Methylation status of long interspersed element-1 in advanced gastric cancer and its prognostic implication.

Abstract

ABSTRACT

Reportedly, pyrosequencing methylation assay might produce inconsistent results between paired snap-frozen and formalin-fixed paraffin embedded archival tissue samples. In this study, we assayed methylation level of four individual CpG sites of L1 using pyrosequencing and found that methylation level of CpG site 3 alone was not different between paired snap-frozen and formalin-fixed paraffin-embedded tissue samples. We aimed to determine whether low methylation status of L1 in CpG site 3 is associated with prognosis of gastric cancer patients. We analyzed 434 formalin-fixed paraffin-embedded tissue samples of advanced gastric cancer for their methylation status in L1 using pyrosequencing and correlated L1 methylation level with their clinicopathological features. Older age at onset, male, tumor location at antrum or lower body, intestinal type, and lymphatic or venous invasion were associated with lower methylation level of L1 in CpG site 3 (nucleotide position, 318 of X58075). Methylation level of L1 in CpG site 3 was significantly lower in microsatellite-stable and EBV-negative gastric cancers than in EBV-positive or microsatellite instability-high gastric cancers. Low methylation status of L1 correlated independently with shorter overall survival and disease-free survival time. Our findings indicate that discrepancy in the methylation level of L1 between fresh tissue and formalin-fixed paraffin-embedded tissue samples depends on CpG sites and that methylation status of L1 CpG site 3 could be utilized as prognositic parameter in advanced gastric cancer.