Tumors of Lynch syndrome (LS) patients display high levels of microsatellite instability (MSI), which results from complete loss of DNA mismatch repair (MMR), in line with Knudson’s two-hit hypothesis. Why some organs, in particular those of the gastrointestinal (GI) tract, are prone to tumorigenesis in LS remains unknown. We hypothesized that MMR is haploinsufficient in certain tissues, compromising microsatellite stability in a tissue-specific manner before tumorigenesis. Using mouse genetics, we tested how levels of MLH1, a central MMR protein, affect age- and tissue-specific microsatellite stability in vivo and whether elevated MSI is detectable prior to loss of MMR function and to neoplastic growth.

To assess putative tissue-specific MMR haploinsufficiency, we determined relevant molecular phenotypes (MSI, Mlh1 promoter methylation status, MLH1 protein and RNA levels) in jejuna of Mlh1^+/− mice and compared them to those in spleen, as well as to MMR-proficient and -deficient controls (Mlh1^+/+ and Mlh1^−/− mice). While spleen MLH1 levels of Mlh1^+/− mice were, as expected, approximately 50 % compared to wildtype mice, MLH1 levels in jejunum varied substantially between individual Mlh1^+/− mice and moreover, decreased with age. Mlh1^+/− mice with soma-wide Mlh1 promoter methylation often displayed severe MLH1 depletion in jejunum. Reduced (but still detectable) MLH1 levels correlated with elevated MSI in Mlh1^+/− jejunum. MSI in jejunum increased with age, while in spleens of the same mice, MLH1 levels and microsatellites remained stable. Thus, MLH1 expression levels are particularly labile in intestine of Mlh1^+/− mice, giving rise to tissue-specific MSI long before neoplasia. A similar mechanism likely also operates also in the human GI epithelium and could explain the wide range in age-of-onset of LS-associated tumorigenesis.

Lynch 综合征 (LS) 患者的肿瘤显示出高水平的微卫星不稳定性 (MSI)，这是由于 DNA 错配修复 (MMR) 完全丧失所致，这与 Knudson 的两次打击假说一致。为什么某些器官，特别是胃肠 (GI) 道的器官，在 LS 中容易发生肿瘤仍然未知。我们假设 MMR 在某些组织中是单倍不足的，在肿瘤发生之前以组织特异性方式损害微卫星稳定性。使用小鼠遗传学，我们测试了 MLH1（一种中心 MMR 蛋白）的水平如何影响体内年龄和组织特异性微卫星稳定性，以及在 MMR 功能丧失和肿瘤生长之前是否可检测到升高的 MSI。

为了评估假定的组织特异性 MMR 单倍体不足，我们确定了Mlh1 ^+/-小鼠空肠中的相关分子表型（MSI、Mlh1启动子甲基化状态、MLH1 蛋白和 RNA 水平），并将它们与脾脏以及 MMR-精通和缺陷对照（Mlh1 ^+/+和Mlh1 ^-/-小鼠）。而脾MLH1水平MLH1 ^{+ / -}小鼠，如所预期，约50％相比，野生型小鼠，在空肠MLH1水平个体之间基本上改变MLH1 ^{+ / -}小鼠中，而且，随着年龄减少。MLH1 ^{+ / -}具有全体细胞Mlh1启动子甲基化的小鼠通常在空肠中表现出严重的 MLH1 耗竭。MLH1 水平降低（但仍可检测）与Mlh1 ^+/-空肠中升高的 MSI 相关。空肠中的 MSI 随着年龄增长而增加，而在相同小鼠的脾脏中，MLH1 水平和微卫星保持稳定。因此，MLH1表达水平是特别不稳定的在肠MLH1 ^{+ / -}小鼠中，瘤形成之前长引起组织特异性MSI。类似的机制也可能在人 GI 上皮中起作用，并且可以解释 LS 相关肿瘤发生的发病年龄范围广泛。