Diversity in colorectal cancer biology is associated with variable responses to 
standard chemotherapy. We aimed to identify and validate DNA hypermethylated 
genes as predictive biomarkers for irinotecan treatment of metastatic CRC 
patients. Candidate genes were selected from 389 genes involved in DNA Damage 
Repair by correlation analyses between gene methylation status and drug response 
in 32 cell lines. A large series of samples (n=818) from two phase III clinical 
trials was used to evaluate these candidate genes by correlating methylation 
status to progression-free survival after treatment with first-line single-agent 
fluorouracil (Capecitabine or 5-fluorouracil) or combination chemotherapy 
(Capecitabine or 5-fluorouracil plus irinotecan (CAPIRI/FOLFIRI)). In the 
discovery (n=185) and initial validation set (n=166), patients with methylated 
Decoy Receptor 1 (DCR1) did not benefit from CAPIRI over Capecitabine treatment 
(discovery set: HR=1.2 (95%CI 0.7-1.9, p=0.6), validation set: HR=0.9 (95%CI 
0.6-1.4, p=0.5)), whereas patients with unmethylated DCR1 did (discovery set: 
HR=0.4 (95%CI 0.3-0.6, p=0.00001), validation set: HR=0.5 (95%CI 0.3-0.7, 
p=0.0008)). These results could not be replicated in the external data set 
(n=467), where a similar effect size was found in patients with methylated and 
unmethylated DCR1 for FOLFIRI over 5FU treatment (methylated DCR1: HR=0.7 (95%CI 
0.5-0.9, p=0.01), unmethylated DCR1: HR=0.8 (95%CI 0.6-1.2, p=0.4)). In 
conclusion, DCR1 promoter hypermethylation status is a potential predictive 
biomarker for response to treatment with irinotecan, when combined with 
capecitabine. This finding could not be replicated in an external validation 
set, in which irinotecan was combined with 5FU. These results underline the 
challenge and importance of extensive clinical evaluation of candidate 
biomarkers in multiple trials.