Alpha-Thalassemia
Summary
Clinical characteristics.
Alpha-thalassemia (α-thalassemia) has two clinically significant forms: hemoglobin Bart hydrops fetalis (Hb Bart) syndrome (caused by deletion/inactivation of all four α-globin genes; --/--), and hemoglobin H (HbH) disease (most frequently caused by deletion/inactivation of three α-globin genes; --/-α).
- Hb Bart syndrome, the more severe form, is characterized by prenatal onset of generalized edema and pleural and pericardial effusions as a result of congestive heart failure induced by severe anemia. Extramedullary erythropoiesis, marked hepatosplenomegaly, and a massive placenta are common. Death usually occurs in the neonatal period.
- HbH disease has a broad phenotypic spectrum: although clinical features usually develop in the first years of life, HbH disease may not present until adulthood or may be diagnosed only during routine hematologic analysis in an asymptomatic individual. The majority of individuals have enlargement of the spleen (and less commonly of the liver), mild jaundice, and sometimes thalassemia-like bone changes. Individuals with HbH disease may develop gallstones and experience acute episodes of hemolysis in response to infections or exposure to oxidant drugs.
Diagnosis/testing.
The diagnosis of Hb Bart syndrome is established in a fetus with characteristic hematologic and hemoglobin (Hb) findings and molecular genetic testing that identifies biallelic pathogenic variants in both HBA1 and HBA2 that result in deletion or inactivation of all four α-globin alleles.
The diagnosis of HbH disease is established in a proband with hematologic and Hb findings and molecular genetic testing that identifies biallelic pathogenic variants in HBA1 and HBA2 that result in deletion or inactivation of three α-globin alleles.
Management.
Treatment of manifestations: Hb Bart syndrome: intrauterine blood transfusions, improved transfusion strategies, and rarely curative hematopoietic stem cell transplant may allow survival of children. HbH disease: while most individuals are clinically well and survive without any treatment, occasional red blood cell transfusions may be needed during hemolytic or aplastic crises. Red blood cell transfusions are very rarely needed for severe anemia affecting cardiac function and erythroid expansion that results in severe bone changes and extramedullary erythropoiesis. In contrast, persons with non-deletional HbH disease may be more severely affected and transfusion dependent.
Prevention of primary manifestations: Because of the severity of Hb Bart syndrome, the occasional presence of congenital anomalies, and the risk for maternal complications, prenatal testing and early termination of pregnancies at risk have usually been considered. However, recent advances in intrauterine and postnatal therapy have increased treatment options, thus complicating the ethical issues for health care providers and families facing an affected pregnancy.
Prevention of secondary complications: Monitor individuals with HbH disease for hemolytic/aplastic crisis during febrile episodes; in those who require chronic red blood cell transfusions, iron chelation therapy should be instituted; for those who are not red blood cell transfusion dependent, iron chelation with deferasirox can be considered to reduce liver iron concentration.
Surveillance: For HbH disease, hematologic evaluation every six to 12 months; assessment of growth and development in children every six to 12 months; monitoring of iron load with serum ferritin concentration and periodic quantitative measurement of liver iron concentration.
Agents/circumstances to avoid: In persons with HbH disease: inappropriate iron therapy and oxidant drugs (i.e., the same drugs to be avoided by individuals with glucose-6-phosphate dehydrogenase deficiency).
Evaluation of relatives at risk: Test the sibs of a proband as soon as possible after birth for HbH disease so that monitoring can be instituted.
Pregnancy management: Complications reported in pregnant women with HbH disease include worsening anemia, preeclampsia, congestive heart failure, and threatened miscarriage; monitoring for these issues during pregnancy is recommended.
Genetic counseling.
Alpha-thalassemia is usually inherited in an autosomal recessive manner.
Hb Bart syndrome. At conception, each sib of a proband with Hb Bart syndrome has a 25% chance of having Hb Bart syndrome (e.g., --/--), a 50% chance of having α-thalassemia trait with deletion or inactivation of two α-globin genes in cis (e.g., --/αα), and a 25% chance of being unaffected and not a carrier.
HbH disease. The risk to sibs of a proband depends on genotype of the parents.
Carrier testing. Family members, members of ethnic groups at risk, and gamete donors should be considered for carrier testing. Couples who are members of populations at risk for α-thalassemia trait with a two-gene deletion in cis (--/αα) can be identified prior to pregnancy as being at risk of conceiving a fetus with Hb Bart syndrome.
Prenatal and preimplantation genetic testing may be carried out for couples who are at high risk of having a fetus with Hb Bart syndrome or for a pregnancy in which one parent is a known α-thalassemia carrier with a two-gene deletion in cis (--/αα) when the other parent is either unknown or unavailable for testing.
Diagnosis
Suggestive Findings
Alpha-thalassemia (α-thalassemia) has two clinically significant forms: hemoglobin Bart hydrops fetalis (Hb bart) syndrome (deletion/inactivation of all four α-globin genes; --/--), and hemoglobin H (HbH) disease (most frequently caused by deletion/inactivation of three α-globin genes; --/-α) (see Figure 1).
Figure 1.
Schematic presentation of the chromosomal location of the α-globin gene cluster on chromosome 16p. The genes are indicated as boxes; gene symbols are above and the hemoglobin is expressed below. The α-globin regulatory region (MCS-R1 to (more...)
Hb Bart syndrome should be suspected in the following:
- An at-risk fetus with increased nuchal thickness, thickened placenta, increased cerebral media artery velocity, and increased cardiothoracic ratio on ultrasonography examination at 13-14 weeks' gestation
- A fetus with generalized edema, ascites, and pleural and pericardial effusions on ultrasonography examination at 22-28 weeks' gestation
HbH disease should be suspected in an infant or child with the following clinical or newborn screening findings:
- Clinical findings
- Mild jaundice
- Hepatosplenomegaly
- Mild thalassemia-like bone changes (e.g., hypertrophy of the maxilla, bossing of the skull, and prominence of the malar eminences)
- Newborn screening findings. Hb Bart >15% at birthNote: (1) Newborn screening for sickle cell disease offered by several states/countries may detect Hb Bart in the newborn with α-thalassemia. (2) Reference ranges may vary among laboratories performing newborn screening. (3) Low concentrations of Hb Bart (1%-8%) are indicative of the carrier states, and while this finding usually does not indicate a need for further evaluation of the newborn, genetic counseling may be recommended for the parents of the newborn [Ferguson 2018, Fogel et al 2018].
Establishing the Diagnosis
The diagnosis of Hb Bart syndrome is established in a fetus based on the following:
- Hematologic findings
- Red blood cell indices. Severe macrocytic hypochromic anemia, in the absence of ABO or Rh blood group incompatibility (See Table 1.)
- Reticulocytosis. Variable; may be >60%
- Peripheral blood smear with large, hypochromic red cells, severe anisopoikilocytosis, and numerous nucleated red cells
- Hemoglobin analysis that reveals decreased amounts or complete absence of hemoglobin A and increased amounts of Hb Bart (See Table 2.)
- Molecular genetic testing that identifies biallelic pathogenic variants in both HBA1 and HBA2 that result in deletion or inactivation of all four α-globin alleles (e.g., homozygous deletion of both HBA1 and HBA2 on both chromosomes; --/--; see Table 3), which confirms the diagnosis and allows for family studies
The diagnosis of HbH disease is established in a proband based on the following:
- Hematologic findings
- Red blood cell indices. Mild-to-moderate (rarely severe) microcytic hypochromic hemolytic anemia (See Table 1.)
- Moderate reticulocytosis (3%-6%)
- Peripheral blood smear with anisopoikilocytosis, and very rarely nucleated red blood cells (i.e., erythroblasts)
- Red blood cell supravital stain showing HbH inclusions (β4 tetramers) in 5%-80% of erythrocytes following incubation of fresh blood smears with 1% brilliant cresyl blue for one to three hours
- Hemoglobin analysis that reveals presence of 0.8%-40% HbH and 60%-90% hemoglobin A (See Table 2.)
- Molecular genetic testing that identifies biallelic pathogenic variants in both HBA1 and HBA2 that result in deletion or inactivation of three α-globin genes (e.g., a 2-α-globin-deletion allele in trans with a one α-globin-deletion allele; --/-α3.7) (see Table 3), which confirms the diagnosis and allows for family studies
Hematologic Findings
Table 1.
Red Blood Cell Indices in Individuals with Hb Bart Syndrome and HbH Disease
| Red Blood Cell Indices 1 | Normal | Affected | ||
|---|---|---|---|---|
| Male | Female | Hb Bart syndrome 2 | HbH disease 3 | |
| Mean corpuscular volume (MCV, fl) | 89.1±5.01 | 87.6±5.5 | 136±5.1 | Children: 56±5 |
| Adults: 61±4 | ||||
| Mean corpuscular hemoglobin (MCH, pg) | 30.9±1.9 | 30.2±2.1 | 31.9±9 | 18.4±1.2 |
| Hemoglobin (Hb, g/dL) | 15.9±1.0 | 14.0±0.9 | 3-8 | Male: 10.9±1.0 |
| Female: 9.5±0.8 | ||||
- 1.
Reference ranges may vary among laboratories.
- 2.
Vaeusorn et al [1985]
- 3.
Galanello et al [1992]
Hemoglobin Analysis
If available, qualitative and quantitative hemoglobin (Hb) analysis by weak-cation high-performance liquid chromatography identifies the amount and type of Hb present. The Hb pattern in α-thalassemia varies by α-thalassemia type (see Table 2). The Hb types most relevant to α-thalassemia are:
- Hemoglobin A (HbA). Two α-globin chains and two β-globin chains (α2β2)
- Hemoglobin F (HbF). Two α-globin chains and two γ-globin chains (α2γ2)
- Hemoglobin Bart (Hb Bart). Four γ-globin chains (γ4)
- Hemoglobin H (HbH). Four β-globin chains (β4)
- Hemoglobin A2 (HbA2). Two α-globin chains and two δ-globin chains (α2δ2)
- Hemoglobin Portland. Two ζ-globin chains and two γ-globin chains (ζ2γ2)
Table 2.
Hemoglobin Patterns in Alpha-Thalassemia
| Hemoglobin Type 1 | Normal | Affected | |
|---|---|---|---|
| Hb Bart syndrome 2 | HbH disease 3 | ||
| HbA | 96%-98% | 0 | 60%-90% |
| HbF | <1% | 0 | <1.0% |
| Hb Bart | 0 | 85%-90% | 2%-5% |
| HbH | 0 | 0 | 0.8%-40% |
| HbA2 | 2%-3% | 0 | <2.0% |
| Hb Portland | 0 | 10%-15% | 0 |
- 1.
Reference ranges may vary among laboratories.
- 2.
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).
- 3.
Deletion or inactivation of three α-globin chains
Note: Hematologic testing to identify alpha-thalassemia trait and alpha-thalassemia silent carrier status is addressed in Genetic Counseling.
Molecular Genetic Testing
Molecular testing approaches can include targeted deletion analysis for common deletions of HBA1 and HBA2, sequence analysis of HBA1 and HBA2, and deletion/duplication analysis of HBA1, HBA2, and the regulatory region multispecies conserved sequence 2 (MCS-R2; previously called HS-40) for uncommon deletions. See Figure 1.
Note: Multiple ligation-dependent probe amplification (MLPA) assay specifically designed for α-globin locus has been described.
Targeted deletion analysis for common deletions of HBA1 and HBA2 can be performed first.
- Common two α-globin-gene deletions include the following:
- Southeast Asian deletion (--SEA)
- Filipino deletion (--FIL)
- Mediterranean deletion (--MED)
Note: (1) These common deletions are typically founder variants (see Prevalence). (2) More than 20 different deletions ranging from ~6 kb to >300 kb and removing both α-globin genes (and sometimes embryonic HBZ) have been reported (see Farashi & Harteveld [2018] Figure 4 and Table A, Locus-Specific Databases). - Common single α-globin-gene deletions include:
- 3.7-kb deletion (-α3.7) deletion
- 4.2-kb deletion (-α4.2) deletion
Note: In addition to these two common deletions, other deletions involving a single α-globin gene have been reported.
Sequence analysis of HBA1 and HBA2 can be performed if a common deletion was not identified.
Note: "Non-deletion" or "trait" HBA2 variant alleles are designated as (αNDα/) or (αTα/), respectively; HBA1 variant alleles are designated as (ααND /) or (ααT /), respectively (see Molecular Genetics).
Gene-targeted deletion analysis MLPA of HBA1, HBA2, and the MCS-R2 regulatory region located 40 kb upstream from the α-globin cluster can be performed to detect uncommon deletions associated with α-thalassemia if pathogenic variants have not been identified by targeted deletion analysis or sequence analysis [Kipp et al 2011].
Further testing for genes associated with genetic disorders similar to α-thalassemia, such as ATRX and HBB (see Differential Diagnosis), may also be considered if clinically indicated.
Table 3.
Molecular Genetic Testing Used in Alpha-Thalassemia
| Gene 1 | Proportion of α-Thalassemia Attributed to Pathogenic Variants in Gene | Proportion of Pathogenic Variants 2 Detectable by Method | ||
|---|---|---|---|---|
| Sequence analysis 3 | Gene-targeted deletion/duplication analysis 4 | |||
| Common deletions | Other deletions | |||
| HBA1 & HBA2 | >98% | ~15% | ~85% | <5% |
| MCS-R2 locus 5 | <1% | <1% | ||
- 1.
See Table A. Genes and Databases for chromosome locus and protein.
- 2.
See Molecular Genetics for information on allelic variants detected in these genes.
- 3.
Sequence analysis detects variants that are benign, likely benign, of uncertain significance, likely pathogenic, or pathogenic. Variants may include small intragenic deletions/insertions and missense, nonsense, and splice site variants; typically, exon or whole-gene deletions/duplications are not detected. For issues to consider in interpretation of sequence analysis results, click here.
- 4.
Methods used to detect common, rare, or previously undescribed deletions/duplications within the α-globin gene cluster and regulatory elements may include Gap PCR, MLPA (also known as break-point PCR), chromosomal microarray analysis (CMA) using oligonucleotide or SNP arrays, and next-generation sequencing (NGS) for analysis of deletion breakpoints [Kipp et al 2011, Clark et al 2017]. Note that methods such as Southern blotting, quantitative PCR, and long-range PCR have been used in the past.
- 5.
See Sollaino et al [2010] and Nomenclature.
Clinical Characteristics
Clinical Description
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. Affected fetuses are either delivered stillborn at 30-40 weeks' gestation or die soon after birth.
The main clinical features are generalized edema and pleural and pericardial effusions as a result of congestive heart failure induced by severe anemia. Notably, red cells with Hb Bart have an extremely high oxygen affinity and are incapable of effective oxygen delivery. Extramedullary erythropoiesis, marked hepatosplenomegaly, and a massive placenta are common.
Retardation in brain growth, hydrocephalus, cardiovascular deformities, and urogenital defects have been reported.
A very small number of newborns survive following intrauterine transfusions and repeated frequent transfusions after birth.
Maternal complications during pregnancy commonly include preeclampsia, polyhydramnios or oligohydramnios, antepartum hemorrhage, and premature delivery.
HbH disease. The phenotype of HbH disease varies; however, clinical features are usually only diagnosed during routine hematologic analysis in an asymptomatic individual.
The majority of individuals have enlargement of the spleen and less commonly of the liver, mild jaundice, and sometimes mild-to-moderate thalassemia-like skeletal changes (e.g., hypertrophy of the maxilla, bossing of the skull, and prominence of the malar eminences) that affect the facial features. Leg ulcers are rare.
Individuals with HbH disease may develop gallstones and experience acute episodes of hemolysis in response to oxidant drugs and infections. Rarely, infection with parvovirus B19 can cause an aplastic crisis.
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].
Significant iron overload is uncommon but has been reported in older individuals, usually resulting from repeated blood transfusions or increased iron absorption [Taher et al 2012].
Genotype-Phenotype Correlations
The phenotype of the α-thalassemia syndromes depends on the degree of α-globin chain deficiency relative to β-globin chain production. The correlation between α-thalassemia pathogenic variants, α-globin mRNA levels, α-globin synthesis, and clinical manifestations of α-thalassemia is well documented.
Hb Bart syndrome
- Most often caused by large deletions on both alleles (--/--)
- Rarely, an individual with Hb Bart syndrome will have a non-deletion variant (--/αND-).
HbH disease
- Most often caused by a large deletion on one allele in trans with a single α-globin-gene deletion (--/-α) or other non-deletion inactivating variant (--/αNDα or --/ααND)
- Individuals homozygous for HBA2 pathogenic variants (αNDα/αNDα) may have HbH disease.
- Individuals who are homozygous or compound heterozygous for highly unstable α-globin gene variants may have HbH disease.
- Rarely, HbH disease is caused by compound heterozygosity for an MCS-R2 (see Nomenclature and Figure 1) deletion and an additional α-gene deletion [(αα)MCS-R2/-α] [Coelho et al 2010, Sollaino et al 2010].
Nomenclature
The α-thalassemia carrier states have been classified on the basis of the total globin protein produced from each of the two α-globin genes and by the number of globin genes that are missing or abnormal (see Table 4).
Table 4.
Carrier State Nomenclature
| Number of Deleted/ Inactivated α-Globin Genes | Nomenclature Based on # of Deleted/ Inactivated α- Globin Genes | Haplotype (i.e., cis or trans) 1 | Genotype Example | Nomenclature Based on Protein 2 | Carrier State Terminology | |
|---|---|---|---|---|---|---|
| Symbol | Definition | |||||
| 1 | α-thalassemia silent carrier 3 | NA | -α/αα 4 | α+ | Some α-globin protein is produced from one chromosome 16. | α-thalassemia silent carrier |
| 2 | α-thalassemia trait/carrier 3 | Cis | --/αα | α0 | Zero α-globin protein is produced from one chromosome 16. | α0 trait (α0-thalassemia) |
| Trans | -α/-α | α+ | Some α-globin protein is produced from each of two chromosomes 16. | α+-thalassemia trait | ||
- 1.
Cis: both α-globin genes on one chromosome 16 are deleted or inactivated; trans: one α-globin gene on one chromosome 16 is deleted or inactivated by a non-deletion variant.
- 2.
HBA2 encodes two to three times more globin than HBA1.
- 3.
Lehmann & Carrell [1984]
- 4.
The most common genotypes are the -α3.7 and –α4.2 deletion alleles (see Table 6 and Table 10).
Genotype nomenclature. In the expression αα/αα, the first alpha in each pair (αα/αα) typically refers to HBA2 and the second alpha in each pair (αα/αα) to HBA1.
The terms "α-thalassemia 1" and "α-thalassemia 2" (referring to α-thalassemia silent carrier and α-thalassemia trait, respectively) are no longer in use [Weatherall et al 1988].
MCS-R2, a multispecies conserved sequence previously known as HS-40, is a cis-acting regulatory element about 40 kb upstream of HBZ that is required for α-globin expression [reviewed by Farashi & Harteveld 2018] (see Figure 1).
Prevalence
Since the early 1960s, prevalence of α-thalassemia has been determined in several populations using the percent of Hb Bart in cord blood. However, because not all newborns with α-thalassemia (mainly α-thalassemia silent carriers) have increased Hb Bart, the prevalence of α-thalassemia derived from this measure may be underestimated.
Data that are more precise have been obtained using molecular testing. For detailed references for the frequency of α-thalassemia in each population, see Piel & Weatherall [2014].
Africa
The highest allele frequency (0.30-0.40) of the -α3.7 allele has been observed in the equatorial belt including Nigeria, Ivory Coast, and Kenya.
The two α-globin-gene deletion in cis (--/αα) has been reported very rarely in North Africa and in the African American population.
The Mediterranean
Alpha-thalassemia trait caused by -α3.7/-α3.7 is common, with the highest allele frequency reported in Sardinia (0.18) and the lowest in Spain.
The two α-globin-gene deletion in cis (--/αα) is very rare (0.002); thus, Hb Bart hydrops fetalis is only rarely reported.
A remarkable aspect of α-thalassemia variants identified in the Mediterranean population is the heterogeneity of variants, particularly the non-deletion variants.
The Arabian Peninsula
Frequency of the -α3.7 allele (causing α-thalassemia trait) varies from 0.01 to 0.67, with the highest values being observed in Oman.
The two α-globin-gene deletion in cis (--/αα) is extremely rare.
India
Alpha-thalassemia trait reaches very high allele 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 -α3.7 allele and the -α4.2 allele variably contribute to incidence of α-thalassemia trait.
The two α-globin-gene deletion in cis (--/αα) is very rare.
Southeast Asia
Alpha0-thalassemia alleles (--SEA, --THAI, --FIL) and α+-thalassemia alleles (-α) are very common, causing a major public health burden.
Alpha-thalassemia caused by HbConstant Spring (HbCS) alleles is also common.
The incidence of Hb Bart hydrops fetalis is expected to be in the range of 0.5-5:1,000 births and HbH disease the range of 4-20:1,000 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.
Some α-thalassemias have unusual mutation mechanisms; for example, some affected individuals on the island of Vanuatu who have normal α-globin genes without deletions or variants have a variant in a regulatory element that creates a GATA-1 site and activates a cryptic promoter [De Gobbi et al 2006].
The two α-globin-gene deletion in cis (--/αα) is very rare.
Differential Diagnosis
Hydrops Fetalis
Hydrops fetalis is associated with many conditions in addition to Hb Bart, including immune-related disorders (e.g., alloimmune hemolytic disease, Rh isoimmunization), fetal cardiac anomalies, chromosome 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.
Hemoglobin H (HbH) Disease
Hemolytic 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 supravital stain; and (4) absence of morphologic or enzymatic changes characteristic of other forms of inherited hemolytic anemia (e.g., hereditary spherocytosis/elliptocytosis, G6PD deficiency). See EPB42-Related Hereditary Spherocytosis.
Alpha-thalassemia X-linked intellectual disability (ATRX) syndrome is characterized by distinctive craniofacial features, genital anomalies, severe developmental delays, hypotonia, intellectual disability, and mild-to-moderate anemia secondary to α-thalassemia. Craniofacial abnormalities include small head circumference, telecanthus or widely spaced eyes, short nose, tented vermilion of the upper lip, and thick or everted vermilion of the lower lip with coarsening of the facial features over time. Although all individuals with ATRX syndrome have a normal 46,XY karyotype, genital anomalies range from hypospadias and undescended testes, to severe hypospadias and ambiguous genitalia, to normal-appearing female genitalia. Global developmental delays are evident in infancy and some affected individuals never walk independently or develop significant speech. Affected individuals do not reproduce. ATRX syndrome is caused by a hemizygous ATRX variant in affected males and inherited in an X-linked manner.
An unknown percent of 46,XY individuals with ATRX syndrome have a mild form of HbH disease, evident as HbH inclusions (β4 tetramers) in erythrocytes following incubation of fresh blood smears with 1% brilliant cresyl blue. In ATRX syndrome, the α-globin gene cluster and the MCS-R1-4 regulatory