The DNA mismatch repair protein MutS alpha recognizes wrongly incorporated DNA bases and initiates their correction during DNA replication. Dysfunctions in mismatch repair lead to a predisposition... Show moreThe DNA mismatch repair protein MutS alpha recognizes wrongly incorporated DNA bases and initiates their correction during DNA replication. Dysfunctions in mismatch repair lead to a predisposition to cancer. Here, we study the homozygous mutation V63E in MSH2 that was found in the germline of a patient with suspected constitutional mismatch repair deficiency syndrome who developed colorectal cancer before the age of 30. Characterization of the mutant in mouse models, as well as slippage and repair assays, shows a mildly pathogenic phenotype. Using cryogenic electron microscopy and surface plasmon resonance, we explored the mechanistic effect of this mutation on MutS alpha function. We discovered that V63E disrupts a previously unappreciated interface between the mismatch binding domains (MBDs) of MSH2 and MSH6 and leads to reduced DNA binding. Our research identifies this interface as a 'safety lock' that ensures high-affinity DNA binding to increase replication fidelity. Our mechanistic model explains the hypomorphic phenotype of the V63E patient mutation and other variants in the MBD interface. Show less
Pieters, W.; Hugenholtz, F.; Kos, K.; Cammeraat, M.; Moliej, T.C.; Kaldenbach, D.; ... ; Riele, H. te 2022
The gut microbiota strongly impacts the development of sporadic colorectal cancer (CRC), but it is largely unknown how the microbiota affects the pathogenesis of mismatch-repair-deficient CRC in... Show moreThe gut microbiota strongly impacts the development of sporadic colorectal cancer (CRC), but it is largely unknown how the microbiota affects the pathogenesis of mismatch-repair-deficient CRC in the context of Lynch syndrome. In a mouse model for Lynch syndrome, we found a nearly complete loss of intestinal tumor development when animals were transferred from a conventional "open" animal facility to specific-pathogen-free (SPF) conditions. Using 16S sequencing we detected large changes in microbiota composition between the two facilities. Transcriptomic analyses of tumor-free intestinal tissues showed signs of strong intestinal inflammation in conventional mice. Whole exome sequencing of tumors developing in Msh2-Lynch mice revealed a much lower mutational load in the single SPF tumor than in tumors developing in conventional mice, suggesting reduced epithelial proliferation in SPF mice. Fecal microbiota transplantations with conventional feces altered the immune landscape and gut homeostasis, illustrated by increased gut length and elevated epithelial proliferation and migration. This was associated with drastic changes in microbiota composition, in particular increased relative abundances of different mucus-degrading taxa such as Desulfovibrio and Akkermansia, and increased bacterial-epithelial contact. Strikingly, transplantation of conventional microbiota increased microsatellite instability in untransformed intestinal epithelium of Msh2-Lynch mice, indicating that the composition of the microbiota influences the rate of mutagenesis in MSH2-deficient crypts. Show less
Houlleberghs, H.; Dekker, M.; Lusseveld, J.; Pieters, W.; Ravesteyn, T. van; Verhoef, S.; ... ; Riele, H. te 2020
BackgroundInactivating mutations in the MLH1 DNA mismatch repair (MMR) gene underlie 42% of Lynch syndrome (LS) cases. LS is a cancer predisposition causing early onset colorectal and endometrial... Show moreBackgroundInactivating mutations in the MLH1 DNA mismatch repair (MMR) gene underlie 42% of Lynch syndrome (LS) cases. LS is a cancer predisposition causing early onset colorectal and endometrial cancer. Nonsense and frameshift alterations unambiguously cause LS. The phenotype of missense mutations that only alter a single amino acid is often unclear. These variants of uncertain significance (VUS) hinder LS diagnosis and family screening and therefore functional tests are urgently needed. We developed a functional test for MLH1 VUS termed 'oligonucleotide-directed mutation screening' (ODMS).MethodsThe MLH1 variant was introduced by oligonucleotide-directed gene modification in mouse embryonic stem cells that were subsequently exposed to the guanine analogue 6-thioguanine to determine whether the variant abrogated MMR.ResutsIn a proof-of-principle analysis, we demonstrate that ODMS can distinguish pathogenic and non-pathogenic MLH1 variants with a sensitivity of >95% and a specificity of >91%. We subsequently applied the screen to 51 MLH1 VUS and identified 31 pathogenic variants.ConclusionODMS is a reliable tool to identify pathogenic MLH1 variants. Implementation in clinical diagnostics will improve clinical care of patients with suspected LS and their relatives. Show less
Childhood brain tumours may be due to germline bi-allelic mismatch repair gene mutations in MLH1, MSH2, MSH6 or PMS2. These mutations can also lead to colorectal neoplasia and haematological... Show moreChildhood brain tumours may be due to germline bi-allelic mismatch repair gene mutations in MLH1, MSH2, MSH6 or PMS2. These mutations can also lead to colorectal neoplasia and haematological malignancies. Here we review this syndrome and present siblings with early-onset rectal adenoma and papillary glioneural brain tumour, respectively, due to novel germline bi-allelic PMS2 mutations. Identification of mismatch repair protein defects can lead to early diagnosis of this condition. In addition, assays for these defects may help to classify brain tumours for research protocols aimed at targeted therapies. Show less
