Coincidence cloning. Taking the coincidences out of genome analysis

Non-methylated Genomic Sites Coincidence Cloning (NGSCC): an approach to large scale analysis of hypomethylated CpG patterns at predetermined genomic loci

We have developed a new approach to the analysis of hypomethylated CpG patterns within predetermined, megabase long, genome regions. The approach, which we term Non-methylated Genomic Sites Coincidence Cloning (NGSCC), includes three main steps. First, total genomic DNA is digested with a methylation sensitive restriction endonuclease, such as HpaII or HhaI. Then the fragments corresponding to the genomic area of interest are selected. To this end the fragmented genome DNA is hybridized with a mixture of clones (BACs, cosmids etc.) representing a given region and digested with the same restriction enzyme(s). A special version of the coincidence cloning procedure was developed to make this hybridization selection highly efficient and specific. Finally, fragments of the locus under study are mapped and sequenced. The technique proved to be efficient and specific. As a test, it was applied to the analysis of hypomethylated CpG patterns along the 1-Mb D19S208-COX7A1 (Chr 19q13.12) locus, on human chromosome 19, in normal testis and in seminoma tissues. Some differences in the distribution of hypomethylated CpGs between the two tissues were demonstrated. The methylation profiles in both tissues revealed a clear trend to clustering of non-methylated sites. We also analyzed the expression of genes located within hypomethylated clusters in both tissues. It was shown that, whereas the expression of some of the genes investigated was correlated with hypomethylation of the region, other genes were expressed regardless of their methylation status. NGSCC thus promises to be a useful approach for the analysis of the role of dynamic epigenetic factors in genome function.