Disease characteristics. Beta-thalassemia is characterized by reduced synthesis of the hemoglobin subunit beta (hemoglobin beta chain) that results in microcytic hypochromic anemia, an abnormal peripheral blood smear with nucleated red blood cells, and reduced amounts of hemoglobin A (HbA) on hemoglobin analysis. Individuals with thalassemia major have severe anemia and hepatosplenomegaly; they usually come to medical attention within the first two years of life. Without treatment, affected children have severe failure to thrive and shortened life expectancy. Treatment with a regular transfusion program and chelation therapy, aimed at reducing transfusion iron overload, allows for normal growth and development and extends life expectancy into the third to fifth decade. Individuals with thalassemia intermedia present later and have milder anemia that only rarely requires transfusion. These individuals are at risk for iron overload secondary to increased intestinal absorption of iron as a result of ineffective erythropoiesis.
Diagnosis/testing. The diagnosis of β-thalassemia relies on measuring red blood cell indices that reveal microcytic hypochromic anemia, nucleated red blood cells on peripheral blood smear, hemoglobin analysis that reveals decreased amounts of HbA and increased amounts of hemoglobin F (HbF) after age 12 months, and the clinical severity of anemia. Molecular genetic testing of the gene encoding the hemoglobin subunit beta (HBB) is available in clinical laboratories and may be useful for predicting the clinical phenotype in some cases as well as presymptomatic diagnosis of at-risk family members and prenatal diagnosis.
Management. Treatment of manifestations: Thalassemia major: Regular transfusions correct the anemia, suppress erythropoiesis, and inhibit increased gastrointestinal absorption of iron. The only available definitive cure is bone marrow transplantation from an HLA-identical sib or cord blood transplantation from a related donor. Thalassemia intermedia: symptomatic therapy based on splenectomy in most patients, sporadic red cell transfusions in some, and folic acid supplementation. Prevention of primary manifestations: See Treatment of manifestations. Prevention of secondary complications: Prevent transfusional iron overload by adequate iron chelation. Surveillance: Thalassemia major: Monitor the effectiveness/side effects of transfusion therapy and chelation therapy in patients of all ages by monthly physical examination, assess liver function tests bimonthly, determine serum ferritin concentration every three months, evaluate growth and development every six months, evaluate eyes, hearing, heart, endocrine function (thyroid, endocrine pancreas, parathyroid, adrenal, pituitary), and liver (ultrasound examination) annually. In adults: bone densitometry to assess for osteoporosis, serum alpha-fetoprotein concentration for early detection of hepatocarcinoma in those with hepatitis C and iron overload; regular gallbladder echography for early detection of cholelithiasis for those at risk. Agents/circumstances to avoid: alcohol consumption, iron-containing preparations. Testing of relatives at risk: If prenatal diagnosis has not been utilized and if the disease-causing mutations have been identified in an affected family member, offer molecular genetic testing to at-risk sibs to allow early diagnosis and appropriate treatment.
Genetic counseling. The β-thalassemias are inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Heterozygotes may be slightly anemic but are clinically asymptomatic. Carriers are often referred to as having thalassemia minor (or β-thalassemia minor). Prenatal testing for pregnancies at increased risk is possible if the disease-causing mutations in the family are known.
Thalassemia major is suspected in an infant or child younger than age two years with severe microcytic anemia and hepatosplenomegaly. Untreated, affected children usually manifest failure to thrive and expansion of the bone marrow to compensate for ineffective erythropoiesis.
Thalassemia intermedia is suspected in individuals who present at a later age with a milder anemia that only rarely requires treatment with blood transfusion.
Red blood cell indices show microcytic anemia (Table 1).
Red Blood Cell Index | Normal 1 | Affected | Carrier 1 | |
---|---|---|---|---|
Male | Female | β-Thal Major | β-Thal Minor | |
Mean corpuscular volume (MCV fl) | 89.1±5.01 | 87.6±5.5 | 50-70 | <79 |
Mean corpuscular hemoglobin (MCH pg) | 30.9±1.9 | 30.2±2.1 | 12-20 | <27 |
Hemoglobin (Hb g/dL) | 15.9±1.0 | 14.0±0.9 | <7 | Males: 11.5-15.3 Females: 9.1-14 |
1. Data from Galanello et al (1979)
Peripheral blood smear
Affected individuals demonstrate the red blood cell (RBC) morphologic changes of microcytosis, hypochromia, anisocytosis, poikilocytosis (spiculated tear-drop and elongated cells), and nucleated red blood cells (i.e., erythroblasts). The number of erythroblasts is related to the degree of anemia and is markedly increased following splenectomy.
Carriers demonstrate reduced mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) (Table 1), and RBC morphologic changes that are less severe than in affected individuals. Erythroblasts are normally not seen.
Qualitative and quantitative hemoglobin analysis (by cellulose acetate electrophoresis and DE-52 microchromatography or HPLC) identifies the amount and type of hemoglobin present. The following hemoglobin (Hb) types are most relevant toβ-thalassemia:
Hemoglobin A (HbA): two globin alpha chains and two globin beta chains (α2 β2)
Hemoglobin F (HbF): two globin alpha chains and two globin gamma chains (α2 γ2)
Hemoglobin A2 (HbA2): two globin alpha chains and two globin delta chains α2 δ2)
The Hb pattern in β-thalassemia varies by β-thalassemia type (Table 2).
Hemoglobin Type | Normal 1 | Affected | Carrier | |
---|---|---|---|---|
βº-Thal Homozygotes 2 | β+-Thal Homozygotes or β+/βº compound heterozygotes 3 | β-Thal Minor | ||
HbA | 96%-98% | 0 | 10%-30% | 92%-95% |
HbF | <1% | 95%-98% | 70%-90% | 0.5%-4% |
HbA2 | 2%-3% | 2%-5% | 2%-5% | >3.5% |
1. Data from Telen & Kaufman (1999)
2. βº-thalassemia: complete absence of globin beta chain production
3. β+-thalassemia: variable degree of reduction of globin beta chain synthesis
Hemoglobin electrophoresis and HPLC also detect other hemoglobinopathies (S, C, E, OArab, Lepore) that may interact with β-thalassemia.
Bone marrow examination is usually not necessary for diagnosis of affected individuals. The bone marrow is extremely cellular, mainly as a result of marked erythroid hyperplasia, with a myeloid/erythroid ratio reversed from the normal (3 or 4) to 0.1 or less.
In vitro synthesis of radioactive labeled globin chains in affected individuals reveals the following
βº-thalassemia: a complete absence of globin beta chains and a marked excess of globin alpha chains compared with globin gamma chains. Theα/γ ratio is greater than 2.0.
β+-thalassemia: a variable degree of reduction of globin beta chains resulting in severe (thalassemia major) to mild (thalassemia intermedia) clinical phenotypes. The imbalance of the α/β and γ ratio is similar to that in βº-thalassemia major.
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Gene. Mutations in HBB are associated with β-thalassemia (see Differential Diagnosis).
Clinical testing
Targeted mutation analysis. The β-thalassemias can be caused by more than 200 different HBB gene mutations [Huisman et al 1997, globin.cse.psu.edu]; however, the prevalent molecular defects are limited in each at-risk population (see Table 4). This phenomenon has greatly facilitated molecular genetic testing.
Commonly occurring mutations of the HBB gene are detected by a number of PCR-based procedures. The most commonly used methods are reverse dot blot analysis or primer-specific amplification, with a set of probes or primers complementary to the most common mutations in the population from which the affected individual originated [Old et al 2005].
Other methods based on real-time PCR or microarray technology because of their reproducibility, rapidity, and easy handling are potentially suitable for the routine clinical laboratory [Vrettou et al 2003, Ye et al 2007].
Mutation scanning/sequence analysis. If targeted mutation analysis fails to detect the mutation, mutation scanning or sequence analysis can be used to detect mutations in the HBB coding region (mutations in the non-coding region would not be detected by this analysis). Sensitivity of both mutation scanning and sequence analysis is 99%.
Note: Deletions of variable extent of the β gene or of the HBB cluster that result in β-thalassemia or in the complex β-thalassemias called γδβ-thalassemia and δβ-thalassemia are rare causes of β-thalassemia and testing that detects deletions is available clinically.
Table 3 summarizes molecular genetic testing for this disorder.
Test Method | Mutations Detected | Mutation Detection Frequency 1 | Test Availability |
---|---|---|---|
Targeted mutation analysis | HBB mutation panels vary by laboratory | Variable depending on mutations included in panel and individual's ethnicity | Clinical
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Mutation scanning/ sequence analysis | Sequence variants in HBB coding region | 99% |
Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.
Confirmation of the diagnosis of beta-thalassemia in a proband requires identification of the disease-causing point mutations in HBB. The appropriate order for molecular genetic testing:
Prognostication. Distinguishing thalassemia major from thalassemia intermedia at the molecular level for the purpose of prognostication requires defining of the HMM mutations and evaluating for coinheritance of those genetic determinants able to sustain a continuous production of gamma globin chains (HbF) in adult life or able to reduce the alpha/non-alpha globin chain imbalance, such as alpha-thalassemia (see Genotype-Phenotype Correlations).
Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.
Note: Carriers are heterozygotes for an 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 mutation in the family.
Other phenotypes associated with mutations in the HBB gene are sickle cell disease caused by A>T substitution at codon 6 (p.Glu6Val) (c.17A>T, Genbank NM_000518.4) and other nucleotide substitutions responsible for hemoglobin variants (globin.cse.psu.edu).
The phenotypes of the homozygous β-thalassemias include thalassemia major and thalassemia intermedia. The clinical severity of the β-thalassemia syndromes depends on the extent of globin alpha chain/non-globin alpha chain (i.e., β+ γ) imbalance. The non-assembled globin alpha chains that result from unbalanced globin alpha chain/non-globin alpha chain synthesis precipitate in the form of inclusions. These globin alpha chain inclusions damage the erythroid precursors in the bone marrow and in the spleen, causing ineffective erythropoiesis. Individuals with thalassemia major usually come to medical attention within the first two years of life; they subsequently require regular red blood cell transfusions to survive. Those who present later and rarely require transfusion are said to have thalassemia intermedia.
β-thalassemia major. Clinical presentation of thalassemia major occurs between ages six and 24 months. Affected infants fail to thrive and become progressively pale. Feeding problems, diarrhea, irritability, recurrent bouts of fever, and progressive enlargement of the abdomen caused by splenomegaly may occur. If the diagnosis of thalassemia major is established at this stage and if a regular transfusion program that maintains a minimum Hb concentration of 95 to 105 g/L is initiated, growth and development are normal until age ten to 11 years.
After age ten to 11 years, affected individuals are at risk of developing severe complications related to iron overload, depending on their compliance with chelation therapy (see Management). Complications of iron overload in children include growth retardation and failure of sexual maturation as well as those complications observed in adults with HFE-associated hereditary hemochromatosis: involvement of the heart (dilated myocardiopathy and pericarditis), liver (fibrosis and cirrhosis), and endocrine glands (resulting in diabetes mellitus and insufficiency of the parathyroid, thyroid, pituitary, and, less commonly, adrenal glands). In individuals who have been regularly transfused, iron overload results mainly from transfusions. Other complications are hypersplenism, chronic hepatitis (resulting from infection with the viruses that cause hepatitis B and/or hepatitis C), cirrhosis (from iron overload and chronic hepatitis), HIV infection, venous thrombosis, and osteoporosis. The risk for hepatocellular carcinoma is increased secondary to liver viral infection, iron overload, and longer survival [Borgna-Pignatti et al 2004].
The survival of individuals who have been well transfused and treated with appropriate chelation extends beyond age 30 years. Myocardial disease caused by transfusional siderosis is the most important life-limiting complication of iron overload in beta-thalassemia. In fact, cardiac complications are reported to cause 71% of the deaths in individuals withβ-thalassemia major [ Borgna-Pignatti et al 2004].
The classic clinical picture of thalassemia major is presently only seen in some developing countries, in which the resources for carrying out long-term transfusion programs are not available. The most relevant features of untreated or poorly transfused individuals are growth retardation, pallor, jaundice, brown pigmentation of the skin, poor musculature, genu valgum, hepatosplenomegaly, leg ulcers, development of masses from extramedullary hematopoiesis, and skeletal changes that result from expansion of the bone marrow. These skeletal changes include deformities of the long bones of the legs and typical craniofacial changes (bossing of the skull, prominent malar eminence, depression of the bridge of the nose, tendency to a mongoloid slant of the eye, and hypertrophy of the maxillae, which tends to expose the upper teeth). Individuals who have not been regularly transfused usually die before the third decade. Individuals who have been poorly transfused are also at risk for complications of iron overload.
β-thalassemia intermedia. Clinical features are pallor, jaundice, cholelithiasis, liver and spleen enlargement, moderate to severe skeletal changes, leg ulcers, extramedullary masses of hyperplastic erythroid marrow, a tendency to develop osteopenia and osteoporosis, and thrombotic complications resulting from iron accumulation and hypercoagulable state resulting from the lipid membrane composition of the abnormal red blood cells [Eldor & Rachmilewitz 2002]. By definition, transfusions are not required or only occasionally required. Iron overload occurs mainly from increased intestinal absorption of iron caused by ineffective erythropoiesis. The associated complications of iron overload present later, but may be as severe as those seen in individuals with thalassemia major who depend on transfusions.
Any inherited or acquired condition that reduces the alpha/non-alpha globin chain imbalance in β-thalassemia results in a lesser degree of globin alpha chain precipitation and leads to a mild β-thalassemia phenotype [Galanello & Cao 1998].
One of the most common and consistent mechanisms is homozygosity or compound heterozygosity for two β+-thalassemia mild and silent mutations (see Table 5).
In contrast, compound heterozygosity for a mild/silent β+ and a severe mutation produces a variable phenotype, ranging from thalassemia intermedia to thalassemia major. Therefore, the presence of this genotype does not predict a mild phenotype. Hemoglobin E (HbE), which is a thalassemic structural variant, characterized by the presence of an abnormal structure as well as biosynthetic defect, should be included in this group. The nucleotide substitution at codon 26, producing the HbE variant (α2 β226 E>K), activates a potential cryptic RNA splice region, resulting in alternative splicing at this position. The homozygous state for HbE results in a mild hemolytic microcytic anemia. Compound heterozygosity for β-thalassemia and HbE result in a wide range of often severe but sometimes mild or even clinically asymptomatic clinical phenotypes.
The clinical picture resulting from homozygosity for β+-thalassemia or homozygosity for βº-thalassemia mutations may be ameliorated by coinheritance of mutations in the gene encoding the globin alpha chain associated with α-thalassemia, which reduces the output of the genes encoding the globin alpha chains and therefore decreases the alpha/non-alpha globin chain imbalance. Because coinheritedα-thalassemia does not always produce a consistent effect, it cannot be used to predict phenotype.
The coinheritance of some genetic determinants able to sustain a continuous production of globin gamma chains (HbF) in adult life may also reduce the extent of alpha/non-alpha globin chain imbalance:
The β-thalassemia mutation per se increases the globin gamma chain (HbF) output. This occurs in the following two situations:
δβº-thalassemia is caused by deletions of variable size in the HBB gene cluster.
Deletions remove only the 5' region of the HBB promoter, which also results in high levels of HbA2.
Co-transmission of hereditary persistence of fetal hemoglobin (HPFH), which is the result of single point mutations of the hemoglobin Gγ (HBG2) or hemoglobin Aγ (HBG1) gene promoter. The most common is a single-base substitution C to T at position 158 upstream of the transcription start site of the HBG2 gene, which is silent in normal individuals and in β-thalassemia heterozygotes, but leads to increased HbF production in individuals with erythropoietic stress, as occurs in homozygousβ-thalassemia. This HBG2 mutation, sometimes referred to as Gγ-158 C>T, is officially designated c.-210C>T (reference sequence NM_00184.2). It is in linkage disequilibrium (in cis configuration) with the HBB mutations (see Table 4 and Table 5). This explains the mild phenotype that may result from the inheritance of these mutations.
Coinheritance of heterocellular HPFH may or may not be linked to the HBB gene cluster. To date, three loci have been mapped: one at Xq22.2-q22.3, one on 6q22.3-q23.1, and one on 8q, which interacts with the HBG2 c.-210C>T mutation). It is likely that many others also exist.
Other modifying factors. The clinical phenotype of homozygous β-thalassemia may also be modified by the coinheritance of other genetic factors mapping outside the β-globin gene cluster. The best known of these modifying genes is the mutation causing Gilbert disease [i.e., (TA)7 configuration instead of the (TA)6 in the TATA box of the gene encoding uridin-diphosphoglucuronyltransferase, which, when combined with thalassemia major or thalassemia intermedia, leads to increased jaundice and increased risk of gallstones [Galanello et al 2001]. Less defined modifying factors are genes coding for HFE-associated hereditary hemochromatosis and genes involved in bone metabolism.
In some instances, heterozygous β-thalassemia may lead to the phenotype of thalassemia intermedia instead of the asymptomatic carrier state. Known molecular mechanisms include the following:
Heterozygosity for mutations in HBB that result in hyper-unstable hemoglobins (dominant β-thalassemia), which precipitate in the red cell membrane together with unassembled hemoglobin alpha chains, resulting in markedly ineffective erythropoiesis. Most of these HBB mutations lie in the third exon and lead to the production of a markedly unstable Hb variant often not detectable in peripheral blood.
Compound heterozygosity for typical β-thalassemia mutations and the triple or (less frequently) quadruple alpha gene arrangement (αα/αα or ααα/ααα or αααα/αα may increase the imbalance in the ratio of globin alpha/non-globin alpha chains.
β-thalassemia is prevalent in populations in the Mediterranean, Middle East, Transcaucasus, Central Asia, Indian subcontinent, and Far East. It is also common in populations of African heritage. The highest incidences are reported in Cyprus (14%), Sardinia (12%), and southeast Asia.
The high gene frequency of β-thalassemias in these regions is most likely related to the selective pressure from malaria [Flint et al 1998]. This distribution is quite similar to that of endemic Plasmodium falciparum malaria. However, because of population migration and, in a limited part, the slave trade, β-thalassemia is now also common in northern Europe, North and South America, the Caribbean, and Australia.
For current information on availability of genetic testing for disorders included in this section, see GeneTests Laboratory Directory. —ED.
β-thalassemia associated with other features. In rare instances the β-thalassemia defect does not lie in HBB or in the β-globin gene cluster. In instances in which the β-thalassemia trait is associated with other features, the molecular lesion has been found either in the gene encoding the transcription factor TFIIH (β-thalassemia trait associated with xeroderma pigmentosum and tricothiodystrophy) or in the X-linked transcription factor GATA-1 (X-linked thrombocytopenia with thalassemia) (see GATA1-Related Cytopenia) [Viprakasit et al 2001, Freson et al 2002].
Few conditions share similarities with homozygous β-thalassemia.
The very rare, so-called dominant β-thalassemias or thalassemic hemoglobinopathies result in an abnormal hyper-unstable protein product. The presence of hyper-unstable hemoglobin should be suspected in any individual with thalassemia intermedia when both parents are hematologically normal or in families with a pattern of autosomal dominant transmission of the thalassemia intermedia phenotype. HBB gene sequencing establishes the diagnosis.
The genetically determined sideroblastic anemias are easily differentiated because of ring sideroblasts in the bone marrow and variably elevated serum concentration of erythrocyte protoporphyrin. Most sideroblastic anemia is associated with defects in the heme biosynthetic pathway, especiallyδ-aminolevulinic acid synthase (see also X-Linked Sideroblastic Anemia and Ataxia).
Congenital diserythropoietic anemias do not have high HbF and do have other distinctive features, such as multinuclearity of the red blood cell precursors.
A few acquired conditions associated with high HbF (juvenile chronic myeloid leukemia, aplastic anemia) may be mistaken for β-thalassemia, even though they have very characteristic hematologic features.
The initial step following diagnosis of β-thalassemia is to distinguish thalassemia intermedia from thalassemia major (see Testing Strategy). The diagnosis of thalassemia major implies the need for a regular transfusion program; the diagnosis of thalassemia intermedia implies the need for intermittent transfusions on an as-needed basis.
A comprehensive review of the management of thalassemia major and thalassemia intermedia has been published by Thalassemia International Federation and is available at the TIF Web site [Eleftheriou 2000].
Thalassemia major. Regular transfusions correct the anemia, suppress erythropoiesis, and inhibit increased gastrointestinal absorption of iron. Before starting the transfusions, it is absolutely necessary to carry out hepatitis B vaccination and perform extensive red blood cell antigen typing, including Rh, Kell, Kidd, and Duffy and serum immunoglobulin determination, the latter of which detects individuals with IgA deficiency who need special (repeatedly washed) blood unit preparation before each transfusion. The transfusion regimen is designed to obtain a pre-transfusion Hb concentration of 95-100 g/L. Transfusions are usually given every two to three weeks.
Thalassemia intermedia. Treatment of individuals with thalassemia intermedia is symptomatic and based on splenectomy and folic acid supplementation. Treatment of extramedullary erythropoietic masses is based on radiotherapy, transfusions, or, in selected cases, hydroxyurea (with a protocol similar to that used for sickle cell disease). Hydroxyurea also increases globin gamma chains and may have other undefined mechanisms. Because individuals with thalassemia intermedia may develop iron overload from increased gastrointestinal absorption of iron or from occasional transfusions, chelation therapy is started when the serum ferritin concentration exceeds 300 µg/L.
Bone marrow transplantation
Bone marrow transplantation (BMT) from an HLA-identical sib represents an alternative to traditional transfusion and chelation therapy. If BMT is successful, iron overload may be reduced by repeated phlebotomy, thus eliminating the need for iron chelation.
The outcome of BMT is related to the pretransplantation clinical conditions, specifically the presence of hepatomegaly, extent of liver fibrosis, and magnitude of iron accumulation. In children who lack the above risk factors, disease-free survival is over 90% [Gaziev & Lucarelli 2003]. A lower survival rate of approximately 60% is reported in individuals with all three risk factors.
Chronic graft-versus-host disease (GVHD) of variable severity may occur in 5%-8% of individuals.
BMT from unrelated donors has been carried out on a limited number of individuals with β-thalassemia. Provided that selection of the donor is based on stringent criteria of HLA compatibility and that individuals have limited iron overload, results are comparable to those obtained when the donor is a compatible sib [La Nasa et al 2005]. However, because of the limited number of individuals enrolled, further studies are needed to confirm these preliminary findings.
Cord blood transplantation. Cord blood transplantation from a related donor offers a good probability of a successful cure and is associated with a low risk of GVHD [Locatelli et al 2003, Walters et al 2005]. For couples who have already had a child with thalassemia and who undertake prenatal diagnosis in a subsequent pregnancy, prenatal identification of HLA compatibility between the affected child and an unaffected fetus allows collection of placental blood at delivery and the option of cord blood transplantation to cure the affected child [Orofino et al 2003]. On the other hand, in case of an affected fetus and a previous normal child, the couple may decide to continue the pregnancy and pursue BMT later, using the normal child as the donor.
Early detection of anemia, the primary manifestation of the disease, allows early appropriate treatment and monitoring.
Transfusional iron overload. The most common secondary complications are those related to transfusional iron overload, which can be prevented by adequate iron chelation. After ten to 12 transfusions, chelation therapy is initiated with desferrioxamine B (DFO) administered five to seven days a week by 12-hour continuous subcutaneous infusion via a portable pump. Recommended dosage depends on the individual's age and the serum ferritin concentration. Young children start with 20-30 mg/kg/day, increasing up to 40 mg/kg/day after age five to six years. The maximum dose is 50 mg/kg/day after growth is completed. The dose may be reduced if serum ferritin concentration is low. By maintaining the total body iron stores below critical values (i.e., hepatic iron concentration <7.0 mg per gram of dry weight liver tissue), desferrioxamine B therapy prevents the secondary effects of iron overload, resulting in a consistent decrease in morbidity and mortality [Borgna-Pignatti et al 2004].
Ascorbate repletion (daily dose not to exceed 100-150 mg) increases the amount of iron removed after DFO administration.
Side effects of DFO chelation therapy are more common in the presence of relatively low iron burden and include ocular and auditory toxicity, growth retardation, and, rarely, renal impairment and interstitial pneumonitis. DFO administration also increases susceptibility to Yersinia infections. The major drawback of DFO chelation therapy is low compliance resulting from complications of administration.
In clinical practice, the effectiveness of DFO chelation therapy is monitored by routine determination of serum ferritin concentration. However, serum ferritin concentration is not always reliable for evaluating iron burden because it is influenced by other factors, the most important being the extent of liver damage.
Determination of liver iron concentration in a liver biopsy specimen shows a high correlation with total body iron accumulation and is the gold standard for evaluation of iron overload. However, (1) liver biopsy is an invasive technique involving the possibility (though low) of complications; (2) liver iron content can be affected by hepatic fibrosis, which commonly occurs in individuals with iron overload and HCV infection; and (3) irregular iron distribution in the liver can lead to possible false-negative results [Clark et al 2003].
In recent years, MRI techniques for assessing iron loading in the liver and heart have improved [Anderson et al 2001, Wood et al 2004, St Pierre et al 2005]. T2 and T2* parameters have been validated for liver iron concentration. Cardiac T2* is reproducible, is applicable between different scanners, correlates with cardiac function, and relates to tissue iron concentration [Anderson et al 2001, Wood et al 2004]. Clinical utility of T2* in monitoring individuals with siderotic cardiomyopathy has been demonstrated [Anderson et al 2004]. Calibration of T2* in the heart will be available in the near future.
Magnetic biosusceptometry (SQUID), which gives a reliable measurement of hepatic iron concentration, is another option [Fischer et al 2003]; however, magnetic susceptometry is presently available only in a limited number of centers worldwide.
Two other chelators have been introduced into clinical use: deferiprone and desferrioxamine.
Deferiprone (L-1), a bidentated oral chelator, available for several years in many countries, is administered in a dose of 75-100 mg/kg/day. The main side effects of deferiprone therapy include neutropenia, agranulocytosis, arthropathy, and gastrointestinal symptoms [Cohen et al 2003] that demand close monitoring. Recent findings seem to exclude any correlation between deferiprone treatment and progression of liver fibrosis [Wanless et al 2002]. The effect of deferiprone on liver iron concentration may vary among the individuals treated. However, results from independent studies suggest that deferiprone may be more cardioprotective than desferrioxamine; compared to those being treated with DFO, individuals being treated with deferiprone have better myocardial MRI pattern and less probability of developing (or worsening pre-existing) cardiac disease [Anderson et al 2002, Piga et al 2003]. These retrospective observations have been confirmed in a prospective study [Pennell et al 2006].
After many years of controversy, deferiprone is emerging as a useful iron chelator equivalent/alternative to desferrioxamine.
Deferasirox recently became available for clinical use in patients with thalassemia. It is effective in adults and children and has a defined safety profile that is clinically manageable with appropriate monitoring. The most common treatment-related adverse events are gastrointestinal disorders, skin rash, and a mild, non-progressive increase in serum creatinine concentration. Post-marketing experience and several phase IV studies will further evaluate the safety and efficacy of deferasirox.
New strategies of chelation using a combination of desferrioxamine and deferiprone have been effective in individuals with severe iron overload; toxicity was manageable [Wonke et al 1998, Wu et al 2004, Tanner et al 2007].
Cardiac disease. In the past few years, particular attention has been directed to the early diagnosis and treatment of cardiac disease because of its critical role in determining the prognosis of individuals with β-thalassemia. Assessment of myocardial siderosis and monitoring of cardiac function combined with intensification of iron chelation can result in excellent long-term prognoses [Anderson et al 2004, Davis et al 2004].
Osteoporosis. Osteoporosis is a common complication in adults with thalassemia major and intermedia [Voskaridou & Terpos 2004]. Its origin is multifactorial, making it difficult to manage. Treatment involves appropriate hormonal replacement, an effective regimen of transfusion and iron chelation, calcium and vitamin D administration, and regular physical activity. Specific treatment with bisphosphonates has been attempted with promising results in several studies and confirmed in a randomized trial [Voskaridou et al 2006].
For individuals with thalassemia major, follow-up to monitor the effectiveness of transfusion therapy and chelation therapy and their side effects includes the following:
Physical examination every month by a physician familiar with the affected individual and the disease
Assessment of liver function tests (serum concentration of ALT) every two months
Determination of serum ferritin concentration every three months
Assessment of growth and development every six months
Annual
Ophthalmologic and audiologic examinations
Complete cardiac evaluation, and evaluation of thyroid, endocrine pancreas, parathyroid, adrenal, and pituitary function
Liver ultrasound evaluation and determination of serum alpha-fetoprotein concentration in adults with hepatitis C and iron overload for early detection of hepatocarcinoma
Bone densitometry to assess for osteoporosis in the adult
Regular gallbladder echography for early detection of cholelithiasis [Galanello et al 2001], particularly in individuals with the Gilbert syndrome genotype [i.e., presence of the (TA)7/(TA)7 motif in the promoter of the UGT1A gene]
The following should be avoided:
Alcohol consumption, which in individuals with liver disease has a synergistic effect with iron-induced liver damage
Iron-containing preparations
If prenatal diagnosis has not been utilized and if the disease-causing mutations have been identified in an affected family member, it is appropriate to offer molecular genetic testing to at-risk sibs to allow early diagnosis and appropriate treatment.
See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.
New chelation strategies, including the combination or alternate treatment with the available chelators, are under investigation.
Induction of fetal hemoglobin synthesis can reduce the severity of β-thalassemia by improving the imbalance between alpha and non-alpha globin chains. Several pharmacologic compounds including 5-azacytidine, decytabine, and butyrate derivatives have had encouraging results in clinical trials [Pace & Zein 2006]. These agents induce Hb F by different mechanisms that are not yet well defined. Their potential in the management of β-thalassemia syndromes is under investigation.
The efficacy of hydroxyurea treatment in individuals with thalassemia is still unclear. Hydroxyurea is used in persons with thalassemia intermedia to reduce extramedullary masses, to increase hemoglobin levels, and, in some cases, to improve leg ulcers. A good response, correlated with particular polymorphisms in the beta-globin cluster (i.e., C>T at -158 G gamma), has been reported in individuals with transfusion dependence [Bradai et al 2003, Yavarian et al 2004]. However, controlled and randomized studies are warranted to establish the role of hydroxyurea in the management of thalassemia syndromes.
The possibility of correction of the molecular defect in hematopoietic stem cells by transfer of a normal gene via a suitable vector or by homologous recombination is being actively investigated [Sorrentino & Niehuis 2001]. The most promising results in the mouse model have been obtained with lentiviral vectors [Rivella et al 2003, Puthenveetil et al 2004].
Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.
Attempts at in utero transplantation using the father as a haploidentical donor have consistently failed.
Genetics clinics are a source of 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.
Support groups have been established for individuals and families to provide information, support, and contact with other affected individuals. The Resources section may include disease-specific and/or umbrella support organizations.
Genetic counseling is the process of providing individuals and families with information on the nature, inheritance, and implications of genetic disorders to help them make informed medical and personal decisions. The following section deals with genetic risk assessment and the use of family history and genetic testing to clarify genetic status for family members. This section is not meant to address all personal, cultural, or ethical issues that individuals may face or to substitute for consultation with a genetics professional. To find a genetics or prenatal diagnosis clinic, see the GeneTests Clinic Directory.
The β-thalassemias are inherited in an autosomal recessive manner.
Parents of a proband
The parents of an affected child are obligate heterozygotes and, therefore, carry a single copy of a disease-causing HBB mutation.
Heterozygotes are clinically asymptomatic but occasionally slightly anemic. Carriers are often referred to as having thalassemia minor (orβ-thalassemia minor).
Sibs of a proband
At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Once an at-risk sib is known to be unaffected, the chance of his/her being a carrier is 2/3.
Heterozygotes are clinically asymptomatic but occasionally slightly anemic. Carriers are often referred to as having thalassemia minor (orβ-thalassemia minor).
Offspring of a proband
Each child of a proband inherits one copy of a disease-causing HBB mutation from the affected parent and thus is an obligate heterozygote.
Because of the high carrier rate for β-thalassemia in certain populations (see Prevalence), the offspring of an affected individual and a reproductive partner from one of the high-prevalence areas are at increased risk forβ-thalassemia.
Given the high carrier rate for β-thalassemia in these populations, it is appropriate to offer carrier testing to the partner of a proband with β-thalassemia.
Other family members of a proband. Each sib of the proband's parents is at a 50% risk of being a carrier.
Individuals who should be considered for carrier detection:
Family members
Gamete donors
Members of at-risk ethnic groups (see Table 4) [Cao et al 1997]
Hematologic testing. The carrier state is often referred to as β-thalassemia minor. Carriers are often identified by analysis of red blood cell indices (Table 1), which shows microcytosis (low MCV) and reduced content of Hb per red cell (low MCH), and by quantitative Hb analysis (Table 2), which displays HbA2 greater than 3.5%.
Pitfalls in carrier identification by hematologic testing:
Coinheritance of α-thalassemia, which may normalize the red blood cell indices. However, in α/β double heterozygotes, the HbA2 concentration remains in the β-thalassemia carrier range and thus is of diagnostic value.
Coinheritance of δ-thalassemia, which reduces to normal the increased Hb A2 levels typical of the β-thalassemia carrier state. Double heterozygosity for δ- and β-thalassemia can be distinguished from the most common α-thalassemia carrier state by globin chain synthesis or globin gene analysis.
Confusion of α-thalassemia carriers with β-thalassemia carriers, resulting from microcytosis and hypochromia. However, α-thalassemia carriers are easily distinguished by normal HbA2 levels (see Alpha-Thalassemia).
Silent HBB mutations, i.e., very mild mutations associated with consistent residual output of hemoglobin beta chains and with normal RBC indices and normal or borderline HbA2. However, homozygosity for silent mutations or compound heterozygosity for a silent HBB mutation and a typical HBB mutation result in mild non-transfusion-dependent forms of β-thalassemia.
Molecular genetic testing. When the hematologic analysis is abnormal, molecular genetic testing of HBB is performed to identify the disease-causing mutation. Mild and silent β-thalassemia mutations, which may result in attenuated forms of the disease, are identified and thus may lead to improved genetic counseling of couples at risk.
Individuals at increased risk. Because of the high carrier rate for HBB mutations in certain populations and the availability of genetic counseling and prenatal diagnosis, population screening is ongoing in several at-risk populations in the Mediterranean [Cao et al 2002]. Carrier testing relies on hematologic analysis. When the hematologic analysis indicates a β-thalassemia carrier state, molecular genetic testing of HBB can be performed to identify a disease-causing mutation. If both partners of a couple have the HBB disease-causing mutation, each of their offspring has a 1/4 risk of being affected. Through genetic counseling and the option of prenatal testing, such a couple can opt to bring to term only those pregnancies in which the fetus is unaffected.
See Management, Testing of Relatives at Risk 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 are affected or are carriers.
DNA banking. DNA banking is the storage of DNA (usually 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 in situations in which the reasons of a particular phenotype are not defined. See
for a list of laboratories offering DNA banking.
High-risk pregnancies
Molecular genetic testing. Prenatal diagnosis for pregnancies at increased risk is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at approimately 15-18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation. Both disease-causing alleles must be identified before prenatal testing can be performed.
Preimplantation genetic diagnosis. Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutations have been identified. For laboratories offering PGD, see
.
Analysis of fetal cells in maternal blood. Prenatal diagnosis by analysis of fetal cells in maternal blood is not yet available, but is being investigated on a research basis [Mavrou et al 2007].
Analysis of fetal DNA in maternal plasma for the presence of the father's mutation may lead to prenatal exclusion of homozygous β-thalassemia. This testing is not yet clinically available, but is being investigated on a research basis with promising results [Lo 2005].
Indeterminate-risk pregnancies. An indeterminate risk pregnancy is one in which:
One parent is a definite heterozygote and the other parent has a β-thalassemia-like hematologic picture, but no HBB mutation has been identified by sequence analysis.
A mother is a known heterozygote and the father is unknown or unavailable for testing, especially if the father belongs to a population at risk.
In either instance, the options for prenatal testing should be discussed in the context of formal genetic counseling. In indeterminate-risk pregnancies, the prenatal testing strategy is analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at approximately 15-18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation for the known HBB mutation. If the known HBB mutation is present, analysis of globin chain synthesis is performed on a fetal blood sample obtained by percutaneous umbilical blood sampling (PUBS) at approximately 18-21 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.
Information in the Molecular Genetics tables is current as of initial posting or most recent update. —ED.
Gene Symbol | Chromosomal Locus | Protein Name |
---|---|---|
HBB | 11p15.5 | Hemoglobin subunit beta |
Data are compiled from the following standard references: Gene symbol from HUGO; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from Swiss-Prot.
Gene Symbol | Locus Specific | Entrez Gene | HGMD |
---|---|---|---|
HBB | HBB | 3043 (MIM No. 141900) | HBB |
For a description of the genomic databases listed, click here.
Note: HGMD requires registration.
Normal allelic variants: The HBB gene, which spans 1.6 kb, contains three exons and both 5' and 3' untranslated regions. The HBB gene is regulated by an adjacent 5' promoter, which contains a TATA, CAAT, and duplicated CACCC boxes, and an upstream regulatory element dubbed the locus control region (LCR). A number of transcription factors regulate the function of the HBB gene, the most important of which is the erythroid Kruppel-like factor (EKLF), which binds the proximal CACCC box and whose knockout in the mouse leads to a thalassemia-like clinical picture. The HBB gene is contained within the HBB gene cluster, which also includes the genes encoding the delta globin chain, A gamma and G gamma chains, and a pseudo HBB gene.
Pathologic allelic variants: β-thalassemias are heterogeneous at the molecular level. More than 200 disease-causing mutations have been identified to date. The large majority of mutations are simple single-nucleotide substitutions or deletion or insertion of oligonucleotides leading to a frameshift. Rarely, theβ-thalassemias are the result of gross gene deletion.
Despite marked molecular heterogeneity, the prevalent molecular defects are limited in each at-risk population (see Table 4), in which four to ten mutations usually account for most of the HBB disease-causing alleles.
DNA Nucleotide Change 1, 2 (Aliases 3 ) | Protein Amino Acid Change 1, 2 | At-Risk Populations | Detection Rate |
---|---|---|---|
c.-136C>G (-87C>G) | -- | Mediterranean | 91% to 95% |
c.92+1G>A (IVS1-1G>A) | -- | ||
c.92+6T>C (IVS1-6T>C) | -- | ||
c.93-21G>A (IVS1-110G>A) | -- | ||
c.118C>T (cd39C>T) | p.Gln39X | ||
c.316-106C>G (IVS2-745C>G) | -- | ||
c.25_26delAA (cd8-AA) | p.Lys8ValfsX13 | Middle East | |
c.27_28insG (cd8/9+G) | p.Ser9ValfsX13 | ||
c.92+5G>C (IVS1-5G>C) | -- | ||
c.118C>T (cd39C>T) | p.Gln39X | ||
c.135delC (cd44-C) | p.Phe45LeufsX15 | ||
c.315+1G>A (IVS2-1G>A) | -- | ||
c.-78A>G (-28A>G) | -- | Thai | |
c.52A>T (17A>T) | p.Lys17X | ||
c.59A>G (19A>G) | p.Asn19Ser (Hb Malay) | ||
c.92+5G>C (IVS1-5G>C) | -- | ||
c.124_127delTTCT (41/42-TTCT) | p.Phe42LeufsX17 | ||
c.316-197C>T (IVS2-654C>T) | -- | ||
c.-78A>G (-28A>G) | -- | Chinese | |
c.52A>T (17A>T) | p.Lys17X | ||
c.124_127delTTCT (41/42-TTCT) | p.Phe42LeufsX17 | ||
c.316-197C>T (IVS2-654C>T) | -- | ||
c.-138C>T (-88C>T) | -- | African / African American | 75% to 80% |
c.-79A>G (-29A>G) | -- | ||
c.92+5G>C (IVS1-5G>T) | -- | ||
c.75T>A (cd24T>A) | p.Gly24Gly | ||
c.316-2A>G (IVS11-849A>G) | -- | ||
c.316-2A>C (IVS11-849A>C) | -- |
See Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www.hgvs.org).
1. Reference sequence is NM_000518 (www.ncbi.nlm.nih.gov/Genbank/index.html)
2. The DNA nucleotide change designations follow current nomenclature guidelines. However, because the initiating methionine is not part of the mature beta-globin protein, the long-standing convention of numbering the amino acids is to begin with the next amino acid (Val). For consistence with the literature and the Globin Gene Server (globin.cse.psu.edu), the amino acid numbering in this table follows that convention.
3. Variant designations that do not conform to current naming conventions
βº-thalassemia (complete absence of hemoglobin subunit beta production) alleles result from nonsense, frameshift, or (sometimes) splicing mutations.
β+-thalassemia alleles (residual output of globin beta chains) are produced by mutations in the promoter area (either the CACCC or the TATA box), the polyadenylation signal, or the 5' or 3' untranslated region, or by splicing abnormalities.
The complex β-thalassemias (delta-beta- and gamma-delta-beta-thalassemia) result from deletion of variable extent of the HBB gene cluster.
β-thalassemia may also be produced by deletion of the LCR, leaving intact the HBB gene itself.
In rare instances, the β-thalassemia defect lies outside the β-globin gene cluster.
Silent mutations, which are characterized by normal hematologic findings and defined only by a mildly unbalanced α/β-globin chain synthesis ratio, result from mutation of the distal CACCC box, the 5' unbalanced region, the polyadenylation signal, and some splicing defects (see Table 5).
Table 5. Mild and Silent HBB Mutations Causing β-Thalassemia
Aliases 1 | Standard Naming Conventions 2, 3 | |||
---|---|---|---|---|
Mutation Type or Location | Mild β+ | Silent | DNA Nucleotide Change (Protein Amino Acid Change) | |
Transcriptional mutants in the proximal CACC box | -90 C>T -88 C>T -88 C>A -87 C>T -87 C>G -87 C>A -86 C>T -86 C>G | c.-140C>T c.-138C>T c.-138C>A c.-137C>T c.-137C>G c.-137C>A c.-136C>T c.-136C>G | ||
-101 C>T -92 C>T | c.-151C>T c.-142C>T | |||
TATA box | -31 A>G -30 T>A -29 A>G | c.-81A>G c.-80T>A c.-79A>G | ||
5' UTR | +22 G>A +10 -T +33 C>G | c.-29G>A c.-41de>T c.-18C>G | ||
+1' A>C | c.-50A>C | |||
Alternative splicing | cd19 A>C (Hb Malay)
cd24 T>A | c.56A>G (p.Asn19Ser) 4
c.72T>A (p.Gly24Gly) 4 | ||
cd27 G>T (Hb Knossos) | c.82G>T (p.Ala27Ser) 4 | |||
Consensus splicing | IVS1-6 T>C | c.91+6T>C | ||
Intron | IVS2-844 C>G | c.316-7C>G | ||
3' UTR | +6 C>G | c.*6C>G | ||
Poly A site | AACAAA AATGAA | c.*110T>C c.*111A>G | ||
AATAAG | c.*113A>G | |||
Mild βº | Frameshift | cd6-A cd8-AA | c.17delA (p.Glu6GlyfsX12) 4 c.25_26delAA(p.Lys8ValfsX13) 4 |
1. Nonstandard variant designations in common use (globin.cse.psu.edu)
2. See Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www.hgvs.org).
3. Reference sequence is NM_000518 (www.ncbi.nlm.nih.gov/Genbank)
4. The DNA nucleotide change designations follow current nomenclature guidelines. However, because the initiating methionine is not part of the mature beta-globin protein, the long-standing convention of numbering the amino acids is to begin with the next amino acid (Val). For consistency with the literature and the Globin Gene Server (globin.cse.psu.edu), the amino acid numbering in this table follows that convention.
For more information, see Genomic Databases table.
Normal gene product: The HBB gene encodes hemoglobin subunit beta. The heterodimeric protein HbA is made up of two globin alpha chains and two globin beta chains.
Abnormal gene product: βº-thalassemia results from the absence of globin beta chains. In β+-thalassemia, the globin beta chain output is reduced to a variable extent, but the globin beta chains have a normal sequence.
GeneReviews provides information about selected national organizations and resources for the benefit of the reader. GeneReviews is not responsible for information provided by other organizations. Information that appears in the Resources section of a GeneReview is current as of initial posting or most recent update of the GeneReview. Search GeneTests for this disorder and select
for the most up-to-date Resources information.—ED.
Cooley's Anemia Foundation
330 Seventh Avenue Suite 900
New York NY 10001
Phone: 800-522-7222
Fax: 212-279-5999
Email: info@cooleyanemia.org
www.cooleysanemia.org
National Library of Medicine Genetics Home Reference
Beta thalassemia
NCBI Genes and Disease
Thalassemia
Thalassaemia International Federation
PO Box 28807
2083 Nicosia
Cyprus
Phone: 22 319129
Fax: 22 314552
Email: info@thalassaemia.org.cy
www.thalassaemia.org.cy
Teaching Case-Genetic Tools
Cases designed for teaching genetics in the primary care setting.
Case 36. A One-Year-Old with Hemoglobin E/Beta-Thalassemia
Medical Genetic Searches: A specialized PubMed search designed for clinicians that is located on the PubMed Clinical Queries page.
No specific guidelines regarding genetic testing for this disorder have been developed.
23 October 2007 (me) Comprehensive update posted to live Web site
19 June 2005 (me) Comprehensive update posted to live Web site
4 April 2003 (ac) Revision
18 March 2003 (tk) Comprehensive update posted to live Web site
28 September 2000 (me) Review posted to live Web site
March 2000 (ac) Original submission