Genomic approaches to identifying targets for treating β hemoglobinopathies

Abstract

Sickle cell disease and β thalassemia are common severe diseases with little effective pathophysiologically-based treatment. Their phenotypic heterogeneity prompted genomic approaches to identify modifiers that ultimately might be exploited therapeutically. Fetal hemoglobin (HbF) is the major modulator of the phenotype of the β hemoglobinopathies. HbF inhibits deoxyHbS polymerization and in β thalassemia compensates for the reduction of HbA. The major success of genomics has been a better understanding the genetic regulation of HbF by identifying the major quantitative trait loci for this trait. If the targets identified can lead to means of increasing HbF to therapeutic levels in sufficient numbers of sickle or β-thalassemia erythrocytes, the pathophysiology of these diseases would be reversed. The availability of new target loci, high-throughput drug screening, and recent advances in genome editing provide the opportunity for new approaches to therapeutically increasing HbF production.

Fig. 1

a Arrangement of the β- and α-globin gene clusters and their regulatory regions, The LCR (locus control region) and HS-40 are the major enhancers of expression within the HBB and HBA gene clusters, respectively. HbA is a tetramer of normal α- and β-globin chains. b. The expression of the globin genes changes throughout development. Embryonic ε globin is produced in the embryo, fetal γ-globin during most of gestation and the major adult β globin from mid-gestation onwards. Not shown are the α-globin-like ζ globin genes and the α-globin genes whose expression starts early in embryogenesis. c. Classification of hemoglobinopathies and thalassemia. Hemoglobinopathies result from mutations that change the primary structure of globin. The most common examples are HbS (HBB glu6val), HbC (HBB glu6lys), and HbE (HBB glu26lys). Rare structural variants affect the oxygen delivery functions of the molecule, its stability and its resistance to oxidation. Thalassemia is caused by mutations that affect transcription and translation of any globin gene by nearly all possible mechanisms. They lead to decreased or absent production of a globin subunit; α and β thalassemia are most common. In all thalassemias the phenotype is a consequence of imbalanced synthesis of globin subunits allowing globin unincorporated into a tetramer to precipitate and otherwise damage the erythrocyte. About 1600 structural variants and thalassemia mutations have been cataloged in The Hemoglobin Variant Database [1]. All thalassemias and hemoglobinopathies can interact in various ways and many different compound heterozygous conditions occur. (Adapted from [51])

Fig. 2

HbF Gene Expression is controlled by Cis- and Trans-acting elements. Shown, not to scale is the LCR, the γ-globin genes and the β-globin gene. γ-Globin and α globin (not shown) form HbF while β-globin and α globin form adult HbA. The major known transcription factors that have been implicated in hemoglobin switching are shown along with some of their interactions. One model holds that BCL11A participates as part of complexes of transcription factors like those shown to regulate the HbF to HbA switch. KLF1 binds the BCL11A promoter activating its expression and as shown has a dual effect switching by directly on activating HBB while repressing HBG2 and HBG1 indirectly by activating BCL11A. The cartoon does not illustrate the 3 dimensional interactions and the chromosome dynamics that include histone modification and methylation of critical regions of the HBB gene cluster.that are integral components of the transcription process. (Figure provided by and adapted from Orkin, SH, From GWAS-identified locus to reversing the fetal hemoglobin switch: Functional and genetic validation, in, Genomics: Gene discovery and clinical applications for cardiovascular, lung, and blood diseases. Sept. 2011, NIH, Bethesda, MD)