Is elevated microsatellite alterations at selected tetranucleotide repeats (EMAST)-negative/MSI-high colorectal cancer a distinct subtype of the disease?

Background and Objectives: Microsatellite instability (MSI) plays a prognostic and predictive role in colorectal cancer (CRC). Elevated microsatellite alterations at selected tetranucleotide repeats (EMAST), a novel type of MSI, was recently identified. Methods: A retrospective analysis of a prospective cohort database was performed. Patients who attempted curative surgery for MSI-high (MSI-H) CRC and had available testing results of EMAST were included for analysis. The difference in clinical characteristics, immunohistochemistry profile, and 3-year recurrence-free and overall survival between EMAST-negative and EMAST-positive tumors was measured. Results: EMAST status was successfully evaluated in 86 cases among patients who received EMAST testing, and only 16.3% (14/86) of these patients were EMAST-negative/MSI-H. Patients with EMAST-negative tumors were younger; their tumors exhibited well differentiation, less venous invasion, and greater mutS homolog 3 expression. There was no distant metastasis or cancer-specific death among EMAST-negative patients. Yet no statistically significant difference was found between the two groups in 3-year overall or recurrence-free survival. Conclusions: Patients with EMAST-negative/MSI-H CRC seem to have different clinicopathological characteristics. Future large-scale studies could clarify the role of EMAST genotype as a sub-classifier of MSI-H CRC.