Strong Prognostic Value of Microsatellite Instability in Intestinal Type Non-cardia Gastric Cancer

BACKGROUND: The clinical role of microsatellite instability (MSI) in gastric cancer (GC) is controversial. A large series of patients submitted to respective surgery for primary GC with a long follow-up time was evaluated. METHODS: 472 patients with prospectively collected frozen samples of normal mucosa and tumor tissue stored in a biological tissue bank were included. Microsatellite analysis was evaluated using 5 quasi monomorphic mononucleotide repeats (BAT-26, BAT-25, NR-24, NR-21, and NR-27). The presence of MSI in 2 or more loci was classified as MSI-H, whereas all other cases were included in the microsatellite-stable (MSS) group. RESULTS: MSI-H phenotype was found in 111 of 472 patients (23.5%). MSI-H status was related significantly with older age, female gender, non-cardia location, WHO histotype, non-cardia Lauren intestinal type, and less advanced stages. Cancer-related 5-year survival was significantly higher in MSI-H versus MSS group (67.6% vs. 35%, p < 0.001). Stratified analysis revealed a significant impact of MSI on prognosis in non-cardia tumors of intestinal type or tubular/poorly differentiated histology, particularly in stages II and III; multivariate Cox regression analysis confirmed MSS status as a strong predictor of poor prognosis (hazard ratio 2.65, 95% CI 1.56-4.51, p < 0.001) in non-cardia intestinal type. No prognostic value of MSI in the diffuse-mixed type and signet-ring cell/mucinous histotypes was observed. CONCLUSIONS: MSI was confirmed as a significant predictor of long term outcome in a large series of GC with a long follow-up time, but the prognostic value is limited to selected histotypes of non-cardia tumors.