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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