ARTICLE ABSTRACTPurpose: WRN promoter CpG island hypermethylation in colorectal cancer has been reported to increase sensitivity to irinotecan-based therapies. We aimed to characterize methylation of the WRN promoter, determine the effect of WRN promoter hypermethylation upon expression, and validate a previous report that WRN promoter hypermethylation predicts improved outcomes for patients with metastatic colorectal cancer (mCRC) treated with irinotecan-based therapy.Experimental Design: WRN methylation status was assessed using methylation-specific PCR and bisulfite sequencing assays. WRN expression was determined using qRT-PCR and Western blotting. WRN methylation status was correlated with overall survival (OS) and progression-free survival (PFS) in 183 patients with mCRC. Among these patients, 90 received capecitabine monotherapy as first-line therapy, and 93 received capecitabine plus irinotecan (CAPIRI) therapy as part of the CAIRO phase III clinical trial.Results: WRN mRNA and WRN protein expression levels were low in colorectal cancer cell lines and in primary colorectal cancer and were largely independent of WRN methylation status. Patients with methylated WRN colorectal cancer had a shorter OS compared with patients who had unmethylated WRN colorectal cancer (HR = 1.6; 95% confidence interval [CI], 1.2–2.2; P = 0.003). Patients with unmethylated WRN showed a significantly longer PFS when treated with CAPIRI compared with capecitabine alone (HR = 0.48; 95% CI, 0.32–0.70; P = 0.0001). In contrast, patients did not benefit from adding irinotecan to capecitabine when WRN was methylated (HR = 1.1; 95% CI, 0.69–1.77; P = 0.7).Conclusions:WRN expression is largely independent of WRN promoter hypermethylation in colorectal cancer. Moreover, we could not validate the previous finding that WRN promoter hypermethylation predicts improved clinical outcomes of mCRC treated with irinotecan-based therapy and found instead the opposite result. Clin Cancer Res; 22(18); 4612–22. ©2016 AACR.