Mediator contributes to IgH locus VDJ rearrangements by promoting usage of most distal V segments

INTRODUCTION The immunoglobulin heavy(IgH)chain locus includes a large cluster of V,D and J segments spanning thousands of kilobases in mammals and its expression implies long-range 3D-regulation of various gene modifications.1 V(D)J recombination occurs during early B-cell ontogeny.2 It is regulated by multiple transcription factors and cis-regulatory elements,and most notably by the intronic 5′Eμelement located in the JH to Cμintron.3 The intergenic control region 1(IGCR1)located between the VH and D gene clusters inhibits rearrangement of the most DH-proximal VH gene segments,while promoting usage of distal VH gene segments.4 Other downstream elements of the locus,flanking its 3′end might have more subtle roles:while the large 3′regulatory region(3′RR)shows strong internal synergies2 and behaves as a superenhancer in mature B cells.Its deletion showed no influence on the VDJ repertoire and rather increased transcription of unrearranged V segments.3–5 Insulators binding CTCF farther downstream of the 3′RR were shown,however,to promote usage of distal VH segments.6 Altogether,there are strong indications that elements involved in long-distance regulation of transcription might also influence VDJ recombination.In-frame splicing of VDJ exons is also needed for full Ig expression.7 Given the functional importance of gene architecture and long-range interactions for the IgH locus physiology,we explored the dependence of the VDJ repertoire on Med1,a known actor of long-range enhancer interactions.