Let-7 miRNAs repress HIC2 to regulate BCL11A transcription and hemoglobin switching.

The switch from fetal (HBG) to adult (HBB) -globin gene transcription in erythroid cells serves as a paradigm for a complex, clinically relevant developmental gene regulatory program. We previously identified HIC2 as a regulator of the switch by inhibiting the transcription of BCL11A, a key repressor of HBG production. HIC2 is highly expressed in fetal cells but the mechanism of its regulation is unclear. Here we report that HIC2 developmental expression is controlled by miRNAs, as loss of global miRNA biogenesis through DICER1 depletion leads to upregulation of HIC2 and HBG mRNA. We identified the adult-expressed let-7 miRNA as a direct posttranscriptional regulator of HIC2. Ectopic expression of let-7 in fetal cells lowered HIC2 levels while inhibition of let-7 in adult erythroblasts increased HIC2 production, culminating in decommissioning of a BCL11A erythroid enhancer and reduced BCL11A transcription. HIC2 depletion in let-7-inhibited cells restored BCL11A-mediated repression of HBG. Together these data establish that fetal hemoglobin silencing in adult erythroid cells is under the control of a miRNA mediated inhibitory pathway (let-7 ⊣ HIC2 ⊣ BCL11A ⊣ HBG).