A DNase I hypersensitive site near the murine gamma1 switch region contributes to insertion site independence of transgenes and modulates the amount of transcripts induced by CD40 ligation

Mechanism and regulation of class switch recombination

Antibody class switching occurs in mature B cells in response to antigen stimulation and costimulatory signals. It occurs by a unique type of intrachromosomal deletional recombination within special G-rich tandem repeated DNA sequences [called switch, or S, regions located upstream of each of the heavy chain constant (C(H)) region genes, except Cdelta]. The recombination is initiated by the B cell-specific activation-induced cytidine deaminase (AID), which deaminates cytosines in both the donor and acceptor S regions. AID activity converts several dC bases to dU bases in each S region, and the dU bases are then excised by the uracil DNA glycosylase UNG; the resulting abasic sites are nicked by apurinic/apyrimidinic endonuclease (APE). AID attacks both strands of transcriptionally active S regions, but how transcription promotes AID targeting is not entirely clear. Mismatch repair proteins are then involved in converting the resulting single-strand DNA breaks to double-strand breaks with DNA ends appropriate for end-joining recombination. Proteins required for the subsequent S-S recombination include DNA-PK, ATM, Mre11-Rad50-Nbs1, gammaH2AX, 53BP1, Mdc1, and XRCC4-ligase IV. These proteins are important for faithful joining of S regions, and in their absence aberrant recombination and chromosomal translocations involving S regions occur.

Locus control regions

Locus control regions (LCRs) are operationally defined by their ability to enhance the expression of linked genes to physiological levels in a tissue-specific and copy number-dependent manner at ectopic chromatin sites. Although their composition and locations relative to their cognate genes are different, LCRs have been described in a broad spectrum of mammalian gene systems, suggesting that they play an important role in the control of eukaryotic gene expression. The discovery of the LCR in the beta-globin locus and the characterization of LCRs in other loci reinforces the concept that developmental and cell lineage-specific regulation of gene expression relies not on gene-proximal elements such as promoters, enhancers, and silencers exclusively, but also on long-range interactions of various cis regulatory elements and dynamic chromatin alterations.

Localization of the 3' IgH locus elements that effect long-distance regulation of class switch recombination

Four transcriptional enhancers lie downstream of the immunoglobulin heavy chain locus: Calpha3'/hs3a, hs1,2, hs3b, and hs4. Although individually weak, these elements have strong transcriptional synergies when combined and they altogether behave as a locus control region. Previous knockout experiments in the 3' region have shown that both hs3a and hs1,2 are dispensable for normal expression and rearrangement of the IgH locus but that their replacement with a transcribed neo gene severely affects class switch recombination. Here we show that even in the absence of a neo gene, joint deletion of the last two 3' enhancers, hs3b and hs4, severely impairs germline transcription and class switching to most isotypes and may in addition affect mu gene expression in resting B cells.