Rada Cristina, Williams Gareth T, Nilsen Hilde, Barnes Deborah E, Lindahl Tomas, Neuberger Michael S
Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom.
Curr Biol. 2002 Oct 15;12(20):1748-55. doi: 10.1016/s0960-9822(02)01215-0.
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.
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.
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.
我们之前提出，激活诱导的脱氨酶（AID）在免疫球蛋白基因座内的位点将胞嘧啶脱氨为尿嘧啶会引发抗体多样化。多样化的模式（阶段1或2高突变、基因转换或类别转换重组）被认为取决于dU/dG损伤的修复模式。一种主要的修复模式涉及切除尿嘧啶，在小鼠中至少有四种不同的酶可以完成这项活动。
仅UNG尿嘧啶-DNA糖基化酶的缺陷就足以扭曲小鼠的高突变途径。在ung(-/-)动物中，dC/dG对处的突变显著偏向于转换（95%），这表明无碱基位点（可诱导颠换）的产生受到了抑制。dA/dT对处的替换模式不受影响。类别转换重组受到了显著但并非完全的抑制。
这些结果为抗体多样化的DNA脱氨模型在类别转换以及高突变方面提供了有力支持，并且在该模型的背景下表明：（i）UNG是负责处理免疫球蛋白基因座内程序性dU/dG损伤的主要小鼠DNA糖基化酶；（ii）突变的第二个（偏向dA/dT的）阶段可能是由起始dU/dG损伤的识别引发的；（iii）类别转换重组很大程度上是通过无碱基位点的形成进行的，尽管（iv）存在一条不依赖UNG的类别转换重组途径，这可能反映了另一种尿嘧啶-DNA糖基化酶的作用，但也可能由一种不同的修复途径来解释，例如一种涉及MSH2/MSH6对dU/dG损伤识别的途径。
