Evolutionary Dynamics of Chromatin Structure and Duplicate Gene Expression in Diploid and Allopolyploid Cotton.

Polyploidy is a prominent mechanism of plant speciation and adaptation, yet the mechanistic understandings of duplicated gene regulation remain elusive. Chromatin structure dynamics are suggested to govern gene regulatory control. Here, we characterized genome-wide nucleosome organization and chromatin accessibility in allotetraploid cotton, Gossypium hirsutum (AADD, 2n = 4X = 52), relative to its two diploid parents (AA or DD genome) and their synthetic diploid hybrid (AD), using DNS-seq. The larger A-genome exhibited wider average nucleosome spacing in diploids, and this intergenomic difference diminished in the allopolyploid but not hybrid. Allopolyploidization also exhibited increased accessibility at promoters genome-wide and synchronized cis-regulatory motifs between subgenomes. A prominent cis-acting control was inferred for chromatin dynamics and demonstrated by transposable element removal from promoters. Linking accessibility to gene expression patterns, we found distinct regulatory effects for hybridization and later allopolyploid stages, including nuanced establishment of homoeolog expression bias and expression level dominance. Histone gene expression and nucleosome organization are coordinated through chromatin accessibility. Our study demonstrates the capability to track high-resolution chromatin structure dynamics and reveals their role in the evolution of cis-regulatory landscapes and duplicate gene expression in polyploids, illuminating regulatory ties to subgenomic asymmetry and dominance.

Dynamic nucleosome remodeling mediated by YY1 underlies early mouse development.

Nucleosome positioning can alter the accessibility of DNA-binding proteins to their cognate DNA elements, and thus its precise control is essential for cell identity and function. Mammalian preimplantation embryos undergo temporal changes in gene expression and cell potency, suggesting the involvement of dynamic epigenetic control during this developmental phase. However, the dynamics of nucleosome organization during early development are poorly understood. In this study, using a low-input MNase-seq method, we show that nucleosome positioning is globally obscure in zygotes but becomes well defined during subsequent development. Down-regulation of the chromatin assembly in embryonic stem cells can partially reverse nucleosome organization into a zygote-like pattern, suggesting a possible link between the chromatin assembly pathway and fuzzy nucleosomes in zygotes. We also reveal that YY1, a zinc finger-containing transcription factor expressed upon zygotic genome activation, regulates the de novo formation of well-positioned nucleosome arrays at the regulatory elements through identifying YY1-binding sites in eight-cell embryos. The YY1-binding regions acquire H3K27ac enrichment around the eight-cell and morula stages, and YY1 depletion impairs the morula-to-blastocyst transition. Thus, our study delineates the remodeling of nucleosome organization and its underlying mechanism during early mouse development.

Aberrant nucleosome organization in mouse SCNT embryos revealed by ULI-MNase-seq.

Somatic cell nuclear transfer (SCNT) can reprogram terminally differentiated somatic cells into totipotent embryos, but with multiple defects. The nucleosome positioning, as an important epigenetic regulator for gene expression, is largely unexplored during SCNT embryonic development. Here, we mapped genome-wide nucleosome profiles in mouse SCNT embryos using ultra-low-input MNase-seq (ULI-MNase-seq). We found that the nucleosome-depleted regions (NDRs) around promoters underwent dramatic reestablishment, which is consistent with the cell cycle. Dynamics of nucleosome position in SCNT embryos were delayed compared to fertilized embryos. Subsequently, we found that the aberrant gene expression levels in inner cell mass (ICM) were positively correlated with promoter NDRs in donor cells, which indicated that the memory of nucleosome occupancy in donor cells was a potential barrier for SCNT-mediated reprogramming. We further confirmed that the histone acetylation level of donor cells was associated with the memory of promoter NDRs. Our study provides insight into nucleosome reconfiguration during SCNT preimplantation embryonic development.

Loop-seq: A high-throughput technique to measure the mesoscale mechanical properties of DNA.

The local mechanical properties of the DNA polymer influence molecular processes in biology that require mechanical deformations of DNA. Lack of suitable high-throughput experimental techniques had precluded measuring how these properties might vary with sequence along the vast lengths of genomes. Here, we present a detailed protocol for a recently developed experimental technique called loop-seq, which measures at least one local mechanical property of DNA-its propensity to cyclize-in genome-scale throughput. Loop-seq has been used to obtain experimentally derived genome-wide maps of a physical property of DNA. Such measurements have revealed that diverse DNA-deforming processes involved in chromatin organization at various genomic loci are regulated by the genetically encoded, sequence-dependent variations in the mechanical properties of DNA.

NucHMM: a method for quantitative modeling of nucleosome organization identifying functional nucleosome states distinctly associated with splicing potentiality.

We develop a novel computational method, NucHMM, to identify functional nucleosome states associated with cell type-specific combinatorial histone marks and nucleosome organization features such as phasing, spacing and positioning. We test it on publicly available MNase-seq and ChIP-seq data in MCF7, H1, and IMR90 cells and identify 11 distinct functional nucleosome states. We demonstrate these nucleosome states are distinctly associated with the splicing potentiality of skipping exons. This advances our understanding of the chromatin function at the nucleosome level and offers insights into the interplay between nucleosome organization and splicing processes.

Q-Nuc: a bioinformatics pipeline for the quantitative analysis of nucleosomal profiles.

Nucleosomal profiling is an effective method to determine the positioning and occupancy of nucleosomes, which is essential to understand their roles in genomic processes. However, the positional randomness across the genome and its relationship with nucleosome occupancy remains poorly understood. Here we present a computational method that segments the profile into nucleosomal domains and quantifies their randomness and relative occupancy level. Applying this method to published data, we find on average ~ 3-fold differences in the degree of positional randomness between regions typically considered "well-ordered", as well as an unexpected predominance of only two types of domains of positional randomness in yeast cells. Further, we find that occupancy levels between domains actually differ maximally by ~ 2-3-fold in both cells, which has not been described before. We also developed a procedure by which one can estimate the sequencing depth that is required to identify nucleosomal positions even when regional positional randomness is high. Overall, we have developed a pipeline to quantitatively characterize domain-level features of nucleosome randomness and occupancy genome-wide, enabling the identification of otherwise unknown features in nucleosomal organization.

Dynamic Reorganization of Nucleosome Positioning in Somatic Cells after Transfer into Porcine Enucleated Oocytes.

The nucleosome, the fundamental structural unit of chromatin, is a critical regulator of gene expression. The mechanisms governing changes to nucleosome occupancy and positioning during somatic cell reprogramming remain poorly understood. We established a method for generating genome-wide nucleosome maps of porcine embryonic fibroblasts (PEF), reconstructed 1-cell embryos generated by somatic cell nuclear transfer (SCNT), and fertilized zygotes (FZ) using MNase sequencing with only 1,000 cells. We found that donor PEF chromatin, especially X chromosome, became more open after transfer into porcine oocytes and nucleosome occupancy decreased in promoters but increased in the genic regions. Nucleosome arrangements around transcriptional start sites of genes with different expression levels in somatic cells tended to become transcriptionally silent in SCNT; however, some pluripotency genes adopted transcriptionally active nucleosome arrangements. FZ and SCNT had similar characteristics, unlike PEF. This study reveals the dynamics and importance of nucleosome positioning and chromatin organization early after reprogramming.

Whole-exome sequencing reveals critical genes underlying metastasis in oesophageal squamous cell carcinoma.

Oesophageal squamous cell carcinoma (ESCC) is one of the most lethal cancers, owing to a high frequency of metastasis. However, little is known about the genomic landscape of metastatic ESCC. To identify the genetic alterations that underlie ESCC metastasis, whole-exome sequencing was performed for 41 primary tumours and 15 lymph nodes (LNs) with metastatic ESCCs. Eleven cases included matched primary tumours, synchronous LN metastases, and non-neoplastic mucosa. Approximately 50-76% of the mutations identified in primary tumours appeared in the synchronous LN metastases. Metastatic ESCCs harbour frequent mutations of TP53, KMT2D, ZNF750, and IRF5. Importantly, ZNF750 was recurrently mutated in metastatic ESCC. Combined analysis from current and previous genomic ESCC studies indicated more frequent ZNF750 mutation in diagnosed cases with LN metastasis than in those without metastasis (14% versus 3.4%, n = 629, P = 1.78 × 10-5 ). The Cancer Genome Atlas data further showed that ZNF750 genetic alterations were associated with early disease relapse. Previous ESCC studies have demonstrated that ZNF750 knockdown strongly promotes proliferation, migration, and invasion. Collectively, these results suggest a role for ZNF750 as a metastasis suppressor. TP53 is highly mutated in ESCC, and missense mutations are associated with poor overall survival, independently of pathological stage, suggesting that these missense mutations have important functional impacts on tumour progression, and are thus likely to be gain-of-function (GOF) mutations. Additionally, mutations of epigenetic regulators, including KMT2D, TET2, and KAT2A, and chromosomal 6p22 and 11q23 deletions of histone variants, which are important for nucleosome assembly, were detected in 80% of LN metastases. Our study highlights the important role of critical genetic events including ZNF750 mutations, TP53 putative GOF mutations and nucleosome disorganization caused by genetic lesions seen with ESCC metastasis. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.