Characterization of a methylation imprint in the Prader-Willi syndrome chromosome region.

In adult human tissues, a HpaII and a CfoI restriction site at the PW71 (D15S63) locus in the Prader-Willi syndrome region on chromosome 15 are methylated on the maternal chromosome, but unmethylated on the paternal chromosome. The HpaII site is part of a sequence with high homology to the long terminal repeat of human endogenous retroviruses. Another HpaII site at the PW71 locus is methylated on both chromosomes. Sperm DNA carries the adult paternal methylation pattern. Oocyte DNA could not be studied. In chorion, placenta and tumor DNA, both HpaII sites are unmethylated. These findings suggest that the PW71 methylation imprint is established in the germline and that extraembryonic tissues and tumors are hypomethylated.