A histone code for regulating V(D)J recombination

A multifunctional element in the mouse Igκ locus that specifies repertoire and Ig loci subnuclear location

Nonbiased V gene usage for V(D)J joining is essential for providing an optimal immune system, but no cis-acting sequence with this function has been uncovered. We previously identified a recombination silencer and heterochromatin targeting element in the Vκ-Jκ intervening sequence of germline Igκ transgenes, which we termed Sis. We now have generated Sis knockout mice in the endogenous locus. Intriguingly, Sis(-/-) mice exhibit a skewed Igκ repertoire with markedly decreased distal and enhanced proximal Vκ gene usage for primary rearrangement, which is associated with reduced occupancy of Ikaros and CCCTC-binding factor in the Vκ-Jκ intervening sequence in pre-B cells, proteins believed to be responsible for dampening the recombination of nearby Vκ genes and altering higher-order chromatin looping. Furthermore, monoallelic heterochromatin localization is significantly reduced in Sis(-/-) mice for Igκ in cis and IgH loci in trans in pre-B cells. Because Sis(-/-) mice still allelically excluded Igκ and IgH loci and still exhibited IgL isotype exclusion, we concluded that stable localization at pericentromeric heterochromatin is neither necessary nor sufficient for the establishment or maintenance of allelic exclusion. Hence, Sis is a novel multifunctional element that specifies repertoire and heterochromatin localization to Ig genes.

The Igκ gene enhancers, E3' and Ed, are essential for triggering transcription

The mouse Igκ gene locus has three known transcriptional enhancers: an intronic enhancer (Ei), a 3' enhancer (E3'), and a further downstream enhancer (Ed). Previous studies on B lymphocytes derived from mutant embryonic stem cells have shown that deletion of either Ei or E3' significantly reduces Igκ gene rearrangement, whereas the combined deletion of both Ei and E3' eliminates such recombination. Furthermore, deletion of either E3' or Ed significantly reduces rearranged Igκ gene transcription. To determine whether the combined presence of both E3' and Ed are essential for Igκ gene expression, we generated homozygous double knockout (DKO) mice with targeted deletions in both elements. Significantly, homozygous DKO mice were unable to generate κ(+) B cells both in bone marrow and the periphery and exhibited surface expression almost exclusively of Igλ-chains, despite the fact that they possessed potentially functional rearranged Igκ genes. Compared with their single-enhancer-deleted counterparts, Igκ loci in homozygous DKO mice exhibited dramatically reduced germline and rearranged gene transcription, lower levels of gene rearrangement and histone H3 acetylation, and markedly increased DNA methylation. This contributed to a partial developmental block at the pre-B cell stage of development. We conclude that the two downstream enhancers are essential in Igκ gene expression and that Ei in homozygous DKO mice is incapable of triggering Igκ gene transcription. Furthermore, these results reveal unexpected compensatory roles for Ed in E3' knockout mice in triggering germline transcription and Vκ gene rearrangements to both Jκ and RS elements.