Standardization of DNA amount for bisulfite conversion for analyzing the methylation status of LINE-1 in lung cancer

Highly methylated Long Interspersed Nucleotide Elements 1 (LINE-1) constitute approximately 20% of the human genome, thus serving as a surrogate marker of global genomic DNA methylation. To date, there is still lacking a consensus about the precise location in LINE-1 promoter and its methylation threshold value, making challenging the use of LINE-1 methylation as a diagnostic, prognostic markers in cancer. This study reports on a technical standardization of bisulfite-based DNA methylation analysis, which ensures the complete bisulfite conversion of repeated LINE-1 sequences, thus allowing accurate LINE-1 methylation value. In addition, the study also indicated the precise location in LINE-1 promoter of which significant variance in methylation level makes LINE-1 methylation as a potential diagnostic biomarker for lung cancer. A serial concentration of 5-50-500 ng of DNA from 275 formalin-fixed paraffin-embedded lung tissues were converted by bisulfite; methylation level of two local regions (at nucleotide position 300-368 as LINE-1.1 and 368-460 as LINE-1.2) in LINE-1 promoter was measured by real time PCR. The use of 5 ng of genomic DNA but no more allowed to detect LINE-1 hypomethylation in lung cancer tissue (14.34% versus 16.69% in non-cancerous lung diseases for LINE-1.1, p < 0.0001, and 30.28% versus 32.35% for LINE-1.2, p < 0.05). Our study thus highlighted the optimal and primordial concentration less than 5 ng of genomic DNA guarantees the complete LINE-1 bisulfite conversion, and significant variance in methylation level of the LINE-1 sequence position from 300 to 368 allowed to discriminate lung cancer from non-cancer samples.