Immunoglobulin isotype switching is inhibited and somatic hypermutation perturbed in UNG-deficient mice.
BACKGROUND
We have previously proposed that deamination of cytosine to uracil at sites within the immunoglobulin loci by activation-induced deaminase (AID) triggers antibody diversification. The pattern of diversification (phase 1 or 2 hypermutation, gene conversion, or switch recombination) is viewed as depending on the mode of resolution of the dU/dG lesion. A major resolution mode involves excising the uracil, an activity that at least four different enzymes can accomplish in the mouse.


RESULTS
Deficiency in UNG uracil-DNA glycosylase alone is sufficient to distort the pathway of hypermutation in mice. In ung(-/-) animals, mutations at dC/dG pairs are dramatically shifted toward transitions (95%), indicating that the generation of abasic sites (which can induce transversions) has been inhibited. The pattern of substitutions at dA/dT pairs is unaffected. Class-switch recombination is substantially, but not totally, inhibited.


CONCLUSIONS
The results provide strong support for the DNA deamination model for antibody diversification with respect to class-switching as well as hypermutation and, in the context of this model, suggest that (i) UNG is the major mouse DNA glycosylase responsible for processing the programmed dU/dG lesions within the immunoglobulin locus; (ii) the second (dA/dT-biased) phase of mutation is probably triggered by recognition of the initiating dU/dG lesion; and (iii) switch recombination largely proceeds via formation of an abasic site, although (iv) an UNG-independent pathway of switch recombination exists, which could reflect action by another uracil-DNA glycosylase but might alternatively be explained by a distinct pathway of resolution, for example, one involving MSH2/MSH6 recognition of the dU/dG lesion.
Rada
Cristina
C
Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom. car@mrc-lmb.cam.ac.uk
Williams
Gareth T
GT
Nilsen
Hilde
H
Barnes
Deborah E
DE
Lindahl
Tomas
T
Neuberger
Michael S
MS
D000818:Animals; D000916:Antibody Diversity; D045647:DNA Glycosylases; D017578:Immunoglobulin Class Switching / Q000187:drug effects / Q000235:genetics / Q000276:immunology*; D007074:Immunoglobulin G / Q000097:blood; D007132:Immunoglobulin Isotypes / Q000097:blood; D015847:Interleukin-4 / Q000494:pharmacology; D008070:Lipopolysaccharides / Q000494:pharmacology; D051379:Mice; D018345:Mice, Knockout; D016296:Mutagenesis; D009699:N-Glycosyl Hydrolases / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism*; D027041:Somatic Hypermutation, Immunoglobulin / Q000235:genetics*; D051981:Uracil-DNA Glycosidase
D007074:Immunoglobulin G; D007132:Immunoglobulin Isotypes; D008070:Lipopolysaccharides; D015847:Interleukin-4; D045647:DNA Glycosylases; D009699:N-Glycosyl Hydrolases; D051981:Uracil-DNA Glycosidase