Cancer predisposition in hereditary non-polyposis colon cancer (HNPCC) is caused by defects in DNA mismatch repair (MMR). Mismatch recognition is attributed to two heterodimeric protein complexes: MutSalpha (refs 2, 3, 4, 5), a dimer of MutS homologues MSH2 and MSH6; and MutSbeta (refs 2,7), a dimer of MSH2 and MSH3. These complexes have specific and redundant mismatch recognition capacity. Whereas MSH2 deficiency ablates the activity of both dimers, causing strong cancer predisposition in mice and men, loss of MSH3 or MSH6 (also known as GTBP) function causes a partial MMR defect. This may explain the rarity of MSH6 and absence of MSH3 germline mutations in HNPCC families. To test this, we have inactivated the mouse genes Msh3 (formerly Rep3 ) and Msh6 (formerly Gtmbp). Msh6-deficient mice were prone to cancer; most animals developed lymphomas or epithelial tumours originating from the skin and uterus but only rarely from the intestine. Msh3 deficiency did not cause cancer predisposition, but in an Msh6 -deficient background, loss of Msh3 accelerated intestinal tumorigenesis. Lymphomagenesis was not affected. Furthermore, mismatch-directed anti-recombination and sensitivity to methylating agents required Msh2 and Msh6, but not Msh3. Thus, loss of MMR functions specific to Msh2/Msh6 is sufficient for lymphoma development in mice, whereas predisposition to intestinal cancer requires loss of function of both Msh2/Msh6 and Msh2/Msh3.

Objective-Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a multi-system disorder characterized by progressive cyst formation in the kidneys. Serious complications of ADPKD are intracranial and aortic aneurysms. The condition is mainly caused by mutations in the PKD1 or PKD2 gene. We have carefully analyzed vascular remodeling in hypomorphic Pkd1 nl/nL mouse model with dissecting aneurysms in the aorta. Methods and Results-Quantitative real-time polymerase chain reaction revealed that in the aorta the expression of normal Pkd1 is reduced to approximately 26%. Using (immuno)histochemistry we have characterized the pathogenetic sequence for dissecting aneurysm formation. The aorta shows regions with accumulation of matrix components between the elastin lamellae. This is followed by increased numbers of smooth muscle cells and locally weakening of the media.In the intima, accumulation of matrix components and detachment of endothelial cells from the elastin lamellae results in a tear. The combination of weak media and a tear in the intima leads to rupture of the vessel wall resulting in intramural bleeding. Conclusions-The Pkd1nl/nl mouse reveals that polycystin1 is implicated in maintenance of the vessel wall structural integrity, and it is a useful model for dissecting aneurysm formation studies. A utosomal Dominant Polycystic Disease (ADPKD) is one of the most common genetic diseases. ADPKD affects 1 in 1000 individuals and is characterized by progressive development of renal cysts and an increased risk of aneurysm formation. The prevalence of cerebral aneurysms in ADPKD patients is approximately 10-fold higher than in the general population and about 27% among ADPKD patients with a family history for aneurysms. 1,2 The few clinical studies done to determine the frequency of aortic aneurysms in ADPKD, suggest a prevalence varying from 1% to 10% in ADPKD patients. [3][4][5] Furthermore, a gene expression profile study revealed a decrease of PKD1 expression in dissected human aortas. 6 ADPKD has been associated with mutations in the PKD1 or PKD2 gene, encoding the proteins polycystin1 and polycystin2. Both genes are expressed in the smooth muscle cells (SMCs) and endothelial cells (ECs) of the blood vessel. 7-9 At present, however, hardly any histopathologic analyses on dissections and aneurysms in ADPKD have been performed.Several Pkd1 and Pkd2 knock-out mouse models have been generated, however knock-out embryos die at embryonic days 13.5 to 14.5 from a primary cardiovascular defect. 9 -12 These embryos show edema, focal vascular leaks, and hemorrhage. The fact that these mice die in utero withholds the possibility to study aneurysm formation in ADPKD. Recently, we have generated a Pkd1 hypomorphic mouse model, Pkd1 nl/nl , with reduced Pkd1 transcripts in the kidneys. 13 Pkd1 nl/nl mice are viable, and the majority of the animals die within 1 to 2 months after birth. These mice show dissecting aneurysms and cysts in kidneys, liver, and pancreas, which is in line with the pathogenic features found in the...

No abstract

Loss of DNA mismatch repair has been found in tumors associated with the familial cancer predisposition syndrome HNPCC (hereditary non-polyposis colorectal cancer) and a subset of sporadic cancers. MSH2 deficiency abolishes the action of the mismatch repair system, resulting in a phenotype which is characterized by an increased accumulation of base substitutions and frameshifts, enhanced recombination between homologous but non-identical DNA sequences, and tolerance to the cytotoxic effects of methylating agents. In this study we describe an embryonic stem cell line in which the level of MSH2 protein is 10-fold reduced compared to that in wild-type cells. Remarkably, these MSH2-low cells were as resistant to killing by methylating agents as cells completely lacking MSH2, while they had retained almost maximal mismatch repair capacity as judged from their anti-mutagenic and anti-recombinogenic capacity and the absence of microsatellite instability. In contrast, MSH2-low cells were highly sensitive to methylation-damage induced mutagenesis. Thus, 10-fold reduced MSH2 protein levels render cells resistant to the toxic and highly sensitive to the mutagenic effects of methylating agents. This condition is not manifested by microsatellite instability and may have implications for both the etiology and treatment of cancer.