MSI-low is an intermediate type between MSI-high and MSS in esophagogastric junction adenocarcinoma.

Abstract

44 Background: High level of microsatellite instability (MSI-high) is an actionable molecular status in oncology, informing tumor response to immune checkpoint inhibitor. However, little is known about MSI-low. The aim of this study is to unveil the characteristics of MSI-low tumor in esophagogastric junction (EGJ) adenocarcinoma. Methods: Using 372 cases with chemo-naive EGJ adenocarcinoma tissue, MSI testing and Epstein Barr Virus (EBV)-DNA detection were performed by DNA fragment analysis (BAT25, BAT26, BAT40, D2S123, D5S346, and D17S250) and real-time PCR, respectively. MSI-high was defined as having two or more unstable markers, MSI-low as one, and microsatellite stable (MSS) as none. MSI status was compared with clinicopathological and molecular status including epigenetic alteration in MLH-1, LINE-1, and CpG methylator phenotype (CIMP), TP53 status (sequencing and immunohistological expression), Ki-67 index, intra-/peri-tumoral lymphocyte counts (CD8+ or FOXP3+), combined positive score (CPS) of PD-L1 expression. Results: MSI-low was detected in 29, MSI-high in 28, and EBV (+) in 9 cases. Three cases with MSI-low showed EBV (+), but MSI-high and EBV (+) were mutually exclusive. In 363 EBV (-) cases, MSI-low was associated with younger age (MSI-high 71.7 ± 11.3, MSI-low 61.3 ± 12.5, and MSS, 65.0 ± 12.3 years old, P = 0.003). Notably, MSI-low exhibited an intermediate type between MSI-high and MSS, in terms of no. of lymph node metastasis, and tumor immune microenvironment [no. of intra-/peri-tumoral CD8+ cells, no. of intra-/peri-tumoral FOXP3+ cells, and CPS of PD-L1 (all P among the 3 groups < 0.05)]. In contrast to MSI-high tumors, MSI-low tumor was not associated with epigenetic alteration in any of MLH-1, LINE-1, or CIMP status. In survival analysis, 3-year disease-specific survival rate of MSI-low was better than that of MSS tumors, and similar to MSI-high (92.2% for MSI-low, 92.2% for MSI-high, and 69.6% for MSI-low). This trend was also observed in time to recurrence (P = 0.01). Conclusions: MSI-low cases exhibited an intermediate immune microenvironment between MSI-high and MSS, and favorable outcome. Our results may implicate MSI-low as a predictive biomarker for immune checkpoint inhibitor.