Activation of the N-myc2 oncogene by woodchuck hepatitis virus integrationin the linked downstream b3n locus in woodchuck hepatocellular carcinoma

In the woodchuck hepatitis Virus (WHV)/woodchuck model for hepatitis B viru s-induced hepatocellular carcinoma, frequent activation of N-myc oncogenes 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 r eported that a WHV integration cloned from a liver tumor was located in a c hromosomal locus already described by others as the site of WHV integration in another hepatocellular carcinoma. On this basis, the locus, named b3n, was defined as a recurrent site of WHV integration. A scaffold or matrix at tachment region (S/MAR) element was subsequently shown to be located in thi s locus similar to 1 kb from the WHV insertion sites. S/MARs are genetic el ements involved both in structural and functional organization of chromosom al DNA and in stimulation of gene expression. in the present work, we inves tigated the possibility that an N-myc gene might be affected by integration in b3n. Analysis of a liver tumor harboring WHV integration in this locus showed N-myc2 overexpression. By restriction analysis, the b3n locus was sh own to be located downstream of N-myc2, so the known sites of viral inserti on in b3n were similar to 11 kb downstream of the N-myc2 promoter. Although these data support that WHV insertion in 63n activates N-myc2, the mechani sms previously described to be involved in N-myc2 activation do not appear to properly account for activation in this subset of WHV integrations. Avai lable data suggest that activation of N-myc2 by WHV integration in b3n migh t he mediated by the S/MAR located near the WHV insertion. (C) 1999 Academi c Press.