Abstract 3231: Prospective study of genomic 5-methylcytosine and LINE-1 methylation levels of leukocyte DNA and colorectal cancer risk
Authors: Huang Wen-Yi , Purdue Mark P. , Su L. Joseph , Katki Hormuzd A. , Moore Lee E. , Yegnasubramanian Srinivasan , Berndt Sonja I. 
Published: August 2018

Abstract
Global hypomethylation in peripheral blood leukocyte DNA, measured by the reduction of genome-wide methylated cytosine (5-mC) or the long interspersed nuclear element-1 (LINE-1) level, has been associated with an increased risk of colorectal cancer, mainly in retrospective studies. Little is known as to how these global methylation markers relate in individuals and their associations with colorectal cancer risk in a prospective study setting.
We studied leukocyte 5-mC and LINE-1 methylation levels in prospectively collected blood specimens from 404 cases and 528 controls who were free of colorectal cancer at blood collection from the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Leukocyte 5-mC level was determined by an HPLC/Tandem Mass Spectrometry method and expressed as the relative amount of methyl- to total cytosine residues (%5-mC). LINE-1 methylation level was measured by McrBC digestion followed by real-time PCR of the undigested/unmethylated LINE1 promoter sequences. In our analysis, we excluded cases diagnosed within one year of blood draw to minimize the potential for reverse causation from prevalent disease. We estimated odds ratios (ORs) and 95% confidence intervals (CIs) for tertiles of methylation using multivariable logistic regression, adjusting for age, sex, and other potential confounders.
We observed a significant association between low LINE-1 methylation and colorectal cancer risk (1st vs. 3rd tertile: OR=1.63, 95% CI=1.10-2.41; P trend=0.01) and a near-significant association with low %5-mC methylation (OR=1.37, 95% CI=0.92-2.03; P trend=0.15). The two markers were not correlated (Spearman's r=0.01). Comparing subjects with low methylation levels (1st tertiles) at both markers with those having the highest methylation level (3rd tertile) at either marker, we observed a stronger significant association (OR=2.49, 95% CI=1.62-3.81; P trend=0.0001). Interestingly, we also found that the association between natural folate intake and colorectal cancer risk was modified by %5-mC or two markers combined (P interaction= 0.0003 and 0.02, respectively). Among individuals with the highest methylation level at either marker, high natural folate intake (3rd tertile) was associated with a reduced risk of colorectal cancer compared to low (1st tertile) folate intake (OR=0.49, 95% CI=0.25-0.96); no association was observed among individuals with lower levels of methylation. No significant risk modification was found for other suspected risk factors, such as smoking, obesity, and aspirin/ibuprofen use.
This prospective investigation showed that reduced LINE-1 methylation in leukocyte DNA, especially in conjunction with reduced 5-mC methylation, was associated with increased colorectal cancer risk. Folate may protect against colorectal carcinogenesis through mechanisms involving adequate DNA methylation in the genome.
Citation Format: Wen-Yi Huang, Mark P. Purdue, L. Joseph Su, Hormuzd A. Katki, Lee E. Moore, Srinivasan Yegnasubramanian, Sonja I. Berndt. Prospective study of genomic 5-methylcytosine and LINE-1 methylation levels of leukocyte DNA and colorectal cancer risk [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3231.