Control of fetal hemoglobin: new insights emerging from genomics and clinical implications

Abstract

Increased levels of fetal hemoglobin (HbF, alpha(2)gamma(2)) are of no consequence in healthy adults, but confer major clinical benefits in patients with sickle cell anemia (SCA) and beta thalassemia, diseases that represent major public health problems. Inter-individual HbF variation is largely genetically controlled, with one extreme caused by mutations involving the beta globin gene (HBB) complex, historically referred to as pancellular hereditary persistence of fetal hemoglobin (HPFH). These Mendelian forms of HPFH are rare and do not explain the common form of heterocellular HPFH which represents the upper tail of normal HbF variation, and is clearly inherited as a quantitative genetic trait. Genetic studies have identified three major quantitative trait loci (QTLs) (Xmn1-HBG2, HBS1L-MYB intergenic region on chromosome 6q23, and BCL11A on chromosome 2p16) that account for 20-50% of the common variation in HbF levels in patients with SCA and beta thalassemia, and in healthy adults. Two of the major QTLs include oncogenes, emphasizing the importance of cell proliferation and differentiation as an important contribution to the HbF phenotype. The review traces the story of HbF quantitative genetics that uncannily mirrors the changing focus in genetic methodology, from candidate genes through positional cloning, to genome-wide association, that have expedited the dissection of the genetic architecture underlying HbF variability. These genetic results have already provided remarkable insights into molecular mechanisms that underlie the hemoglobin 'switch'.

Figure 1.

Population distribution of the F cell trait. The main panel shows the positively-skewed, continuous distribution of % F-cell measurements. Approximately 10% of the population at the upper end of the distribution has >4.5% F cells (FC) (corresponding to ∼0.6% HbF), and are designated as having heterocellular hereditary persistence of fetal hemoglobin (hHPFH). The panels below indicate F cell values at the two extremes of the distribution. FC were detected using anti-γ globin antibody on fixed erythrocytes smears or by fluorescence-activated cell sorting (FACS) after labeling of the intracellular HbF (24).

Figure 2.

Large-scale association mapping of the three main HbF/FC QTLs in patients and population samples. Shown are the SNPs representing the strongest association signal in each study as referenced. For the chromosome 2 (C) and 6 (B) QTLs, the signals cluster well, and the implicated SNP is highly dependent on the study design (e.g. GWAS or saturation genotyping). In the HBS1L-MYB region on chromosome 6, tight linkage disequilibrium complicates high-resolution mapping. For the HBB cluster (A), the Sardinian population seems to present a genetic scenario different from Northern Europeans such as the British twins shown here. Evidence from smaller-scale studies (data not shown) points to an effect of rs7482144 (the Xmn1-HBG2 site) in sickle cell patients as well. Genome distances and feature sizes are from the UCSC genome browser (NCBI 36.1), the ideograms, with permission, from Michelle M Le Beau, Chicago.

Figure 3.

Proposed mechanisms of fetal hemoglobin modulation (adapted from Stamatoyannopoulos, G. Experimental Hematology 2005; 33: 259–271). (A) Normal fetal and adult hemoglobin are produced by cells from the same lineage, i.e. hemoglobin switching occurs through a change in transcription program within the same cell. A change in transcription program occurs with erythropoietic differentiation, from one that predominantly supports γ globin expression to one that supports only β globin expression. FC represents the progeny of erythroid progenitor cells that terminate early before the change in transcription program occurs. Two plausible mechanisms of HbF modulation have emerged from biological studies of the QTLs. (B) Direct effect on γ globin gene expression (BCL11A is proposed as a developmental stage-specific repressor); and (C) Indirect via perturbation of kinetics of erythropoiesis—HMIP 2 contains a distal regulatory locus that controls MYB expression, whose levels affect differentiation of erythropoiesis. An accelerated erythropoiesis (mimicking stress erythropoiesis) leads to the release of more progenitor cells that are still synthesizing significant amounts of HbF.