Thalassemia

Any of a group of inherited disorders of erythrocytes, resulting from absence or deficiency in one or more of the polypeptide chains of hemoglobin, the protein-iron complex in erythrocytes facilitating oxygen transport in our body

Overview
Thalassemias are hemoglobinopathies, genetic disorders of hemoglobin, the metalloprotein complex conveying oxygen and carbon dioxide in the human body. Depending on the defective component one can differentiate between &#945;-, &#946;-, and &#948;-thalassemia and compound thalassemias, which might, moreover, occur in combination with each other or with related defects. In adults, hemoglobin is composed of a central iron ion, two &#945;-globin chains, and two &#946;-globin chains, so that &#945;- and &#946;-thalassemias have the greatest health impact in adult life. The degree of reduction in synthesis and the genotype, including modifying factors, determine the corresponding clinical picture, which may vary from barely detectable hematologic abnormality to severe and fatal anemia. Thalassemia signs include anemia (See cytopenia) and instability of erythrocytes, vasoocclusions, organ damage and organ iron overload. Palliative treatments include regular blood transfusions and chelation (binding and removal) of the resulting excess iron. A cure can presently be achieved by bone marrow transplants from compatible donors and may be established in the future by gene therapy, i.e. the correction and reimplantation of the patient's own hematopoietic stem cells.

Epidemiology
The &beta;-thalassaemias are distributed widely amongst Mediterranean populations, in the Middle East, in parts of India and Pakistan and throughout South-East Asia, and are much less common in Africa, except for some isolated pockets in West Africa and in parts of North Africa. The &alpha;-thalassemias have their greatest prevalence throughout Africa, in the Caribbean and in Central Asia. Owing to the continual migration of populations from one area to another, however, there is virtually no country in the world now in which thalassemia does not affect some percentage of the inhabitants. Molecular analysis of the thalassemia genes has demonstrated a striking heterogeneity. Approximately 200 different &beta;-thalassemia mutations have been reported and yet, population studies indicate that probably only 20 &beta;-thalassemia alleles account for more than 80% of the &beta;-thalassemia mutations in the world. Similarly, the &alpha;-thalassemias show a high level of heterogeneity but locally enriched mutations. These observations are caused by geographic clustering, where each population has a few very common mutations, together with a varying number of rare ones. Furthermore, the pattern of mutations is different in each population. Even when the same mutation occurs in different populations, it is usually found together with a different globin gene RFLP haplotype. It is likely, therefore, that different thalassemia mutations have arisen independently in different populations and achieved their high frequency by selection. Although some movement of the thalassemia genes may have occurred between populations by drift, little doubt exists that independent mutation and selection have been the major factors responsible for the world distribution of the &alpha;-, &beta;- and minor thalassaemias. Population studies suggest that one reason for the wide distribution of the thalassemia polymorphism is protection of heterozygotes against Plasmodium falciparum malaria. There are numerous studies showing statistically significant correlation of thalassemias and malaria resistance, while other have approached the analysis of the mechanism underlying this protection.

Etiology and pathogenesis
Human hemoglobin production is highly developmentally regulated. The human adult hemoglobin is synthesized in the red blood cells and its major function is O2 transport from the lungs to the tissues. It consists of a major component, hemoglobin A (Hb A), and a minor component, Hb A2, which constitutes about 2.5% of the total. During intrauterine life, several embryonic hemoglobins are present. The structure of these hemoglobins is similar, with each consisting of two separate pairs of identical globin chains. Except for some of the embryonic hemoglobins, all the normal human hemoglobins have one pair of &alpha; chains: in Hb A they are combined with &#946; chains (&alpha;2&beta;2,) in Hb A2 with &#916; chains (&alpha;2&#916;2), and in fetal hemoglobin, Hb F, with &#947; chains (&alpha;2&#947;2). Before the eighth week of intrauterine life, there are three embryonic hemoglobins: Hb Gower 1 (&#950;2&#949;2), Hb Gower 2 (&alpha;2&#949;2) and Hb Portland (&#950;2&#947;2). The &#946;-like globin chains are controlled by a gene cluster on chromosome 11 in which the different genes are arranged in the order 5’-&#949;-G&#947;-A&#947;-&#968;&#946;-&#916;-&beta;-3’. The &alpha;-like gene cluster is on chromosome 16, p13.3, and the genes are arranged in the order 5’-&#950;-&#968;&#950;-&#968;&alpha;2-&#968;&alpha;1-&alpha;2-&alpha;1-&#952;-3’. Types of thalassemia are defined according to the globin chains and number of alleles (gene copies) affected by mutation, and the severity of the chain defect(s). The common and clinically important types are &#946;-, &#916;&#946;- and &alpha;-thalassemia. For &beta;-thalassemia there are two types of mutations, &beta;0 in which no &beta;-globin chains are produced, and &beta;+ in which &beta;-globin chains are produced at a reduced rate. Some types of &beta;-thalassemia are designated &beta;++ to indicate that the defect in &beta;-chain production is particularly mild. While two, virtually identical alpha; genes exist in the diploid human genome, resulting in four alpha; gene copies, only one &beta; gene and hence only two &beta; alleles are present. In the homozygous state (both allele are affected by the same defect) or compound heterozygous state (each allele has a different defect, but both are defective) for &beta;-thalassemia, &beta;-globin chain synthesis is either absent or markedly reduced. This results in the accumulation, within erythroid cells, of excessive amounts of &alpha;-globin chains, owing to a near-absence of binding partners for the the abundant &alpha; chains. The free &alpha;-chains are unable to form viable tetramers and instead precipitate in the red cell precursors, forming inclusion bodies. These &alpha;-chain inclusions can be demonstrated by both light and electron microscopy in the erythroid precursor cells in the bone marrow as well as in peripheral red cells. They are responsible for the intramedullary (i.e. in-bone-marrow) destruction of the erythroid precursors and hence the ineffective erythropoiesis that characterizes all &beta;-thalassemias. Although the anemia in &beta;-thalassemia is primarily due to ineffective erythropoiesis, there is also a hemolytic component, which is related to the destruction of mature erythrocytes containing &alpha;-globin chain inclusions in the circulation. The clinical severity can be modulated by several factors: principally, the nature of the &beta;-thalassemia mutation; coinheritance of &alpha;-thalassemia, which will lead to a reduction in the excess of &alpha;-chain pool and inclusion body formation; and coinheritance of genetic factors which increase &#947;-chain production, since &#947;-chains will combine with the excess &alpha;-chains to form Hb F.

Clinical and laboratory features
The most clinically severe form of &beta;-thalassaemia is called thalassemia major. Some patients have a milder clinical picture which is characterized by a later onset and either no transfusion requirement or much reduced dependency on blood donations. This condition is called &beta;-thalassemia intermedia. &beta;-thalassemia minor is the term used to describe the heterozygous carrier state for &beta;-thalassemia, which is largely symptomless. Infants affected by &beta;-thalassemia are well at birth, owing to the abundance of Hb F in their blood. Anaemia usually develops during the first few months of life, when Hb F content would naturally decrease and be replaced by Hb A, and becomes progressively more severe. The infants fail to thrive and may have feeding problems, bouts of fever and diarrhoea, and other gastrointestinal symptoms. If an accurate diagnosis is made at this stage and the infant is started on a regular blood-transfusion regimen, subsequent growth and development may be relatively normal, at least over the next decade. The well-transfused thalassemic child remains relatively asymptomatic until the age of 10–11 years. Few of the complications of the disorder occur during childhood, and the disease presents a problem only when the effects of iron overloading resulting from ineffective erythropoiesis and from repeated blood transfusions become apparent at the end of the first decade. In the absence of additional therapeutic interventions (see below), the first observable change is often a failure or reduction of the pubertal growth spurt, and this is usually associated with failure of sexual maturation. Throughout their teenage years children suffer from a variety of complications due to endocrine deficiency and nearly all develop cardiac symptoms in the latter half of their second decade. However, the inadequately transfused child develops the typical features of Cooley’s anemia. Growth is stunted, and with bossing of the skull and overgrowth of the maxillary region the face gradually assumes a 'mongoloid' appearance. The liver and spleen are enlarged and the pigmentation of the skin increases. Many features of a hypermetabolic state, with fever, wasting and hyperuricemia, may develop. The clinical course is characterized by severe anemia with frequent complications. These include recurrent infections associated with worsening of anaemia, spontaneous fractures and other complications of bone due to massive bone-marrow expansion, folic-acid deficiency, a bleeding tendency, hypersplenism, gallstones, leg ulcers and a variety of syndromes owing to tumor masses resulting from extramedullary hematopoiesis (i.e. formation of blood cells outside the bone marrow). Iron overload is again manifested by the absence of the pubertal growth spurt and a failure of the menarche (i.e. the first menstrual period). These inadequately transfused patients present complications in vital organs much earlier than patients with regular blood transfusions, and death, usually from cardiac failure, occurs in the second decade. The heterozygous state of &beta;-thalassemia is not usually associated with any clinical disability except in periods of stress, such as pregnancy or during severe infection, when a moderate degree of anaemia may be found. Hemoglobin values are usually in the normal range. The most consistent findings are the small, poorly hemoglobinised red cells (mean cell hemoglobin of 20–22 pg and mean cell volume of 50–70 fl). The red cell indices and the Hb A2 level, which is increased from 3.5 to 7, are particularly useful in screening for heterozygous carriers of thalassemia in population surveys.

Management
The basic management plan has not changed over the years and includes blood transfusion, iron chelation, folic acid and splenectomy if necessary. The effectiveness and efficiency of the regimens has greatly increased and the quality of life and survival rates of patients have made impressive progress.

Transfusion
Transfusion dependency characterizes &beta;-thalassemia patients (with a few exceptions who present with the intermediate picture and are not discussed here). The blood transfusion scheme that has proved most beneficial is so-called 'hypertransfusion' where the patient's hemoglobin is maintained at an average of 12 g/dL. Endogenous bone marrow production of red cells is suppressed and bone marrow expansion does not occur, so the characteristic bone changes and facies are prevented. Red cells with abnormal inclusions do not enter the peripheral circulation and hypersplenism is prevented. Also, the constantly high hemoglobin level does not allow excessive gastrointestinal absorption of iron. The quality of life is much improved.

Splenectomy
In the past, splenectomy was a regular feature of the management of thalassemics. Now it is only occasionally needed and usually in older patients, provided of course that a high transfusion regimen is adhered to. The main reason for splenectomy was to reduce blood requirements which increased with hypersplenism. Splenectomy, especially if carried out at an early age, carries with it a longterm risk of infections, especially pneumococcal. These patients should be given prophylactic penicillin and pneumococcal vaccine. Platelet aggregation with microembolism affecting lung function has also been described and can be prevented by aspirin or dipyridamole.

Iron chelation
Iron chelation is the most expensive form of therapy and the most difficult for the patients to accept and adhere to. It is of great importance, however, as the prevention of complications of hemosiderosis to vital organs and compliance to treatment over the years has been shown to improve survival. The best results are obtained by subcutaneously infusing desferrioxamine over an 8–10 h period for 5 days per week; vitamin C is given orally on the days when desferrioxamine is given as this has been shown to increase the amount of iron that is excreted. The effectiveness of chelation therapy can be monitored approximately by observing the urinary excretion of iron and the serum ferritin levels.

Folic acid
Folic acid is an essential part of treatment, covering the increased needs of an overactive marrow.

Hepatitis B vaccination
Hepatitis B vaccines are available for new patients or those serologically negative to the virus. This prophylaxis is particularly important for multitransfused patients in Mediterranean and Middle East countries where many blood donors are carriers of hepatitis B virus.

Bone marrow transplantation
Bone marrow transplantation from HLA-identical siblings (i.e. siblings with compatible human leukocyte antigen, the major blood surface markers) has been increasingly adopted for the cure of hemoglobinopathies. Since 1981, more than 1500 bone marrow transplants have been performed in many centres worldwide. Results have improved remarkably since the first reports owing to the use of cyclosporin, more effective treatment of cytomegalovirus infection, improvement in aseptic techniques, and the evolution of systematic antibiotic therapy.

Gene therapy
Pending.

Prevention
The hemoglobinopathies constitute a major public health problem internationally and particularly in the developing world, which has the least resources for coping with the problem. Many of the above innovations are too expensive and present practical difficulties to countries where the prevalence of thalassemia is high. Approx. 250 million people (4.5% of the world population) carry a potentially pathologic hemoglobin gene and annually 300,000 infants are born with a major hemoglopinopathy. It is apparent that prevention of the disease is of primary importance not only to reduce the burden on the health services but also to give a better chance of survival to existing patients. Preventive programs, in countries such as Cyprus, Greece and Italy, consisting of public education, population screening, genetic counseling and prenatal diagnosis, have been effective in reducing the birth rate of &beta;-thalassaemia major. However, their implementation in densely populated countries with low economic resources is difficult. In vitro fertilisation, embryo biopsy and single-cell genetic analysis now make it possible to screen human embryos at the preimplantation stage of development for the genetic defects causing a number of inherited diseases. Pre-implantation genetic diagnosis allows the selection of unaffected embryos for implantation in the uterus. The method is costly and technically demanding and is therefore mainly used for couples with infertility problems or with repeated affected pregnancies.

Conclusion
Thalassaemia is a serious genetic disease affecting a large number of individuals worldwide. Survival and quality of life is still poor in many parts of the world. Prevention programmes are essential in areas of high prevalence, since effective cure through gene therapy is still in the development phase. Other approaches to treatment, e.g. the use of Hb F modulators, are still experimental.