Long-Distance Regulation of Fetal Vδ Gene Segment TRDV4 by the Tcrd Enhancer

Abstract

Murine Tcra and Tcrd gene segments are organized into a single genetic locus (Tcra/Tcrd locus) that undergoes V(D)J recombination in CD4−CD8− double-negative (DN) thymocytes to assemble Tcrd genes and in CD4+CD8+ double-positive thymocytes to assemble Tcra genes. Recombination events are regulated by two developmental stage-specific enhancers, Eδ and Eα. Effects of Eα on Trca/Tcrd locus chromatin have been well documented, but effects of Eδ have not. In this regard, Eα acts over long distances to activate many Vα and Jα segments for recombination in double-positive thymocytes. However, in DN thymocytes, it is unclear whether Eδ functions over long distances to regulate Vδ gene segments or functions only locally to regulate Dδ and Jδ gene segments. In this study, we analyzed germline transcription, histone modifications, and recombination on wild-type and Eδ-deficient alleles in adult and fetal thymocytes. We found that Eδ functions as a local enhancer whose influence is limited to no more than ∼10 kb in either direction (including Dδ, Jδ, and TRDV5 gene segments) in adult DN thymocytes. However, we identified a unique long-distance role for Eδ promoting accessibility and recombination of fetal Vδ gene segment TRDV4, over a distance of 55 kb, in fetal thymocytes. TRDV4 recombination is specifically repressed in adult thymocytes. We found that this repression is enforced by a developmentally regulated loss of histone acetylation. Constitutively high levels of a suppressive modification, histone H3 lysine 9 dimethylation, may contribute to repression as well.