A cohort study of tumoral LINE-1 hypomethylation and prognosis in colon cancer

Abstract

Genome-wide DNA hypomethylation plays has an important role in genomic instability and colorectal carcinogenesis. However, the relationship between cellular DNA methylation level and patient outcome remains uncertain. Using 643 colon cancers in two independent prospective cohorts, we quantified DNA methylation in repetitive long interspersed nucleotide element-1 (LINE-1) elements using pyrosequencing, which is a good indicator of global DNA methylation level. We used Cox proportional hazard models to calculate hazard ratios (HRs) of colon cancer-specific and overall mortality, adjusting for patient and tumoral features, including CpG island methylator phenotype (CIMP). Statistical tests were two-sided. LINE-1 hypomethylation was linearly associated with a statistically significant increase in colon cancer-specific mortality (for a 30% decrease in LINE-1 methylation: multivariable HR = 2.37, 95% confidence interval [CI] = 1.42 to 3.94; P(trend) < .001) and overall mortality (multivariable HR = 1.85, 95% CI = 1.25 to 2.75; P(trend) = .002). The association was consistent across the two independent cohorts and strata of clinical and molecular characteristics, including sex, age, tumor location, stage, and CIMP, microsatellite instability, KRAS, BRAF, p53, and chromosomal instability status. In conclusion, tumoral LINE-1 hypomethylation is independently associated with shorter survival among colon cancer patients.

Figure 1

Long interspersed nucleotide element-1 (LINE-1) methylation level and patient survival in colon cancer. Colon cancer samples from 640 participants in the Health Professionals Follow-up Study and the Nurses’ Health Study were assayed for LINE-1 methylation by pyrosequencing. A) Kaplan–Meier analysis for colon cancer–specific survival (≥75% LINE-1 methylation: survival probability at 5 years = 0.85, 95% CI = 0.69 to 0.93; 60%–75% methylation: survival probability at 5 years = 0.83, 95% CI = 0.79 to 0.87; 45%–60% methylation: survival probability at 5 years = 0.74, 95% CI = 0.68 to 0.80; and <45% methylation: survival probability at 5 years = 0.54, 95% CI = 0.35 to 0.70). B) Kaplan–Meier analysis for overall survival (≥75% LINE-1 methylation: survival probability at 5 years = 0.80, 95% CI = 0.64 to 0.89; 60%–75% methylation: survival probability at 5 years = 0.77, 95% CI = 0.72 to 0.81; 45%–60% methylation: survival probability at 5 years = 0.71, 95% CI = 0.64 to 0.76; and <45% methylation: survival probability at 5 years = 0.48, 95% CI = 0.30 to 0.64). Vertical bars, 95% confidence intervals at 2-year intervals. P values (two-sided) were calculated using the log-rank test. C) Smoothing spline plot of unadjusted hazard ratios (HRs) for colon cancer–specific mortality according to LINE-1 methylation level (%). D) Smoothing spline plot of unadjusted HRs for overall mortality according to LINE-1 methylation level (%). Hatched lines, 95% confidence intervals.