Epigenetic consequences of foreign DNA insertions: de novo methylation and global alterations of methylation patterns in recipient genomes.

The insertion of foreign DNA into mammalian or plant genomes is a frequent event in biology. My laboratory has pursued a long-standing interest in the structure of integrated adenovirus genomes and in the mechanism of foreign DNA insertions in mammalian cells. The long-term consequences of the integration of alien DNA are only partly known, and even less well understood are the mechanisms that bring them about. Evidence from viral systems has contributed to the realization that foreign DNA insertions entail a complex of sequelae that have also become apparent in non-viral systems: (i) The de novo methylation of integrated foreign DNA sequences has frequently been observed. (ii) Alterations of DNA methylation patterns in the recipient genome at and remote from the site of foreign DNA insertion have been demonstrated but it remains to be investigated how generally this phenomenon occurs. Many viral genomes find and have found entry into the genomes of present-day organisms. A major portion of mammalian genomes represents incomplete retroviral genomes that frequently have become permanently silenced by DNA methylation. It is still unknown how and to what extent the insertion of retroviral or retrotransposon sequences into established genomes has altered and shaped the methylation and transcription profiles of present day genomes. An additional reason for concern about the effects of foreign DNA integration is the fact that in all fields of molecular biology and medicine, the generation of transgenic or transgenomic cells and organisms has become a ubiquitously applied experimental technique.