PO-330 Investigating tissue- and age-specific microsatellite instability in mice with different levels of MLH1, a central mismatch repair protein

Abstract

IntroductionLynch syndrome (LS) is an inherited condition in which individuals harbour germline mutations in one or more of the DNA mismatch repair (MMR) genes, typically MLH1 or MSH2, predisposing them to cancers, in particular non-polyposis colorectal cancer. Tumours in LS are characterised by high microsatellite instability (MSI), which reflects defective MMR. MSI in colorectal cancer results from epigenetic inactivation or loss of heterozygosity (LOH) of the wild-type MMR allele, leading to absence of MMR proteins. However, some evidence suggests haploinsufficiency of MMR proteins, i.e. heterozygosity may lead to loss of genome integrity. For instance, Mlh1 +/-mice display increased tumour incidence and individuals with LS show subtle MSI in their peripheral blood leukocytes.Our aim is to understand how MMR protein levels affect in vivo tandem DNA repeat instability. A key outstanding question is why certain tissues, in particular the intestine and colon in humans, are especially prone to MSI. We hypothesise that normal levels of MMR proteins are particularly critical for genome stability in these tissues.Material and methodsWe are using mouse genetics to find out how the levels of MMR proteins contribute to in vivo tandem DNA repeat instability. We are analysing tissue- and age- specific MSI in mice expressing different levels of MMR proteins. We are using single molecule PCR followed by capillary electrophoresis to investigate microsatellite instability. Data is being analysed using Fragman R package. Our group holds a National Animal Experiment Board license to perform animal experiments.Results and discussionsOur preliminary results in old Mlh1 +/-mice show a very subtle but detectable MSI increase in small intestine compared to Mlh1+/+mice, the observed difference being predominantly due to an increase in deletion-mutant alleles.ConclusionOur result show subtle increase in MSI in macroscopically normal intestine of Mlh1 +/-mice, which indicates inactivation of just one Mlh1 allele is sufficient to induce genome instability.