Elements between the IgH variable (V) and diversity (D) clusters influence antisense transcription and lineage-specific V(D)J recombination

Ig and T-cell receptor (TCR) variable-region gene exons are assembled from component variable (V), diversity (D) and joining (J) gene segments during early B and T cell development. The RAG1/2 endonuclease initiates V(D) J recombination by introducing DNA double-strand breaks at borders of the germ-line segments. In mice, the Ig heavy-chain (IgH) locus contains, from5' to 3', several hundred V-H gene segments, 13 D segments, and 4 J(H) segments within a several megabase region. In developing B cells, IgH variable-region exon assembly is ordered with D to J(H) rearrangement occurring on both alleles before appendage of a V-H segment. Also, IgH V-H to DJ(H) rearrangement does not occur in T cells, even though DJ(H) rearrangements occur at low levels. In these contexts, V(D)J recombination is controlled by modulating substrate gene segment accessibility to RAG1/2 activity. To elucidate control elements, we deleted the 100-kb intergenic region that separates the V-H and D clusters (generating Delta V-H-D alleles). In both B and T cells, Delta V-H-D alleles initiated high-level antisense and, at lower levels, sense transcription from within the downstream D cluster, with antisense transcripts extending into proximal V-H segments. In developing T lymphocytes, activated germ-line antisense transcription was accompanied by markedly increased IgH D-to-J(H) rearrangement and substantial V-H to DJ(H) rearrangement of proximal IgH V-H segments. Thus, the V-H-D intergenic region, and likely elements within it, can influence silencing of sense and antisense germ-line transcription from the IgH D cluster and thereby influence targeting of V(D)J recombination.