Tissue-specific methylation of a CpG island in transgenic mice.

Clustering of CpG dinucleotides in CpG-rich islands is a characteristic feature of mammalian genomes. Such CpG islands are frequently associated with genes and usually hypomethylated, regardless of the gene activity. This is the case for the CpG island of the murine Thy-1 gene. A transgenic line containing multiple copies of a truncated, concatemeric CpG island from the Thy-1.1 allele (Thy-1.2 background) showed that a stable fraction (approx. 0.20) became fully methylated in somatic tissues of homozygous mice with respect to testable restriction sites, while the remaining copies were methylation-free, i.e., this methylation appears to be an 'all-or-none' phenomenon. DNA from extraembryonic tissues (placenta and yolk sac) and epididymal sperm showed, however, an even higher degree of methylation in two distinct patterns. In the extraembryonic tissue, partial methylation of each copy was seen, whereas in sperm a high degree of 'all-or-none' methylation (greater than 0.35) was observed.