3D genome organization and its study in livestock breeding

Eukaryotic genomes are hierarchically packaged into cell nucleus,affecting gene regulation.The genome is organized into multiscale structural units,including chromosome territories,compartments,topologically associating domains(TADs),and DNA loops.The identification of these hierarchical structures has benefited from the development of experimental approaches,such as 3C-based methods(Hi-C,ChIA-PET,etc.),imaging tools(2D-FISH,3D-FISH,Cryo-FISH,etc.)and ligation-free methods(GAM,SPRITE,etc.).In recent two decades,numerous studies have shown that the 3D organization of genome plays essential roles in multiple cellular processes via various mechanisms,such as regulating enhancer activity and promoter-enhancer interactions.However,there are relatively few studies about the 3D genome in livestock species.Therefore,studies for exploring the function of 3D genomes in livestock are urgently needed to provide a more comprehensive understanding of potential relationships between the genome and production traits.In this review,we summarize the recent advances of 3D genomics and its biological functions in human and mouse studies,drawing inspiration to explore the 3D genomics of livestock species.We then mainly focus on the biological functions of 3D genome organization in muscle development and its implications in animal breeding.

Reorganization of 3D genome architecture across wild boar and Bama pig adipose tissues

Background:A growing body of evidence has revealed that the mammalian genome is organized into hierarchical layers that are closely correlated with and may even be causally linked with variations in gene expression.Recent studies have characterized chromatin organization in various porcine tissues and cell types and compared them among species and during the early development of pigs.However,how chromatin organization differs among pig breeds is poorly understood.Results:In this study,we investigated the 3D genome organization and performed transcriptome characterization of two adipose depots(upper layer of backfat[ULB]and greater omentum[GOM])in wild boars and Bama pigs;the latter is a typical indigenous pig in China.We found that over 95%of the A/B compartments and topologically associating domains(TADs)are stable between wild boars and Bama pigs.In contrast,more than 70%of promoterenhancer interactions(PEIs)are dynamic and widespread,involving over a thousand genes.Alterations in chromatin structure are associated with changes in the expression of genes that are involved in widespread biological functions such as basic cellular functions,endocrine function,energy metabolism and the immune response.Approximately 95%and 97%of the genes associated with reorganized A/B compartments and PEIs in the two pig breeds differed between GOM and ULB,respectively.Conclusions:We reported 3D genome organization in adipose depots from different pig breeds.In a comparison of Bama pigs and wild boar,large-scale compartments and TADs were mostly conserved,while fine-scale PEIs were extensively reorganized.The chromatin architecture in these two pig breeds was reorganized in an adipose depotspecific manner.These results contribute to determining the regulatory mechanism of phenotypic differences between Bama pigs and wild boar.

Architectural proteins for the formation and maintenance of the 3D genome

Eukaryotic genomes are densely packaged into hierarchical three-dimensional(3D) structures that contain information about gene regulation and many other biological processes. With the development of imaging and sequencing-based technologies, 3D genome studies have revealed that the high-order chromatin structure is composed of hierarchical levels, including chromosome territories, A/B compartments, topologically associated domains, and chromatin loops. However, how this chromatin architecture is formed and maintained is not completely clear. In this review, we introduce experimental methods to investigate the 3D genome, review major architectural proteins that regulate 3D chromatin organization in mammalian cells, such as CTCF(CCCTC-binding factor), cohesin, lamins, and transcription factors, and discuss relevant mechanisms such as phase separation.

Unraveling the 3D Genome Architecture in Plants:Present and Future

The eukaryotic genome has a hierarchicalthree-dimensional(3D)organization with functional implications for DNA replication,DNA repair,and transcriptional regulation.Over the past decade,scientists have endeavored to elucidate the spatial characteristics and functions of plant genome architecture using high-throughput chromatin conformation capturing technologies such as Hi-C,ChlA-PET,and HiChIP.Here,we systematically review current understanding of chromatin organization in plants at multiple scales.We also discuss the emerging opinions and concepts in 3D genome research,focusing on state-of-the-art 3D genome techniques,RNA-chromatin interactions,liquid-liquid phase separation,and dynamic chromatin alterations.We propose the application of single-cell/single-molecule multi-omics,multiway(DNA-DNA,DNA-RNA,and RNA-RNA interactions)chromatin conformation capturing methods,and proximity ligation-independent 3D genome-mapping technologies to explore chromatin organization structure and function in plants.Such methods could reveal the spatial interactions between trait-related SNPs and their target genes at various spatiotemporal resolutions,and elucidate the molecular mecha-nisms of the interactions among DNA elements,RNA molecules,and protein factors during the formation of key traits in plants.

Toward precise CRISPR DNA fragment editing and predictable 3D genome engineering

Ever since gene targeting or specific modification of genome sequences in mice was achieved in the early 1980s,the reverse genetic approach of precise editing of any genomic locus has greatly accelerated biomedical research and biotechnology development.In particular,the recent development of the CRISPR/Cas9 system has greatly expedited genetic dissection of 3D genomes.CRISPR gene-editing outcomes result from targeted genome cleavage by ectopic bacterial Cas9 nuclease followed by presumed random ligations via the host double-strand break repair machineries.Recent studies revealed,however,that the CRISPR genomeediting system is precise and predictable because of cohesive Cas9 cleavage of targeting DNA.Here,we synthesize the current understanding of CRISPR DNA fragment-editing mechanisms and recent progress in predictable outcomes from precise genetic engineering of 3D genomes.Specifically,we first briefly describe historical genetic studies leading to CRISPR and 3D genome engineering.We then summarize different types of chromosomal rearrangements by DNA fragment editing.Finally,we review significant progress from precise ID gene editing toward predictable 3D genome engineering and synthetic biology.The exciting and rapid advances in this emerging field provide new opportunities and challenges to understand or digest 3D genomes.

Integrative Analysis of Genome,3D Genome,and Transcriptome Alterations of Clinical Lung Cancer Samples

Genomic studies of cancer cell alterations,such as mutations,copy number variations(CNVs),and translocations,greatly promote our understanding of the genesis and development of cancers.However,the 3D genome architecture of cancers remains less studied due to the complexity of cancer genomes and technical difficulties.To explore the 3D genome structure in clinical lung cancer,we performed Hi-C experiments using paired normal and tumor cells harvested from patients with lung cancer,combining with RNA sequenceing analysis.We demonstrated the feasibility of studying 3D genome of clinical lung cancer samples with a small number of cells(1×10^(4)),compared the genome architecture between clinical samples and cell lines of lung cancer,and identified conserved and changed spatial chromatin structures between normal and cancer samples.We also showed that Hi-C data can be used to infer CNVs and point mutations in cancer.By integrating those different types of cancer alterations,we showed significant associations between CNVs,3D genome,and gene expression.We propose that 3D genome mediates the effects of cancer genomic alterations on gene expression through altering regulatory chromatin structures.Our study highlights the importance of analyzing 3D genomes of clinical cancer samples in addition to cancer cell lines and provides an integrative genomic analysis pipeline for future larger-scale studies in lung cancer and other cancers.

3D genome organization in the central nervous system,implications for neuropsychological disorders

Chromosomes in eukaryotic cell nuclei are highly compacted and finely organized into hierarchical threedimensional(3 D) configuration. In recent years, scientists have gained deeper understandings of 3 D genome structures and revealed novel evidence linking 3 D genome organization to various important cell events on the molecular level. Most importantly, alteration of 3 D genome architecture has emerged as an intriguing higher order mechanism that connects disease-related genetic variants in multiple heterogenous and polygenic neuropsychological disorders, delivering novel insights into the etiology. In this review, we provide a brief overview of the hierarchical structures of 3 D genome and two proposed regulatory models,loop extrusion and phase separation. We then focus on recent Hi-C data in the central nervous system and discuss 3 D genome alterations during normal brain development and in mature neurons. Most importantly,we make a comprehensive review on current knowledge and discuss the role of 3 D genome in multiple neuropsychological disorders, including schizophrenia, repeat expansion disorders, 22 q11 deletion syndrome, and others.

Evidence of constraint in the 3D genome for trans-splicing in human cells

Fusion transcripts are commonly found in eukaryotes, and many aberrant fusions are associated with severe diseases, including cancer. One class of fusion transcripts is generated by joining separate transcripts through trans-splicing. However, the mechanism of trans-splicing in mammals remains largely elusive. Here we showed evidence to support an intuitive hypothesis that attributes trans-splicing to the spatial proximity between premature transcripts. A novel trans-splicing detection tool(TSD) was developed to reliably identify intra-chromosomal trans-splicing events(i TSEs) from RNA-seq data. TSD can maintain a remarkable balance between sensitivity and accuracy, thus distinguishing it from most state-of-the-art tools. The accuracy of TSD was experimentally demonstrated by excluding potential false discovery from mosaic genome or template switching during PCR. We showed that i TSEs identified by TSD were frequently found between genomic regulatory elements, which are known to be more prone to interact with each other. Moreover, i TSE sites may be more physically adjacent to each other than random control in the tested human lymphoblastoid cell line according to Hi-C data. Our results suggest that trans-splicing and 3 D genome architecture may be coupled in mammals and that our pipeline, TSD, may facilitate investigations of trans-splicing on a systematic and accurate level previously thought impossible.

）eveloping biolmaging and quantitative methods to study 3D genome

The recent advances in chromosome configuration capture （3C）-based series molecular methods and optical super- resolution （SR） techniques offer powerful tools to investigate three dimensional （3D） genomic structure in prokaryotic and eukaryotic cell nucleus. In this review, we focus on the progress during the last decade in this exciting field. Here we at first introduce briefly genome organization at chromosome, domain and sub-domain level, respectively; then we provide a short introduction to various super-resolution microscopy techniques which can be employed to detect gcnome 3D structure. We also reviewed the progress of quantitative and visualization tools to evaluate and visualize chromatin interactions in 3D genome derived from Hi-C data. We end up with the discussion that imaging methods and 3C-based molecular methods are not mutually exclusive -- actually they arc complemental to each other and can be combined together to study 3D genome organization.

Dynamic chromatin architectures provide insights into the genetics of cattle myogenesis

Background Sharply increased beef consumption is propelling the genetic improvement projects of beef cattle in China.Three-dimensional genome structure is confirmed to be an important layer of transcription regulation.Although genome-wide interaction data of several livestock species have already been produced,the genome structure states and its regulatory rules in cattle muscle are still limited.Results Here we present the first 3D genome data in Longissimus dorsi muscle of fetal and adult cattle(Bos taurus).We showed that compartments,topologically associating domains(TADs),and loop undergo re-organization and the structure dynamics were consistent with transcriptomic divergence during muscle development.Furthermore,we annotated cis-regulatory elements in cattle genome during myogenesis and demonstrated the enrichments of promoter and enhancer in selection sweeps.We further validated the regulatory function of one HMGA2 intronic enhancer near a strong sweep region on primary bovine myoblast proliferation.Conclusions Our data provide key insights of the regulatory function of high order chromatin structure and cattle myogenic biology,which will benefit the progress of genetic improvement of beef cattle.

3D disorganization and rearrangement of genome provide insights into pathogenesis of NAFLD by integrated Hi-C, Nanopore, and RNA sequencing

The three-dimensional(3D)conformation of chromatin is integral to the precise regulation of gene expression.The 3D genome and genomic variations in non-alcoholic fatty liver disease(NAFLD)are largely unknown,despite their key roles in cellular function and physiological processes.Highthroughput chromosome conformation capture(Hi-C),Nanopore sequencing,and RNA-sequencing(RNA-seq)assays were performed on the liver of normal and NAFLD mice.A high-resolution 3D chromatin interaction map was generated to examine different 3D genome hierarchies including A/B compartments,topologically associated domains(TADs),and chromatin loops by Hi-C,and whole genome sequencing identifying structural variations(SVs)and copy number variations(CNVs)by Nanopore sequencing.We identified variations in thousands of regions across the genome with respect to 3D chromatin organization and genomic rearrangements,between normal and NAFLD mice,and revealed gene dysregulation frequently accompanied by these variations.Candidate target genes were identified in NAFLD,impacted by genetic rearrangements and spatial organization disruption.Our data provide a high-resolution 3D genome interaction resource for NAFLD investigations,revealed the relationship among genetic rearrangements,spatial organization disruption,and gene regulation,and identified candidate genes associated with these variations implicated in the pathogenesis of NAFLD.The newly findings offer insights into novel mechanisms of NAFLD pathogenesis and can provide a new conceptual framework for NAFLD therapy.

The 3D organization of genome in the nucleus: from the nucleosome to the 4D nucleome

The genetic information,stored in the linear sequences of DNA,encodes all of the genes that a living organism uses to produce proteins and RNAs essential for various cellular functions.In eukaryotes,however,genomic DNA is hierarchically and efficiently packaged into multiple-level chromatin structures within the nucleus,including the nucleosome,30 nm fibers,chromatin loops,topology associated domains(TADs).

A Key to Genome Maze in 3D

How a genome with linear length over meters is compacted into the micrometer-sized nucleus of higher eukaryotic cells, and how this compaction affects and is affected by the genome functionalities have puzzled biologists for decades. There are mainly two classes of technologies that are dedicated to the probing of genome spatial organization. Image-based analyses of three-dimensional （3D） fluorescence in situ hybridization （FISH） [1] have been widely used in genome spatial research. This technology family has contributed to many early land- mark discoveries of 3D genome （e.g., finding that chromo- somes occupy distinct non-overlapping territories in interphase [2]）, and continues to assist biologists in zooming into the genome spatial structure [3]. The other technology family is 3D genome mapping, which takes advantage of the next-generation sequencing （NGS） technology to sample the proximity ligated genome fragments [4,5] as chromatin interac- tions. Chromatin interaction analysis by paired-end tag sequencing （ChIA-PET） and high-throughput chromosome conformation capture （Hi-C） are two representative technolo- gies in this class [4,5]. ChIA-PET detects chromatin interac- tions mediated by specific protein factors, thus it is more specific and has higher resolution in comparison to Hi-C, which captures all chromatin contacts comprehensively. Since the introduction of ChIA-PET and Hi-C seven years ago [6,7], more detailed spatial genome architectures have been revealed,such as topologically associated domains （TAD） [8] and clus- tered gene promoters for transcription [9]. However, both Hi-C and ChlA-PET technologies face challenges including data noise stemming from complicated experimental protocols, exponential explosion in the sequencing depth required for higher resolution analysis, and large starting cell numbers. Some endeavors have been made to address one or some of such challenges. For instance, DNase I or micrococcal nucle- ase have been used to substitute restriction enzymes for chro- matin fragmentation, enabling higher resolution in chromatin contact mapping [10,11].

3D genomic organization in cancers

Background:The hierarchical three-dimensional(3D)architectures of chromatin play an important role in fundamental biological processes,such as cell differentiation,cellular senescence,and transcriptional regulation.Aberrant chromatin 3D structural alterations often present in human diseases and even cancers,but their underlying mechanisms remain unclear.Results:3D chromatin structures(chromatin compartment A/B,topologically associated domains,and enhancerpromoter interactions)play key roles in cancer development,metastasis,and drug resistance.Bioinformatics techniques based on machine learning and deep learning have shown great potential in the study of 3D cancer genome.Conclusion:Current advances in the study of the 3D cancer genome have expanded our understanding of the mechanisms underlying tumorigenesis and development.It will provide new insights into precise diagnosis and personalized treatment for cancers.

Many facades of CTCF unified by its coding for three-dimensional genome architecture

CCCTC-binding factor(CTCF)is a multifunctional zinc finger protein that is conserved in metazoan species.CTCF is consistently found to play an important role in many diverse biological processes.CTCF/cohesin-mediated active chromatin‘loop extrusion’architects three-dimensional(3D)genome folding.The 3D architectural role of CTCF underlies its multifarious functions,including developmental regulation of gene expression,protocadherin(Pcdh)promoter choice in the nervous system,immunoglobulin(Ig)and T-cell receptor(Tcr)V(D)J recombination in the immune system,homeobox(Hox)gene control during limb development,as well as many other aspects of biology.Here,we review the pleiotropic functions of CTCF from the perspective of its essential role in 3D genome architecture and topological promoter/enhancer selection.We envision the 3D genome as an enormous complex architecture,with tens of thousands of CTCF sites as connecting nodes and CTCF proteins as mysterious bonds that glue together genomic building parts with distinct articulation joints.In particular,we focus on the internal mechanisms by which CTCF controls higher order chromatin structures that manifest its many fa?ades of physiological and pathological functions.We also discuss the dichotomic role of CTCF sites as intriguing3D genome nodes for seemingly contradictory‘looping bridges’and‘topological insulators’to frame a beautiful magnificent house for a cell’s nuclear home.

HiC-3DViewer： a new tool to visualize Hi-C data in 3D space

Background： Although significant progress has been made to map chromatin structure at unprecedented resolution and scales, we are short of tools that enable the intuitive visualization and navigation along the three-dimensional （3D） structure of chromatins. The available tools people have so far are generally script-based or present basic features that do not easily enable the integration of genomic data along with 3D chromatin structure, hence, many scientists find themselves in the obligation to hack tools designed for other purposes such as tools for protein structure study. Methods： We present HiC-3DViewer, a new browser-based interactive tool designed to provide an intuitive environment for investigators to facilitate the 3D exploratory analysis of Hi-C data along with many useful annotation functionalities. Among the key features of HiC-3DViewer relevant to chromatin conformation studies, the most important one is the 1D-to-2D-to-3D mapping, to highlight genomic regions of interest interactively. This feature enables investigators to explore their data at different levels/angels. Additionally, investigators can superpose different genomic signals （such as ChIP-Seq, SNP） on the top of the 3D structure. Results： As a proof of principle we applied HiC-3DViewer to investigate the quality of Hi-C data and to show the spatial binding of GATA1 and GATA2 along the genome. Conclusions： As a user-friendly tool, HiC-3DViewer enables the visualization of inter/intra-chromatin interactions and gives users the flexibility to customize the look-and-feel of the 3D structure with a simple click. HiC-3DViewer is implemented in Javascript and Python, and is freely available at： http：//bioinfo.au.tsinghua.edu.cn/member/nadhir/ HiC3DViewer/. Supplementary information （User Manual, demo data） is also available at this website.

Computational inference of physical spatia organization of eukaryotic genomes

Chromosomes are packed in the cell＇s nucleus, and chromosomal conformation is critical to nearly all intranuclear biological reactions, including gene transcription and DNA replication. Nevertheless, chromosomal conformation is largely a mystery in terms of its formation and the regulatory machinery that accesses it. Results： Thanks to recent technological developments, we can now probe ehromatin interaction in substantial detail, boosting research interest in modeling genome spatial organization. Here, we review the current computational models that simulate chromosome dynamics, and explain the physical and topological properties of chromosomal conformation, as inferred from these newly generated data. Conclusion： Novel models shall be developed to address questions beyond averaged structure in the near further.

Advances in technologies for 3D genomics research

The spatial structure of the orderly organized chromatin in the nucleus has important roles in maintaining normal cell function and in regulation of gene expression, and the high-throughput Hi-C and Ch IA-PET methods have been widely used in various biological studies for determining potential spatial genome structures and their functions. However, there are still great difficulties and challenges in three-dimensional(3D) genomics research. More efficient, economical, and unbiased approaches to studying 3D genomics need to be developed for more widespread and easier applications. Here, we review the most recent studies on new 3D genomics research technologies, such as improvements of the traditional Hi-C and Ch IA-PET methods, new approaches based on non-proximal-ligation strategies, and imaging-based methods improved in recent years. Especially, we review the CRISPR-based methods for functional validations in 3D genomics, which could be the forthcoming directions. We hope this review can show some insights into the potential improvements for future 3D genomics.

Mapping Multi-factor-mediated Chromatin Interactions to Assess Dysregulation of Lung Cancer-related Genes

Studies on the lung cancer genome are indispensable for developing a cure for lung cancer.Whole-genome resequencing,genome-wide association studies,and transcriptome sequencing have greatly improved our understanding of the cancer genome.However,dysregulation of longrange chromatin interactions in lung cancer remains poorly described.To better understand the three-dimensional(3D)genomic interaction features of the lung cancer genome,we used the A549 cell line as a model system and generated high-resolution chromatin interactions associated with RNA polymerase II(RNAPII),CCCTC-binding factor(CTCF),enhancer of zeste homolog 2(EZH2),and histone 3 lysine 27 trimethylation(H3K27me3)using long-read chromatin interaction analysis by paired-end tag sequencing(ChIA-PET).Analysis showed that EZH2/H3K27me3-mediated interactions further repressed target genes,either through loops or domains,and their distributions along the genome were distinct from and complementary to those associated with RNAPII.Cancer-related genes were highly enriched with chromatin interactions,and chromatin interactions specific to the A549 cell line were associated with oncogenes and tumor suppressor genes,such as additional repressive interactions on FOXO4 and promoter–promoter interactions between NF1 and RNF135.Knockout of an anchor associated with chromatin interactions reversed the dysregulation of cancer-related genes,suggesting that chromatin interactions are essential for proper expression of lung cancer-related genes.These findings demonstrate the 3D landscape and gene regulatory relationships of the lung cancer genome.

Prediction of chromatin looping using deep hybrid learning(DHL)

Background:With the development of rapid and cheap sequencing techniques,the cost of whole-genome sequencing(WGS)has dropped significantly.However,the complexity of the human genome is not limited to the pure sequenceand additional experiments are required to learn the human genome's influence on complex traits.One of the most exciting aspects for scientists nowadays is the spatial organisation of the genome,which can be discovered using spatial experiments(e.g.,Hi-C,ChIA-PET).The information about the spatial contacts helps in the analysis and brings new insights into our understanding of the disease developments.Methods:We have used an ensemble of deep learning with classical machine learning algorithms.The deep learning network we used was DNABERT,which utilises the BERT language model(based on transformers)for the genomic function.The classical machine learning models included support vector machines(SVMs),random forests(RFs),and K-nearest neighbor(KNN).The whole approach was wrapped together as deep hybrid learning(DHL).Results:We found that the DNABERT can be used to predict the ChIA-PET experiments with high precision.Additionally,the DHL approach has increased the metrics on CTCF and RNAPII sets.Conclusions:DHL approach should be taken into consideration for the models utilising the power of deep learning.While straightforward in the concept,it can improve the results significantly.