Hepatocellular carcinoma in WHV/N-myc2 transgenic mice: oncogenic mutations of beta-catenin and synergistic effect of p53 null alleles

The intronless N-myc2 gene was originally identified as the major target of hepatitis virus insertion in woodchuck liver tumors. Here we report that t ransgenic mice carrying the N-myc2 gene controlled by woodchuck hepatitis v irus (WHV) regulatory sequences are highly predisposed to fiver cancer. In a WHV/N-myc2 transgenic line, hepatocellular carcinomas or adenomas arose i n over 70% of mice, despite barely detectable expression of the methylated transgene in liver cells, Furthermore, a transgenic founder carrying unmeth ylated transgene sequences succumbed to a large liver tumor by the age of t wo months, demonstrating the high oncogenicity of the woodchuck N-myc2 retr oposon, Stabilizing mutations or deletions of beta-catenin were found in 25 % of liver tumors and correlated with reduced tumor latency (P<0.05), confi rming the important role of beta-catenin activation in MSc-induced tnmorige nesis, The ability of the tumor suppressor gene p53 to cooperate with N-myc 2 in liver cell transformation was tested by introducing a p53-null allele into WHV/N-myc2 transgenic mice. The loss of one p53 allele in transgenic a nimals markedly accelerated the onset of liver cancer (P=0.0001), and most tumors of WHV/N-myc2 p53+/Delta mice harbored either a deletion of the wt p 53 allele or a beta-catenin mutation. These findings provide direct evidenc e that activation of N-myc2 and reduction of p53 levels act synergistically during multistage carcinogenesis in vivo and suggest that different geneti c pathways may underlie liver carcinogenesis initiated by a myc transgene.