New Insights into the Genome Organization of Yeast Double-Stranded RNA LBC Viruses

The yeasts Torulaspora delbrueckii (Td) and Saccharomyces cerevisiae (Sc) may show a killer phenotype that is encoded in dsRNA M viruses (V-M), which require the helper activity of another dsRNA virus (V-LA or V-LBC) for replication. Recently, two TdV-LBCbarr genomes, which share sequence identity with ScV-LBC counterparts, were characterized by high-throughput sequencing (HTS). They also share some similar characteristics with Sc-LA viruses. This may explain why TdV-LBCbarr has helper capability to maintain M viruses, whereas ScV-LBC does not. We here analyze two stretches with low sequence identity (LIS I and LIS II) that were found in TdV-LBCbarr Gag-Pol proteins when comparing with the homologous regions of ScV-LBC. These stretches may result from successive nucleotide insertions or deletions (indels) that allow compensatory frameshift events required to maintain specific functions of the RNA-polymerase, while modifying other functions such as the ability to bind V-M (+)RNA for packaging. The presence of an additional frameshifting site in LIS I may ensure the synthesis of a certain amount of RNA-polymerase until the new compensatory indel appears. Additional 5′- and 3′-extra sequences were found beyond V-LBC canonical genomes. Most extra sequences showed high identity to some stretches of the canonical genomes and can form stem-loop structures. Further, the 3′-extra sequence of two ScV-LBC genomes contains rRNA stretches. The origin and possible functions of these extra sequences are here discussed.

Complete genome sequence of a new quadrivirus infecting a member of the genus Thelonectria

A novel dsRNA virus named “Thelonectria quadrivirus 1” (TQV1) was found in a member of the genus Thelonectria (Ascomycota), isolated from a root associated with stem collar necrosis of Fraxinus excelsior L. The complete genome of TQV1 is composed of four segments, each containing a single ORF on the positive sense RNA. The sequence of the 5´ (5´-(C/T)ACGAAAAA-3´) and 3´termini (5´AT(T/G)AGCAATG(T/C)GC(G/A)CG-3’) of dsRNA 1 (4876 bp), dsRNA 2 (4312 bp), dsRNA 3 (4158 bp), and dsRNA 4 (3933 bp) are conserved. Based on its genome organization and phylogenetic position, TQV1 is suggested to be a new member of the family Quadriviridae. This is the first report of a mycovirus infecting a member of the genus Thelonectria.

Deciphering the Role of 3D Genome Organization in Breast Cancer Susceptibility

Cancer risk by environmental exposure is modulated by an individual’s genetics and age at exposure. This age-specific period of susceptibility is referred to as the “Window of Susceptibility” (WOS). Rats have a similar WOS for developing breast cancer. A previous study in rat identified an age-specific long-range regulatory interaction for the cancer gene, Pappa, that is associated with breast cancer susceptibility. However, the global role of three-dimensional genome organization and downstream gene expression programs in the WOS is not known. Therefore, we generated Hi-C and RNA-seq data in rat mammary epithelial cells within and outside the WOS. To systematically identify higher-order changes in 3D genome organization, we developed NE-MVNMF that combines network enhancement followed by multitask non-negative matrix factorization. We examined three-dimensional genome organization dynamics at the level of individual loops as well as higher-order domains. Differential chromatin interactions tend to be associated with differentially up-regulated genes with the WOS and recapitulate several human SNP-gene interactions associated with breast cancer susceptibility. Our approach identified genomic blocks of regions with greater overall differences in contact count between the two time points when the cluster assignments change and identified genes and pathways implicated in early carcinogenesis and cancer treatment. Our results suggest that WOS-specific changes in 3D genome organization are linked to transcriptional changes that may influence susceptibility to breast cancer.

MyoD is a 3D genome structure organizer for muscle cell identity

The genome exists as an organized, three-dimensional (3D) dynamic architecture, and each cell type has a unique 3D genome organization that determines its cell identity. An unresolved question is how cell type-specific 3D genome structures are established during development. Here, we analyzed 3D genome structures in muscle cells from mice lacking the muscle lineage transcription factor (TF), MyoD, versus wild-type mice. We show that MyoD functions as a “genome organizer” that specifies 3D genome architecture unique to muscle cell development, and that H3K27ac is insufficient for the establishment of MyoD-induced chromatin loops in muscle cells. Moreover, we present evidence that other cell lineage-specific TFs might also exert functional roles in orchestrating lineage-specific 3D genome organization during development.

Contribution of 3D genome topological domains to genetic risk of cancers: a genome-wide computational study

Background Genome-wide association studies have identified statistical associations between various diseases, including cancers, and a large number of single-nucleotide polymorphisms (SNPs). However, they provide no direct explanation of the mechanisms underlying the association. Based on the recent discovery that changes in three-dimensional genome organization may have functional consequences on gene regulation favoring diseases, we investigated systematically the genome-wide distribution of disease-associated SNPs with respect to a specific feature of 3D genome organization: topologically associating domains (TADs) and their borders. Results For each of 449 diseases, we tested whether the associated SNPs are present in TAD borders more often than observed by chance, where chance (i.e., the null model in statistical terms) corresponds to the same number of pointwise loci drawn at random either in the entire genome, or in the entire set of disease-associated SNPs listed in the GWAS catalog. Our analysis shows that a fraction of diseases displays such a preferential localization of their risk loci. Moreover, cancers are relatively more frequent among these diseases, and this predominance is generally enhanced when considering only intergenic SNPs. The structure of SNP-based diseasome networks confirms that localization of risk loci in TAD borders differs between cancers and non-cancer diseases. Furthermore, different TAD border enrichments are observed in embryonic stem cells and differentiated cells, consistent with changes in topological domains along embryogenesis and delineating their contribution to disease risk. Conclusions Our results suggest that, for certain diseases, part of the genetic risk lies in a local genetic variation affecting the genome partitioning in topologically insulated domains. Investigating this possible contribution to genetic risk is particularly relevant in cancers. This study thus opens a way of interpreting genome-wide association studies, by distinguishing two types of disease-associated SNPs: one with an effect on an individual gene, the other acting in interplay with 3D genome organization.

A spatial stochastic model reveals the role of supercoiling in transcription regulation

In Escherichia coli, translocation of RNA polymerase (RNAP) during transcription introduces supercoiling to DNA, which influences the initiation and elongation behaviors of RNAP. To quantify the role of supercoiling in transcription regulation, we develop a spatially resolved supercoiling model of transcription, describing RNAP-supercoiling interactions, topoisomerase activities, stochastic topological domain formation, and supercoiling diffusion in all transcription stages. This model establishes that transcription-induced supercoiling mediates the cooperation of co-transcribing RNAP molecules in highly expressed genes. It reveals that supercoiling transmits RNAP-accessible information through DNA and enables different RNAP molecules to communicate within and between genes. It thus predicts that a topological domain could serve as a transcription regulator, generating substantial transcription bursting and coordinating communications between adjacent genes in the domain. The model provides a quantitative platform for further theoretical and experimental investigations of how genome organization impacts transcription.

Deeply hidden genome organization directly mediated by SATB1

Mammalian genomes are organized by multi-layered chromatin folding. Whether and how three-dimensional genome organization contributes to cell-type specific transcription remains unclear. Here we uncover genome architecture formed by specialized sequences, base-unpairing regions (BURs), bound to a nuclear architectural protein, SATB1. SATB1 regulates cell-type specific transcription that underlies changes in cellular phenotypes. We developed a modified ChIP-seq protocol that stringently purifies genomic DNA only with its directly-associated proteins and unmasked previously-hidden BURs as direct SATB1 targets genome-wide. These SATB1-bound BURs are mutually exclusive from CTCF binding sites, and SATB1 is dispensable for CTCF/cohesion-mediated topologically associated domains (TADs). Instead, BURs largely overlap with lamina associated domains (LADs), and the fraction of BURs tethered to the SATB1 protein network in the nuclear interior is cell type-dependent. Our results reveal TAD-independent chromatin folding mediated by BUR sequences, which serve as genome architecture landmarks targeted by SATB1, to regulate cell-type specific gene expression.

Dynamic interplay between structural variations and 3D chromosome organization in pancreatic cancer

Structural variations (SVs) are the greatest source of variation in the genome and can lead to oncogenesis. However, the identification and interpretation of SVs in human pancreatic cancer remain largely undefined due to technological limitations. Here, we investigate the spectrum of SVs and three-dimensional (3D) chromatin architecture in human pancreatic ductal epithelial cell carcinogenesis by using state-of-the-art long-read single-molecule real-time (SMRT) and high-throughput chromosome conformation capture (Hi-C) sequencing techniques. We find that the 3D genome organization is remodeled and correlated with gene expressional change. The bulk remodeling effect of cross-boundary SVs in the 3D genome partly depends on intercellular genomic heterogeneity. Meanwhile, contact domains tend to minimize these disrupting effects of SVs within local adjacent genomic regions to maintain overall stability of 3D genome organization. Moreover, our data also demonstrates complex genomic rearrangements involving two key driver genes CDKN2A and SMAD4, and elucidates their influence on cancer-related gene expression from both linear view and 3D perspective. Overall, this study provides a valuable resource and highlights the impact, complexity and dynamicity of the interplay between SVs and 3D genome organization, which further expands our understanding of pathogenesis of SVs in human pancreatic cancer.