Region specific histone modifications in class switch recombination
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Class switch recombination (CSR) rearranges the antibody variable (V) region from mu to one of the downstream gamma, epsilon or alpha constant (C) regions in the immunoglobulin heavy chain locus, which results in the expression of that V region as an IgG, IgE or IgA isotype. The genomic DNA recombines between the switch (S) regions that are 5' to each of the C regions. Point mutations are introduced into the S regions during CSR. However, the exact role and detailed mechanism of targeting and regulating each of the molecular events during CSR remains unclear. I hypothesize that region specific modifications play a role in CSR targeting.;We found that in primary splenic mouse B cells histone H3 and H4 are constitutively hyperacetylated in the donor Smu region. The downstream candidate Sgamma regions are hypoacetylated in the resting B cells. The stimuli that induce germline transcripts (GLTs) of the Sgamma regions also induce hyperacetylation of the associated histones. Furthermore, we observe a significantly increased frequency of mutation in the hyperacetylated DNA segments. In addition, time course experiments reveal that the pattern of association of RNA polymerase II with S regions is much like that of H3 hyperacetylation. Histone H3 trimethylated at lysine 9 was also found in the Smu region constitutively, as well as in the downstream recipient Sgamma region.;Collectively, our data suggests that H3 hyperacetylation reflects the transcriptional activity of a given switch region. Increased transcriptional activity in the S regions creates more substrates for AID. Presumably, more AID-dependent double-stranded breaks that are required for the recombination are also generated in these hyperacetylated segments. However, additional factors must be required at this step to determine if the switch regions will undergo recombination and mediate switching. We also identified H3 lysine 9 trimethylation as a novel histone modification in the S regions, which very likely reflects the transcriptional activity and/or switching specificity, related with the target S regions. In summary, hyperacetylation, H3 lysine 9 trimethylation, increased transcription and AID-dependent mutations very likely are required but are not sufficient for recombination between the S regions.