Institut de Recherches Cliniques de Montréal, Division of Immunity and Viral Infections, Montréal, H2W 1R7 Québec, Canada.
Nucleic Acids Res. 2013 Mar 1;41(5):3032-46. doi: 10.1093/nar/gks1470. Epub 2013 Jan 11.
The mammalian antibody repertoire is shaped by somatic hypermutation (SHM) and class switch recombination (CSR) of the immunoglobulin (Ig) loci of B lymphocytes. SHM and CSR are triggered by non-canonical, error-prone processing of G/U mismatches generated by activation-induced deaminase (AID). In birds, AID does not trigger SHM, but it triggers Ig gene conversion (GC), a 'homeologous' recombination process involving the Ig variable region and proximal pseudogenes. Because recombination fidelity is controlled by the mismatch repair (MMR) system, we investigated whether MMR affects GC in the chicken B cell line DT40. We show here that Msh6(-/-) and Pms2(-/-) DT40 cells display cell cycle defects, including genomic re-replication. However, although IgVλ GC tracts in MMR-deficient cells were slightly longer than in normal cells, Ig GC frequency, donor choice or the number of mutations per sequence remained unaltered. The finding that the avian MMR system, unlike that of mammals, does not seem to contribute towards the processing of G/U mismatches in vitro could explain why MMR is unable to initiate Ig GC in this species, despite initiating SHM and CSR in mammalian cells. Moreover, as MMR does not counteract or govern Ig GC, we report a rare example of 'homeologous' recombination insensitive to MMR.
哺乳动物的抗体库是由 B 淋巴细胞免疫球蛋白（Ig）基因座的体细胞高频突变（SHM）和类别转换重组（CSR）形成的。SHM 和 CSR 是由激活诱导脱氨酶（AID）产生的 G/U 错配的非canonical、易错处理触发的。在鸟类中，AID 不会触发 SHM，但它会触发 Ig 基因转换（GC），这是一种涉及 Ig 可变区和近端假基因的“同源”重组过程。由于重组保真度受错配修复（MMR）系统控制，我们研究了 MMR 是否会影响鸡 B 细胞系 DT40 中的 GC。我们在这里表明，Msh6(-/-)和 Pms2(-/-) DT40 细胞显示细胞周期缺陷，包括基因组再复制。然而，尽管 MMR 缺陷细胞中的 IgVλ GC 区略长于正常细胞，但 Ig GC 频率、供体选择或每个序列的突变数量没有改变。发现禽类 MMR 系统与哺乳动物的 MMR 系统不同，在体外似乎不会促进 G/U 错配的处理，这可以解释为什么尽管在哺乳动物细胞中启动了 SHM 和 CSR，但 MMR 不能在该物种中启动 Ig GC。此外，由于 MMR 不能对抗或控制 Ig GC，我们报告了一个罕见的“同源”重组对 MMR 不敏感的例子。
