Sequences and structures at hepadnaviral integration: recombination sites implicate topoisomerase I in hepadnaviral DNA rearrangements and integration.

We have previously shown that eukaryotic topoisomerase I (topo I) can mediate hepadnaviral integration in vitro. To investigate further the possible in vivo significance of topo I in hepadnaviral integration and to detect additional important factors, we have generated an extensive compilation of hepadnaviral recombination sites from chronically infected liver tissues and hepatocellular carcinomas. These sequences were subjected to various established sequence and structural analyses. Our investigation provides evidence that topo I can mediate hepadnaviral integration and rearrangement in vivo. During integration, free ends can be exposed to other nuclear enzymes, resulting in the addition of 'filler DNA'. In other cases, junctional homologies between viral and cellular DNA may facilitate integration. Structural analysis suggests that torsional stress may act on the cellular target sites, possibly promoting the integration process. A mechanism is proposed by which hepadnavirus integration into the host chromosomes is primarily mediated by topo I.