ABSTRACT
Microsatellite instability (MSI) is categorized by mutation frequency: high MSI (MSI‐H), low MSI (MSI‐L), and microsatellite stable (MSS). MSI‐H tumors have a distinct immunogenic phenotype, with immunotherapies using checkpoint inhibitors already approved for the treatment of MSI‐H gastroesophageal adenocarcinoma (GEA); this is not observed for MSI‐L or MSS. Here, we tested the hypothesis that MSI‐L tumors are also a distinct phenotype and potentially immunogenic. MSI‐PCR assays (BAT25, BAT26, BAT40, D2S123, D5S346 and D17S250) were performed on 363 Epstein‐Barr virus‐negative, surgically resected esophagogastric junction (EGJ) adenocarcinoma samples. Tumors were characterized as MSI‐H (≥2 markers), MSI‐L (1 marker), or MSS (0 markers). CD8+ cell counts, PD‐L1 and HER2 expression levels, TP53 mutations, epigenetic alterations, and prognostic significance were also examined. All pathological and molecular experiments were conducted using serial, whole‐tumor sections of chemo‐naïve surgical specimens. MSI‐H and MSI‐L were assigned to 28 (7.7%) and 24 (6.6%) cases, respectively. Compared with MSS cases, MSI‐L cases had significantly higher intratumoral CD8+ cell infiltration (P=0.048) and favorable EGJ cancer‐specific survival (multivariate hazard ratio=0.35, 95% CI, 0.12–0.82; P=0.012). MSI‐L tumors were also significantly associated with TP53‐truncating mutations as compared with MSI‐H (P=0.009) and MSS (P=0.012) cases, and this trend was also observed in GEA data from The Cancer Genome Atlas (TCGA). Indel mutational burden among TCGA MSI‐L tumors was significantly higher than that of MSS tumors (P=0.016). These results suggest that MSI‐L tumors may have a distinct tumor phenotype and be potentially immunogenic in EGJ adenocarcinoma.This article is protected by copyright. All rights reserved.
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