Inactivation of imprinted genes induced by cellular stress and tumorigenesis

Cellular proliferation under stressful conditions may result in permanent genetic and epigenetic changes. Using primary mouse embryonic fibroblasts, we have completed a screening test to identify gene expression changes triggered when cells proliferate under stress. In this manner, we have discovered a novel phenomenon that consists of the rapid and coordinated silencing of genes subject to imprinting, including Cdkn1c, Igf2, H19, Ndn1, Grb10, and Meg3. This generalized silencing of imprinted genes is independent of the stress-responsive tumor suppressors p53, p19(Arf), and p16(Ink4a), and it is also independent of the oxidative culture conditions and the stress response known as "mouse embryonic fibroblast senescence". In the case of Cdkn1c and H19, their silencing is associated with unscheduled de novo methylation of the normally expressed allele at their corresponding CpG island promoters, thus resulting in biallelic methylation. Finally, we provide evidence for frequent de novo methylation of Cdkn1c in a variety of murine cancer types. Altogether, our data support the concept that silencing of imprinted genes, including methylation of Cdkn1c, constitutes an epigenetic signature of cellular stress and tumorigenesis.