A 220-nucleotide deletion of the intronic enhancer reveals an epigenetic hierarchy in immunoglobulin heavy chain locus activation

A tissue-specific transcriptional enhancer, E mu, has been implicated in developmentally regulated recombination and transcription of the immunoglobulin heavy chain (IgH) gene locus. We demonstrate that deleting 220 nucleotides that constitute the core E. results in partially active locus, characterized by reduced histone acetylation, chromatin remodeling, transcription, and recombination, whereas other hallmarks of tissue-specific locus activation, such as loss of H3K9 dimethylation or gain of H3K4 dimethylation, are less affected. These observations define E mu-independent and E mu-dependent phases of locus activation that reveal an unappreciated epigenetic hierarchy in tissue-specific gene expression.