Application of msh3 immunohistochemistry and tetranucleotide microsatellite analysis to identify Elevated Microsatellite Alterations at Selected Tetranucleotide repeats (EMAST) in colorectal tumours

Abstract

Background and aims: Reduced function of the DNA mismatch repair enzyme MSH3 in tumour cells results in elevated microsatellite alterations at selected tetranucleotide repeats (EMAST). In contrast to microsatellite instability (MSI) caused by MLH1/PMS2/MSH2/MSH6 defects, EMAST in CRCs is a marker of poor prognosis. This study examined the prevalence of EMAST-positive CRCs in a Western Australian population and critically evaluated immunohistochemical and molecular detection methods. Methods and results: MSH3 immunostaining of 250 archival CRCs identified aberrant MSH3 expression (focal or widespread reduction of nuclear MSH3, cytoplasmic MSH3 mis-localisation) in 66% cases. Tetranucleotide MSI (EMAST) was detected in 46% cases, a proportion consistent with limited previous reports. Discordance between MSH3 immunostaining and molecular detection of tetranucleotide MSI resulted from over-detection of abnormal MSH3 immunostaining, which is focal and confounded by common tissue damage (crush, autolysis). Summary and conclusions: Aberrant MSH3 function in CRCs results from elevated inflammatory signalling, a potentially modifiable risk factor that may be targeted to reduce or prevent disease relapse. A combination of MSH3 immunostaining and molecular methods can be used to identify EMAST-positive CRCs, however further antibody development and establishment of consistent tetranucleotide panels will improve the reliability and reproducibility of its detection in routine practice.