Prognostic and Predictive Roles of High-Degree Microsatellite Instability in Colon Cancer: A National Cancer Institute–National Surgical Adjuvant Breast and Bowel Project Collaborative Study

PURPOSE: The role of high-degree microsatellite instability (MSI-H) as a marker to predict benefit from adjuvant chemotherapy remains unclear. PATIENTS AND METHODS: To help define its impact, we conducted an analysis of National Surgical Adjuvant Breast and Bowel Project (NSABP) patients who were randomly assigned to a surgery-alone group (untreated cohort) and patients assigned to an adjuvant fluorouracil (FU) -treated group (treated cohort). MSI-H and other potential markers were assessed (TGF-BRII, p53, thymidylate synthase, and Ki67). RESULTS: In all, 98 (18.1%) of 542 patients exhibited MSI-H, and there was a strong inverse relationship between MSI-H and mutant p53 status (P < .001). The prognostic analyses showed increased recurrence-free survival (RFS) for MSI-H patients versus MSS/MSI-L patients (P = .10), but showed no difference in overall survival (OS; P = .67). There was a potential interaction between MSI-H and mutant p53 in terms of improved RFS (P = .03). In the predictive marker analysis, we observed no interaction between MSI status and treatment for either RFS (P = .68) or OS (P = .62). Hazard ratios (HR) for RFS for MSI-H versus MSS/MSI-L patients were 0.77 (95% CI, 0.40 to 1.48) in the untreated-patients group and 0.60 (95% CI, 0.30 to 1.19) in the treated-patients group. HRs for OS were 0.82 (95% CI, 0.44 to 1.51) and 1.02 (95% CI, 0.56 to 1.85) for the respective groups. There was a trend toward improved RFS in patients with MSI-H and mutant p53. CONCLUSION: These results do not support the use of MSI-H as a predictive marker of chemotherapy benefit.