At the genetic level, sickle cell disease is unambiguous, resulting from mutation of a single nucleotide (A→G) that introduces an amino acid substitution (valine for glutamic acid) in the β subunit of hemoglobin. At the clinical level, however, the disease is phenotypically diverse, ranging from asymptomatic to debilitating. An important determinant of clinical severity is the patient’s level of fetal hemoglobin (HbF, α2γ2) as the γ subunit of HbF competes with sickle-β for binding to the α chain. Normally, in a process called hemoglobin switching, synthesis of the γ-chain is stopped approximately six months after birth at the same time that β-chain synthesis is initiated. A group of rare conditions called hereditary persistence of fetal hemoglobin (HPFH) are characterized by continued synthesis of high levels of HbF in adult life. Patients with both sickle cell disease and HPFH have been identified, and as anticipated, they have a clinically benign phenotype (those with at least 25 percent HbF are neither anemic nor subject to vaso-occlusive complication). Moreover, no deleterious effects are observed in patients who are homozygous for HPFH, even when 100 percent of the hemoglobin produced is HbF. This observation indicates that preventing or reversing hemoglobin switching would be a safe approach to treating sickle cell disease (and β-thalassemia). For this reason, determining the molecular mechanisms that regulate expression of HbF has been the ultimate quest for a number of investigators.
Interestingly, elevated levels of HbF are seen in otherwise normal individuals, and epidemiological studies have shown that adult HbF expression is inherited as a quantitative trait. The field of HbF investigation has been invigorated by two recent genome-wide association studies that identified three major loci that account for ~20 percent of the variation in HbF levels and predict the clinical severity of sickle cell disease and β-thalassemia.1-3 The sequence variant with the greatest effect was located in an intron of BCL11A on chromosome 2p15, and the product of the gene is a zinc-finger protein. Through a series of rigorous, compelling experiments, Sankaran, et al., from Stuart Orkin’s lab at Children’s Hospital Boston, showed that the HbF-high BCL11A genotype is associated with reduced expression of the gene and that expression of full-length forms of BCL11A (apparently influenced by sequence variants) is restricted to adult erythroid cells. In vitro experiments demonstrated that down-regulating BCL11A expression in primary adult erythroid cells leads to enhanced HbF expression. Finally, Sankaran, et al. produced the “smoking gun” by showing that BCL11A occupies several discrete sites in the β-globin gene cluster, indicating a direct role for BCL11A in globin gene regulation.
In Arthurian legend, the Holy Grail is the cup or platter used by Jesus at the Last Supper. Obtaining it was the ultimate quest because of its religious significance and miraculous power. Over time, the grail has come to represent other more prosaic things, but finding it is always the highest goal, worthy of the pursuer’s best effort. Have Sankaran, et al., informed by the powerful genome-wide association studies,1-3 come into possession of the hemoglobin grail (i.e., the basis of hemoglobin switching)? If not, the hemoglobin grail appears to be within reach, and finding it brings with it the possibility of developing strategies for ameliorating the severity of diseases (sickle cell disease and β-thalassemia) that affect millions worldwide.
