Centro Di Endocrinologia e Oncologia; CNR, Napoli
We have previously observed that the expression of the Igf2 and H19 genes was activated in an experimental model of liver carcinogenesis, their imprinting status was conserved and about one third of the hepatocellular carcinomas analyzed had lost the maternally inherited alleles, suggesting that Igf2 and H19 or other linked imprinted genes had a role in the progression of these neoplasms. We have further analyzed the activation of Igf2 and H19 in the course of liver carcinogenesis by means of in situ hybridization procedure and observed that their expression patterns partially coincided. Both Igf2 and H19 were activated in the larger and more advanced neoplastic lesions, while H19 was also expressed in less defined areas outside the nodules, suggesting that the two genes differed in their kinetics of activation during carcinogenesis. By using a mouse line carrying the deletion of an endodermal enhancer located 3' of the H19 gene (kindly provided by Dr. S. Tilghman), we showed that this cis-acting element is required on the paternal chromosome for the activation of the Igf2 gene during liver carcinogenesis and that its deletion causes a significant delay in the development of the hepatocellular carcinomas. We have also observed that the deletion of the structural H19 gene and 10 Kb of its 5' flanking, but not that of only the transcribed sequence (mice kindly provided by Drs. S. Tilghman and L. Dandolo), weakened about 10-fold the activation of the Igf2 allele in cis in the preneoplastic stages of liver carcinogenesis. This indicates the presence of an element required for Igf2 activation in the deleted region. Interestingly, this element appears to be dispensable for the same gene in the majority of the hepatocellular carcinomas developed at later stages.