B cell development arrest upon insertion of a neo gene between JH and Emu: promoter competition results in transcriptional silencing of germline JH and complete VDJ rearrangements.

Previous targeting experiments within the IgH locus have shown that V(D)J recombination was affected by an insertion of a neo gene within E(mu) upstream of the core enhancer, but not by insertions downstream of the enhancer. Similarly, class switch recombination to a given (C) gene was affected only by interposition of neo in between that gene and the 3' IgH enhancers. Here we show that insertion of neo upstream E(mu) only marginally impairs V(D)J recombination, but results in an altered D and J(H) gene usage and completely blocks transcription of the germline J(H) region and the rearranged VDJ segments. Although transcriptional silencing of J(H) occurs upstream of the insertion and results in the lack of mature B cells in homozygous mutant animals, IgH transcription is maintained downstream of the insertion together with neo transcription and can be up-regulated by LPS stimulation or upon fusion with plasmacytoma cells. Altogether these data argue for a polarized "neo effect" involving promoter competition and further show that V(D)J rearrangement can be uncoupled from transcription.