International Immunology, Vol. 15, No. 4, pp. 477-481,
April 2003
© 2003 Japanese Society for Immunology
Ig gene somatic hypermutation in mice defective for DNA polymerase
proofreading
1 Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL 60637, USA 2 Present address: Department of Pediatrics, University of California at San Francisco, San Francisco, CA 94143, USA 3 Present address: Department of Pathology, University of Washington, Seattle, WA 98195, USA
Correspondence to: U. Storb; E-mail stor{at}midway.uchicago.edu
Transmitting editor: E. A. Clark
This study is an investigation of the possible role of DNA polymerase (pol)
with an inactivated exonuclease (exo) in somatic hypermutation (SHM). Analysis of endogenous heavy chain transcripts revealed no difference in mutation frequency and pattern between exo/, exo+/ and exo+/+ mice. The lack of an effect of the pol
exo mutation on SHM could be due to: (i) normally pol
is used in SHM, but the exo is prevented from proofreading, (ii) normally pol
is used, but the decrease in fidelity of the exo pol does not increase hypermutation frequency enough to be detected, and (iii) pol
is not used in SHM. Based on the finding in the exo/ mice and the current understanding of the process of SHM, it is concluded that pol
is not normally involved in creating the mutations. The majority of the mutated sequences obtained in this study, including many from the exo/ mice, were from genes which had switched to a
heavy chain class. Thus, the pol
proofreading activity is not required for class switch recombination (CSR). Genealogical trees observed with multiple mutated sequences of various Ig classes show that CSR and SHM occur intermingled during expansion of a cell clone, raising the possibility that they may occur at the same time.
Keywords: class switch recombination, polymerase
exonuclease, somatic hypermutation
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