Genomic DNA is folded into a higher-order structure that regulates transcription and maintains genomic stability. Although progress has been made on understanding biochemical characteristics of epigenetic modifications in cancer, the in-situ higher-order folding of chromatin structure during malignant transformation remains largely unknown. Here, using optimized stochastic optical reconstruction microscopy (STORM) for pathological tissue (PathSTORM), we uncover a gradual decompaction and fragmentation of higher-order chromatin folding throughout all stages of carcinogenesis in multiple tumor types, and prior to tumor formation. Our integrated imaging, genomic, and transcriptomic analyses reveal functional consequences in enhanced transcription activities and impaired genomic stability. We also demonstrate the potential of imaging higher-order chromatin disruption to detect high-risk precursors that cannot be distinguished by conventional pathology. Taken together, our findings reveal gradual decompaction and fragmentation of higher-order chromatin structure as an enabling characteristic in early carcinogenesis to facilitate malignant transformation, which may improve cancer diagnosis, risk stratification, and prevention.

Background The FACT complex is a conserved histone chaperone with critical roles in transcription and histone deposition. FACT is essential in pluripotent and cancer cells, but otherwise dispensable for most mammalian cell types. FACT deletion or inhibition can block induction of pluripotent stem cells, yet the mechanism through which FACT regulates cell fate decisions remains unclear. Results To explore the mechanism for FACT function, we generated AID-tagged murine embryonic cell lines for FACT subunit SPT16 and paired depletion with nascent transcription and chromatin accessibility analyses. We also analyzed SPT16 occupancy using CUT&RUN and found that SPT16 localizes to both promoter and enhancer elements, with a strong overlap in binding with OCT4, SOX2, and NANOG. Over a timecourse of SPT16 depletion, nucleosomes invade new loci, including promoters, regions bound by SPT16, OCT4, SOX2, and NANOG, and TSS-distal DNaseI hypersensitive sites. Simultaneously, transcription of Pou5f1 (encoding OCT4), Sox2, Nanog, and enhancer RNAs produced from these genes’ associated enhancers are downregulated. Conclusions We propose that FACT maintains cellular pluripotency through a precise nucleosome-based regulatory mechanism for appropriate expression of both coding and non-coding transcripts associated with pluripotency.

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The mammalian FACT complex is a highly conserved histone chaperone with essential roles in transcription elongation, histone deposition, and maintenance of stem cell state. FACT is essential for viability in pluripotent cells and cancer cells, but otherwise dispensable for most mammalian cell types. FACT deletion or inhibition can block reprogramming of fibroblasts to induced pluripotent stem cells, yet the molecular mechanisms through which FACT regulates cell fate decisions remain unclear. To determine the mechanism by which FACT regulates stem cell identity, we used the auxin-inducible degron systems to deplete murine embryonic stem cells of FACT subunit SPT16 and subjected depleted cells to genome-wide factor localization, nascent transcription analyses, and genome-wide nucleosome profiling. Inducible depletion of SPT16 reveals a critical role in regulating targets of the master regulators of pluripotency: OCT4, KLF4, MYC, NANOG, and SOX2. Depletion of SPT16 leads to increased nucleosome occupancy at genomic loci occupied by these transcription factors, as well as gene-distal regulatory sites defined by DNaseI hypersensitivity. This heightened occupancy suggests a mechanism of nucleosome filling, wherein the sites typically maintained in an accessible state by FACT are occluded through loss of FACT-regulated nucleosome spacing. 20% of transcription arising from gene-distal regions bound by these factors is directly dependent on FACT, and putative gene targets of these non-coding RNAs are highly enriched for pluripotency in pathway analyses. Upon FACT depletion, transcription of Pou5f1 (OCT4), Sox2, and Nanog are downregulated, suggesting that FACT not only co-regulates expression of the encoded proteins' targets, but also the pluripotency factors themselves. We find that FACT maintains cellular pluripotency through a complex regulatory network of both coding and non-coding transcription.