Reactivation of Developmentally Silenced Globin Genes by Forced Chromatin Looping

•Tethering Ldb1 to embryonic or fetal globin genes activates them in adult erythroblasts•Activation of embryonic or fetal globin genes reduces adult type globin expression•Tethered Ldb1 reconfigures enhancer-promoter contacts commensurate with gene expression•Forced chromatin looping presents a novel therapeutic strategy for sickle cell anemia

SummaryDistal enhancers commonly contact target promoters via chromatin looping. In erythroid cells, the locus control region (LCR) contacts β-type globin genes in a developmental stage-specific manner to stimulate transcription. Previously, we induced LCR-promoter looping by tethering the self-association domain (SA) of Ldb1 to the β-globin promoter via artificial zinc fingers. Here, we show that targeting the SA to a developmentally silenced embryonic globin gene in adult murine erythroblasts triggers its transcriptional reactivation. This activity depends on the LCR, consistent with an LCR-promoter looping mechanism. Strikingly, targeting the SA to the fetal γ-globin promoter in primary adult human erythroblasts increases γ-globin promoter-LCR contacts, stimulating transcription to approximately 85% of total β-globin synthesis, with a reciprocal reduction in adult β-globin expression. Our findings demonstrate that forced chromatin looping can override a stringent developmental gene expression program and suggest a novel approach to control the balance of globin gene transcription for therapeutic applications.

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