A Functional Element Necessary for Fetal Hemoglobin Silencing

Abstract

BACKGROUND An improved understanding of the regulation of the fetal hemoglobin genes holds promise for the development of targeted therapeutic approaches for fetal hemoglobin induction in the β-hemoglobinopathies. Although recent studies have uncovered trans-acting factors necessary for this regulation, limited insight has been gained into the cis-regulatory elements involved. METHODS We identified three families with unusual patterns of hemoglobin expression, suggestive of deletions in the locus of the β-globin gene (β-globin locus). We performed array comparative genomic hybridization to map these deletions and confirmed breakpoints by means of polymerase-chain-reaction assays and DNA sequencing. We compared these deletions, along with previously mapped deletions, and studied the trans-acting factors binding to these sites in the β-globin locus by using chromatin immunoprecipitation. RESULTS We found a new (δβ)0-thalassemia deletion and a rare hereditary persistence of fetal hemoglobin deletion with identical downstream breakpoints. Comparison of the two deletions resulted in the identification of a small intergenic region required for γ-globin (fetal hemoglobin) gene silencing. We mapped a Kurdish β0-thalassemia deletion, which retains the required intergenic region, deletes other surrounding sequences, and maintains fetal hemoglobin silencing. By comparing these deletions and other previously mapped deletions, we elucidated a 3.5-kb intergenic region near the 5′ end of the δ-globin gene that is necessary for γ-globin silencing. We found that a critical fetal hemoglobin silencing factor, BCL11A, and its partners bind within this region in the chromatin of adult erythroid cells. CONCLUSIONS By studying three families with unusual deletions in the β-globin locus, we identified an intergenic region near the δ-globin gene that is necessary for fetal hemoglobin silencing. (Funded by the National Institutes of Health and others.)

Corepressor-dependent silencing of fetal hemoglobin expression by BCL11A

“…However, BCL11A does not bind detectably to the γ-globin promoter in erythroid chromatin, suggesting that its mode of action is more complex than simply blocking transcription at the proximal promoter. This observation is consistent with a role of BCL11A in promoting long-range chromosomal interactions within the β-globin locus (6,7,9). Therefore, in an effort to elucidate further the precise molecular mechanisms by which BCL11A coordinates the switch, it is relevant to identify systematically BCL11A-interacting partner proteins, and use functional and genetic approaches to assess their individual roles in HbF regulation and erythroid cell maturation.…”

The Switch from Fetal to Adult Hemoglobin

“…By mapping a variety of deletions within the human b-globin locus that either result in db-thalassemia, with modest increases in HbF and some remaining globin chain imbalance present, or HPFH, with robust increases in HbF and balanced globin chain synthesis, it was shown that an N3-kb region upstream of the d-globin gene is necessary for silencing of the g-globin genes ( Fig. 2) (Sankaran et al 2011c). Interestingly, this region harbors binding sites for BCL11A, along with its partners such as GATA1 and HDAC1.…”

Transcriptional divergence and conservation of human and mouse erythropoiesis

“…However, there are examples where aspects of mouse erythropoiesis are inconsistent with human erythropoiesis. For example, humans express a fetal hemoglobin not found in the mouse, and their globin gene regulation pattern differs significantly (5)(6)(7). Mutations that impair erythropoiesis in humans are often not faithfully recapitulated in mouse models (8)(9)(10)(11).…”