Conserved Cis-Acting Range Extender Elements Mediate Long-Range Developmental Enhancer Activity in Mammals

Long-range developmental enhancers can regulate cell-type specific target gene expression over distances that exceed hundreds of thousands of base pairs in mammals. Although higher-order chromatin organization provides support for such long-range enhancer—promoter interactions, the genetic factors that regulate these interactions during mammalian development remain largely unexplored. To address this central question, we replaced a limb enhancer, located >800,000 base pairs from its target promoter, with equivalent limb enhancers of similar strength from other genomic loci. We used this replacement strategy to generate six knock-in mouse strains, with enhancers from four distinct genomic loci. All enhancers drove strong expression in developing limb buds when placed adjacent to a reporter gene. However, when transplanted to a distal location, all four enhancers failed to activate remote gene expression and distal limb outgrowth, suggesting that proximally-active enhancers cannot activate transcription remotely. Including a highly conserved, but previously uncharacterized, sequence found adjacent to a long-range limb enhancer rescued gene expression and limb outgrowth. This cis-acting Range EXtender (REX) element does not have classical enhancer activity but is necessary and sufficient for enhancer–promoter communication over very long distances in vivo. The REX element contains highly conserved [C/T]AATTA consensus sequences that match binding preferences of LIM-homeodomain (LHX) transcription factors. We used multiomic single-cell ATAC-seq/RNA-seq profiling to identify [C/T]AATTA homeodomain motifs next to thousands of long-range limb enhancers. The presence of these motifs positively correlates with E–P distance genome-wide. Our data support a model where REX elements facilitate target gene activation by remote enhancers over very long distances in mammalian genomes.