Abstract:
Objective This study aimed to investigate the possibility that smoking may promote cancer development via LINE-1 hypomethylation. Material and methods The LINE-1 methylation in clinically normal oral mucosa of current smokers was compared to that of non-smokers by using combined bisulphite restriction analysis. Each LINE-1 sequence was categorised into 4 patterns depending on the methylation status and location of 2 CpG dinucleotides from 5’ to 3’; which included mCmC, uCuC, mCuC and uCmC. Of these, mC and uC represent methylated and unmethylated CpG, respectively. Results Despite there was no significant difference in the overall LINE-1 methylation level, the percentages of some methylation patterns were different. The %mCmC and %uCuC increased, while the %mCuC decreased in current smokers (p=0.002, 0.015 and <0.0001, respectively). Additionally, the lower %mCuC still persisted in persons who had stopped smoking for over 1 year (p=0.001). The %mCuC also decreased in the higher pack-year smokers (p=0.028). Interestingly, the uCuC could rise from mCuC to uCmC, while mCmC could rise from mCuC only. We further analysed expression microarrays from the airway epithelia of smokers and found that smoking-associated intragenic LINE-1 sporadically repressed or activated host genes, compared to genes that do not contain LINE-1. Conclusion The smoking paradoxically increase or decrease LINE1 methylation of certain loci. Hypomethylated LINE-1 loci induced by smoking led to the same consequences as those associated with cancer.
