Rapid and inexpensive preparation of genome-wide nucleosome footprints from model and non-model organisms

Predicting the effect of CRISPR-Cas9-based epigenome editing

Epigenetic regulation orchestrates mammalian transcription, but functional links between them remain elusive. To tackle this problem, we use epigenomic and transcriptomic data from 13 ENCODE cell types to train machine learning models to predict gene expression from histone post-translational modifications (PTMs), achieving transcriptome-wide correlations of ~ 0.70 - 0.79 for most cell types. Our models recapitulate known associations between histone PTMs and expression patterns, including predicting that acetylation of histone subunit H3 lysine residue 27 (H3K27ac) near the transcription start site (TSS) significantly increases expression levels. To validate this prediction experimentally and investigate how natural vs. engineered deposition of H3K27ac might differentially affect expression, we apply the synthetic dCas9-p300 histone acetyltransferase system to 8 genes in the HEK293T cell line and to 5 genes in the K562 cell line. Further, to facilitate model building, we perform MNase-seq to map genome-wide nucleosome occupancy levels in HEK293T. We observe that our models perform well in accurately ranking relative fold-changes among genes in response to the dCas9-p300 system; however, their ability to rank fold-changes within individual genes is noticeably diminished compared to predicting expression across cell types from their native epigenetic signatures. Our findings highlight the need for more comprehensive genome-scale epigenome editing datasets, better understanding of the actual modifications made by epigenome editing tools, and improved causal models that transfer better from endogenous cellular measurements to perturbation experiments. Together these improvements would facilitate the ability to understand and predictably control the dynamic human epigenome with consequences for human health.

Increased nuclear factor I-mediated chromatin access drives transition to androgen receptor splice variant dependence in prostate cancer

Androgen receptor (AR) splice variants, of which ARv7 is the most common, are increased in castration-resistant prostate cancer, but the extent to which they drive AR activity is unclear. We generated a subline of VCaP cells (VCaP16) that is resistant to the AR inhibitor enzalutamide (ENZ). AR activity in VCaP16 is driven by ARv7, independently of full-length AR (ARfl), and its cistrome and transcriptome mirror those of ARfl in VCaP cells. ARv7 expression increases rapidly in response to ENZ, but there is a delay in gaining chromatin binding and transcriptional activity, which is associated with increased chromatin accessibility. AR and nuclear factor I (NFI) motifs are most enriched at more accessible sites, and NFIB/X knockdown greatly diminishes ARv7 function. These findings indicate that ARv7 can drive the AR program but that its activity is dependent on adaptations that increase chromatin accessibility to enhance its intrinsically weak chromatin binding.

Osmotic disruption of chromatin induces Topoisomerase 2 activity at sites of transcriptional stress

Transcription generates superhelical stress in DNA that poses problems for genome stability, but determining when and where such stress arises within chromosomes is challenging. Here, using G1-arrested S. cerevisiae cells, and employing rapid fixation and ultra-sensitive enrichment, we utilise the physiological activity of endogenous topoisomerase 2 (Top2) as a probe of transcription-induced superhelicity. We demonstrate that Top2 activity is surprisingly uncorrelated with transcriptional activity, suggesting that superhelical stress is obscured from Top2 within chromatin in vivo. We test this idea using osmotic perturbation-a treatment that transiently destabilises chromatin in vivo-revealing that Top2 activity redistributes within sub-minute timescales into broad zones patterned by long genes, convergent gene arrays, and transposon elements-and also by acute transcriptional induction. We propose that latent superhelical stress is normally absorbed by the intrinsic topological buffering capacity of chromatin, helping to avoid spurious topoisomerase activity arising within the essential coding regions of the genome.

The histone chaperone Spt6 controls chromatin structure through its conserved N-terminal domain

The disassembly and reassembly of nucleosomes by histone chaperones is an essential activity during eukaryotic transcription elongation. This highly conserved process maintains chromatin integrity by transiently removing nucleosomes as barriers and then restoring them in the wake of transcription. While transcription elongation requires multiple histone chaperones, there is little understanding of how most of them function and why so many are required. Here, we show that the histone chaperone Spt6 acts through its acidic, intrinsically disordered N-terminal domain (NTD) to bind histones and control chromatin structure. The Spt6 NTD is essential for viability and its histone binding activity is conserved between yeast and humans. The essential nature of the Spt6 NTD can be bypassed by changes in another histone chaperone, FACT, revealing a close functional connection between the two. Our results have led to a mechanistic model for dynamic cooperation between multiple histone chaperones during transcription elongation.

The RPD3L deacetylation complex is required for facultative heterochromatin repression in Neurospora crassa

Repression of facultative heterochromatin is essential for developmental processes in numerous organisms. Methylation of histone H3 lysine 27 (H3K27) by Polycomb repressive complex 2 is a prominent feature of facultative heterochromatin in both fungi and higher eukaryotes. Although this methylation is frequently associated with silencing, the detailed mechanism of repression remains incompletely understood. We utilized a forward genetics approach to identify genes required to maintain silencing at facultative heterochromatin genes in Neurospora crassa and identified three previously uncharacterized genes that are important for silencing: sds3 (NCU01599), rlp1 (RPD3L protein 1; NCU09007), and rlp2 (RPD3L protein 2; NCU02898). We found that SDS3, RLP1, and RLP2 associate with N. crassa homologs of the Saccharomyces cerevisiae Rpd3L complex and are required for repression of a subset of H3K27-methylated genes. Deletion of these genes does not lead to loss of H3K27 methylation but increases acetylation of histone H3 lysine 14 at up-regulated genes, suggesting that RPD3L-driven deacetylation is a factor required for silencing of facultative heterochromatin in N. crassa, and perhaps in other organisms.

Increased chromatin accessibility mediated by nuclear factor I drives transition to androgen receptor splice variant dependence in castration-resistant prostate cancer

Androgen receptor (AR) splice variants, of which ARv7 is the most common, are increased in prostate cancer (PC) that develops resistance to androgen signaling inhibitor drugs, but the extent to which these variants drive AR activity, and whether they have novel functions or dependencies, remain to be determined. We generated a subline of VCaP PC cells (VCaP16) that is resistant to the AR inhibitor enzalutamide (ENZ) and found that AR activity was independent of the full-length AR (ARfl), despite its continued high-level expression, and was instead driven by ARv7. The ARv7 cistrome and transcriptome in VCaP16 cells mirrored that of the ARfl in VCaP cells, although ARv7 chromatin binding was weaker, and strong ARv7 binding sites correlated with higher affinity ARfl binding sites across multiple models and clinical samples. Notably, although ARv7 expression in VCaP cells increased rapidly in response to ENZ, there was a long lag before it gained chromatin binding and transcriptional activity. This lag was associated with an increase in chromatin accessibility, with the AR and nuclear factor I (NFI) motifs being most enriched at these more accessible sites. Moreover, the transcriptional effects of combined NFIB and NFIX knockdown versus ARv7 knockdown were highly correlated. These findings indicate that ARv7 can drive the AR program, but that its activity is dependent on adaptations that increase chromatin accessibility to enhance its intrinsically weak chromatin binding.

Skeletal muscle differentiation induces wide-ranging nucleosome repositioning in muscle gene promoters

In a previous report, we demonstrated that Cbx1, PurB and Sp3 inhibited cardiac muscle differentiation by increasing nucleosome density around cardiac muscle gene promoters. Since cardiac and skeletal muscle express many of the same proteins, we asked if Cbx1, PurB and Sp3 similarly regulated skeletal muscle differentiation. In a C2C12 model of skeletal muscle differentiation, Cbx1 and PurB knockdown increased myotube formation. In contrast, Sp3 knockdown inhibited myotube formation, suggesting that Sp3 played opposing roles in cardiac muscle and skeletal muscle differentiation. Consistent with this finding, Sp3 knockdown also inhibited various muscle-specific genes. The Cbx1, PurB and Sp3 proteins are believed to influence gene-expression in part by altering nucleosome position. Importantly, we developed a statistical approach to determine if changes in nucleosome positioning were significant and applied it to understanding the architecture of muscle-specific genes. Through this novel statistical approach, we found that during myogenic differentiation, skeletal muscle-specific genes undergo a set of unique nucleosome changes which differ significantly from those shown in commonly expressed muscle genes. While Sp3 binding was associated with nucleosome loss, there appeared no correlation with the aforementioned nucleosome changes. In summary, we have identified a novel role for Sp3 in skeletal muscle differentiation and through the application of quantifiable MNase-seq have discovered unique fingerprints of nucleosome changes for various classes of muscle genes during myogenic differentiation.

Biased eviction of variant histone H3 nucleosomes triggers biofilm growth in Candida albicans

IMPORTANCE

Candida albicans lives as a commensal in most healthy humans but can cause superficial skin infections to life-threatening systemic infections. C. albicans also forms biofilms on biotic and abiotic surfaces. Biofilm cells are difficult to treat and highly resistant to antifungals. A specific set of genes is differentially regulated in biofilm cells as compared to free-floating planktonic cells of C. albicans. In this study, we addressed how a variant histone H3VCTG, a previously identified negative regulator of biofilm formation, modulates gene expression changes. By providing compelling evidence, we show that biased eviction of H3VCTG nucleosomes at the promoters of biofilm-relevant genes facilitates the accessibility of both transcription activators and repressors to modulate gene expression. Our study is a comprehensive investigation of genome-wide nucleosome occupancy in both planktonic and biofilm states, which reveals transition to an open chromatin landscape during biofilm mode of growth in C. albicans, a medically relevant pathogen.

Simultaneous Mapping of DNA Binding and Nucleosome Positioning with SpLiT-ChEC

The organization of chromatin - including the positions of nucleosomes and the binding of other proteins to DNA - helps define transcriptional profiles in eukaryotic organisms. While techniques like ChIP-Seq and MNase-Seq can map protein-DNA and nucleosome localization separately, assays designed to simultaneously capture nucleosome positions and protein-DNA interactions can produce a detailed picture of the chromatin landscape. Most assays that monitor chromatin organization and protein binding rely on antibodies, which often exhibit nonspecific binding, and/or the addition of bulky adducts to the DNA-binding protein being studied, which can affect their expression and activity. Here, we describe SpyCatcher Linked Targeting of Chromatin Endogenous Cleavage (SpLiT-ChEC), where a 13-amino acid SpyTag peptide, appended to a protein of interest, serves as a highly-specific targeting moiety for in situ enzymatic digestion. The SpyTag/SpyCatcher system forms a covalent bond, linking the target protein and a co-expressed MNase-SpyCatcher fusion construct. SpyTagged proteins are expressed from endogenous loci, whereas MNase-SpyCatcher expression is induced immediately before harvesting cultures. MNase is activated with high concentrations of calcium, which primarily digests DNA near target protein binding sites. By sequencing the DNA fragments released by targeted MNase digestion, we found that this method recovers information on protein binding and proximal nucleosome positioning. SpLiT-ChEC provides precise temporal control that we anticipate can be used to monitor chromatin under various conditions and at distinct points in the cell cycle.

Tup1 is critical for transcriptional repression in Quiescence in S. cerevisiae

Upon glucose starvation, S. cerevisiae shows a dramatic alteration in transcription, resulting in wide-scale repression of most genes and activation of some others. This coincides with an arrest of cellular proliferation. A subset of such cells enters quiescence, a reversible non-dividing state. Here, we demonstrate that the conserved transcriptional corepressor Tup1 is critical for transcriptional repression after glucose depletion. We show that Tup1-Ssn6 binds new targets upon glucose depletion, where it remains as the cells enter the G0 phase of the cell cycle. In addition, we show that Tup1 represses a variety of glucose metabolism and transport genes. We explored how Tup1 mediated repression is accomplished and demonstrated that Tup1 coordinates with the Rpd3L complex to deacetylate H3K23. We found that Tup1 coordinates with Isw2 to affect nucleosome positions at glucose transporter HXT family genes during G0. Finally, microscopy revealed that a quarter of cells with a Tup1 deletion contain multiple DAPI puncta. Taken together, these findings demonstrate the role of Tup1 in transcriptional reprogramming in response to environmental cues leading to the quiescent state.

The ACF chromatin-remodeling complex is essential for Polycomb repression

Establishing and maintaining appropriate gene repression is critical for the health and development of multicellular organisms. Histone H3 lysine 27 (H3K27) methylation is a chromatin modification associated with repressed facultative heterochromatin, but the mechanism of this repression remains unclear. We used a forward genetic approach to identify genes involved in transcriptional silencing of H3K27-methylated chromatin in the filamentous fungus Neurospora crassa. We found that the N. crassa homologs of ISWI (NCU03875) and ACF1 (NCU00164) are required for repression of a subset of H3K27-methylated genes and that they form an ACF chromatin-remodeling complex. This ACF complex interacts with chromatin throughout the genome, yet association with facultative heterochromatin is specifically promoted by the H3K27 methyltransferase, SET-7. H3K27-methylated genes that are upregulated when iswi or acf1 are deleted show a downstream shift of the +1 nucleosome, suggesting that proper nucleosome positioning is critical for repression of facultative heterochromatin. Our findings support a direct role of the ACF complex in Polycomb repression.