Somatic hypermutation and class switch recombination in Msh6(-/-)Ung(-/-) double-knockout mice.
Somatic hypermutation (SHM) and class switch recombination (CSR) are initiated by activation-induced cytosine deaminase (AID). The uracil, and potentially neighboring bases, are processed by error-prone base excision repair and mismatch repair. Deficiencies in Ung, Msh2, or Msh6 affect SHM and CSR. To determine whether Msh2/Msh6 complexes which recognize single-base mismatches and loops were the only mismatch-recognition complexes required for SHM and CSR, we analyzed these processes in Msh6(-/-)Ung(-/-) mice. SHM and CSR were affected in the same degree and fashion as in Msh2(-/-)Ung(-/-) mice; mutations were mostly C,G transitions and CSR was greatly reduced, making Msh2/Msh3 contributions unlikely. Inactivating Ung alone reduced mutations from A and T, suggesting that, depending on the DNA sequence, varying proportions of A,T mutations arise by error-prone long-patch base excision repair. Further, in Msh6(-/-)Ung(-/-) mice the 5' end and the 3' region of Ig genes was spared from mutations as in wild-type mice, confirming that AID does not act in these regions. Finally, because in the absence of both Ung and Msh6, transition mutations from C and G likely are "footprints" of AID, the data show that the activity of AID is restricted drastically in vivo compared with AID in cell-free assays.
Shen
Hong Ming
HM
Department of Molecular Genetic and Cell Biology, University of Chicago, Chicago, IL 60637, USA.
Tanaka
Atsushi
A
Bozek
Grazyna
G
Nicolae
Dan
D
Storb
Ursula
U
D000818:Animals; D020137:Base Pair Mismatch; D003564:Cytidine Deaminase / Q000378:metabolism*; D045647:DNA Glycosylases / Q000172:deficiency* / Q000502:physiology; D053843:DNA Mismatch Repair; D004268:DNA-Binding Proteins / Q000172:deficiency* / Q000502:physiology; D017578:Immunoglobulin Class Switching; D051379:Mice; D018345:Mice, Knockout; D027041:Somatic Hypermutation, Immunoglobulin; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
D004268:DNA-Binding Proteins; C102997:Msh6 protein, mouse; D045647:DNA Glycosylases; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D003564:Cytidine Deaminase