Background

Thalassemia was first recognized by Cooley and Lee in 1925¹⁸ as a form of severe anemia associated with splenomegaly and bone changes in children. The term thalassemia is derived from the Greek φαλασσα (the sea) since many of the early cases came from the Mediterranean region. However, it is now clear that the disorder is not limited to the countries around the Mediterranean but occurs throughout the world, being also prevalent in the tropical and subtropical regions including the Middle East, parts of Africa, the Indian subcontinent and Southeast Asia. It appears that heterozygotes for thalassemia are protected from the severe effects of falciparum malaria and natural selection has increased and maintained their gene frequencies in these malarious regions.

The thalassemias are classified into α, β, δβ, γδβ, δ, γ and εγδβ thalassemias according to the type of globin chain(s) that is reduced. The two major categories are the α and β thalassemias while the rare forms include the δβ, γδβ and εγδβ thalassemias. Hereditary persistence of fetal hemoglobin (HPFH) refers to the group of disorders in which the switch from fetal to adult hemoglobin production is altered and high levels of fetal hemoglobin are synthesized in individuals. Because of their concomitant increase in HbF levels, the δβ and γδβ thalassemias are often considered with the HPFH syndromes. In many populations the α and β thalassemias coexist with a variety of different structural hemoglobin variants. In these populations it is quite common to inherit a combination of α and/or β thalassemia and/or structural hemoglobin variant genes; these complex interactions give rise to an extremely wide spectrum of clinical phenotypes which together constitute the thalassemia syndromes.¹⁰

Most thalassemias are inherited in a Mendelian recessive fashion. Heterozygotes are normally symptomless, although they can often be recognized by simple hematological analysis. Severely affected individuals have inherited two copies of mutant hemoglobin gene, homozygotes for α or β thalassemia, or compound heterozygotes for different molecular forms of α or β thalassemia and a hemoglobin variant. It has been estimated that about 300 000 individuals severely affected with thalassemia are born each year, posing a heavy burden on the health services.¹⁰ Due to recent population movements, these hemoglobin disorders have become an important part of clinical practice in all countries, including the UK.¹⁹

Genetic basis of disease

Molecular analysis of the β thalassemia genes has demonstrated a striking heterogeneity. Although almost 300 β thalassemia alleles (including deletions) have been characterized, population studies indicate that probably only 20 β thalassemia alleles account for >80% of the β thalassemia mutations in the whole world.^1,10 This is because in each of the high-frequency areas, only a few (4–6) mutations are common, reflecting the local selection from malaria, with a varying number of rare ones. Each of these populations has its own unique group of mutations.

Unlike α thalassemia, in which deletions in the α-globin gene cluster account for most of the mutations, the molecular defects causing β thalassemia are usually point mutations involving one (or a limited number of nucleotides) within the β gene or its immediate flanking regions.^1,10,20

These point mutations involve the critical sequences that interfere with gene function at either the transcriptional or the post-transcriptional level, including translation (Fig. 9.2). Approximately half of these mutations completely inactivate the β gene with no β-globin production causing β° thalassemia. Other mutations allow the production of some β-globin, and are classified as β⁺ or β⁺⁺ thalassemia, depending on the degree of quantitative reduction in the output of the β chains. The β-globin chains that are synthesized are usually structurally normal. Mutations affecting the conserved sequences in the 5′ promoter, i.e. TATA box, proximal CACCC and distal CACCC box, typically cause a 70–80% reduction in promoter activity and are often very mild. Mutations affecting the polyadenylation signal (AATAAA) at the 3′ end, also generally result in a mild β⁺ thalassemia phenotype.

Fig. 9.2 Point mutations causing β thalassemia. The β globin gene is represented by 3 exons (gray) interrupted by 2 introns with the 5′ and 3′ untranslated regions (UTRs, striped boxes). The vertical lines within the open rectangular boxes represent the sites of the different mutations which can be found in the UTRs, exons and introns, and causing typical recessively inherited β thalassemia. Mutations in the hatched rectangular boxes are dominantly inherited. They include mutations that lead to premature termination due to frameshifts or nonsense codons, insertions or deletions of intact codons and amino acid substitutions.

A few β thalassemia mutations are ‘silent’; carriers do not have any evident hematological phenotypes with near normal red cell indices and HbA₂ levels, the only abnormality being an imbalanced globin chain synthesis. These β thalassemia mutations have usually been ascertained by finding individuals with intermediate forms of β thalassemia resulting from compound heterozygosity for one typical β thalassemia mutation in combination with a very mild β⁺ thalassemia allele. In this case, one parent has typical β° thalassemia trait and the other, apparently normal. Overall, the ‘silent’ β° thalassemia alleles are uncommon except for the –101 C→T mutation which has been observed fairly frequently in the Mediterranean region where it interacts with a variety of more severe β thalassemia mutations to produce milder forms of β thalassemia.²¹

The β° thalassemia alleles are mostly caused by premature termination of translation, either by single base substitution to a nonsense codon, or through a frameshift mutation. Studies show that the different in-phase termination mutants exhibit a ‘positional’ effect and are subjected to a surveillance mechanism (nonsense mediated RNA decay or NMD) to prevent the accumulation of mutant mRNAs coding for truncated peptides.^22,23 Frameshifts and nonsense mutations that result in premature termination early in the sequence (in exon 1 and 2) are associated with minimal amounts of mutant β-mRNA.²⁴ In such cases, no β chain is produced from the mutant allele, resulting in a phenotype of typical heterozygous β thalassemia in the carrier. However, some mutations that produce in-phase terminations later in the β sequence, in exon 3 and the 3′ half of exon 2²⁵ escape NMD, and are associated with substantial amounts of abnormal β-mRNA from the mutant allele. The mutant β-mRNA leads to a synthesis of abnormal β chain variants that are often highly unstable and non-functional, and are not able to form viable tetramers, thus effectively causing a functional deficiency of β-globin chain.¹ Not only are these abnormal β-globin chain variants non-functional, but they prove to be an additional nuisance as they precipitate in the erythroid precursors, overloading the proteolytic mechanism and accentuating the ineffective erythropoiesis. Carriers for such β thalassemia alleles have fairly severe anemias, hence the term ‘dominantly inherited β thalassemia’. For a detailed review of the dominantly inherited β thalassemias, see Thein & Wood 2009¹ and Thein 1999.²⁶

β thalassemia is rarely caused by deletions. Of these, only the 619 bp deletion at the 3′ end of the β gene is common, but even that is restricted to the Sind populations of India and Pakistan where it constitutes ~30% of the β thalassemia alleles.^1,27 The other deletions, although extremely rare, are of particular clinical interest because they are associated with an unusually high level of HbA₂ in heterozygotes. The mechanism underlying the markedly elevated levels of HbA₂ and the variable increases in HbF in heterozygotes for these deletions is postulated to be related to the removal of the 5′ promoter region of the β-globin gene which removes competition for the upstream β-LCR and limiting transcription factors, resulting in an increased interaction of the LCR with the γ and δ genes in cis, thus enhancing their expression (Fig. 9.3). This mechanism may also explain the unusually high HbA₂ levels associated with the promoter mutations at positions –88 and –29.

Fig. 9.3 Deletions causing β thalassemia. The deletions that remove the promoter region are associated with unusually high HbA₂ levels and variably increased HbF levels in the heterozygous state.

Unusual causes of β thalassemia, although rare, illustrate the numerous molecular mechanisms of down-regulating the β-globin gene.¹ They include transposable elements which may disrupt and inactivate human genes. The insertion of such an element, a retrotransposon of the LI family into intron 2 of the β-globin gene, has been reported to cause β⁺ thalassemia.²⁸ Mutations in other genes distinct from the β-globin complex (trans-acting mutations) can down-regulate β-globin expression. Such trans-acting mutations have been described affecting the XPD protein that is part of the general transcription factor TF11H,²⁹ and the erythroid-specific GATA-1 protein.³⁰ Somatic deletions of the β-globin gene have been reported as the contributory cause for the unusually severe anemia in three unrelated families of French and Italian origin^31,32 (see intermediate forms of β thalassemia). Unipaternal isodisomy of chromosome 11p15.5 that encompassed the β-globin gene cluster contributed to thalassemia major in a Chinese patient.³³

Pathophysiology

The molecular defects in β thalassemias result in absent or reduced β chain production while α chain synthesis proceeds at a normal rate. This imbalance in globin synthesis in β thalassemia gives rise to excess α chains which are extremely unstable and precipitate in the red cell precursors forming inclusion bodies (Fig. 9.4). These inclusions interfere with the red cell maturation and are responsible for the intramedullary destruction of the erythroid precursors and hence the ineffective erythropoiesis that characterizes all β thalassemias. The anemia of β thalassemia results from a combination of underproduction of hemoglobin and ineffective erythropoiesis and, ultimately, correlates well with the degree of α- to non α-globin chain imbalance and the amount of free α chains. The complications of splenomegaly, bone disease, endocrine and cardiac damage are related to the severity of anemia and the hypercatabolic state, and the subsequent degree of iron loading resulting from the increased gastrointestinal iron absorption and from repeated blood transfusion.

Fig. 9.4 Diagrammatic representation of the pathophysiology of β thalassemia.

Genotype-phenotype correlation

Typically, β thalassemia is inherited as haploinsufficient Mendelian recessives. The most severe end of the clinical spectrum, β° thalassemia, is characterized by the complete absence of HbA (α₂β₂) and results from the inheritance of two β° thalassemia alleles (homozygous or compound heterozygous states). This normally results in the transfusion-dependent state of β thalassemia major and, at worst, the patients present within 6 months of life with profound anemia and, if not treated with regular blood transfusions, die within their first 2 years. Individuals who have inherited a single β thalassemia allele, whether β° or β⁺, have thalassemia trait. They are clinically asymptomatic but may have a mild anemia with characteristic hypochromic microcytic red blood cells, elevated levels of HbA₂ (α₂δ₂) and variable increases of HbF (α₂γ₂). Inheritance of two β thalassemia alleles, however, does not always lead to thalassemia major; many patients with two β thalassemia alleles have a milder disease, ranging from a condition that is slightly less severe than transfusion-dependence to one that is asymptomatic and often mistaken for thalassemia trait. The diverse collection of phenotypes between the two extremes of thalassemia major and trait constitute the clinical syndrome of thalassemia intermedia.

The heterozygous states for β thalassemia also show a tremendous phenotypic diversity, comparable to that for the inheritance of two β thalassemia alleles. In some cases, the β thalassemia allele is so mild that it is phenotypically ‘silent,’ with no anemia or hematological abnormalities. In others, the heterozygous state causes a phenotype almost as severe as the major forms, that is, the β thalassemia allele is dominantly inherited.

Unraveling the molecular basis of the clinical diversity in the intermediate state has provided tremendous insights on genotype-phenotype correlation and clues for the development of molecular therapy for the β thalassemias.

The main genetic interactions that result in a phenotype of thalassemia intermedia are summarized in Table 9.1. Thalassemia intermedia can result from the inheritance of one or two β thalassemia alleles. In 60–90% of the cases^34–36 the patients have inherited two β thalassemia alleles and the reduced disease severity can be explained by the inheritance of the milder forms (β⁺⁺ and ‘silent’ β thalassemia alleles) that allow the production of a significant proportion of β-globin chains. Co-inheritance of a single α-globin gene deletion (αα/α−) has very little effect on β° thalassemia, but individuals with two α-globin gene deletions (α−/α− or αα/−−) and β⁺ thalassemia have a mild disease requiring intermittent transfusions. The role of increased HbF response as an ameliorating factor becomes evident in the group of thalassemia intermedia patients who are mildly affected despite having minimal amounts or no HbA (α2β2), and without α thalassemia. Three major QTLs – Xmn1-Gγ, HBS1L-MYB on chromosome 6q, and BCL11A on chromosome 2p – recently identified in genome-wide association studies (GWAS) have been shown to impact HbF levels, not only in healthy individuals, but also patients with β thalassemia and sickle cell disease (SCD).^13,37,38 In Sardinia, BCL11A and HBS1L-MYB with α thalassemia accounts for 75% of variable phenotypic severity of β thalassemia,³⁹ and in Thailand, the three HbF QTLs are associated with both disease severity and HbF levels in HbE/β thalassemia.⁴⁰

Table 9.1 Thalassemia intermedia: the common genetic interactions that underlie the phenotype of thalassemia intermedia

Other HbF determinants within the cluster are related to the mutation itself. Small deletions or mutations that involve the promoter sequence of the β-globin gene are associated with variable increases in HbF and unusually high HbA₂ levels, and reflect the competition between the γ and β globin gene promoters for interaction with the upstream β-LCR. Hence, although such deletions cause a complete absence of β-globin product, the severity of the phenotype is offset by the concomitant increase in hemoglobin F.

Inheritance of single copies of β thalassemia gene can also lead to thalassemia intermedia. In the majority of cases this is caused by the co-inheritance of extra α-globin genes, the severity of outcome depends on the total number of α-globin genes inherited (from one or two copies of triplicated (/ααα), or quadruplicated (/αααα) α-globin complexes), and the type of β thalassemia mutation (β° or β⁺).^41,42 More recently, another mechanism of inheriting extra α-globin genes has been described which involved segmental duplication of the whole α-globin gene cluster.^43,44 The additional α-globin genes have no phenotype in normal people but the small excess of α chains in heterozygous β thalassemia appears to tip the balance, crossing the critical threshold of α-globin excess with phenotypic consequences.

In a number of cases, the unusually severe heterozygous state is associated with a normal α-globin genotype. In such cases, the β thalassemia mutation itself leads to the synthesis of highly unstable, structurally abnormal β-globin chain variants.¹⁷ The hyperunstable β chain variants are rapidly destroyed in the erythroid precursors, giving rise to a functional deficiency of β chains and simulating a phenotype of β thalassemia. The non-functional β chain variants together with the unmatched α-globin chains aggravate the ineffective erythropoiesis causing a disease phenotype even when present in a single copy. Hence, the term ‘dominantly inherited β thalassemia’.

Finally, although rare, somatic mosaicisms of the β-globin gene in β thalassemia heterozygotes have been reported causing thalassemia intermedia. The individuals from three different families had moderately severe anemia despite being constitutionally heterozygous for a common β thalassemia mutation; they also had a normal α (αα/αα) genotype.^31,32 Subsequent investigations revealed that these individuals had a somatic deletion including the β-globin complex, on the other chromosome 11p15, in a subpopulation (~80%) of erythroid cells giving rise to a somatic mosaic: about 20% of the erythroid cells were heterozygous with one normal copy of β-globin gene and the rest hemizigous (i.e. without any normal β-globin gene). The sum total of the β-globin product is thus about 80% less than the asymptomatic β thalassemia trait. These unusual cases once again illustrate that the severity of anemia in β thalassemia reflects the quantitative deficiency of β-globin production.

Given the differences in the spectrum of β thalassemia mutations and differences in the frequency of the different α thalassemia variants, the relative importance of these genetic factors would vary accordingly in different population groups. It is also important to note that the genotypic factors are not mutually exclusive.

Tertiary modifiers affect complications of the disease; the severity of osteopenia and osteoporosis, iron loading and jaundice may be affected by polymorphisms of genes involved in the metabolic pathways concerned with these complications.⁴⁵ Bone mass is a quantitative trait under strong genetic control involving multiple quantitative trait loci (QTLs) and the QTLs implicated include estrogen receptor gene, vitamin D receptor (VDR), collagen type a1 genes and transforming growth factor β1 (TGFβ1).⁴⁶ The elevations of bilirubin and incidence of gallstones in thalassemia, as in other hemolytic anemias, are influenced by a polymorphic variant (seven (TA) repeats) in the promoter of the uridine diphosphate-glucuronosyltransferase 1A (UGT1A1) gene, also referred to as Gilbert’s syndrome.^47,48 Iron loading in β thalassemia is also variable and results not just from blood transfusion but also from increased iron absorption. Variants in the HFE gene have a modulating effect on iron absorption,⁴⁹ and as other genes in iron homeostasis become uncovered, it is likely that there will be genetic variants in these loci that influence the different degrees of iron loading in β thalassemia.⁵⁰ For instance, recent genome wide association studies have identified variants in HFE, and the TMPRSS6 (transmembrane protease serine 6 gene that regulates hepcidin expression) associated with iron status, erythrocyte volume and concentration.^51–53

Diagnosis

In β thalassemia major, untransfused hemoglobin levels are usually less than 5 g/dl but can be as low as 2–3 g/dl. Mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) are low, with a very wide red cell distribution width (RDW), marked anisopoikilocytosis, target cell formation, and basophilic stippling. Poorly hemoglobinized nucleated red cells are frequently found in the peripheral blood, and may reach very high levels after splenectomy. The reticulocyte count is elevated but less than expected for the degree of anemia in keeping with the ineffective erythropoiesis. A bone marrow aspirate is not essential to make the diagnosis, but if performed shows marked erythroid hyperplasia characterized by poorly hemoglobinized normoblasts and dyserythropoiesis. Supravital staining (e.g. methyl violet) shows ragged inclusions in many of the erythroid precursor cells; similar inclusions are found in the peripheral red blood cells after splenectomy. Immunoelectron microscopy confirms that these inclusions consist of precipitated α-globin chains (Figs 9.5 and 9.6). Increased iron deposition is also seen in the bone marrow; the majority of the iron granules are randomly distributed. Biochemical evidence of hemolysis such as elevated bilirubin, aspartate transaminase (AST) and lactate dehydrogenase (LDH) with a normal alanine transaminase (ALT) and progressive iron loading is observed. Other biochemical changes may include evidence of diabetes and endocrine dysfunction such as parathyroid or thyroid insufficiency.

Fig. 9.5 Electron micrograph of a late erythroblast from a homozygote for β thalassemia. The cytoplasm contains multiple small rounded masses of electron-dense material, some of which have fused together to form larger masses. The masses probably consist of precipitated α chains. Uranyl acetate and lead citrate. × 11 625.

(Courtesy of Professor SN Wickramasinghe).

Fig. 9.6 Bone marrow macrophage from a case of homozygous β thalassemia. The macrophage contains a phagocytosed late erythroblast within which intracytoplasmic α chain precipitates can be recognized. Uranyl acetate and lead citrate. × 11 150.

(Courtesy of Professor SN Wickramasinghe).

Hemoglobin analysis is needed to confirm the diagnosis, typically using electrophoretic or chromatographic techniques. The findings on Hb electrophoresis vary with the β thalassemia genotype and are informative only in the previously untransfused patient. Absence of HbA in pretransfused samples confirms a diagnosis of β° thalassemia, and the hemoglobin consists of F and A₂ only. In β⁺ thalassemia (homozygous or compound heterozygotes), a variable amount of HbA is present. The HbF is usually elevated and varies from 10 to almost 100% of the total hemoglobin in the case of β° thalassemia. The HbA₂ level is of no diagnostic value. In vitro globin chain biosynthesis of peripheral blood reticulocytes or bone marrow shows globin chain imbalance with a marked excess of α over β and γ chain production. Obviously, in β° thalassemia there is a complete absence of β-globin chain synthesis.

The heterozygous state for β thalassemia (β° or β⁺) is remarkably uniform hematologically. Anemia, if present, is mild and the diagnosis is based on a low mean corpuscular volume (MCV) and mean cell hemoglobin (MCH) accompanied by an increased proportion of HbA₂, from 3.5 to 5.5%, with the exception of a subgroup that has a normal level of HbA₂. Globin chain biosynthesis shows α chains in excess of about twofold. The hematological and hemoglobin electrophoresis profiles are usually sufficient to make a diagnosis of β thalassemia, although DNA analysis, where available, is often used to confirm the diagnosis.

Normal HbA₂ heterozygous β thalassemia may be difficult to distinguish hematologically from heterozygous α thalassemia since both cases have low MCVs and low MCHs, and normal HbA₂ levels. The distinction is made by globin chain biosynthesis (where available) and DNA analysis; family studies are useful. Type 1 normal HbA₂ β thalassemia, otherwise known as ‘silent’ β thalassemia, has almost normal red cell indices; it results from very mild mutations which cause only a minimal deficit in β chain production. The other group of normal HbA₂ β thalassemia (type 2) results from the co-inheritance of δ thalassemia in cis or in trans to the β thalassemia gene; in the latter, family studies show independent segregation of the thalassemia indices and normal HbA₂ levels.

Clinical features and management

The clinical phenotypes of β thalassemia range from very severe (major) to a completely silent carrier state, with a huge range of intermediate phenotypes (thalassemia intermedia) between the two ends of the spectrum.¹⁰

Thalassemia major. In many developed countries, infants affected with severe forms of β thalassemia will be first identified by neonatal screening programs, before the development of any symptoms; antenatal screening of the parents and possibly prenatal diagnosis may have identified the infant to be at risk of severe β thalassemia before birth. The severely affected child is likely to require regular red blood cell transfusions. To meet the aims of correcting the anemia and suppressing the abnormal endogenous erythroid hyperplasia, the trough hemoglobin levels need to be above 9.5 g/dl. In practice, this means maintaining pretransfusion hemoglobin of 9–10 g/dl.

Inadequately transfused children develop the typical features of Cooley’s anemia. They show marked retardation of growth and development with progressive hepatosplenomegaly. A typical ‘thalassemic’ facies develops with frontal bossing, prominent cheek bones and protruding upper jaw due to erythroid hyperplasia in the skull and facial bones. Radiography of the skull shows the typical ‘hair-on-end’ appearance. The long bones and phalanges become rarefied from marrow expansion and show a lacy, trabecular pattern on radiography. These changes may be associated with repeated pathological fractures. Occasionally the expanding marrow extends from the rib or vertebrae and forms large paraspinal extramedullary masses. The massive marrow expansion causes a hypermetabolic state accompanied by intermittent fevers and weight loss. Gallstones and leg ulcers are common complications. Without any transfusion, death occurs within the first 2 years. ‘Palliative’ transfusion allows the child to live somewhat longer but the bony deformities remain unchanged and the child ultimately succumbs to an overwhelming infection. If these children survive to puberty, they develop complications of iron overload. Iron accumulation results from an increased rate of gastrointestinal absorption as well as that derived from the blood transfusions.

Adequately transfused children grow and develop normally until early puberty. Iron overload inevitably complicates regular blood transfusions, and progress of their disease then depends on whether they have received regular iron chelation. If not, they begin to show signs of progressive hepatic, cardiac and endocrine disturbances including diabetes, hypoparathyroidism and delayed or absent secondary sexual development. The endocrinopathies and cardiac disease are ascribed to the labile, more toxic forms of iron that appear in cells and plasma, referred to as non-transferrin bound iron (NTBI). Throughout their teenage life these children suffer from a variety of complications due to different endocrine deficiencies. Unless iron overload is controlled by regular chelation therapy, death results in the second or third decade, from acute or intractable congestive cardiac failure. Children who are adequately transfused and fully compliant with iron chelation therapy grow and develop normally, with few or no skeletal abnormalities, and achieve sexual maturity. Even within this group there is a high frequency of growth retardation and retarded sexual maturity, with variable complications relating to iron metabolism, bone disease, endocrine abnormalities and liver disease.⁵⁴

Iron chelation is usually started after 1 year of monthly blood transfusions.^55,56 Currently, three iron chelating agents are available for use: desferrioxamine (DFO), deferiprone and deferasirox. DFO has been in use since the 1970s and is known to be safe and effective; side-effects seem to occur when the drug is used in high doses or when iron stores are low. The main problem with DFO is that it has to be given parenterally, and due to its short half-life, the regimen involves continuous subcutaneous infusions given over 8 hours using a syringe driver or balloon pump. Deferiprone was the first oral iron chelator to be used; it was licensed in Europe in 1999. Arthropathy and agranulocytosis (which can be fatal) are potential serious side-effects, and patients taking deferiprone are recommended to have weekly blood counts. Deferiprone seems to be particularly effective in removing cardiac iron, a therapeutic effect ascribed to its low molecular weight. Deferiprone is increasingly used in combination with DFO.⁵⁷ Deferasirox is a once-daily oral iron chelator, approved in the USA and Europe since the mid-2000s.⁵⁸ Large clinical trials have demonstrated its efficacy in removing liver iron in thalassemia, with increasing evidence that it also removes cardiac iron. All patients on regular blood transfusion and iron chelation therapy should be monitored regularly with a yearly review for assessment of growth, including tests for endocrine and cardiac function, iron loading (serum ferritin, MRI for liver iron concentration) and adverse effects related to therapy.⁵⁶

Bone marrow transplantation (BMT) is considered the treatment of choice if there is a HLA-identical sibling. BMT should be considered only if it is clear that the child is transfusion-dependent and should be considered early as the success of BMT is reduced as the child gets older, with increasing iron overload and iron-related organ damage.⁵⁹

Thalassemia intermedia. Thalassemia intermedia (TI) includes patients with a wide range of clinical phenotypes. To a large extent, whether a patient is classified as thalassemia intermedia or major depends on a doctor deciding if the patients would benefit from regular blood transfusions; this decision is based not only on clinical factors (such as failure to grow and frequent infections) but also on non-clinical factors such as experience of the doctor, availability of blood and wishes of the patient.