JH6 downstream intronic sequence is dispensable for RNA polymerase II accumulation and somatic hypermutation of the variable gene in Ramos cells

DNA flexibility can shape the preferential hypermutation of antibody genes

Antibody-coding genes accumulate somatic mutations to achieve antibody affinity maturation. Genetic dissection using various mouse models has shown that intrinsic hypermutations occur preferentially and are predisposed in the DNA region encoding antigen-contacting residues. The molecular basis of nonrandom/preferential mutations is a long-sought question in the field. Here, we summarize recent findings on how single-strand (ss)DNA flexibility facilitates activation-induced cytidine deaminase (AID) activity and fine-tunes the mutation rates at a mesoscale within the antibody variable domain exon. We propose that antibody coding sequences are selected based on mutability during the evolution of adaptive immunity and that DNA mechanics play a noncoding role in the genome. The mechanics code may also determine other cellular DNA metabolism processes, which awaits future investigation.

Promoter Proximity Defines Mutation Window for VH and VΚ Genes Rearranged to Different J Genes

Somatic hypermutation induced by activation-induced deaminase (AID) occurs at high densities between the Ig V gene promoter and intronic enhancer, which encompasses DNA encoding the rearranged V gene exon and J intron. It has been proposed that proximity between the promoter and enhancer defines the boundaries of mutation in V regions. However, depending on the J gene used, the distance between the promoter and enhancer is quite variable and may result in differential targeting around the V gene. To examine the effect of distance in mutation accumulation, we sequenced 320 clones containing different endogenous rearranged V genes in the IgH and Igκ loci from Peyer's patch B cells of mice. Clones were grouped by their use of different J genes. Distances between the V gene and enhancer ranged from ∼2.3 kb of intron DNA for rearrangements using J1, ∼2.0 kb for rearrangements using J2, ∼1.6 kb for rearrangements using J3 (H) or 4 (κ), and 1.1 kb for rearrangements using J4 (H) or 5 (κ). Strikingly, >90% of intron mutations occurred within 1 kb downstream of the J gene for both H and κ clones, regardless of which J gene was used. Thus, there is no evidence that the intron sequence or enhancer plays a role in determining the extent of mutation. The results indicate that V region intron mutations are targeted by their proximity to the promoter, suggesting they result from AID interactions with RNA polymerase II over a 1-kb region.

What Targets Somatic Hypermutation to the Immunoglobulin Loci?

One of the most profound enigmas in B cell biology is how activation-induced deaminase (AID) is targeted to a very small region of DNA in the immunoglobulin loci. Two specific regions are singled out: the variable region of 2 kb that contains rearranged genes on the heavy, κ light, and λ light chain loci, and the switch region of ∼4 kb that contains an extensive stretch of G:C rich DNA on the heavy chain locus. Transcription is required for AID recruitment; however, many genes are also highly transcribed and do not undergo the catastrophic mutagenesis that occurs in variable and switch regions. The DNA sequences of these regions cause RNA polymerase II to accumulate for an extended distance of 2-4 kb. The stalled polymerases then recruit the transcription cofactor Spt5, and AID, which deaminates cytosines to uracils in exposed transcription bubbles. Thus, the immunoglobulin loci are unique in that a favorable combination of DNA sequences and 3' transcription enhancers make them the perfect storm for AID-induced somatic hypermutation.