The H19 endodermal enhancer is required for Igf2 activation and tumor formation in experimental liver carcinogenesis

The expression of the linked but reciprocally imprinted Igf2 and H19 genes is activated in adult liver in the course of tumor development. By in situ hybridization analysis we have shown that both the Igf2 and H19 RNAs are ex pressed in the majority of the neoplastic nodules, and that hepatocellular carcinomas are developed in an experimental model of liver carcinogenesis. H19 is also highly activated in smaller and less distinct hyperplastic regi ons, The few neoplastic areas showing Igf2 but no H19 RNA display loss of t he maternally inherited allele at the Igf2/H19 locus. These data are compat ible with the existence of a common activation mechanism of these two genes during liver carcinogenesis and with a stronger H19 induction in the pre-n eoplastic lesions. By using mice carrying a deletion of the H19 endodermal enhancer, we show that this regulatory element is necessary for the activat ion of the lgf2 and H19 genes upon induction of liver carcinogenesis. Furth ermore, multiple sites of the H19 endodermal enhancer region become hyperse nsitive to DNase I when the carcinogenesis process is induced. Lastly, live r tumors developed in mice paternally inheriting the H19 enhancer deletion are found to have marked growth delays, increased frequency of apoptotic nu clei, and lack of Igf2 mRNA expression, thus indicating that this regulator y element plays a major role in the progression of liver carcinogenesis, si nce it is required for the activation of the anti-apoptotic lgf2 gene.