Hepadnavirus integration: mechanisms of activation of the N-myc2 retrotransposon in woodchuck liver tumors.

In persistent hepadnavirus infections, a distinctive feature of woodchuck hepatitis virus (WHV) is the coupling of frequent viral integrations into myc family genes with the rapid onset of primary liver tumors. We have investigated the patterns of WHV DNA insertion into N-myc2, a newly identified retroposed oncogene, in woodchuck hepatomas resulting from either natural or experimental infections. In both cases, integrated viral sequences were preferentially associated with the N-myc2 locus. In more than 40% of the woodchuck tumors analyzed, viral insertion sites were clustered in a 3-kb region upstream of N-myc2 or in the 3' noncoding region. Insertion of WHV sequences homologous to the human hepatitis B virus enhancers, either upstream or downstream of the N-myc2 coding domain, was associated with the production of normal N-myc2 mRNA or hybrid N-myc2-WHV transcripts, initiated at the normal N-myc2 transcriptional start site. Transient-transfection assays with different N-myc2-WHV constructs in HepG2 cells demonstrated that the viral enhancers could efficiently activate the N-myc2 promoter. These results, showing that cis activation of preferred cellular targets through enhancer insertion is a common strategy for tumor induction by WHV, emphasize the previously noted similarities between hepadnaviruses and nonacute oncogenic retroviruses.