Thalassemia

Abstract

Thalassemia is the most common form of inherited anemia worldwide. The World Health Organization reports suggest that about 60 000 infants are born with a major thalassemia every year. Although individuals originating from the tropical belt are most at risk, it is a growing global health problem due to extensive population migrations. Despite important advances on curative approaches such as stem cell transplantation and promising results of gene therapy, blood transfusions and iron chelation still remain as cornerstones of disease management. The purpose of this article is to focus on mainly the clinical aspects and management of beta-thalassemia major.

Keywords:

• Beta-thalassemia
• Blood transfusion
• Iron overload
• Iron chelators

Introduction

Thalassemia is the most common form of inherited anemia worldwide which is characterized by the decreased or abolished production of either the alpha-like (alpha-thalassemia) or the beta-like (beta-thalassemia) globin chains that are produced to form hemoglobin tetramers (alpha₂gamma₂, HbF; alpha₂beta₂, HbA; alpha₂delta₂, HbA₂) during the fetal and postnatal life.1

In 1925, thalassemia was first recognized in its severe (named Cooley’s anemia) and milder (named La Malattia di Rietti-Greppi-Micheli) forms independently in the US and Italy, which are today known as thalassemia major (TM) and thalassemia intermedia (TI).2,3 Over the next 20 years, it became clear that thalassemia had been described for homozygous or compound heterozygous states for a recessively inherited microcytic mild anemia terms as thalassemia minor.4 Subsequently, it has been recognized that thalassemia resulting from defects in the production of alpha- and beta-globin chains of hemoglobin, lead to the most common monogenic disease in humans not confined to the Mediterranean, but widespread throughout the Africa, Middle East, Southeast Asia, and Western Pacific region.5

Epidemiology and Prevention

The World Health Organization report suggests that about 1·5% of the world’s population might be carriers of thalassemia and that about 60 000 infants are born with a major thalassemia including homozygous beta-thalassemia, E/beta-thalassemia, homozygous alpha⁰ thalassemia and HbH disease every year.6 The extensive studies strongly indicated that the carrier status for either alpha- or beta-thalassemia offers protection against the Falciparum malaria, which explains high carrier frequency in areas where malaria has been endemic.7

Long established screening programs for detecting carriers in the population with genetic counseling and the option of prenatal diagnosis at risk pregnancies have resulted in a marked reduction in the rate of affected birth in Mediterranean countries. Systematic carrier screening has recently been established in parts of Middle East and Asia countries.6 Following identification of both disease-causing alleles, prenatal diagnosis can be performed by analysis of DNA extracted from fetal cells obtained by either amniocentesis or chorionic villus sampling (at 15–18 weeks and 10–12 weeks of gestation respectively). Preimplantation genetic diagnosis would also be an option for getting pregnant to an unaffected child. It is expected that the analysis of circulating fetal DNA in maternal blood will play an increasingly important role in the future practice of prenatal diagnosis.8

The Molecular Pathology and Pathophysiology

Severe beta-thalassemia in which both beta-genes (one on each copy of chromosome 11) are affected (beta^T/beta^T) usually becomes manifest during the first year of life, when synthesis of fetal hemoglobin decline but switching to adult hemoglobin cannot be allowed, because of diminished synthesis of beta-globin chains to partner the alpha-globin chains. However, heterozygote beta-thalassemia in which one of two beta-genes is affected (beta/beta^T) usually is not associated with a clinical significance. The major determinant of the severity of beta-thalassemia is the extent of alpha-/non-alpha-globin chain imbalance, which is mainly determined by the molecular defects in the beta-genes, in which more than 200 point mutations and some deletions have been described, resulting in either reduction (beta⁺-thalassemia) or absence (beta⁰-thalassemia) of beta-globin chain synthesis. Any factor capable of reducing the alpha-/non-alpha-globin chain imbalance in a subject with affected beta-genes may have an ameliorating effect on the clinical picture. The most important modifiers on disease severity are the co-inheritance of alpha-thalassemia that is resulted with reducing alpha chain output or a genetic determinant that is able to sustain a continuous production of gamma chains in adult life caused by point mutations at G-gamma or A-gamma promoters (−158 C3T G-gamma; −196 C3T A-gamma).9

The insufficient synthesis of beta-globin chains is resulted with a relative excess of alpha-globin chains which precipitate in erythroid precursors and lead to oxidative damage of the cell membrane, thereby resulting in ineffective erythropoiesis and in mature red cells causing hemolysis. This primary pathology leads to severe anemia which causes tissue hypoxia stimulating erythropoietin synthesis, erythroid marrow expansion, and splenomegaly. Marrow expansion results in characteristic bone deformities and osteopenia and also leads to increased iron absorbtion from the gut which is ultimately resulted with iron overload.10 Recent progress in understanding regulation of iron homeostasis has contributed greatly in understanding how hypoxia and ineffective erythropoiesis mediate increased iron absorbtion in thalassemia.11 Increased iron absorbtion in non-transfused patients with TI can be up to 5–10 times normal (0·1 mg/kg/day), which is mainly deposited in hepatocytes. Regular blood transfusions are the predominant cause of iron overload (0·3–0·5 mg/kg/day) that is mainly deposited in macrophages in patients with TM. However, once iron loading capacity of macrophages is overwhelmed, transferrin becomes saturated and plasma non-transferrin bound iron appears. Non-transferrin bound iron is taken up excessively by the cells via uncontrolled uptake mechanisms, such as calcium and zinc channels, and contributes to expansion of labile iron pools which becomes available to participate in the generation of free radicals. Tissue iron overload is resulted with iron induced liver disease, endocrine complications and inevitably death from iron induced cardiomyopathy if untreated.12

Clinical and Hematological Features

Beta-thalassemia carriers (beta/beta^T) are clinically asymptomatic individuals who have characteristic hematological features including normal/slightly reduced hemoglobin concentration, increased red cell count, microcytosis, and hypochromia. The hemoglobin pattern reveals increased HbA₂>3·5% associated with variable amount of HbF 0·5–4%. However, silent carriers show normal hematological features and hemoglobin pattern. Homozygosity of silent alleles with beta⁺ or beta⁰ alleles gives rise to mild to moderate forms of TI.

Homozygosity or compound heterozygosity for beta-thalassemia (beta^T/beta^T) is most commonly presented with severe microcytic, hypochromic hemolytic anemia (Hb<7 g/dl) during the first year of life, but usually later than 3 months old. Reticulocytosis, as well as increased number of nucleated red cells, anisocytosis, and poikilocytosis in blood smear are prominent features. The hemoglobin pattern varies related with the type of beta-thalassemia. Patients with beta⁰/beta⁰ thalassemia show a hemoglobin pattern characterized by the absence of HbA and relative increase of HbF (95–98%). These patients come to medical attention during the first year of life and require life-long regular transfusions. Patients with beta⁰/beta⁺ or beta⁺/beta⁺ thalassemia have a residual beta-globin synthesis where HbA is 10–30% and HbF 70–90%. These patients presenting later onset of disease (2–4 years old) and capable of maintaining hemoglobin of 7–9 g/dl without regular transfusions show a markedly heterogeneous hematological picture and wide clinical spectrum with varying degrees of splenomegaly and skeletal changes.13

Management of Beta-TM

Transfusion regimen

Although life-long transfusion therapy is the cornerstone of the treatment for most patients with homozygous beta-thalassemia, the decision for initiation of regular transfusion should be taken carefully. Regular transfusion regimen should only be started if the patient cannot maintain hemoglobin level of ⩾7 g/dl and/or suffers from growth impairment and/or shows progressively increase in spleen size. Once transfusion decision is established, regular transfusion program with a target pretransfusion hemoglobin of 9–9·5 g/dl is maintained by using packed red blood cells for not only correction of anemia, but suppression of erythropoiesis to provide the prevention of skeletal deformities and splenomegaly and inhibition of increased gastrointestinal iron absorbtion.10 Splenectomy is indicated when blood consumption is above 200–250 ml/kg/year of packed red blood cells.14 Before attributing the increment in blood consumption to hypersplenism, the presence of delayed hemolytic transfusion reaction and poor quality of transfused units should be excluded.

Iron Chelation Therapy

Objectives of iron chelation therapy

Iron gained by regular blood transfusions is required to be removed by iron chelation therapy for preventing iron toxicity in TM. The primary objective of iron chelation is to maintain body iron at safe levels at all times, but once iron is accumulated, the objective of iron chelation is to reduce tissue iron to the safe levels.12

When chelation should be started and what are the safe levels of body iron burden?

Initiation of chelation has been traditionally determined based on experience with desferrioxamine (DFO). DFO chelation has been started following 10–20 red cell transfusions and when serum ferritin exceeds 1000 μg/l.10 Although this traditional threshold is currently applied to other chelation practices, there is uncertainty whether chelation can be safely started earlier with other chelators [deferiprone (DFP) and deferasirox (DFX)] or this should be desired.

A proper monitoring of chelation has of importance for measuring response rate to a chelation regimen and providing dose adjustments to enhance chelation efficacy but avoid toxicity. Chelation therapy should be maintained at liver iron concentration (LIC) of about 3·2–7 mg iron/g dry weight (d.w.) (normal ranges 0·6–1·2 mg iron/g d.w.) which was associated with normal survival without complications of iron overload in subjects with non-transfusional iron overload. Although, the serum ferritin levels corresponding to targeted LIC have not been clearly defined, in practice, serum ferritin is maintained at between 500 and 1000 μg/l.10 Cardiac magnetic resonance (T2*) was found superior to serum ferritin and LIC in identifying patients at high risk of developing heart failure and arrhythmia from myocardial siderosis, which is responsible for most of the deaths in TM. It has been demonstrated that there is a progressive and significant decline in left ventricular ejection fraction below a myocardial T2* of 20 milliseconds.15 Assessment of cardiac iron and management of TM patients by considering cardiac risks has been strongly recommended in clinical practice.

Properties, effectiveness, and unwanted effects of iron chelators

Three chelators are currently available for the purpose of preventing and removing iron overload.

DFO is administered prolonged (8–12 hours) subcutaneous infusion at least 5 days a week. Adequate doses of DFO are 20–40 mg/kg/day in children and 30–60 mg/kg/day in adults. Although it is a highly effective iron chelator, compliance to administration of DFO remained as the major problem. Clinical data show that siderotic heart failure is reversible with 24-hour intravenous DFO infusion via indwelling catheter and myocardial T2* improves in concert with function during recovery.16 Most of the toxic side effects of DFO on impaired growth and skeletal changes are observed in children when treatment starts early (<2 years), at low body iron burden (serum ferritin<1000 μg/l) and relatively higher doses of chelator (>40 mg/kg). DFO-related retinopathy (loss of visual acuity, field defects, and defects in color vision) and ototoxicity (symmetric and high-frequency sensorineural hearing loss) are also observed at higher doses of chelator at lower serum ferritin levels. Yersinia enterocolitica infections should be suspected in patients on DFO chelation with enterocolitis and fever that prompt antibiotic therapy might prevent life threatening sepsis and shock.10

Oral chelator DFP is a widely used regimen of 75 mg/kg/day at three divided doses up to 100 mg/kg, specifically for patients with TM when DFO is inadequate, intolerable, or unacceptable. There are still limited data available on the use of DFP in children between 6 and 10 years of age, and no data on DFP use in children under 6 years. Probably because of the rapid inactivation by glucuronidation within the liver, DFP shows less impressive effect on liver iron.17 However, the addition of subcutaneous standard dose of DFO (40–50 mg/kg) only twice weekly to daily DFP (75 mg/kg/day) resulted in higher negative iron balance, compared with either drug alone without increasing toxicity.18 A randomized prospective study suggests that DFP at higher doses up to 100 mg/kg/day has superior access to myocardial iron stores compared with DFO.19 Combined therapy of daily DFP with DFO (5 days a week) for simultaneously focusing on liver and heart iron, showed larger improvement in cardiac and liver iron compared to standard DFO therapy.20 Agranulocytosis, occurred in 0·5% of patients, is the most serious side effect of DFP therapy requiring weekly monitoring of neutrophil count. Agranulocytosis is always reversible with discontinuation of DFP and reintroduction of DFP after an initial episode of agranulocytosis is not recommended. Gastrointestinal disturbances (nausea, vomiting, and abdominal pain), arthropathy, increase in liver enzymes and zinc deficiency are more common, but less severe unwanted effects of DFP.12

DFX as once daily oral chelator was approved for the treatment of patients with transfusional iron overload — older than 2 years — as first-line therapy.21 Although the recommending starting dose of DFX is 20 mg/kg/day, dose titration between 10 and 40 mg/kg/day based on transfusional iron intake and body iron burden are required for achieving therapeutic target of maintenance or reduction in iron burden. A long-term prospective study showed continued reduction and normalization of cardiac iron along with a significant decrease in liver iron with a manageable safety profile at DFX doses of above 30 mg/kg over 3 years.22 The most frequent adverse effects are gastrointestinal disturbances (nausea, vomiting, abdominal pain, diarrhea, and constipation), skin rashes, and transient fluctuations in liver enzymes. During the first months of DFX, a mild, dose-dependent, non-progressive increase in serum creatinine has been observed in one-third of patients. These creatinine increases remained within normal ranges, resolved spontaneously or with dose reduction. The cases of acute kidney injury have been reported in the post-marketing surveillance of DFX in patients with severe comorbidity like renal and hepatic impairment, but have not been observed in patients with TM. High-frequency hearing loss and lenticular opacities are also observed less frequently. It is recommended monthly monitorization of serum creatinine, urine protein and liver enzymes, and annual auditory and ophthalmic examinations.12

Since 1999, there has been 71% reduction in annualized death rate from iron overload in TM in the UK. This marked improvement in survival can be attributed to introduction of cardiac T2* to identify myocardial siderosis and appropriate intensification of iron chelation treatment accordingly.23

Stem Cell Transplantation (SCT) and Gene Therapy

Despite therapeutic progress on survival and quality of life in TM, SCT remained as the only curative method available today. Although more than 90% of patients who receive HLA-identical related donor SCT are surviving and 80–90% of them are being disease-free, there are still uncertainties how the curative but potentially lethal SCT can be applied for adult and patients with advanced disease or having matched unrelated donor.24 The major limitation of SCT is the lack of an HLA-identical sibling donor for the majority (70–75%) of affected patients. Gene therapy may overcome the problem of donor availability and provide ultimate cure for all patients with thalassemia.9

Intensive clinical and molecular studies on the management of thalassemia and development of targeted therapies (such as reactivation of HbF production, gene therapy, and cell therapy) show promising results for beta-thalassemia patients. However, struggle for establishing widespread health services covering screening, counseling and prenatal diagnosis is still required to control this increasing global health burden.