Thymic epithelia amplify noise in chromatin accessibility via p53 repression to impose immune tolerance [RNA-seq]
Phenotypic plasticity of somatic cells is a principal feature of vertebrate adaptation as well as a hallmark of tumorigenesis. However, the determinants and mechanisms that regulate the stability of somatic cell identities remain unclear. Here, using the somatic plasticity of thymic epithelial cells – which facilitates the selection of a self-discriminating T cell repertoire – as a physiological model system, we show that stochastic fluctuations in background chromatin accessibility at nucleosome-dense regions are amplified by thymic epithelial cells to ectopically express thousands of genes highly restricted to other specialized cell types. We found broad regions of inaccessible chromatin flanking tissue-specific genes become 'destabilized' during thymic epithelial maturation independently of AIRE-induced transcription of these genes, but concurrently with the repression of the tumor suppressor p53. Augmenting p53 activity in thymic epithelial cells reduced noise in chromatin accessibility at nucleosome-dense regions, inhibited ectopic expression of tissue-specific genes and caused multi-organ autoimmunity. Furthermore, we found p53-regulated fluctuations in background chromatin accessibility in lung adenocarcinoma to be associated with high plasticity states that promote tumor progression. Taken together, our findings establish p53-dependent stabilization of nucleosomal barriers to cellular reprogramming as a fulcrum of cell fate integrity that underlies critical components of immune tolerance induction and oncogenic potential. Overall design: Thymic epithelial cells were isolated using a previously published protocol (Kim & Serwold, Methods in Molecular Biology, 2019) with minor modifications. Briefly, thymi from 4-5 week-old mice were harvested and connective tissue was removed. Stromal tissue was perforated using scissors and incubated with rotation in DMEM-F12 (Sigma D6421) at room temperature for for 10min to liberate thymocytes. Remaining stromal tissue was enzymatically digested (0.5mg/mL Collagenase D (Sigma 11088858001), 0.2mg/mL DNaseI (Sigma 10104159001), 0.5mg/mL Papain (Worthington Biochemical LS003126)). Cells were stained with anti-EpCAM antibodies conjugated to APC-Cy7 and EpCAM+ cells were enriched via positive selection using magnetic anti-Cy7 beads (Miltenyi 130-091-652). Enriched fraction was stained with fluorescently conjugated antibodies for FACS or analysis.  75,000 primary mTECs were FACS-sorted directly into RULT lysis buffer (Qiagen RNEasy Kit) and total RNA was extracted using manufacturer's instructions. mRNA was enriched and RNA-seq libraries were constructed using the Illumina TruSeq Stranded mRNA Kit. Paired-end, dual-index sequencing was performed on the Illumina NovaSeq 6000 platform.