Hemoglobinopathies are the most common single-gene diseases in the world; approximately 5–7 % of the world’s population is a carrier for one of the hemoglobin disorders. The extensive study of these disorders has led to much of our knowledge regarding protein structure and function, mutational mechanisms, gene expression during development, and evolution. Extensive reviews of these disorders are available [13, 14]. A list of the numerous characterized mutant alleles is available at the Globin Gene Server (http://​globin.​cse.​psu.​edu) and the Human Gene Mutation Database (http://​www.​hgmd.​cf.​ac.​uk/​ac/​index.​php).

Hemoglobin (Hb) functions as an oxygen carrier in vertebrate red cells. The protein is a tetramer consisting of two α and two β chains. Each subunit contains a heme prosthetic group that is responsible for the oxygen-binding capacity of the protein. These chains are highly homologous to one another at the primary sequence level as well as in three-dimensional structure. Six different globin chains (α, β, γ, δ, ε, ζ) are found in normal human hemoglobins at various times during development [13–15]. The α and β chains are encoded by separate genetic loci (Fig. 16.2). The α and α-like chains (ζ, Ψζ, Ψα) are clustered on chromosome 16. In addition, two copies of the α-globin genes, α₁ and α₂, are located on each chromosome 16. The β and β-like chains (ε, Gγ, Aγ, Ψβ, δ) are clustered on chromosome 11.

The developmental regulation of these genes is important to the understanding of many of the hemoglobin disorders. The gene order in each cluster is identical to the order of expression during development. In each developmental stage, there is an equimolar production of α and β chains. Hb Gower 1 (ζ₂ε₂)_, Hb Gower 2 (α₂ε₂), and Hb Portland (ζ₂γ₂) are expressed in embryos. In the fetal stage, the Hb is predominantly α₂γ₂ (HbF). In normal adults, the Hb is comprised of approximately 96 % α₂β₂ (HbA), 2.5 % δ₂β₂ (HbA2), and approximately 1 % α₂γ₂ (HbF). In addition to promoters and enhancer elements, the locus control region (LCR) is responsible for developmental expression of these genes. The LCR for the β locus is approximately 20 kb upstream of the ε-globin gene and the LCR for the α locus is approximately 40 kb upstream from the ζ-globin gene. The LCR is required for expression of all the globin genes at the locus [13].

The difference in gene dosage and expression patterns between α and β genes is significant. Each diploid genome encodes four copies of the α gene and two copies of the β gene. Since γ globin is expressed during fetal life and shortly after birth, β globin mutations do not exert their deleterious effects during the prenatal period. In contrast, since α chains are expressed during fetal and postnatal life, mutations in the α chain can cause severe disease during both developmental stages.

The hemoglobinopathies are commonly divided into two broad categories: structural variants, and thalassemias that are characterized by reduced rates of production of α or β chains [13]. These categories are not exclusive, as structural variants, which result in unstable hemoglobins, can be associated with both α- and β-thalassemia phenotypes. Although an extensive number of abnormal hemoglobins have been described, this chapter focuses on two of the most common structural variants, sickle hemoglobin (HbS) and HbC, and the more common forms of α- and β-thalassemia.

Sickle hemoglobin results from a single nucleotide substitution (GAG→GTG) that changes codon 6 of the β-globin gene from a glutamic acid to a valine (p.Glu6Val). Using standard nomenclature this variant should be reported as p.Glu7Val; however, the extensive literature uses the numbering of the mature protein in which the first methionine is cleaved. Homozygosity for this mutation causes SD, an autosomal recessive disorder. SD is a major cause of morbidity and mortality in Africa and in populations with individuals of African descent such as the Mediterranean area, the Middle East, and India [13]. Under the condition of low oxygen tension in the microvasculature, HbS polymerizes into fibers that cause the erythrocytes to sickle. The poor deformability of the sickle red cells and their defective passage in the micro-circulation causes the vasoocclusive events that are the hallmark of SD. The shortened survival of these red cells leads to a chronic hemolytic anemia. The frequency of SD is 1 in 600 in African Americans; the heterozygote frequency in African Americans is approximately 8 %, but the frequency in Africa can be much higher [13]. The frequency of the HbS allele, as well as alleles causing thalassemia, is maintained by their protective effect against malaria. The heterozygous state, sickle cell trait, is clinically normal but may be at risk for vasoocclusive events when exposed to low oxygen tension such as flying at high altitude in an airplane with reduced cabin pressure.

The sickling disorders also include compound heterozygous states with the HbS mutation in association with another β-globin variant such as HbC, or β-thalassemia. Similar to HbS, HbC also has an amino acid substitution at position 6 of the β-globin gene (Glu6Lys). HbC is less soluble than HbA and leads to reduced deformability of the red cells and a mild hemolytic anemia. The allele is frequent in persons of African descent; approximately 3 % of African Americans are carriers [13]. Individuals with β^Sβ^C have sickle cell hemoglobin (SC) disease. SC disease is a milder hemolytic disorder than SD and therefore may go unrecognized until a serious complication occurs.

The thalassemias result from the reduced synthesis or stability of either the α- or β-globin chain. The chains in excess due to the α:β imbalance precipitate within the red blood cells, leading to membrane damage and red cell destruction. The defect produces a hypochromic, microcytic anemia. The population distribution of thalassemia includes the Mediterranean, Middle East, portions of Africa, India, and Southeast Asia; however, the migration of these populations has resulted in the worldwide occurrence of these diseases [13].

The most common forms of α-thalassemia are caused by deletions. The genetics of α-thalassemia are complicated by the fact that each normal chromosome has two α genes and that there are haplotypes with either the loss of one (-α) (α+ thalassemia) or both (–) (α^o thalassemia) copies of the α gene. In addition to the normal genotype, four additional genotypes are possible: silent carrier (αα/-α), α-thalassemia trait (-α/-α or –/αα), HbH (β₄) disease (-α/–), and hydrops fetalis, Hb Barts: γ₄ (–/–). Individuals with HbH disease have a moderately severe hemolytic anemia, while the inheritance of no copies of the α gene (Hb Barts) is incompatible with life. Carriers of α-thalassemia trait have only a mild hypochromic microcytic anemia, but couples with these genotypes are at risk for having a fetus with hydrops fetalis or a child with HbH disease. The (–) genotype, and therefore the risk of hydrops fetalis, is largely restricted to Southeast Asia, while in other groups the (-α) genotype is more common [13, 14]. Distinguishing these two carrier states is important for accurate genetic counseling. The most widely occurring single α-globin gene deletions are -α^3.7 and -α^4.2. The double α-globin gene deletions α-^SEA, α-^FIL, and α-^THAI are common in Southeast Asia while α-^MED and -(α)²⁰ are more common in the Mediterranean area. Nondeletion α-thalassemia alleles are rare, but the α^{Constant Spring}, a mutation in the termination codon of the α₂ gene, is frequently present in Southeast Asia.

The β-thalassemias are mainly inherited in an autosomal recessive manner, although there are some dominant forms. Heterozygotes are asymptomatic but with recognizable hematologic parameters including an elevated HbA₂. Homozygotes and compound heterozygotes of HbE develop a severe anemia within the first year of life as the switch from γ to β chains occurs. In contrast to α-thalassemia, β-thalassemias are heterogeneous at the molecular level. The molecular mechanism of the mutations is equally varied. Most mutations are single-nucleotide substitutions or frameshift mutations. More than 200 disease-causing mutations have been identified in the β-globin gene; however, within each at-risk population there is a set of 4–10 mutations that account for the majority of disease-causing alleles (Table 16.3) [15]. HbE is a β-globin variant (Glu26Lys) that can cause a mild thalassemia phenotype. It is one of the most common Hb variants, and the frequency is high in Southeast Asia. Although HbE homozygotes are asymptomatic, compound heterozygotes with another β-thalassemia allele will have an abnormal phenotype [13]. Although deletions are rare, a 619 base pair (bp) deletion involving the 3′ end of the β-globin gene is common in India and Pakistan, accounting for approximately 30 % of the β-thalassemia alleles [13]. β-Thalassemia also may result from deletion of part of the globin gene cluster (e.g., 5P-thalassemia) or from deletions that start 50–100 kilobases (kb) upstream from the globin gene cluster and extend 3′ into the cluster (εγδβ-thalassemia). Some of these deletions lead to hereditary persistence of fetal hemoglobin. In addition, there are fusion chain variants such as Hb Lepore, a δβ⁺ thalassemia. Mutations either completely (β°-thalassemia) or partially (β⁺-thalassemia) abolish β-chain production. Milder phenotypes have been recognized that can be explained at the molecular level by homozygosity or compound heterozygosity for mild or silent mutations, coinheritance of α-thalassemia, or coinheritance of a genetic determinant that increases the production of the γ chain.