuration (-α/-α); also known as αº-thalassemia4. Deletion or inactivation of one of the α-globin gene (-α/αα); also known as α+-thalassemiaIn HbH disease, bone marrow is extremely cellular, mainly as a result of marked erythroid hyperplasia.Note: Bone marrow examination is usually not necessary for diagnosis of affected individuals.Newborn screening for sickle cell disease offered by several states/countries may detect Hb Bart in the newborn with α-thalassemia (see National Newborn Screening Status Report; pdf).Notes: (1) Newborns with concentrations of Hb Bart greater than 15% need further evaluation (i.e., clinical and hematologic evaluation and molecular genetic testing), as they may develop HbH disease. (2) Low concentrations of Hb Bart (1%-8%) are indicative of the carrier states and usually do not need further evaluation. Reference ranges may very among laboratories performing newborn screening.Molecular Genetic TestingGenes. 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 encoding ζ-globin and a cis-acting regulatory element, HS-40, located 40 kb upstream of HBZ. All regulatory-element and trans-acting mutations causing α-thalassemia also ultimately alter expression of all these genes:HBA1 is the α₁ gene.HBA2 is the α₂ gene; it is 20 kb away from the embryonic ζ gene (Figure 1).FigureFigure 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, (more...)Clinical testingTargeted mutation analysisDeletions. (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 (-α3.7) and the 4.2-kb leftward deletion (-α4.2).
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 embryonic HBZ) 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., -α3.7), the result is HbH disease.Sequence variants such as HbConstant Spring (HbCS), 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.An expanded multiplex gap-PCR genotyping assay has been developed for the simultaneous detection of HbConstant Spring and deletions that give rise to alpha-thalassemia [Kidd et al 2010].Table 3. Summary of Molecular Genetic Testing Used in Alpha-ThalassemiaView in own windowGene SymbolTest MethodMutations DetectedPercent of AllelesMutation Detection Frequency by Test Method 1Test AvailabilityHBA1 and HBA2 Targeted mutation analysisDeletions 2 ~90% 3Variable ClinicalHBA2 sequence variants 4, 5Variable 6Variable 6Sequence analysis HBA1, HBA2 sequence variants 5 ~9%-10% 4 Theoretically 100%Deletion / duplication analysis 7Deletions and duplicationsUnknownVariable 61. The ability of the test method used to detect a mutation that is present in the indicated gene2. 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.3. Varies by population4. Targeted analysis of known sequence variants by restriction endonuclease digestion or other direct DNA methods5. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations; typically, exonic or whole-gene deletions/duplications are not detected.6. Varies by population, testing laboratory, and test method7. Testing that identifies deletions/duplications not readily detectable by sequence analysis of the coding and flanking intronic regions of genomic DNA; included in the variety of methods that may be used are: quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), and chromosomal microarray (CMA) that includes this gene/chromosome segment.Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Testing StrategyThe following screening tests can be used if α-thalassemia is suspected:Red blood cell indicesPeripheral blood smearRed blood cell supravital stain of peripheral bloodQualitative and quantitative hemoglobin analysisTo confirm the diagnosis in a proband. Molecular genetic testingCarrier testing for at-risk relatives. Molecular genetic testing is requested in the parents of individuals with Hb Bart syndrome and HbH disease.Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutations in the family.Genetically Related (Allelic) DisordersAlpha-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 HBA1 and HBA2 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 chromosomal microarray [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 ATRX 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 intellectual disability syndrome is NOT an allelic disorder (see Differential Diagnosis).}

Natural HistoryThe 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. 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.The majority of individuals show microcytic hypochromic hemolytic anemia (Table 1), enlargement of the spleen and less commonly the liver, mild jaundice, and sometimes mild-to-moderate thalassemia-like skeletal changes (such as hypertrophy of the maxilla, bossing of the skull, and prominence of the malar eminences) that mainly affect the facial features.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; in very rare cases hydrops fetalis is present [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 CorrelationsThe 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. The most and least severe (respectively) are: non-deletion HBA2, -α^3.7 (because of compensatory increase of the α-globin gene output from the remaining HBA1) 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 SpringA 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 HbConstant 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, HbConstant Spring (2.6%-11.6%), and Hb Bart.Deletions of the HS-40 regulatory region found approximately 40 kb telomeric to HBZ (Figure 1) cause αº-thalassemia and have been reported in a few families with HbH disease [Higgs 2001]. The phenotype is like that of the deletion type of HbH disease.Hb Bart syndrome results from deletion of four α-globin chains and rarely may involve non-deletion defects.}

Differential DiagnosisHydrops fetalis is associated with many conditions in addition to Hb Bart, including immune-related disorders (alloimmune hemolytic disease or Rh isoimmunization), fetal cardiac anomalies, chromosomal abnormalities, fetal infections, genetic disorders, and maternal and placental disorders. The combination of a hydropic fetus with a very high proportion of Hb Bart, however, is found in no other condition.Note to clinicians: For a patient-specific ‘simultaneous consult’ related to Hb Bart, go to Image , an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).Hemoglobin H diseaseHemolytic anemias. HbH disease can be distinguished from other hemolytic anemias by: (1) microcytosis, which is uncommon in other forms of hemolytic anemia; (2) the fast-moving band (HbH) on hemoglobin electrophoresis; (3) the presence of inclusion bodies (precipitated HbH) in red blood cells after vital stain; and (4) absence of morphologic or enzymatic changes characteristic of other forms of inherited hemolytic anemia (e.g., hereditary spherocytosis/elliptocytosis, G6PD deficiency).Alpha-thalassemia X-linked intellectual disability (ATRX) syndrome is a rare form of α-thalassemia characterized by distinctive craniofacial features, genital anomalies, and severe developmental delays with hypotonia and intellectual disability [Gibbons 2006]. Affected individuals usually have a normal 46,XY karyotype. Genital anomalies, observed in 80% of children, range from hypospadias, micropenis, and undescended testicles to ambiguous genitalia. Global developmental delays are evident in infancy, and some affected individuals never walk independently or develop significant speech. Affected individuals do not reproduce. Inheritance is X-linked.
An unknown percent of affected 46,XY individuals have a mild form of HbH disease, evident as hemoglobin H inclusions (β₄ tetramers) in erythrocytes following incubation of fresh blood smears with 1% brilliant cresyl blue (BCB).
Mutations in ATRX are causative. The ATR-X protein is a novel member of the SWI2/ SNF2 family of molecular motors that remodel chromatin using hydrolysis of ATP as a source of energy. The chromatin remodeling alters the access of DNA to trans-acting factors, thereby influencing transcription, replication, repair, and methylation and thus regulating the expression of a restricted class of genes including the α-globin genes [Picketts et al 1998, Higgs 2004].
Note: In ATRX syndrome, the α-globin gene cluster and the HS-40 regulatory region of chromosome 16 are structurally intact.Acquired mutations in ATRX can arise in myelodysplastic syndrome and cause an acquired form of HbH disease (See Genetically Related Disorders, Acquired α-thalassemia).Note to clinicians: For a patient-specific ‘simultaneous consult’ related to HbH disease, go to .Carrier states (αº-thalassemia and α+-thalassemia)Beta-thalassemia. Whereas microcytosis and hypochromia are present in αº-thalassemia carriers, hematologically manifesting α⁺-thalassemia carriers, and β-thalassemia carriers, β-thalassemia carriers are distinguished by a high percent of HbA2.Iron deficiency anemiaThe αº-thalassemia carrier state and the hematologically evident forms of α⁺-thalassemia can be confused with iron-deficiency anemia because MCV and MCH are lower than normal in both conditions. However, in iron-deficiency anemia, the red blood cell count is decreased, while it is usually increased in αº-thalassemia carriers.Though there is some overlap, individuals with iron deficiency anemia show a marked increase in red blood cell distribution width (RDW), a quantitative measure of RBC anisocytosis. The RDW is usually normal or close to normal in thalassemia.The determination of the RBC zinc protoporphyrin concentration and iron studies (serum iron concentration, transferrin saturation) can be used to diagnose iron deficiency anemia with certainty.Iron deficiency and thalassemia can coexist, complicating diagnosis.Note to clinicians: For a patient-specific ‘simultaneous consult’ related to the following, go to :For αº-thalassemia carriersFor α+-thalassemia carriers

ManagementEvaluations Following Initial DiagnosisTo 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 ManifestationsHb Bart syndrome currently has no effective treatment. However, in some cases noninvasive monitoring by Doppler ultrasonography, intrauterine transfusions, and hematopoietic stem cell transplantation (HSCT) have improved the prognosis of individuals with this disorder [Vichinsky 2009]. These advances in intrauterine and postnatal therapy have resulted in ethical dilemmas for the family and health care provider. HbH diseaseMost 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 ManifestationsHb Bart hydrops fetalis syndromeEarly 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 postnatal 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 ComplicationsHbH diseaseDuring febrile episodes, a clinical evaluation is recommended because of the increased risk of hemolytic/aplastic 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. Use of antimicrobial prophylaxis notwithstanding, a careful clinical evaluation of splenectomized individuals with fever is recommended.SurveillanceHbH diseaseHematologic evaluation every six to 12 months to determine the steady state levels of hemoglobinIn children, assessment of growth and development every six to 12 monthsMonitoring 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 supplementationAgents/Circumstances to AvoidHbH diseaseInappropriate iron therapyOxidant drugs including sulphonamides; some antimalarials because of the risk of hemolytic crisisEvaluation of Relatives at RiskThe 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.Pregnancy ManagementDuring pregnancy several complications have been reported in women with HbH disease, including worsening of anemia (with occasional need of red cell transfusions), preeclampsia, congestive heart failure, and threatened miscarriage [Origa et al 2007]. Monitoring for these possible complications is recommended.Therapies Under InvestigationSearch ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Molecular GeneticsInformation in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Alpha-Thalassemia: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDHBA116p13​.3Hemoglobin subunit alphaHbVar: A Database of Human Hemoglobin Variants and Thalassemias
HBA1 @ LOVDHBA1HBA216p13​.3Hemoglobin subunit alphaHbVar: A Database of Human Hemoglobin Variants and Thalassemias
HBA2 @ LOVDHBA2HBZ16p13​.3Hemoglobin subunit zetaHbVar: A Database of Human Hemoglobin Variants and ThalassemiasHBZData are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Alpha-Thalassemia (View All in OMIM) View in own window 141800HEMOGLOBIN--ALPHA LOCUS 1; HBA1 141850HEMOGLOBIN--ALPHA LOCUS 2; HBA2 142310HEMOGLOBIN--ZETA LOCUS; HBZ 604131ALPHA-THALASSEMIAMolecular Genetic PathogenesisHBA1, HBA2, and HBZAlpha-globin genes are duplicated (HBA1 and HBA2) and lie in the telomeric region of chromosome 16 (16p13.3) within a cluster that also contains an embryonic α-like gene (HBZ) and three pseudogenes (HBZP (ψ-ζ), HBAP1 (ψ-α₁), HBM (ψ-α₂). A θ gene (HBQ1) with an unknown function is located at the 5' end of the cluster (See Figure 1).HBA1 and HBA2 are embedded within two markedly homologous regions that extend for approximately 4 kb. Their sequence homology is maintained by gene conversion and unequal crossover events. In this DNA region, three highly homologous segments, named X, Y, and Z, separated by non-homologous segments, have been defined (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.The expression of HBA1 and HBA2 is regulated by a region (HS-40) located 40 kb upstream from the α-globin cluster (Figure 1). This region contains multiple binding sites for transcriptional factors (NF-E2, GATA-1). The deletion of HS-40 results in an α-thalassemia phenotype, in spite of the structural integrity of both α-globin genes.Normal allelic variants. Both HBA1 and HBA2 have three coding exons. The mRNAs produced by HBA1 and HBA2 have identical coding regions and can be distinguished only by their 3' UTR.Pathologic allelic variants. See Table 4. Deletion of one or both HBA1 and HBA2 is the most common cause of α-thalassemia: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, 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 molecular mechanisms leading to the silencing of either HBA1 or HBA2 include: mutations involving RNA splicing, the poly (A) additional signal, the initiation of mRNA translation, as well as missense mutation of the termination, in-frame deletions, frame-shift mutations, and nonsense mutations. Mutations of α-globin genes that result in the production of hyper-unstable globin variants such as Hb^{Quong Sze} and that are unable to assemble into stable β₄ tetramers and are thus rapidly degraded, may also result in α-thalassemia (Table 1) [Higgs 2001].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.)Table 4. Selected HBA1 and HBA2 Pathologic Allelic Variants View in own windowDNA Nucleotide Change ^{1(Standard Nomenclature 2) Protein Amino-Acid Change 1 or Functional Globin Genes Deleted 3
(Standard Nomenclature 2)Reference Sequences(HBA2:c.2T>C)Alpha2 initiation codon Met>Thr; -α NcoI of HBA2(HBA2:p.Met1Thr)NM_000517​.4
NP_000508​.1(HBA2:c.377T>C)Alpha2 Leu125Pro,
Hb Quong Sze(HBA2:p.Leu126Pro)(HBA2:c.427T>C)Alpha2 142, Stop>Gln
HbConstant Spring (HbCS)
(HBA2:p.X143Glnext32)Codon 30/31 2-bp deletion
(HBA2:c.94_95delAG)The deletion of 2 nucleotides causes a frameshift & premature termination at codon(TAA)
(HBA2:p.Arg32Aspfs*24)HBA2:c.[339C>G ; 340_351delCTCCCCGCCGAG]Alpha2 His112Gln and deletion of codons 113-116 -
Leu-Pro-Ala-Glu,
Hb Lleida
(HBA2:p.His113Gln; p.Leu114_ Glu117del)NM_000517​.4
NP_000508​.1Splicing sites -αHphI α, HphI digestion for the pentanucleotide HBA2 IVS-1 deletion
(HBA2:c.95+2_95+6delTGAGG)--HBA1:c.223G>CAsp74Gly, HbQ-Thailand(HBA1:p.Asp75Gly)NM_000558​.3
NP_000549​.1HBA2:c.[207C>G (or HBA1) or 207C>A]Asn68Lys, HbG Philadelphia(HBA2 or HBA1 p:Asn69Lys)NM_000517​.4
NP_000508​.1PolyA addition site of HBA2
(AATAAA >AATAAG)
(HBA2:c.*+94A>G)Alpha2 αTSaudi-α IN: 2 bp del
(c.[-2_-3delAC; -α3.7] 4)Deletion of HBA2 and of nucleotides that additionally impair translation -α3.7Deletion of HBA2Z84721​.1-α4.2Deletion of HBA2-α5.2 Deletion of HBA2 and 5’ end of HBA1 5-α20.5(g.15164_37864del22701) 6Deletion of HBA2 and 5’ end of HBA1 −−FIL(g.11684_43534del31851) 6Deletion of HBA2 and HBA1 −−MED(g.24664_41064del16401) 6Deletion of HBA2 and HBA1 −−SEA (g.26264_45564del19301) 6Deletion of HBA2 and HBA1 −−THAI(g.10664_44164del33501) 6Deletion of HBA2 and HBA1See Quick Reference for an explanation of nomenclature.1. Globin mutations are given by their conventional nomenclature (globin​.cse.psu.edu/)2. Standard naming conventions of the Human Genome Variation Society (www​.hgvs.org), as listed by the Globin Gene Server (globin​.cse.psu.edu)3. Only functional globin genes involved in the deletion are given; deleted pseudogenes are not listed.4. Denotes two variations in one allele: deletion of AC at -2 and -3 before ATG initiation codon in cis configuration on an -α3.7 deletion allele [Viprakasit et al 2003]5. Pressley et al [1980]6. Coordinates from entries in the Globin Gene Server (globin​.cse.psu.edu); it is not known if all deletions in these categories will have precisely the same nucleotide coordinates.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.Abnormal gene product. The consequence of a single α-globin gene deletion is reduced production of α-globin chains by the affected chromosome (α+-thalassemia). Measurement of α-globin mRNA indicates that the -α4.2 mutation is not associated with a compensatory increase in expression in the remaining HBA1, whereas with the -α3.7 mutation, the remaining HBA1 expression is roughly halfway between that of normal HBA2 and HBA1 (Figure 1).}