PO-389 Epigenomic and mutation determinants of carcinogen-driven primary epithelial cell immortalization

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IntroductionBased on cancer genome sequencing, almost 600 genes have been implicated in cancer development to date, and recently, chromatin structural changes have emerged as a key mechanism for translating exogenous cues, such as carcinogen exposures, to the deregulation of biological processes implicated in tumorigenesis. However, the functional relationship between genetic and epigenetic alterations in the context of early events of carcinogen-driven cell transformation is not well understood.Material and methodsWe employ an in vitro carcinogen exposure system using primary human mammary epithelial cells (HMEC), which utilises a biological barrier bypass to mimic early steps of cell transformation, to introduce genetic and epigenetic alterations and to investigate their functional interplay during cell immortalization. A temporal, integrated (epi)genomic analysis is applied to benzo[a]pyrene (B[a]P)-exposed, immortalised HMEC, involving whole genome sequencing (WGS), analyses of chromatin structure (ChIP-seq, ATAC-seq), and patterns of gene regulation (DNA methylome 850K array, RNA-seq).Results and discussionsWGS of sequentially derived post-stasis and immortalised clones, as well as tumorigenic clones (generated by TP53 insertional mutagenesis of post-stasis lines), identified a prominent B[a]P mutational signature across all clones. While all sequenced clones had comparable SBS counts, we observed a gradual increase of structural variants, most notably copy number variations, from post-stasis to immortal and tumorigenic clones. Reproducibly distinct methylome patterns also correlated closely with the individual phenotypic stages of cell transformation. Chromatin accessibility analysis further showed changes characteristic to individual as well as multiple exposed clones, such as in the breast cancer tumour suppressor gene BRCA2 or the promoter region of PCDH18, a gene frequently inactivated by promoter methylation in colorectal cancer. Cell type-matched histone modification and RNA expression patterns are being established to enable a thorough integrated analysis of genetic and epigenetic determinants of carcinogen-driven HMEC transformation.ConclusionWe propose that in-depth, integrated (epi)genomic analysis of this innovative model system can reveal novel molecular determinants of carcinogen-driven cellular immortalization, thus improving our understanding of cancer causes, development and progression.Funding INCa-INSERM Plan Cancer 2015 Grant.