Published 21 November 2005. doi:10.1084/jem.20051604
Rockefeller University Press, 0022-1007 $8.00
JEM, Volume 202, Number 10, 1443-1454
The very 5' end and the constant region of Ig genes are spared from somatic mutation because AID does not access these regions
Simonne Longerich1,
Atsushi Tanaka1,
Grazyna Bozek2,
Dan Nicolae3, and
Ursula Storb1,2
1 Committee on Immunology, University of Chicago, Chicago, IL 60637
2 Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL 60637
3 Department of Statistics, University of Chicago, Chicago, IL 60637
CORRESPONDENCE Ursula Storb: stor{at}midway.uchicago.edu
Somatic hypermutation (SHM) is restricted to VDJ regions and their adjacent flanks in immunoglobulin (Ig) genes, whereas constant regions are spared. Mutations occur after about 100 nucleotides downstream of the promoter and extend to 12 kb. We have asked why the very 5' and most of the 3' region of Ig genes are unmutated. Does the activation-induced cytosine deaminase (AID) that initiates SHM not gain access to these regions, or does AID gain access, but the resulting uracils are repaired error-free because error-prone repair does not gain access? The distribution of mutations was compared between uracil DNA glycosylase (Ung)-deficient and wild-type mice in endogenous Ig genes and in an Ig transgene. If AID gains access to the 5' and 3' regions that are unmutated in wild-type mice, one would expect an "AID footprint," namely transition mutations from C and G in Ung-deficient mice in the regions normally devoid of SHM. We find that the distribution of total mutations and transitions from C and G is indistinguishable in wild-type and Ung-deficient mice. Thus, AID does not gain access to the 5' and constant regions of Ig genes. The implications for the role of transcription and Ung in SHM are discussed.
Abbreviations used: AID, activation-induced cytosine deaminase; CDR, complementarity-determining region; Msh, mismatch repair; pol, polymerase; SHM, somatic hypermutation; Ung, uracil DNA glycosylase.

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