A high degree of LINE-1 hypomethylation is a unique feature of early-onset colorectal cancer

Abstract

Objective: Early-onset colorectal cancer (CRC) represents a clinically distinct form of CRC that is often associated with a poor prognosis. Methylation levels of genomic repeats such as LINE-1 elements have been recognized as independent factors for increased cancer-related mortality. The methylation status of LINE-1 elements in early-onset CRC has not been analyzed previously.

Design: We analyzed 343 CRC tissues and 32 normal colonic mucosa samples, including 2 independent cohorts of CRC diagnosed ≤ 50 years old (n=188), a group of sporadic CRC >50 years (MSS n=89; MSI n=46), and a group of Lynch syndrome CRCs (n=20). Tumor mismatch repair protein expression, microsatellite instability status, LINE-1 and MLH1 methylation, somatic BRAF V600E mutation, and germline MUTYH mutations were evaluated.

Results: Mean LINE-1 methylation levels (± SD) in the five study groups were early-onset CRC, 56.6% (8.6); sporadic MSI, 67.1% (5.5); sporadic MSS, 65.1% (6.3); Lynch syndrome, 66.3% (4.5) and normal mucosa, 76.5% (1.5). Early-onset CRC had significantly lower LINE-1 methylation than any other group (p<0.0001). Compared to patients with <65% LINE-1 methylation in tumors, those with ≥ 65% LINE-1 methylation had significantly better overall survival (p=0.026, log rank test).

Conclusions: LINE-1 hypomethylation constitutes a potentially important feature of early-onset CRC, and suggests a distinct molecular subtype. Further studies are needed to assess the potential of LINE-1 methylation status as a prognostic biomarker for young people with CRC.

Figure 1. LINE-1 methylation analysis by bisulfite pyrosequencing in different CRC subsets.

Bisulfite pyrosequencing of LINE-1 in colorectal tissues; Normal mucosa (n = 32), early-onset CRC from Argentina (n = 116), early-onset CRC from Spain (n = 70), older onset CRC with microsatellite stability (MSS; n = 89), older onset CRC with microsatellite instability (MSI) associated with MLH1 promoter hypermethylation (n = 46) and Lynch syndrome CRCs (n = 20).The black horizontal bar indicates the mean methylation level.

Figure 2. Kaplan–Meier survival curves depicting the effect of LINE-1 methylation on 3-year overall survival in early-onset CRC patients.

Vertical tick marks indicate censored events. The green line represents survival in CRCs with LINE-1 hypomethylation <65% (n = 92) and the blue line represents LINE-1 methylation ≥65% (n = 23). p value for log rank and Fisher’s exact test are shown.