Up to seven members of the histone H1 family may contribute to chromatin compaction and its regulation in human somatic cells. In breast cancer cells, knock-down of multiple H1 variants deregulates many genes, promotes the appearance of genome-wide accessibility sites and triggers an interferon response via activation of heterochromatic repeats. However, how these changes in the expression profile relate to the re-distribution of H1 variants as well as to genome conformational changes have not been yet studied. Here, we combined ChIP-seq of five endogenous H1 variants with Chromosome Conformation Capture analysis in wild-type and H1.2/H1.4 knock-down T47D cells. The results indicate that H1 variants coexist in the genome in two large groups depending on the local GC content and that their distribution is robust with respect to H1 depletion. Despite the small changes in H1 variants distribution, knock-down of H1 translated into more isolated but de-compacted chromatin structures at the scale of topologically associating domains (TADs). Such changes in TAD structure correlated with a coordinated gene expression response of their resident genes. This is the first report describing simultaneous profiling of five endogenous H1 variants and giving functional evidence of genome topology alterations upon H1 depletion in human cancer cells.

An epigenetic characterization of Giemsa bands (G bands) revealed their utility as epigenetic units to investigate the differential distribution of linker histones. GC content is a strong driver in histone H1 distribution: H1X is enriched at high GC bands and H1.2 is abundant at low GC, compacted bands. Hi‐C analysis showed that TADs with a high H1.2/H1X ratio strongly overlap with B compartment, inaccessible chromatin and low‐GC bands.

The ChIP-seq signal of histone modifications at promoters is a good predictor of gene expression in different cellular contexts, but whether this is also true at enhancers is not clear. To address this issue, we develop quantitative models to characterize the relationship of gene expression with histone modifications at enhancers or promoters. We use embryonic stem cells (ESCs), which contain a full spectrum of active and repressed (poised) enhancers, to train predictive models. As many poised enhancers in ESCs switch towards an active state during differentiation, predictive models can also be trained on poised enhancers throughout differentiation and in development. Remarkably, we determine that histone modifications at enhancers, as well as promoters, are predictive of gene expression in ESCs and throughout differentiation and development. Importantly, we demonstrate that their contribution to the predictive models varies depending on their location in enhancers or promoters. Moreover, we use a local regression (LOESS) to normalize sequencing data from different sources, which allows us to apply predictive models trained in a specific cellular context to a different one. We conclude that the relationship between gene expression and histone modifications at enhancers is universal and different from promoters. Our study provides new insight into how histone modifications relate to gene expression based on their location in enhancers or promoters.

Human somatic cells may contain up to seven members of the histone H1 family contributing to chromatin compaction and its regulation. In breast cancer cells, knock-down (KD) of each H1 variant results in specific gene expression changes. We have previously shown that combined KD of H1.2 and H1.4 (H1 KD) deregulates many genes, promotes the appearance of accessibility sites genome wide and triggers an interferon response via activation of heterochromatic repeats. Here we describe for the first time ChIP-seq profiling of five endogenous H1 variants at the same time, obtaining that H1 variants are differentially distributed at low (H1.2, H1.5, H1.0) and high (H1X, H1.4) GC content regions. Further, we report that H1 KD promotes redistribution of some of the variants and changes on genome architecture. H1 KD decreased topological associating domain (TAD) border definition and interactions, both between TADs and intra-TAD. In addition, many TADs presented a coordinated gene expression response to H1 KD. Up-regulated genes accumulate within TADs with low gene density and high H1.2 content. In conclusion, our data suggests that the equilibrium between distinct histone H1 variants helps maintaining the topological organization of the genome and the proper expression of particular gene programs.

Genome organization plays a pivotal role in transcription, but how transcription factors (TFs) rewire the structure of the genome to initiate and maintain the programs that lead to oncogenic transformation remains poorly understood. Acute promyelocytic leukemia (APL) is a fatal subtype of leukemia driven by a chromosomal translocation between the promyelocytic leukemia (PML) and retinoic acid receptor α (RARα) genes. We used primary hematopoietic stem and progenitor cells (HSPCs) and leukemic blasts that express the fusion protein PML-RARα as a paradigm to temporally dissect the dynamic changes in the epigenome, transcriptome, and genome architecture induced during oncogenic transformation. We found that PML-RARα initiates a continuum of topologic alterations, including switches from A to B compartments, transcriptional repression, loss of active histone marks, and gain of repressive histone marks. Our multiomics-integrated analysis identifies Klf4 as an early down-regulated gene in PML-RARα-driven leukemogenesis. Furthermore, we characterized the dynamic alterations in the Klf4 cis-regulatory network during APL progression and demonstrated that ectopic Klf4 overexpression can suppress self-renewal and reverse the differentiation block induced by PML-RARα. Our study provides a comprehensive in vivo temporal dissection of the epigenomic and topological reprogramming induced by an oncogenic TF and illustrates how topological architecture can be used to identify new drivers of malignant transformation.