Frequent changes in subtelomeric DNA methylation patterns and its relevance to telomere regulation during human hepatocarcinogenesis

Abstract

Subtelomeric chromatin modifications are important regulators of telomere length. We examined the subtelomeric DNA methylation status of 7q, 8q, 17q, 18p, 21q and XpYp in 32 pairs of hepatocellular carcinomas (HCCs) and their adjacent non-HCCs via methylation-specific PCR (quantified as methylation ratio). In addition, 10q was subjected to bisulfite-genomic-sequencing. Telomere length was determined by Southern hybridization. In all cases, the relationship between methylation ratio and telomere length was determined. High levels of methylation ratio were found on chromosomes 7q, 18p and XpYp, whereas 8q 17q and 21q were less methylated in both HCCs and non-HCCs. Compared to non-HCCs, HCCs exhibited a higher methylation ratio on 18p and 21q, and a wider distribution of methylation ratio on 7q, 21q and 10q (p < 0.05). The methylation ratio of 18p and of 21q was negatively and positively correlated with telomere length of HCCs, respectively (p < 0.05). We evaluated changes in methylation pattern between non-HCCs and HCCs. Out of 185 sites, hypermethylation changes from non-HCC to HCC were found at 47 sites and hypomethylation changes at 31 sites. Changes in methylation pattern were observed at three to four sites among six chromosomal sites in 15 patients (47%). There was a tendency toward hypomethylation changes at 7q (p = 0.013) and hypermethylation changes at 21q (p = 0.057) when telomere lengthened from non-HCCs to HCCs. In summary, subtelomeric methylation patterns dynamically changed during hepatocarcinogenesis. Subtelomeric methylation at certain regions was related to telomere lengthening or shortening, suggesting an association between subtelomeric chromatin structure and telomere length regulation in human hepatocarcinogenesis.