The prognostic and predictive value of mismatch repair status in patients with locally advanced rectal cancer following neoadjuvant therapy

Background: We examined the predictive value of mismatch repair (MMR) status in relation to responses to neoadjuvant therapy and the prognosis of locally advanced rectal cancer patients. Methods: A total of 854 consecutive patients with locally advanced rectal cancer with MMR status who underwent neoadjuvant therapy followed by curative surgery between January 2013 and December 2018 were included this retrospective study. MMR status was determined by an analysis of MMR protein expression by immunohistochemistry (IHC). Propensity score matching was performed to reduce imbalances in baseline characteristics. The categorical variables were compared using the Chi-square test or Fisher's exact test. Local recurrence-free survival (LRFS) and disease-free survival (DFS) curves were estimated using the Kaplan-Meier method. Results: Deficient MMR (dMMR) was detected in 63 of the 854 (7.4%) patients. Patients with dMMR had a lower proportion of tumor regression grade (TRG) of 0-1 compared with proficient MMR (pMMR) (28.6% vs. 43.7%; P=0.027). After propensity score matching at 1:4 for patients who received chemotherapy alone, proportion of TRG of 0-1 was observed to be significantly lower in dMMR than pMMR patients (9.1 vs. 30.3%%; P=0.013). For patients who received chemoradiation, after matching, no significant difference in the proportion of TRG 0-1 and the pathological complete response (pCR) rate was observed. The multivariable analysis revealed that patients whose tumors had dMMR had significantly longer DFS than those whose tumors had pMMR [hazards ratio (HR) =0.38, 95% confidence interval (CI): 0.18-0.81, P=0.013]. In the subgroup analysis, dMMR was only a statistically significant prognostic factor for DFS in patients with ypStage II/III (HR =0.38, 95% CI: 0.17-0.86; P=0.020). Conclusions: We found that patients with dMMR responded worse to chemotherapy alone than patients with pMMR, in terms of TRG. Also, dMMR is a good prognostic marker for DFS in patients with ypStage II/III after neoadjuvant therapy.