Laboratory of Molecular Biology and Immunology, National Institute on Aging, National Institutes of Health, Baltimore, MD 21224, USA..
J Exp Med. 2011 Oct 24;208(11):2209-16. doi: 10.1084/jem.20111135. Epub 2011 Oct 3.
Activation-induced deaminase (AID) deaminates cytosine to uracil in immunoglobulin genes. Uracils in DNA can be recognized by uracil DNA glycosylase and abasic endonuclease to produce single-strand breaks. The breaks are repaired either faithfully by DNA base excision repair (BER) or mutagenically to produce somatic hypermutation (SHM) and class switch recombination (CSR). To unravel the interplay between repair and mutagenesis, we decreased the level of x-ray cross-complementing 1 (XRCC1), a scaffold protein involved in BER. Mice heterozygous for XRCC1 showed a significant increase in the frequencies of SHM in Igh variable regions in Peyer's patch cells, and of double-strand breaks in the switch regions during CSR. Although the frequency of CSR was normal in Xrcc1(+/-) splenic B cells, the length of microhomology at the switch junctions decreased, suggesting that XRCC1 also participates in alternative nonhomologous end joining. Furthermore, Xrcc1(+/-) B cells had reduced Igh/c-myc translocations during CSR, supporting a role for XRCC1 in microhomology-mediated joining. Our results imply that AID-induced single-strand breaks in Igh variable and switch regions become substrates simultaneously for BER and mutagenesis pathways.
激活诱导的脱氨酶（AID）可使免疫球蛋白基因中的胞嘧啶脱氨变为尿嘧啶。DNA 中的尿嘧啶可被尿嘧啶 DNA 糖基化酶和无碱基内切酶识别，产生单链断裂。这些断裂可通过 DNA 碱基切除修复（BER）或致突变修复准确修复，产生体细胞超突变（SHM）和类别转换重组（CSR）。为了阐明修复和突变之间的相互作用，我们降低了参与 BER 的 X 射线修复交叉互补基因 1（XRCC1）的水平。XRCC1 杂合子小鼠在派尔集合淋巴结细胞的 Igh 可变区中的 SHM 频率以及 CSR 过程中开关区的双链断裂显著增加。尽管 Xrcc1(+/-) 脾 B 细胞中的 CSR 频率正常，但开关连接处的微同源性长度减少，这表明 XRCC1 也参与了替代非同源末端连接。此外，Xrcc1(+/-) B 细胞在 CSR 期间的 Igh/c-myc 易位减少，支持 XRCC1 在微同源介导连接中的作用。我们的结果表明，AID 诱导的 Igh 可变区和开关区中的单链断裂同时成为 BER 和致突变途径的底物。
