Pathophysiology of thalassemia

INTRODUCTION — Alpha and beta thalassemia are inherited hemoglobinopathies in which impaired production of one type of globin chain (alpha chains in alpha thalassemia; beta chains in beta thalassemia) causes an imbalance in the ratio between alpha and beta (or beta-like) chains, which is normally tightly controlled.

Balanced synthesis is important because the intact hemoglobin A (Hb A) and hemoglobin F (Hb F) tetramers are highly soluble in red blood cell (RBC) cytoplasm, but the unbound alpha and beta chains are not. They precipitate, causing significant cellular damage. These unpaired chains cause problems with RBC maturation and lead to ineffective erythropoiesis, hemolytic anemia, iron overload, and ensuing complications.

This topic discusses the pathophysiology of alpha and beta thalassemia. Separate topic reviews discuss:

●Genetics – (See "Molecular genetics of the thalassemia syndromes".)

●Laboratory testing – (See "Methods for hemoglobin analysis and hemoglobinopathy testing".)

●Clinical manifestations and diagnosis – (See "Diagnosis of thalassemia (adults and children)".)

●Management – (See "Management of thalassemia" and "Iron chelators: Choice of agent, dosing, and adverse effects".)

●Hematopoietic stem cell transplant – (See "Hematopoietic stem cell transplantation for transfusion-dependent thalassemia" and "Thalassemia: Management after hematopoietic cell transplantation".)

TERMINOLOGY AND DISEASE CLASSIFICATION — Thalassemia refers to a spectrum of diseases characterized by reduced or absent production of one or more globin chains (table 1). This can lead to a relative excess of unpaired chains from the unaffected genes, which continue to be produced at normal rates.

Beta thalassemia — Beta thalassemia is caused by variants (mutations) in the hemoglobin beta locus (HBB, beta globin gene) that lead to impaired production of beta globin chains. This leads to an excess of alpha or alpha-type chains.

Beta⁰ variants are those that abolish beta globin production, and beta⁺ variants can produce some beta globin but far less than the normal amount. The clinical phenotype depends on which combination of beta globin variants an individual carries.

Previously, individuals with beta thalassemia were classified (in order of decreasing severity) as having beta thalassemia major, intermedia, and minor. Subsequent classification has shifted to use of the terms transfusion dependent and non-transfusion dependent. (See "Diagnosis of thalassemia (adults and children)", section on 'Overview of subtypes and disease severity'.)

The new terminology has been adopted because it is based on the clinical status of the patient with regard to a major prognostic feature (lifelong transfusion dependence, or not) and cleanly distinguishes these two groups of patients. In contrast, the category called thalassemia intermedia was deemed too broad and vague, ranging from minimal symptomatology to clinical stigmata overlapping with severe transfusion-dependent beta thalassemia.

●Transfusion-dependent – Transfusion-dependent individuals require regular transfusions to mitigate severe anemia and extramedullary hematopoiesis. These individuals were previously referred to as having beta thalassemia major. Individuals who periodically require transfusions for anemia are not considered transfusion dependent, but they may become so over time.

●Non-transfusion-dependent – Non-transfusion-dependent individuals do not require regular transfusions. They may never receive a transfusion, or they may periodically require transfusions for anemia (eg, during pregnancy or an acute infection). These individuals were described as having thalassemia intermedia if anemic and/or showing stigmata of hemolytic anemia or ineffective erythropoiesis, and as having thalassemia minor if minimally symptomatic. However, the term thalassemia minor was also used to describe the almost universally asymptomatic heterozygous state. This confusion was an additional reason for transitioning to the new terminology.

Every individual has two beta globin genes (one from each parent). Beta thalassemia gene variants are classified according to the degree of reduction in beta globin production (table 1).

●Beta⁰ thalassemia refers to pathogenic gene variants that completely abolish beta globin production. Patients who are homozygous or compound heterozygous for beta⁰ thalassemia mutations are more likely to have transfusion-dependent beta thalassemia (previously called beta thalassemia major); they cannot make any beta globin chains and hence cannot make the normal adult hemoglobin (hemoglobin A [Hb A]; alpha2/beta2). These are the patients originally described by Cooley. They often have profound, transfusion-dependent anemia and other manifestations of severe thalassemia. Individuals heterozygous for beta⁰ thalassemia are more likely to have a lower hemoglobin, and a few may even exhibit mild stigmata of hemolysis and ineffective erythropoiesis.

There are exceptions to this general trend, and beta⁰ thalassemia patients can be non-transfusion-dependent. For example, some mutations abolishing beta globin synthesis, such as delta beta thalassemia (also called Hb Lepore syndrome) are associated with significant compensatory production of gamma chains, and therefore, fetal hemoglobin (Hb F) in post-natal life. Coinheritance of alpha thalassemia trait can also reduce chain imbalance, thereby reducing severity. (See "Diagnosis of thalassemia (adults and children)", section on 'Clinical manifestations'.)

●Beta⁺ thalassemia refers to mutations that cause decreased (but not absent) beta globin production. Patients who are homozygous for beta⁺ thalassemia mutations can make some Hb A and tend to be less severely affected than those with beta⁰ thalassemia mutations.

•Individuals with a beta⁺ thalassemia mutation combined with a beta⁰ thalassemia mutation typically have transfusion-dependent beta thalassemia.

•Individuals with two beta⁺ thalassemia mutations typically have transfusion-independent beta thalassemia, although some may be transfusion dependent or may become transfusion dependent later in life.

•Individuals heterozygous for beta⁺ thalassemia inherit a beta thalassemia allele from one parent and a normal beta globin allele from the other parent. These individuals have profound microcytosis but mild or minimal anemia; they are largely asymptomatic and are often identified incidentally (by family testing or on a complete blood count [CBC] done for other reasons). Anemia can become more significant during pregnancy.

The beta thalassemia mutations are discussed in detail separately. (See "Molecular genetics of the thalassemia syndromes".)

Alpha thalassemia — Alpha thalassemia is caused by pathogenic gene variants in the hemoglobin alpha locus (alpha globin genes) that lead to impaired production of alpha globin chains (figure 1). This results in a relative excess of beta and beta-like chains. Depending largely on the alpha globin genotype, individuals with symptomatic alpha thalassemia can either be transfusion independent (common) or transfusion dependent (rare).

Alpha⁰ variants are those that abolish alpha globin production, and alpha⁺ variants can produce some alpha globin but less than the normal amount. Often alpha⁰ variants are those in which an entire alpha globin gene cluster is deleted (deletional variants) whereas alpha⁺ variants may involve point mutations (non-deletional variants). The most common non-deletional alpha globin variant is hemoglobin Constant Spring. (See "Molecular genetics of the thalassemia syndromes".)

In contrast to beta thalassemias, in which beta⁺ variants tend to be less severe, some alpha⁺ alleles such as Hb Constant Spring can actually aggravate severity. This is because the common alpha⁺ alleles encode alpha chains that, when combined with beta chains, create highly unstable hemoglobins that precipitate, adding to the burden of inclusion bodies in the developing and circulating RBCs.

The alpha globin gene cluster has two copies of the alpha globin gene, alpha2 and alpha1. Thus, every individual has four alpha globin genes (two from each parent); the designation "aa/aa" is used to describe four normally functioning alpha globin genes. The clinical phenotype depends on which combination of alpha globin variants an individual carries. There are multiple possible combinations of variants, increasing the complexity of alpha thalassemia classification.

●Hydrops fetalis with Hb Barts – The syndrome of hydrops fetalis is the most severe form of alpha thalassemia, due to deletion or inactivation of all four alpha globin genes (--/--). This syndrome was previously considered to be incompatible with live birth, due to lack of alpha chains that normally appear after the fifth or sixth week of fetal life. However, live births have been reported with antenatal diagnosis and in-utero transfusions, in some cases followed by allogeneic bone marrow transplantation [1,2]. (See "Alpha thalassemia major: Prenatal and postnatal management".)

Normal fetal hemoglobin (Hb F) is a tetramer of alpha chains and gamma chains. In the absence of functional alpha chains, gamma chains homotetramerize to form hemoglobin Barts (Hb Barts). Hb Barts has little propensity to precipitate and form inclusions [3]. However, Hb Barts (as well as homotetramers of beta globin; hemoglobin H [Hb H]) are functionally useless for oxygen delivery due to their high affinity for oxygen (10 times greater than that of Hb A); they are susceptible to oxidant injury [4,5]. They demonstrate no Bohr effect, and they lack a sigmoidal hemoglobin-oxygen dissociation curve due to lack of heme-heme interaction. Instead, their oxyhemoglobin dissociation curves resemble that of myoglobin [6]. (See "Structure and function of normal hemoglobins".)

Zeta chains are an embryonic alpha-like chain. If the genetic variant allows the fetus to synthesize zeta chains, then hemoglobin Portland (Hb Portland; zeta2 gamma2) can be produced, allowing survival in utero until the third trimester [7]. These fetuses are functionally severely anemic, with high-output cardiac failure, anasarca, and capillary leak ("hydrops"), along with enlarged placenta and maternal hypertension and polyhydramnios, despite the fact that the measured hemoglobin concentration may be as high as 10 g/dL due to the presence of Hb Barts and Hb Portland (see "Nonimmune hydrops fetalis"). The red blood cells (RBCs) are hypochromic and microcytic, and erythropoiesis is vastly expanded to compensate for the resulting profound tissue hypoxia, with compensatory extramedullary erythropoiesis in the liver and spleen. A small component of Hb H may also be present in the late stages of gestation, when the fetus begins to synthesize small amounts of beta chains. (See "Structure and function of normal hemoglobins", section on 'Embryonic hemoglobins'.)

●Hb H disease – Hb H disease refers to deletion or inactivation of three of the four alpha globin genes (a-/--). Clinically significant unbalanced globin chain synthesis, with an alpha/beta synthesis ratio of 0.3 to 0.6 (normal, 1.0±0.05), causes accumulation of unpaired gamma chains during fetal development and early infancy and accumulation of unpaired beta chains in adulthood [8,9].

The resulting gamma globin homotetramers in the fetus and infant form Hb Barts, and the resulting beta globin homotetramers in the child and adult form Hb H [4,10,11]. As noted above, Hb Barts and Hb H are functionally useless for oxygen delivery. However, the presence of one functioning alpha globin allele supports enough Hb F synthesis to allow the fetus to survive gestation and enough Hb A synthesis to sustain postnatal life, albeit with significant morbidity.

Hb H disease was identified simultaneously by two groups of investigators in 1955 [12,13]. Affected patients had a hypochromic microcytic anemia of variable severity; target cells on peripheral smear; reticulocytosis; inclusions in a few red blood cells (RBCs) known as Heinz bodies, many more of which could be produced by incubating RBCs with mild oxidant dyes like brilliant cresyl blue, resistance to lysis by hypotonic solutions, and signs of hemolysis, including a shortened ⁵¹Cr RBC survival, splenomegaly, and elevated indirect reacting bilirubin. (See "Red blood cell survival: Normal values and measurement".)

A key finding was the presence of an abnormal hemoglobin, accounting for up to 40 percent of the total hemoglobin, which had more rapid electrophoretic mobility at pH 8.6 than Hb A (figure 2). Upon standing, Hb H turned brown and precipitated, forming aggregates that looked somewhat like the RBC inclusions, a phenomenon accentuated by the addition of mild oxidants.

In deletional Hb H disease, the patient has inherited only a single alpha globin gene (a-/--). In the non-deletional form, the patient has inherited two alpha globin genes from one parent, but one of these carries a non-deletional defect such as a point mutation (aa*/--, where a* represents a variant such as hemoglobin Constant Spring). The most common alpha⁺ mutations generate highly unstable hemoglobins, which add to the inclusion body burden of the RBCs [14,15]. (See "Unstable hemoglobin variants".)

●Alpha thalassemia minor/trait – Inheritance of two normal and two deletional alpha genes, referred to as alpha thalassemia minor or alpha thalassemia-1 trait, can occur with heterozygosity for an alpha⁰ genotype (aa/--) or heterozygosity for an alpha⁺ genotype (a-/a-). These individuals have minimal or no anemia and microcytosis (low mean corpuscular volume [MCV]).

●Alpha thalassemia minima/silent carrier – Inheritance of three normal and one deletional alpha genes (aa/a-) is referred to as alpha thalassemia minima, silent carrier, or alpha thalassemia-2 trait. These individuals often have no clinical or hematologic abnormalities. The diagnosis can be reliably made only via DNA analysis. (See "Methods for hemoglobin analysis and hemoglobinopathy testing", section on 'Molecular genetic (DNA-based) methods'.)

●Complex alpha thalassemia variants – In addition to the variants described above, individuals can have alpha⁰ variants from one parent and alpha⁺ variants from the other parent.

Combinations of hemoglobin variants — Further complexity occurs when an individual carries combinations of alpha and beta globin gene variants, or combinations of alpha or beta thalassemia with another hemoglobin variant such as sickle hemoglobin or Hb C.

As a general rule, alpha thalassemia plus a beta globin variant tends to partially ameliorate the severity of the beta globin variant, whereas beta thalassemia plus a beta globin variant like Hb S tends to produce the clinical phenotype of the other variant, which is frequently slightly milder (sickle-beta thalassemia) because the thalassemia allele lowers the intracellular concentration of hemoglobin, which reduces sickling.

Some structural variants are also associated with reduced globin biosynthesis (eg, Hb E, listed below) or accumulation due to instability (eg, Hb Terra Haute), in which case, coinheritance of a beta thalassemia allele aggravates the clinical phenotype.

●Alpha and beta thalassemia – Individuals with coinheritance of homozygous beta⁺ thalassemia (beta⁺/beta⁺) and alpha thalassemia (-a/-a, --/aa, or -a/aa) will present with intermediate clinical severity. The concomitant reduction of both alpha and beta chains reduces the degree of imbalance relative to what would occur if only one or the other type of chain was affected. (See "Diagnosis of thalassemia (adults and children)", section on 'Clinical manifestations'.)

●Hereditary persistence of fetal hemoglobin – Certain beta globin variants lead to production of higher levels of Hb F, which can serve as a beta globin-like partner with alpha globin, leading to a less severe imbalance in the alpha-to-beta globin ratio in individuals with beta thalassemia (figure 1). (See "Fetal hemoglobin (Hb F) in health and disease", section on 'Hereditary persistence of fetal hemoglobin (HPFH)'.)

●Sickle-beta thalassemia – Co-inheritance of the sickle mutation on one beta globin allele and a thalassemic mutation at the other beta globin allele leads to sickle cell disease, the severity of which depends on the nature of the beta thalassemic mutation (ie, whether it results in reduced [beta⁺] or absent [beta⁰] beta globin production). (See "Overview of compound sickle cell syndromes", section on 'Sickle-beta thalassemia'.)

●Hb E – Hb E results from a beta globin mutation that reduces beta globin production. Compound heterozygosity for Hb E and a beta thalassemia variant is responsible for a large proportion of severe beta thalassemia throughout the world [16]. It occurs at a frequency of 3 to 9 percent in Thailand [17,18]. The clinical phenotype is heterogeneous, with hemoglobin levels ranging from approximately 3 to 14 g/dL; the co-inheritance of an alpha thalassemia variant may modulate the phenotype [16]. Hb E is commonly seen in India and Southeast Asia. The phenotype appears to be especially severe in Sri Lanka [19].

●Dominant thalassemia – Dominant thalassemia refers to several single rare beta thalassemia variants that produce a disease picture in the heterozygous state typically characterized by a non-transfusion-dependent (NTD) beta thalassemia intermedia phenotype and occasionally a transfusion-dependent clinical picture [20]. These autosomal dominant variants are often missense mutations in exon 3 that produce unstable and hyper-unstable globins. These dominant beta thalassemia variants have often been described as de novo mutations in families of dispersed ethnic origins. The majority of cases are not detected by conventional testing, and they therefore require increased clinical awareness and specialized testing. (See "Methods for hemoglobin analysis and hemoglobinopathy testing".)

The clinical phenotype cannot be completely predicted from the globin genotype, as other genetic variation and environmental factors can influence disease severity. This subject is discussed in more detail separately. (See "Hemoglobin variants including Hb C, Hb D, and Hb E" and "Overview of compound sickle cell syndromes", section on 'Sickle-beta thalassemia' and "Overview of compound sickle cell syndromes", section on 'Sickle-alpha thalassemia'.)

Acquired alpha thalassemia in MDS — Individuals with clonal hematopoietic disorders such as myelodysplastic syndrome (MDS) can develop acquired alpha thalassemia in the clonal population; this is also referred to as acquired Hb H disease. (See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)", section on 'Acquired hemoglobin H disease'.)

GLOBIN CHAIN IMBALANCE — Hemoglobin is a tetramer consisting of two heterodimers; each heterodimer has one alpha globin-like chain and one beta globin-like chain (as well as heme). The ratio of alpha- to beta-like globin chains is tightly controlled at 1.00±0.05.

Gene variants in thalassemia disrupt this normal regulation by producing reduced levels of one of the types of globin chains [21,22]. The free (unpaired) alpha or beta chains cannot be incorporated into normal hemoglobin A (Hb A; alpha-beta tetramers). Beta globin production begins at approximately the 35^th to 38^th weeks of gestation and abruptly increases after birth, so that Hb A predominates by approximately six months postnatally. Alpha globin production begins early in fetal development (approximately six weeks of gestation) and persists at high levels throughout life.

Differing properties of alpha and beta chains and their effects on developing red blood cells (RBCs) are summarized in the table (table 2).

●Beta thalassemia – For reasons that are not entirely clear, excess alpha globin chains cannot form soluble homotetramers. They are unstable and begin aggregating as soon as they accumulate in erythroid precursors, producing insoluble aggregates that precipitate adjacent to the RBC membrane [23]. This occurs in very early marrow erythroid precursors, affecting membrane assembly and accelerating programmed cell death.

A different beta-like chain (also called "non-alpha" chain) is used during fetal development and early childhood. The switching between beta-like chains (figure 1) is controlled by a complex set of enhancer and silencer sequences (elements) near the gene loci, along with their cognate transcription factors. These have become targets for therapeutic manipulation to increase postnatal Hb F synthesis in beta chain hemoglobinopathies:

•Embryo – Epsilon chains

•Fetus and neonate – Gamma chains

•Young child through adult – Beta chains (plus a minor component of delta chains, which form Hb A2)

Since beta thalassemia mutations affect beta chains but not embryonic (epsilon) or fetal (gamma) chains, the age of onset of beta thalassemia is in early childhood rather than during fetal development. Symptoms emerge only when Hb F is replaced by adult hemoglobin following the perinatal Hb F to Hb A switch (figure 3).

Some individuals have triplication of alpha globin genes. A case report of three individuals with beta thalassemia and a simultaneous triplication of the alpha globin gene (encoding six rather than four alpha globin chains) described strong enhancement of the beta thalassemia phenotype, further supporting the correlation between the degree of chain imbalance with clinical severity [24].

Conversely, as noted above, coinheritance of an alpha thalassemia variant that reduces alpha chain production in an individual with beta thalassemia results in a less severe phenotype by shifting the ratio of alpha to beta globin closer to 1. (See 'Combinations of hemoglobin variants' above.)

●Alpha thalassemia – In alpha thalassemia, excess beta-like chains can form somewhat soluble tetramers. They do not produce a major inclusion body burden in early- to mid-stage erythroblasts. Ineffective erythropoiesis is thus less marked in most alpha thalassemia patients (relative to beta thalassemia patients). In the fetus and infant, these are predominantly gamma globin tetramers (Hb Barts); in the child and adult, they are predominantly beta globin tetramers (Hb H).

Since alpha chains are required to produce functional hemoglobins during fetal development, alpha thalassemia manifests before birth. Homozygous alpha⁰ thalassemia is generally considered to be incompatible with extrauterine life, with rare exceptions involving intrauterine transfusions. (See "Alpha thalassemia major: Prenatal and postnatal management".)

In individuals with a form of alpha thalassemia caused by the Constant Spring variant (CS) hemoglobin, an increase in ineffective erythropoiesis is probably due to the binding of unstable precipitates of alpha CS, in addition to beta chains, to the membranes of erythroid precursors.

Globin chain imbalance and the resulting unpaired globin chains have a number of effects on the structure and mechanical properties of RBCs. (See 'Effects on the RBC' below.)

EFFECTS ON THE RBC

Oxidative injury — Oxidation is a normal phenomenon in red blood cells (RBCs; hemoglobin is oxidized to methemoglobin at a rate of approximately 0.5 to 3 percent per day); however, in normal RBCs, the oxidized methemoglobin subsequently is reduced (restored to native hemoglobin) via cytochrome b5 reductase [25]. (See "Methemoglobinemia", section on 'How are the levels regulated?'.)

In thalassemic RBCs, unpaired alpha or beta chains with an attached heme moiety are susceptible to oxidation. They are partly proteolyzed to form hemichromes (oxidized and partly catabolized peptide fragments) [26-28]. These iron-containing hemichromes can generate reactive oxygen species (ROS), perhaps by acting as Fenton reagents; in turn they can oxidize adjacent RBC membrane proteins and lipids [29-32].

Oxidant injury can damage RBC membranes indirectly, by affecting globin chains that bind to the membranes, or by directly altering cytoskeletal or integral membrane proteins, as evidenced by loss of free thiols within globin chains and other membrane proteins [33-36]. Oxidant injury to membrane proteins and lipids can have profound effects on membrane properties. (See 'Membrane damage' below.)

Oxidant injury is in turn worsened by free iron, levels of which are in turn increased by the effects of oxidant injury [37] (see 'Iron overload' below). Increased amounts of membrane iron have been found in association with denatured hemoglobin in beta thalassemic cells [38]. Proteolysis of some cytoskeletal and membrane proteins may also occur, although this has not been studied directly.

In vitro incubation of thalassemic RBCs with mild oxidants such as methylene blue and brilliant cresyl blue can enhance hemichrome formation [12,13]. This allows one to visualize these precipitated, mutilated remnants of globin, which have been termed "inclusion bodies." A clinical counterpart is the observation that hemolysis may be exacerbated when individuals with hemoglobin H (Hb H) disease are exposed to oxidant drugs, such as the sulfonamides, or to conditions that predispose to oxidative stress, such as infections [5]. In this sense, Hb H mimics many of the clinical features of unstable hemoglobins. (See "Unstable hemoglobin variants", section on 'Clinical manifestations'.)

In experiments designed to model the oxidant damage, normal RBCs were incubated with the oxidant compounds methylhydrazine (MHZ) and phenylhydrazine (PHZ) [39]. MHZ induced the binding of oxidized beta globin chains to the membrane skeleton, and the membranes became hyperstable, as is typical for severe alpha thalassemia. In contrast, PHZ induced the binding of oxidized alpha globin chains to the membrane skeleton, causing them to become unstable, exactly as seen in the RBCs in severe beta thalassemia. (See 'Membrane damage' below.)

High levels of oxidant stress in thalassemic RBCs is further supported by findings of decreased hexose monophosphate shunt activity, a source of protection against oxidant injury, and increased superoxide dismutase, glutathione peroxidase, catalase, and other enzymes important in oxidant defense [40-42].

Membrane damage — Normal RBC precursors undergo orderly cytoskeletal and membrane assembly that includes spectrin, band 4.1, band 3 (the anion exchanger), and several other proteins, as illustrated in the figure (figure 4) and discussed in detail separately. (See "Red blood cell membrane: Structure and dynamics".)

In vitro studies have demonstrated that in severe beta thalassemia, partially oxidized alpha globin chains are bound to the RBC membrane skeleton [33]. Likewise, in alpha thalassemia, partially oxidized beta globin has been demonstrated to be bound to membrane skeletons (figure 5) [33,43].

Oxidative damage to membrane components, either indirectly through binding of globin chains or through direct oxidation of membrane proteins or lipids (see 'Oxidative injury' above), can affect various membrane properties:

●Asymmetry of the bilayer – Asymmetry between the inner and outer leaflets of the lipid bilayer is a normal property of RBC membranes. (See "Red blood cell membrane: Structure and dynamics", section on 'Lipid bilayer'.)

In thalassemias, normal membrane asymmetry is disrupted, and incorporation of membrane cytoskeletal proteins is frequently disorderly and discontinuous, especially in areas with alpha chain aggregates (in beta thalassemia) or Hb H inclusions (in alpha thalassemia) adjacent to the membrane, and at earlier stages of erythroid development [44]. Several abnormalities are seen, including increased levels of Band 3 (also called anion exchanger 1 [AE1]), which is the major integral membrane protein that stabilizes the RBC membrane and exchanges chloride ions for bicarbonate, along with decreased levels of band 4.1, which stabilizes spectrin-actin interactions [45,46]. Levels of spectrin and its binding partners (eg, ankyrin, actin) may be reduced [46-49]. Membrane preparations from individuals with Hb H disease reveal a large concentration of membrane-bound globin not present in control RBCs (figure 6) [33].

●Cytoskeletal composition – Preparations of integral protein-rich fractions of RBC membranes, referred to as inside-out vesicles (IOVs), have demonstrated that Hb H RBC membranes contain only one-half the amount of spectrin as IOVs from normal RBCs [46]. Free spectrin, actin, and band 4.1 appear in the supernatant wash medium during the preparation of RBC membranes by stepwise hypotonic lysis of alpha thalassemic but not control RBCs [47,50]. The defect in membrane protein composition can be reproduced by adding heme-containing alpha globin chains to normal RBC membranes [51].

●Rigidity – Membranes from Hb H RBCs are two to three times more rigid than normal [50,52]. Membranes from patients with Hb H and Hb Constant Spring (Hb H/CS) are even more rigid [53], and the most rigid membranes are seen in Hb CS/CS [52,53]. Ektacytometric studies suggested that an interaction of the excess beta globin chains in Hb H RBCs with the cytosolic face of the membrane played an important role in the membrane rigidity [50].

Rigidity may retard passage of Hb H RBCs through splenic and hepatic sinusoids. Extreme rigidity could lead to RBC fragmentation in parts of the vasculature in which there is elevated shear stress. This is consistent with RBC fragmentation observed on the peripheral blood smear. (See 'CBC and RBC morphology' below.)

●Stability – Membrane stability is a technical term used to describe the ability of isolated RBC membranes to resist fragmentation under an intense elliptically deforming shear stress [50]. Membrane stability is generally thought to be a function of the RBC membrane skeleton and the interaction of the skeleton with the major transmembrane proteins, band 3 and glycophorin.

•Membrane rigidity is increased in Hb H, further increased in Hb H/CS, and maximally increased in Hb CS/CS [52]. These changes are thought to be due to the membrane-bound beta globin and alpha CS globin chains.

•In contrast, RBC membranes in beta thalassemia are highly friable and unstable, a change induced by partially oxidized alpha globin chains. (See 'Oxidative injury' above.)

These changes in membrane properties are thought to play an important role in the abnormal maturation and decreased deformability of thalassemic RBCs. In erythroid precursors, they may lead to increased apoptosis in the bone marrow. In mature RBCs, they may cause increased membrane expression of phosphatidylserine (PS) and neoantigens, all in turn leading to increased hemolysis, especially in the reticuloendothelial system. Hb H as well as Constant Spring variants (Hb H/CS and Hb CS/CS) RBCs have PS on the outer leaflet of the membrane [54]; the absolute number of such RBCs in the circulation is relatively small, although this may reflect their rapid removal from the circulation by the reticuloendothelial macrophages. (See 'Ineffective erythropoiesis' below and 'Hemolysis' below.)

In both alpha and beta thalassemia, membrane damage produced by the accumulation of excess globin chains (ie, excess alpha globin chains in beta thalassemia and excess beta globin chains in alpha thalassemia) may be reduced by the action of proteases that directly attack and partially destroy these excess globin chains but are incapable of eliminating the toxic chains [55].

Abnormal hydration — RBC hydration refers to the ratio of water to hemoglobin and other solutes. Beta thalassemia RBCs are dehydrated, and alpha thalassemia RBCs are hyperhydrated (table 2).

●In beta thalassemia, RBCs are dehydrated (they have a lower-than-normal ratio of water to solutes) (figure 7). The dehydrated cells have a high mean cell hemoglobin concentration (MCHC) and appear as dense on the peripheral blood smear (picture 1) [50,56,57]. This is thought to be due to excessive activation of the potassium-chloride cotransport system, a pH- and volume-activated ion channel that controls loss of potassium chloride (figure 8), with excess channel activity and ion export (remaining open in beta thalassemic RBCs more so than in normal RBCs) leading to water loss by osmotic mechanism [50,58].

●In severe alpha thalassemia, RBCs are hyperhydrated (they have a higher-than-normal ratio of water to solutes). The proportion of hyperhydrated RBCs varies depending on the specific genotype [53,59]. It is an unproven hypothesis that in alpha thalassemia the potassium-chloride cotransporter closes down early, preventing the usual loss of potassium chloride and water that is part of normal RBC physiology. Hb CS variants are hyperhydrated relative to Hb H RBCs [59,60].

Reduced deformability — Deformability (flexibility) is an essential property that enables RBCs with a diameter of 7 to 8 microns to traverse the capillary circulation, with a lumen diameter of approximately 3 microns, and the reticuloendothelial system, made up of macrophages lining slit-like sinusoids in the spleen (and liver) that are tortuous and narrower still. Reduced deformability delays passage through the microvascular bed and delays transit through the reticuloendothelial system, making them more susceptible to macrophage clearance. This contributes to extravascular hemolysis. (See 'Hemolysis' below.)

Deformability depends on the RBC having a specific surface area-to-volume ratio, relatively low cytoplasmic viscosity, and viscoelastic membrane properties. Poorly deformable RBCs are trapped and destroyed by reticuloendothelial macrophages in the spleen and liver. (See "Red blood cell membrane: Structure and dynamics".)

Factors that reduce deformability in beta thalassemia include RBC dehydration and increased membrane rigidity [50,61]. In alpha thalassemia, Hb H cells would be expected to have increased deformability, and the mechanisms of reduced deformability are less well understood [12,50].

MECHANISMS OF ANEMIA

Overview of anemia mechanisms — There are several mechanisms of anemia in the thalassemias. All of them appear to share a common underlying cause – an imbalance in the ratio of alpha (or alpha-like) and beta (or beta-like) globin chains during erythropoiesis.

In both alpha and beta thalassemias, there is a component of destruction of developing red blood cell (RBC) precursors in the bone marrow (ineffective erythropoiesis) as well as mature RBCs in the circulation (hemolysis). There is an increase in erythropoiesis, but it is insufficient to compensate for the decreased production and increased destruction of RBC precursors and mature RBCs. Since the rate of hemolysis in Hb H is only increased threefold, it is not clear why the marrow, which normally has a fivefold or greater capacity to increase erythropoiesis, does not do so [62-64]. Some elements of ineffective erythropoiesis may play a role.

For reasons that are only partially understood, the relative contributions of ineffective erythropoiesis in the bone marrow and hemolysis in the circulation differ in alpha versus beta thalassemia. In severe beta thalassemia, profound ineffective erythropoiesis in the bone marrow tends to predominate, with a component of peripheral blood hemolysis; in alpha thalassemia, the converse tends to occur, with a predominance of peripheral blood hemolysis and a lesser component of ineffective erythropoiesis in the bone marrow.

Ineffective erythropoiesis

Overview of the process — Ineffective erythropoiesis refers to reduced production of RBCs in the bone marrow due to destruction of mid- to late-stage maturing erythroblasts. It is characterized by a combination of apoptotic cell death and hemolysis of developing RBC precursors. The mechanism is incompletely understood. The pathophysiologic result is a major reduction in the output of reticulocytes into the circulation despite expansion of the developing pool of early erythroid precursors, due to premature destruction of these precursors within the bone marrow.

Ineffective erythropoiesis occurs in both alpha and beta thalassemia (table 2); it is especially important in the pathophysiology of beta thalassemia. Toxic unpaired alpha chains have been demonstrated as early as the proerythroblast stage [65-67]. Free alpha chains were also shown to form dense cytoplasmic aggregates in basophilic, polychromatophilic, and orthochromatophilic erythroblasts (the final stages of RBC precursor development) [43,44,65,66,68,69]. (See "Regulation of erythropoiesis".)

The magnitude of ineffective erythropoiesis was initially underappreciated, until studies using radiolabeled iron were performed that demonstrated that in transfusion-dependent beta thalassemia, intramedullary cell death affected as much as 60 to 75 percent of developing RBC precursors [70]. Subsequently, the extent of apoptosis and erythroid expansion (numbers of developing erythroblasts) were shown to be increased in parallel [71].

Characteristic stages of ineffective erythropoiesis in beta thalassemia include expansion of the erythroid progenitor cell population and accelerated differentiation followed by maturation arrest and apoptosis at the polychromatophilic erythroblast stage (two steps before the reticulocyte) [72].

Although the beta globin inclusions in hemoglobin H (Hb H) disease occur primarily in circulating RBCs as they age, excess beta globin chains also may accumulate and precipitate in erythroid precursors and may contribute to oxidant or other cellular injury that contributes to ineffective erythropoiesis in alpha thalassemia [5,73,74]. The mechanisms are likely similar but to a lesser extent than seen in beta thalassemia.

Causes of ineffective erythropoiesis — Mechanisms of ineffective erythropoiesis include apoptosis and phagocytosis by bone marrow macrophages.

●Apoptosis – A major cause of premature death of erythroid precursors is accelerated apoptosis [69,71,75,76]. One mechanism of apoptosis was shown to involve sequestration of heat shock proteins such as HSP70 by free alpha globin chains in the cytoplasm of the RBC precursors, causing maturation arrest and ultimately cell death [77]. Other proteins that regulate apoptosis such as caspases and cytochromes have been shown to be abnormally phosphorylated in bone marrow erythroblasts of some patients [78]. The extent of erythroid apoptosis correlates very closely with the extent of ineffective erythropoiesis in both alpha and beta thalassemia variants (figure 9) [64].

●Phagocytosis – A second cause of premature cell death involves phagocytosis of developing erythroid precursors by bone marrow macrophages [79]. One of the consequences and hallmarks of apoptosis is the movement of phosphatidylserine (PS) from the inner to the outer leaflet of the membrane phospholipid bilayer. As mentioned above, the movement of PS to the outer leaflet may be one of the signals that promotes removal of thalassemic cells by the reticuloendothelial system. The deposition of IgG on the cell surface may promote phagocytosis in severe alpha thalassemia, as erythroblasts from patients with Hb H disease have excessive amounts of IgG on their surfaces [80]. Mechanisms of phagocytosis in the peripheral blood are discussed below. (See 'Hemolysis' below.)

Consequences of ineffective erythropoiesis — Downstream effects of ineffective erythropoiesis include erythroid expansion with extramedullary hematopoiesis and increased iron absorption:

●Erythroid expansion – In severe forms of beta thalassemia, ineffective erythropoiesis is associated with extramedullary hematopoiesis (production of RBCs in other tissues besides the bone marrow, such as the liver and spleen).

Erythroid expansion is driven by increased erythropoietin levels, although this may be blunted somewhat by inflammatory cytokines. GDF11, a cytokine in the transforming growth factor (TGF)-beta family, promotes the expansion of erythroid progenitor cells. Blockade of GDF11 in an animal model of thalassemia has been shown to correct several features of ineffective erythropoiesis including anemia, hemolysis, and iron overload. Potential therapies that could reduce GDF11 are under development for individuals with thalassemia. These therapies may work in part by reducing apoptosis of erythroid precursors [72]. (See "Regulation of erythropoiesis", section on 'TGF-beta gene family'.)

Erythroid expansion manifests as expansion of the bone marrow cavities that lead to bony distortion of the long bones, head, and facial bones, which are not normally used for erythropoiesis. Erythroid expansion in the spine can cause significant pain and neurologic deficits. (See "Diagnosis of thalassemia (adults and children)", section on 'Skeletal changes'.)

In individuals with severely impaired erythropoiesis, RBC production may occur in extramedullary sites such as the spine, liver, and spleen. (See "Diagnosis of thalassemia (adults and children)", section on 'Hepatosplenomegaly'.)

●Iron overload – Ineffective erythropoiesis also increases iron absorption and leads to increased total body iron. (See "Regulation of iron balance", section on 'Systemic iron homeostasis'.)

Hemolysis — Both alpha and beta thalassemias are characterized by hemolysis in the peripheral blood that shortens their survival to as much as one-third of that of normal RBCs (eg, down to 28 to 37 days to as short as 12 to 19 days) and contributes to anemia [5,11,12,62,81,82]. (See "Red blood cell survival: Normal values and measurement".)

The mechanism is thought to involve membrane damage caused by aggregated, precipitated, and/or oxidized globin chains, as well as resulting changes in the mechanical properties of the RBCs (rigidity, dehydration, reduced deformability) that retard their passage through the sinusoids of the reticuloendothelial system and promote increased phagocytosis by reticuloendothelial macrophages [24,33-35,45,48,81,83]. Slower passage through the microvasculature allows macrophages of the reticuloendothelial organs to carefully scrutinize these RBCs and detect any cell surface signals that could alert the macrophages to retard, bind, and engulf the defective RBC.

Signals for phagocytosis have been studied extensively and may include the following:

●Reduced sialic acid in the membrane [84].

●Exteriorization of phosphatidylserine (PS), which in normal RBCs is confined to the inner membrane leaflet and sequestered from macrophages [85]. As noted above (see 'Membrane damage' above), in some beta thalassemic RBCs, the membrane bilayer is disrupted (presumably by oxidative damage) and PS is exposed on the outer membrane [54,86]. (See "Red blood cell membrane: Structure and dynamics", section on 'Structural organization and dynamic regulation'.)

●Anti-RBC autoantibodies may form as the abnormal membrane proteins are exposed [87]. Oxidative damage from membrane-associated heme, hemichromes, and/or iron may cause membrane proteins to aggregate abnormally and create neoantigens [88,89]. As noted above, increases in membrane IgG and complement components are a likely contributor to the increased phagocytosis of alpha thalassemic RBCs [90]. Macrophages are able to identify the Fc domains of IgG on the RBC's outer membrane surface.

●Increased cytokine production, especially tumor necrosis factor, may activate monocytes and macrophages and make them more phagocytic [91]. Serum concentrations of macrophage/monocyte colony-stimulating factor (M-CSF) are elevated in Hb H disease and correlate inversely with the patient's hemoglobin concentration [92].

The importance of reticuloendothelial clearance of these abnormal cells is illustrated by the observation that individuals with beta thalassemia who have undergone splenectomy (and thus lack this clearance mechanism) have a greater proportion of RBCs with unstable and abnormal membranes [50]. Individuals with alpha thalassemia who have undergone splenectomy have increases in membrane RBC surface IgG levels.

In compound heterozygotes of Hb H disease and Hb Constant Spring (Hb H/CS), peripheral RBC destruction is increased, with a half-time of only 8.3 days (compared with a normal half-time of 26 days using the same assay) [64]. In homozygotes (Hb CS/CS), which should have the phenotype of an alpha thalassemia trait, the half-time is 13.7 days. Hb H/CS RBCs have more membrane associated IgG than either normal or Hb H RBCs [80].

CBC AND RBC MORPHOLOGY — The effects of thalassemia on red blood cell (RBC) structure (see 'Effects on the RBC' above) lead to characteristic abnormalities on the complete blood count (CBC) and morphologic changes on the blood smear. These are summarized briefly below and discussed separately. (See "Diagnosis of thalassemia (adults and children)", section on 'Laboratory testing'.)

●Anemia, RBC count, and reticulocyte count – Anemia (low hemoglobin and hematocrit) is a feature of most forms of thalassemia; mechanisms are multifactorial, as discussed above. (See "Diagnosis of thalassemia (adults and children)", section on 'Anemia' and 'Mechanisms of anemia' above.)

Despite the overall reduction in hemoglobin and hematocrit, the number of RBCs (RBC count) is typically normal-to-increased in thalassemias, as the bone marrow produces increased numbers of small cells. This combination of decreased hemoglobin and hematocrit with increased or normal RBC count is one of the features that leads clinicians to suspect thalassemia rather than iron deficiency when evaluating microcytic anemia. In iron deficiency, there are parallel decreases in hemoglobin, hematocrit, and RBC count. (See "Microcytosis/Microcytic anemia", section on 'Overview'.)

The reticulocyte count may be increased due to increased erythropoiesis [12,13,63]. However, the count is often inappropriately low for the degree of anemia, reflecting the effects of ineffective erythropoiesis, which impedes the output of red cells relative to what one would expect for the degree of erythropoietin stimulation of early progenitors.

Other indicators of increased erythrocyte activity include increased erythropoietin levels and soluble transferrin receptor (sTfR) [93,94].

●RBC indices and morphology – Unbalanced globin chain synthesis leads to a global reduction in hemoglobin per cell (low mean corpuscular hemoglobin).

RBCs in alpha thalassemia are small and under-hemoglobinized, leading to microcytosis and hypochromia (low mean corpuscular volume [MCV] and low mean corpuscular hemoglobin [MCH]). The hypochromia is due to a combination of reduced hemoglobin per cell as well as hyperhydration. The microcytosis occurs due to a reduction in cell volume caused by reduced hemoglobin content.

RBC morphology can be quite abnormal in severe thalassemias, with fragmentation and bizarre shapes. Target cells, teardrop cells, burr cells (echinocytes), and fragmented forms (schistocytes) can be seen. The blood smear images illustrate typical findings in beta thalassemia (picture 1) and alpha thalassemia (picture 2). The severity of the abnormalities depends on disease severity. Nucleated RBCs are also common, especially following splenectomy. Individuals receiving regular transfusions may have populations of normal-appearing RBCs interspersed with thalassemic cells.

Fragmented RBCs are likely to have reduced deformability and greater difficulty undergoing the elliptical deformation that allows them to traverse reticuloendothelial sinusoids; thus, fragmentation is likely to contribute to further hemolysis. (See 'Effects on the RBC' above and 'Hemolysis' above.)

●RBC inclusions – Unpaired globin chains can precipitate and produce intracellular inclusions in developing and mature circulating RBCs. In beta thalassemia, precipitated alpha chains can form large inclusions [21]. In alpha thalassemias, these are composed of beta globin tetramers (hemoglobin H [Hb H]). These inclusions increase in individuals who have undergone splenectomy [5]. They can also be induced by incubating RBCs with mild oxidants such as brilliant cresyl blue or new methylene blue (picture 3) [5,12,13,53].

When RBCs from patients with Hb H disease are separated by density into young, middle aged, and old cells, the level of soluble Hb H is highest in young cells, where the number of inclusion-containing cells is lowest. As the cells age and become denser, soluble Hb H decreases and inclusions increase [5]. The abundance of inclusions correlates with the cytosolic concentration of Hb H [95]. Electron microscopy studies have characterized the inclusions and documented that they are composed of beta globin chains [43,53].

IRON OVERLOAD

Causes of iron overload — Iron overload can occur in beta and alpha thalassemia. There are two main mechanisms:

●Transfusions – Significant total body iron overload can occur after as few as 15 to 20 transfusions. (See "Approach to the patient with suspected iron overload", section on 'Transfusional iron overload'.)

●Ineffective erythropoiesis – Ineffective erythropoiesis, with death of developing red blood cell precursors in the bone marrow, causes a significant increase in intestinal iron absorption by an incompletely understood mechanism. (See "Approach to the patient with suspected iron overload", section on 'Ineffective erythropoiesis'.)

Although the mechanism is incompletely understood, it is thought to involve reduced levels of hepcidin. (See "Regulation of iron balance", section on 'Intestinal iron absorption' and "Regulation of iron balance", section on 'Hepcidin'.)

Both anemia and increased erythropoiesis lead to suppression of hepcidin and, therefore, increased iron absorption. An important proximal regulator is erythroferrone, a member of the C1q-tumor necrosis factor (TNF) family of proteins that is produced by maturing erythroblasts in response to erythropoietin [96,97]. Studies in an animal model suggest that increased erythroferrone may be the principal contributor to excessive iron absorption in thalassemia [98].

Reduced levels of hepcidin lead to increased iron uptake from the intestine and bone marrow macrophages. (See "Regulation of iron balance", section on 'Hepcidin'.)

Thus, in individuals with thalassemia, iron overload can occur even in the absence of transfusions but is greatly exacerbated by transfusions.

Increased iron absorption leads to increases in the levels of storage iron as well as free iron, also called non-transferrin-bound iron (NTBI). Increased storage iron is reflected as increased serum transferrin saturation (TSAT), increased serum ferritin, and increased macrophage iron in the bone marrow [67]. (See "Approach to the patient with suspected iron overload", section on 'Sequence and interpretation of testing'.)

Consequences of iron overload — The consequences of excess total body iron are many-fold, and thought to be principally due to the effects of reactive oxygen species that can affect the following organ systems:

●Heart

●Liver

●Kidneys

●Thyroid

●Gonads

●Pancreas

●Bone marrow

●Pituitary gland

●Parathyroid glands

In a series of 174 patients with transfusion-dependent thalassemia who were receiving regular transfusions and chelation therapy, NTBI was detected in 83 percent; all patients with heart disease had increased NTBI; and all patients without NTBI were free of heart disease [99]. (See 'Organ damage' below and "Approach to the patient with suspected iron overload", section on 'Consequences of excess iron stores'.)

The evaluation and management of excess iron stores in patients with thalassemia are discussed separately. (See "Diagnosis of thalassemia (adults and children)", section on 'Rule out iron deficiency' and "Iron chelators: Choice of agent, dosing, and adverse effects".)

OTHER DISEASE COMPLICATIONS

Hypercoagulable state — Several reports have described an increased risk of thromboembolic complications in thalassemia [100-102]. In a retrospective series of 584 individuals with beta thalassemia intermedia, thrombosis was observed in 82 (14 percent) [103]. (See "Diagnosis of thalassemia (adults and children)".)

The mechanisms are incompletely understood. It has been suggested that increased phosphatidylserine (PS) on the outer leaflet of the red blood cell (RBC) membrane may promote thrombosis, similar to the role of PS in promoting coagulation on the surface of activated platelets [104]. (See "Overview of hemostasis", section on 'Multicomponent complexes'.)

Other hemostatic changes may include alterations in the levels of procoagulant or anticoagulant factors and/or chronic activation of platelets, endothelial cells, or white blood cells [100]. Splenectomy may also increase the risk of thrombosis [105,106]. Similarity to the increased risk of thromboembolism in sickle cell disease has also been suggested. However, as high-quality evidence for the increased risk of thrombophilia is lacking, we manage patients with thalassemia in a manner similar to the general population. Further study of this issue is warranted.

Organ damage — Organ damage in beta thalassemia is due to the combined effects of anemia, chronic hypoxia, iron overload, and possibly other disease features such as a chronic inflammatory state. These complications tend to be more pronounced in beta thalassemia than in alpha thalassemia.

●Kidney disease – Kidney disease may result from extramedullary hematopoiesis involving the kidney, hyperuricemia, and other metabolic effects of increased hematopoietic cell turnover [107]. Iron overload may also be toxic to the kidney. Some iron chelating agents such as deferasirox may also be nephrotoxic. (See "Diagnosis of thalassemia (adults and children)", section on 'Clinical manifestations'.)

Hypercalciuria and kidney stones are common in thalassemia second to endocrine disease, bone disease, and iron chelators [108].

●Heart disease – Heart disease including cardiomyopathy and pulmonary hypertension are multifactorial. Iron toxicity plays a major role in cardiac dysfunction. Other factors may include hemolysis, chronic anemia, endocrinopathy, vascular changes, and pulmonary disease. (See "Diagnosis of thalassemia (adults and children)", section on 'Heart failure and arrhythmias'.)

●Diabetes – Diabetes and other endocrine and metabolic abnormalities may result from iron deposition and oxidative damage to pancreatic cells. (See "Diagnosis of thalassemia (adults and children)", section on 'Endocrine and metabolic abnormalities'.)

●Bone disease – Decreased bone density is common and increases the risk of fracture and skeletal deformities. Contributing factors may include endocrine disorders, iron toxicity, and erythroid proliferation. Additional data suggest that erythroferrone may contribute by modifying the availability of bone morphogenetic protein (BMP) for osteoclast activity [109]. Increased osteoclast activity may be induced by cytokines of the tumor necrosis factor (TNF) family.

Susceptibility to Yersinia infection — Infection with Yersinia enterocolitica is a significant cause of morbidity in patients with thalassemia (as well as other iron overload syndromes such as chronic liver disease and hereditary hemochromatosis) [110,111]. (See "Yersiniosis: Infection due to Yersinia enterocolitica and Yersinia pseudotuberculosis".)

Yersinia enterocolitica is a siderophilic (iron-loving) organism; it contains several pathways to facilitate iron-uptake, which is essential for its growth [112]. (See "Yersiniosis: Infection due to Yersinia enterocolitica and Yersinia pseudotuberculosis".)

Iron chelation with desferrioxamine may make circulating iron more bioavailable to Yersinia. RBC transfusions may also provide a rich source of iron. In a series of 14 individuals with thalassemia who developed a Yersinia infection, 57 percent of the infections occurred within 10 days of blood transfusion [111]. Recognition of these associations and unusual manifestations in these patients, such as an appendicitis-like syndrome, may direct clinicians to earlier anti-yersinial therapy, along with temporary cessation of chelation [110,113]. (See "Yersiniosis: Infection due to Yersinia enterocolitica and Yersinia pseudotuberculosis".)

REDUCED MALARIA RISK — The high prevalence of thalassemia, especially in regions of the world where malaria is (or was) endemic, suggests evolutionary pressure to retain disease variants; this appears to have been driven by a reduction in malarial risk, since thalassemic red blood cells (RBCs) offer innate protection against severe malaria due to Plasmodium falciparum [114-119].

This effect is more pronounced in alpha thalassemia, but beta thalassemia does offer some protection. Malaria resistance appears to be due to reduced parasite multiplication within RBCs rather than reduced parasite invasion [120].

The mechanisms are incompletely understood and may include oxidant injury, persistence of fetal hemoglobin, and enhanced macrophage clearance [114,115,117,120-122]. This subject is discussed in more detail separately. (See "Protection against malaria by variants in red blood cell (RBC) genes".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Sickle cell disease and thalassemias".)

SUMMARY

●Terminology and definitions – Thalassemia refers to a spectrum of diseases characterized by reduced or absent production of one or more globin chains (figure 1). Beta thalassemia is due to impaired production of beta globin chains; alpha thalassemia is due to impaired production of alpha globin chains. Terminology has shifted to the use of transfusion dependent or non-transfusion-dependent as more clinically meaningful. (See 'Terminology and disease classification' above.)

The severity of beta thalassemia may also be affected by other genetic variation that affects the ratio of alpha globin to beta globin, including concomitant alpha thalassemia, hereditary persistence of fetal hemoglobin, concomitant sickle hemoglobin mutation, or concomitant hemoglobin E. (See 'Combinations of hemoglobin variants' above.)

●Globin chain imbalance – Reduced production of one type of globin chain (alpha or beta) disrupts the normally tightly controlled ratio of 1.00±0.05. Beta thalassemia mutations affect beta chains but not embryonic (epsilon) or fetal (gamma) chains; thus, the age of onset of beta thalassemia is in early childhood rather than during fetal development (figure 3). Alpha chains are used during fetal development; thus, alpha thalassemia manifests before birth. In the fetus and during early infancy, excess gamma globin chains form tetramers, creating Hb Barts. In childhood and adulthood, excess beta globin chains form tetramers, creating Hb H. Differences in the pathophysiology of alpha and beta thalassemia are summarized in the table (table 2). (See 'Globin chain imbalance' above.)

●Oxidative injury – Unpaired alpha or beta chains with an attached heme moiety are susceptible to oxidation to form hemichromes, which can generate reactive oxygen species (ROS). These can cause oxidant injury and damage red blood cell (RBC) membranes directly, via effects on cytoskeletal or integral membrane proteins or lipids, or indirectly by affecting the globin chains that bind to the membranes. (See 'Oxidative injury' above.)

●Altered RBC properties – Oxidative damage to membrane components reduces the normal asymmetry between the inner and outer leaflets of the lipid bilayer and may cause increased membrane expression of phosphatidylserine (PS) and neoantigens. Membrane stability is increased in alpha thalassemia and decreased in beta thalassemia. These changes interfere with normal maturation of RBCs and the deformability needed to traverse sinusoids in the liver and spleen. In erythroid precursors, they may lead to increased apoptosis in the bone marrow. In mature RBCs, they may lead to increased hemolysis. (See 'Membrane damage' above and 'Abnormal hydration' above and 'Reduced deformability' above.)

●Mechanisms of anemia – Alpha and beta thalassemias both have a component of ineffective erythropoiesis (destruction of developing RBC precursors in the bone marrow) and hemolysis (destruction of circulating mature RBCs). The relative contributions differ in the different thalassemias (table 2). In beta thalassemia, ineffective erythropoiesis tends to predominate; in severe beta thalassemia this can be associated with extramedullary hematopoiesis (production of RBCs in other tissues besides the bone marrow, such as the liver and spleen). In alpha thalassemia, peripheral blood hemolysis plays a larger role. (See 'Overview of anemia mechanisms' above.)

●CBC and blood smear – The severity of abnormalities on the complete blood count (CBC) and blood smear depends on disease severity. The blood smear images illustrate typical findings in beta thalassemia (picture 1) and alpha thalassemia (picture 2). (See 'CBC and RBC morphology' above.)

•There may be anemia with a normal-to-increased RBC count.

•The reticulocyte count may be increased but is often inappropriately low given the degree of anemia.

•RBCs can be microcytic (low mean corpuscular volume [MCV]) and hypochromic (low mean corpuscular hemoglobin [MCH]).

•Morphology can be quite abnormal with fragmentation, bizarre shapes, target cells, teardrop cells, and burr cells (echinocytes).

•Nucleated RBCs are also common, especially following splenectomy.

•Precipitated globin chains can produce large intracellular inclusions.

•Individuals receiving regular transfusions may have populations of normal-appearing RBCs interspersed with thalassemic cells.

●Iron overload – There are two main mechanisms of iron overload in the thalassemias: transfusions and ineffective erythropoiesis. Iron overload can occur after as few as 15 to 20 transfusions. Ineffective erythropoiesis causes a significant increase in intestinal iron absorption by an incompletely understood mechanism thought to involve reduced levels of hepcidin. Iron overload can affect the heart, liver, kidneys, endocrine organs (thyroid, pancreas, gonads), and bone marrow. (See 'Iron overload' above.)

●Other complications – Other potential complications include a hypercoagulable state; organ damage caused by iron overload, anemia, chronic hypoxia, and possibly chronic inflammatory state; and increased risk of infection with the siderophilic (iron-loving) organism Yersinia enterocolitica. (See 'Other disease complications' above.)

●Clinical implications – (See "Diagnosis of thalassemia (adults and children)" and "Methods for hemoglobin analysis and hemoglobinopathy testing" and "Management of thalassemia" and "Hematopoietic stem cell transplantation for transfusion-dependent thalassemia".)

ACKNOWLEDGMENTS — UpToDate gratefully acknowledges Stanley L Schrier, MD (deceased), who contributed as Section Editor on earlier versions of this topic and was a founding Editor-in-Chief for UpToDate in Hematology.

The UpToDate editorial staff also acknowledges extensive contributions of Donald H Mahoney, Jr, MD, and William C Mentzer, MD, to earlier versions of this and many other topic reviews.