Carcinogen-specific mutational and epigenetic alterations in INK4A, INK4B and p53 tumour-suppressor genes drive induced senescence bypass in normal diploid mammalian cells

Abstract

Immortalization (senescence bypass) is a critical rate-limiting step in the malignant transformation of mammalian somatic cells. Human cells must breach at least two distinct senescence barriers to permit unfettered clonal evolution during cancer development: (1) stress- or oncogene-induced premature senescence (SIPS/OIS), mediated via the p16-Rb and/or ARF-p53-p21 tumour-suppressive pathways, and (2) replicative senescence triggered by telomere shortening. In contrast, because their telomerase is constitutively active, cells from small rodents possess only the SIPS/OIS barrier, and are therefore useful for studying SIPS/OIS bypass in isolation. Dermal fibroblasts from the Syrian hamster (SHD cells) are exceptionally resistant to spontaneous SIPS bypass, but it can be readily induced following exposure to a wide range of chemical and physical carcinogens. Here we show that a spectrum of carcinogen-specific mutational and epigenetic alterations involving the INK4A (p16), p53 and INK4B (p15) genes are associated with induced SIPS bypass. With ionizing radiation, immortalization is invariably accompanied by efficient biallelic deletion of the complete INK4/CDKN2 locus. In comparison, SHD cells immortalized by the powerful polycyclic hydrocarbon carcinogen benzo(a)pyrene display transversion point mutations in the DNA-binding domain of p53 coupled with INK4 alterations such as loss of expression of p15. Epimutational silencing of p16 is the primary event associated with immortalization by nickel, a human non-genotoxic carcinogen. As SIPS/OIS bypass is a prerequisite for the immortalization of normal diploid human epithelial cells, our results with the SHD model will provide a basis for delineating combinations of key molecular changes underpinning this important event in human carcinogenesis.

Epigenetic alterations induced by genotoxic occupational and environmental human chemical carcinogens: A systematic literature review

“…In the same study, IFNγ methylation was not altered, in contrast to the above-mentioned study [50]. Hypermethylation of the promoter region of p16 was evident in B a P-induced primary immortalized Syrian hamster dermal fibroblasts, accompanied by an overexpression of the gene [54]. …”

A Tox21 Approach to Altered Epigenetic Landscapes: Assessing Epigenetic Toxicity Pathways Leading to Altered Gene Expression and Oncogenic Transformation In Vitro

“…The multi-step tumourigenesis model starting with a dysregulated epigenetic progenitor cell is in keeping with observations made in cultured primary Syrian hamster dermal fibroblasts transiently exposed to benzo[ a ]pyrene or nickel chloride (a non-mutagenic carcinogen) [294]. The in vitro exposures generated immortal variants that could be selected for continued growth after senescence of the bulk culture of primary cells (at around 50 passages).…”

Nickel and Epigenetic Gene Silencing

“…Similarly, O 6 -methylguanine DNA methyltransferase (MGMT), an enzyme that repairs O 6 -methylguanine, was silenced by promoter hypermethylation in NiS-transformed human bronchial epithelial (16HBE) cells [47]. Recent studies on the Syrian hamster (SHD) cell lines immortalized by soluble nickel demonstrated that hypermathylation of CpG cluster immediately upstream of p16 exon 1 resulted in p16 gene silencing and consequent cell senescence bypass [48]. …”