Antisense Intergenic Transcription Precedes Igh D-to-J Recombination and Is Controlled by the Intronic Enhancer Eμ

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Abstract
V(D)J recombination is believed to be regulated by alterations in chromatin accessibility to the recombinase machinery, but the mechanisms responsible remain unclear. We previously proposed that antisense intergenic transcription, activated throughout the mouse Igh VH region in pro-B cells, remodels chromatin for VH-to-DJH recombination. Using RNA fluorescence in situ hybridization, we now show that antisense intergenic transcription occurs throughout the Igh DHJH region before D-to-J recombination, indicating that this is a widespread process in V(D)J recombination. Transcription initiates near the Igh intronic enhancer Eμ and is abrogated in mice lacking this enhancer, indicating that Eμ regulates DH antisense transcription. Eμ was recently demonstrated to regulate DH-to-JH recombination of the Igh locus. Together, these data suggest that Eμ controls DH-to-JH recombination by activating this form of germ line Igh transcription, thus providing a long-range, processive mechanism by which Eμ can regulate chromatin accessibility throughout the DH region. In contrast, Eμ deletion has no effect on VH antisense intergenic transcription, which is rarely associated with DH antisense transcription, suggesting differential regulation and separate roles for these processes at sequential stages of V(D)J recombination. These results support a directive role for antisense intergenic transcription in enabling access to the recombination machinery.