Activation of the N-myc2 Oncogene by Woodchuck Hepatitis Virus Integration in the Linked Downstreamb3nLocus in Woodchuck Hepatocellular Carcinoma

Abstract

In the woodchuck hepatitis virus (WHV)/woodchuck model for hepatitis B virus-induced hepatocellular carcinoma, frequent activation of N-myconcogenes by WHV integration has been firmly established. N-myc2, the most frequently affected gene, was reported to be activated by WHV insertion either in the proximity of the gene or in a distant uncoding locus,win. We previously reported that a WHV integration cloned from a liver tumor was located in a chromosomal locus already described by others as the site of WHV integration in another hepatocellular carcinoma. On this basis, the locus, namedb3n, was defined as a recurrent site of WHV integration. A scaffold or matrix attachment region (S/MAR) element was subsequently shown to be located in this locus ∼1 kb from the WHV insertion sites. S/MARs are genetic elements involved both in structural and functional organization of chromosomal DNA and in stimulation of gene expression. In the present work, we investigated the possibility that an N-mycgene might be affected by integration inb3n. Analysis of a liver tumor harboring WHV integration in this locus showed N-myc2 overexpression. By restriction analysis, theb3nlocus was shown to be located downstream of N-myc2, so the known sites of viral insertion inb3nwere ∼11 kb downstream of the N-myc2 promoter. Although these data support that WHV insertion inb3nactivates N-myc2, the mechanisms previously described to be involved in N-myc2 activation do not appear to properly account for activation in this subset of WHV integrations. Available data suggest that activation of N-myc2 by WHV integration inb3nmight be mediated by the S/MAR located near the WHV insertion.