Isolated Changes in Chromatin Accessibility and Enhancer-Promoter Contacts at the β-Globin Locus Distinguish Fetal Hemoglobin Producing F-Cells from a-Cells

Reversal of the developmental switch from fetal (HbF, α 2γ 2) to adult (HbA,α 2β 2) hemoglobin is an important therapeutic approach for sickle cell disease (SCD) and β-thalassemia. It has been noted since the 1950s that a small number of circulating red blood cells, called F-cells, produce elevated levels of HbF; these cells are resistant to sickling and are present in increased numbers in patients with SCD and those treated with pharmacological HbF inducers such as hydroxyurea. Because successful therapy for SCD requires increasing the number of F-cells, it is imperative to understand how these cells arise. This can potentially occur through a shift towards a global fetal-like program, selective variation in levels of known HbF silencers such as BCL11A or LRF, or through discrete epigenetic changes at the β-globin locus. We previously began to address this clinically important question using a novel experimental approach of sorting cultured primary human erythroblasts into HbF-high (F-cell) and HbF-low (A-cell) populations (Khandros et al, Blood 2020). We showed that surprisingly, F-cells from healthy donor primary erythroid cultures have minimal transcriptional differences with A-cells. Unexpectedly, this was also the case when comparing responders (F-cells) and non-responders (A-cells) to treatment with the HbF inducers pomalidomide and hydroxyurea, and there were no differences in the expression of known HbF regulators. We therefore hypothesize that HbF synthesis in F-cells is determined by epigenetic variation confined to the β-globin locus (and not by global changes in the cell fate or nuclear milieu).