Abstract： Objective To investigate the interactive effect of mismatch repair(MMR)protein status and adverse clinicopathological features on prognosis of stage Ⅰ-Ⅲ colon cancer.Methods The retrospective cohort study was conducted.The clinicopathological data of 1 650 patients with colon cancer of stage Ⅰ-Ⅲ who were admitted to 7 hospitals in China from January 2016 to December 2017 were collected.There were 963 males and 687 females,aged 62(53,71)years.Patients were classified as 230 cases of MMR deficiency(dMMR)and 1 420 cases of MMR proficiency(pMMR)based on their MMR protein status.Observation indicators:(1)comparison of clinicopathological characteristics between patients of different MMR protein status;(2)analysis of factors affecting the survival outcomes of patients of dMMR;(3)analysis of factors affecting the survival outcomes of patients of pMMR;(4)interaction analysis of MMR and adverse clinicopathological features on survival outcomes.Measurement data with normal distribution were represented as Mean±SD,and comparison between groups was conducted using the independent t test.Measurement data with skewed distribution were represented as M(Q1,Q3),and comparison between groups was conducted using the Mann-Whitney U test.Count data were described as absolute numbers,and comparison between groups was conducted using the chi-square test or Fisher exact probability.Comparison of ordinal data was conducted using the Mann-Whitney U test.The random forest interpolation method was used for missing values in data interpolation.Univariate analysis was conducted using the COX proportional risk regression model,and multivariate analysis was conducted using the COX stepwise regression with forward method.The coefficient of multiplication interaction effect was obtained using the interaction term coefficient of COX proportional risk regression model.Evaluation of additive interaction effects was conducted using the relative excess risk due to interaction(RERI).Results(1)Comparison of clinicopathological characteristics between patients of different MMR protein status.There were significant differences in age,T staging,the number of lymph node harvest,the number of lymph node harvest＜12,high grade tumor between patients of dMMR and pMMR(P＜0.05).(2)Analysis of factors affecting the survival outcomes of patients of dMMR.Results of multivariate analysis showed that T staging,N staging,the number of lymph node harvest＜12 were independent factors affecting the disease-free survival(DFS)of colon cancer patients of dMMR(hazard ratio=3.548,2.589,6.702,95％confidence interval as 1.460-8.620,1.064-6.301,1.886-23.813,P＜0.05).Age and N staging were independent factors affecting the overall survival(OS)of colon cancer patients of dMMR(hazard ratio=1.073,10.684,95％confidence interval as 1.021-1.126,2.311-49.404,P＜0.05).(3)Analysis of factors affecting the survival outcomes of patients of pMMR.Results of multivariate analysis showed that age,T staging,N staging,vascular tumor thrombus were independent factors affecting the DFS of colon cancer patients of pMMR(hazard ratio=1.018,2.214,2.598,1.549,95％confidence interval as 1.006-1.030,1.618-3.030,1.921-3.513,1.118-2.147,P＜0.05).Age,T staging,N staging,high grade tumor were independent factors affecting the OS of colon cancer patients of pMMR(hazard ratio=1.036,2.080,2.591,1.615,95％confidence interval as 1.020-1.052,1.407-3.075,1.791-3.748,1.114-2.341,P＜0.05).(4)Interaction analysis of MMRand adverse clinicopathological features on survival outcomes.Results of interaction analysis showed that the multiplication interaction effect between the number of lymph node harvest＜12 and MMR protein status was significant on DFS of colon cancer patients(hazard ratio=3.923,95％confidence interval as 1.057-14.555,P＜0.05).The additive interaction effects between age and MMR protein status,between high grade tumor and MMR protein status were significant on OS of colon cancer patients(RERI=-0.033,-1.304,95％confidence interval as-0.049 to-0.018,-2.462 to-0.146).Conclusions There is an interaction between the MMR protein status and the adverse clinicopathological features(the number of lymph node harvest＜12,high grade tumor)on prognosis of colon cancer patients of stage Ⅰ-Ⅲ.In patients of dMMR,the number of lymph node harvest＜12 has a stronger predictive effect on poor prognosis.In patients of pMMR,the high grade tumor has a stronger predictive effect on poor prognosis.