Comment on "Chromosomal Instability and Tumors Promoted by DNA Hypomethylation" and "Induction of Tumors in Mice by Genomic Hypomethylation"

Abstract

It is clear that the data collected in (1, 2) are the result of rather extreme modeling in mice. The authors present data showing large decreases in repetitive element DNA methylation as a result of genetic manipulation, but these decreases may have limited relevance to human neoplasia and therapy. To address this issue, we first wished to ascertain whether the high degree of hypomethylation observed in the genetically manipulated mice is ever observed in human tumors. We examined the methylation of LINE-1 transposable elements and Alu elements using a quantitative COBRA (4) assay. PCR primers were designed based on the conserved Alu and LINE-1 sequences. Previous cloning and sequencing experiments have suggested that, in practice, the assay amplifies about 15,000 distinct Alu elements. Using this assay to quantitate global methylation, we found that in 19 pairs of colon cancer, the mean Alu methylation was identical to adjacent normal colon mucosa (normal: 82.5% methylation, SEM = 1.3%; cancer: 82.3% methylation, SEM = 1.0%; Fig. 1). The extent of Alu element methylation did vary in individual cases from normal to tumor, but 12 tumors showed an increase in Alu methylation (range = 1.2 to 7.4%), compared to only 7 tumors that showed a decrease (range = 2.1 to 12.0%). We did find hypomethylation of LINE-1 in colon cancer, but the mean decrease in methylation was only 7.0% (n = 23; range = 5.2 to 27.8%) compared to uninvolved adjacent mucosa. Normal colon mucosa had a mean LINE-1 methylation of 69.7% (SEM = 2.2%) while colon tumors showed 62.7% (SEM = 1.0%) LINE-1 methylation (Fig. 1).