Stage-resolved Hi-C analyses reveal meiotic chromosome organizational features influencing homolog alignment

Epigenetic priming in the male germline

Epigenetic priming presets chromatin states that allow the rapid induction of gene expression programs in response to differentiation cues. In the germline, it provides the blueprint for sexually dimorphic unidirectional differentiation. In this review, we focus on epigenetic priming in the mammalian male germline and discuss how cellular memories are regulated and inherited to the next generation. During spermatogenesis, epigenetic priming predetermines cellular memories that ensure the lifelong maintenance of spermatogonial stem cells and their subsequent commitment to meiosis and to the production of haploid sperm. The paternal chromatin state is also essential for the recovery of totipotency after fertilization and contributes to paternal epigenetic inheritance. Thus, epigenetic priming establishes stable but reversible chromatin states during spermatogenesis and enables epigenetic inheritance and reprogramming in the next generation.

Principles of chromosome organization for meiotic recombination

In meiotic cells, chromosomes are organized as chromatin loop arrays anchored to a protein axis. This organization is essential to regulate meiotic recombination, from DNA double-strand break (DSB) formation to their repair. In mammals, it is unknown how chromatin loops are organized along the genome and how proteins participating in DSB formation are tethered to the chromosome axes. Here, we identify three categories of axis-associated genomic sites: PRDM9 binding sites, where DSBs form; binding sites of the insulator protein CTCF; and H3K4me3-enriched sites. We demonstrate that PRDM9 promotes the recruitment of MEI4 and IHO1, two proteins essential for DSB formation. In turn, IHO1 anchors DSB sites to the axis components HORMAD1 and SYCP3. We discovered that IHO1, HORMAD1, and SYCP3 are associated at the DSB ends during DSB repair. Our results highlight how interactions of proteins with specific genomic elements shape the meiotic chromosome organization for recombination.

Cooltools: Enabling high-resolution Hi-C analysis in Python

Chromosome conformation capture (3C) technologies reveal the incredible complexity of genome organization. Maps of increasing size, depth, and resolution are now used to probe genome architecture across cell states, types, and organisms. Larger datasets add challenges at each step of computational analysis, from storage and memory constraints to researchers' time; however, analysis tools that meet these increased resource demands have not kept pace. Furthermore, existing tools offer limited support for customizing analysis for specific use cases or new biology. Here we introduce cooltools (https://github.com/open2c/cooltools), a suite of computational tools that enables flexible, scalable, and reproducible analysis of high-resolution contact frequency data. Cooltools leverages the widely-adopted cooler format which handles storage and access for high-resolution datasets. Cooltools provides a paired command line interface (CLI) and Python application programming interface (API), which respectively facilitate workflows on high-performance computing clusters and in interactive analysis environments. In short, cooltools enables the effective use of the latest and largest genome folding datasets.

High resolution maps of chromatin reorganization through mouse meiosis reveal novel features of the 3D meiotic structure

When germ cells transition from the mitotic cycle into meiotic prophase I (MPI), chromosomes condense into an array of chromatin loops that are required to promote homolog pairing and genetic recombination. To identify the changes in chromosomal conformation, we isolated nuclei on a trajectory from spermatogonia to the end of MPI. At each stage along this trajectory, we built genomic interaction maps with the highest temporal and spatial resolution to date. The changes in chromatin folding coincided with a concurrent decline in mitotic cohesion and a rise in meiotic cohesin complexes. We found that the stereotypical large-scale A and B compartmentalization was lost during meiotic prophase I alongside the loss of topological associating domains (TADs). Still, local subcompartments were detected and maintained throughout meiosis. The enhanced Micro-C resolution revealed that, despite the loss of TADs, higher frequency contact sites between two loci were detectable during meiotic prophase I coinciding with CTCF bound sites. The pattern of interactions around these CTCF sites with their neighboring loci showed that CTCF sites were often anchoring the meiotic loops. Additionally, the localization of CTCF to the meiotic axes indicated that these anchors were at the base of loops. Strikingly, even in the face of the dramatic reconfiguration of interphase chromatin into a condensed loop-array, the interactions between regulatory elements remained well preserved. This establishes a potential mechanism for how the meiotic chromatin maintains active transcription within a highly structured genome. In summary, the high temporal and spatial resolution of these data revealed previously unappreciated aspects of mammalian meiotic chromatin organization.

A Hierarchical Mechanotransduction System: From Macro to Micro

Mechanotransduction is a strictly regulated process whereby mechanical stimuli, including mechanical forces and properties, are sensed and translated into biochemical signals. Increasing data demonstrate that mechanotransduction is crucial for regulating macroscopic and microscopic dynamics and functionalities. However, the actions and mechanisms of mechanotransduction across multiple hierarchies, from molecules, subcellular structures, cells, tissues/organs, to the whole-body level, have not been yet comprehensively documented. Herein, the biological roles and operational mechanisms of mechanotransduction from macro to micro are revisited, with a focus on the orchestrations across diverse hierarchies. The implications, applications, and challenges of mechanotransduction in human diseases are also summarized and discussed. Together, this knowledge from a hierarchical perspective has the potential to refresh insights into mechanotransduction regulation and disease pathogenesis and therapy, and ultimately revolutionize the prevention, diagnosis, and treatment of human diseases.

Increased long-distance and homo-trans interactions related to H3K27me3 in Arabidopsis hybrids

In plants, the genome structure of hybrids changes compared with their parents, but the effects of these changes in hybrids remain elusive. Comparing reciprocal crosses between Col × C24 and C24 × Col in Arabidopsis using high-throughput chromosome conformation capture assay (Hi-C) analysis, we found that hybrid three-dimensional (3D) chromatin organization had more long-distance interactions relative to parents, and this was mainly located in promoter regions and enriched in genes with heterosis-related pathways. The interactions between euchromatin and heterochromatin were increased, and the compartment strength decreased in hybrids. In compartment domain (CD) boundaries, the distal interactions were more in hybrids than their parents. In the hybrids of CURLY LEAF (clf) mutants clfCol  × clfC24 and clfC24  × clfCol , the heterosis phenotype was damaged, and the long-distance interactions in hybrids were fewer than in their parents with lower H3K27me3. ChIP-seq data revealed higher levels of H3K27me3 in the region adjacent to the CD boundary and the same interactional homo-trans sites in the wild-type (WT) hybrids, which may have led to more long-distance interactions. In addition, the differentially expressed genes (DEGs) located in the boundaries of CDs and loop regions changed obviously in WT, and the functional enrichment for DEGs was different between WT and clf in the long-distance interactions and loop regions. Our findings may therefore propose a new epigenetic explanation of heterosis in the Arabidopsis hybrids and provide new insights into crop breeding and yield increase.

Multifaceted Roles of Histone Lysine Lactylation in Meiotic Gene Dynamics and Recombination

Male germ cells, which are responsible for producing millions of genetically diverse sperm through meiosis in the testis, rely on lactate as their central energy metabolite. Recent study has revealed that lactate induces epigenetic modification in cells through histone lactylation, a post-translational modification involving the addition of lactyl groups to lysine residues on histones. Here we report dynamic histone lactylation at histone H4-lysine 5 (K5), -K8, and -K12 during meiosis prophase I in mouse spermatogenesis. By profiling genome-wide occupancy of histone H4-K8 lactylation (H4K8la), which peaks at zygotene, our data show that H4K8la mark is observed at the promoters of genes exhibiting active expression with Gene Ontology (GO) functions enriched for meiosis. Notably, our data also demonstrate that H4K8la is closely associated with recombination hotspots, where machinery involved in the processing DNA double-stranded breaks (DSBs), such as SPO11, DMC1, RAD51, and RPA2, is engaged. In addition, H4K8la was also detected at the meiosis-specific cohesion sites (marked by RAD21L and REC8) flanking the recombination hotspots. Collectively, our findings suggest that histone lactylation serves as a novel mechanism through which lactate regulates germ cell meiosis.

3D genome remodeling and homologous pairing during meiotic prophase of mouse oogenesis and spermatogenesis

During meiosis, the chromatin and transcriptome undergo prominent switches. Although recent studies have explored the genome reorganization during spermatogenesis, the chromatin remodeling in oogenesis and characteristics of homologous pairing remain largely elusive. We comprehensively compared chromatin structures and transcriptomes at successive substages of meiotic prophase in both female and male mice using low-input high-through chromosome conformation capture (Hi-C) and RNA sequencing (RNA-seq). Compartments and topologically associating domains (TADs) gradually disappeared and slowly recovered in both sexes. We found that homologs adopted different sex-conserved pairing strategies prior to and after the leptotene-to-zygotene transition, changing from long interspersed nuclear element (LINE)-enriched compartments B to short interspersed nuclear element (SINE)-enriched compartments A. We complemented marker genes and predicted the sex-specific meiotic sterile genes for each substage. This study provides valuable insights into the similarities and distinctions between sexes in chromosome architecture, homologous pairing, and transcriptome during meiotic prophase of both oogenesis and spermatogenesis.

CTCF-mediated 3D chromatin predetermines the gene expression program in the male germline

Spermatogenesis is a unidirectional differentiation process that generates haploid sperm, but how the gene expression program that directs this process is established is largely unknown. Here we determine the high-resolution 3D chromatin architecture of male germ cells during spermatogenesis and show that CTCF-mediated 3D chromatin predetermines the gene expression program required for spermatogenesis. In undifferentiated spermatogonia, CTCF-mediated chromatin contacts on autosomes pre-establish meiosis-specific super-enhancers (SE). These meiotic SE recruit the master transcription factor A-MYB in meiotic spermatocytes, which strengthens their 3D contacts and instructs a burst of meiotic gene expression. We also find that at the mitosis-to-meiosis transition, the germline-specific Polycomb protein SCML2 resolves chromatin loops that are specific to mitotic spermatogonia. Moreover, SCML2 and A-MYB establish the unique 3D chromatin organization of sex chromosomes during meiotic sex chromosome inactivation. We propose that CTCF-mediated 3D chromatin organization enforces epigenetic priming that directs unidirectional differentiation, thereby determining the cellular identity of the male germline.

Improving comparative analyses of Hi-C data via contrastive self-supervised learning

Hi-C is a widely applied chromosome conformation capture (3C)-based technique, which has produced a large number of genomic contact maps with high sequencing depths for a wide range of cell types, enabling comprehensive analyses of the relationships between biological functionalities (e.g. gene regulation and expression) and the three-dimensional genome structure. Comparative analyses play significant roles in Hi-C data studies, which are designed to make comparisons between Hi-C contact maps, thus evaluating the consistency of replicate Hi-C experiments (i.e. reproducibility measurement) and detecting statistically differential interacting regions with biological significance (i.e. differential chromatin interaction detection). However, due to the complex and hierarchical nature of Hi-C contact maps, it remains challenging to conduct systematic and reliable comparative analyses of Hi-C data. Here, we proposed sslHiC, a contrastive self-supervised representation learning framework, for precisely modeling the multi-level features of chromosome conformation and automatically producing informative feature embeddings for genomic loci and their interactions to facilitate comparative analyses of Hi-C contact maps. Comprehensive computational experiments on both simulated and real datasets demonstrated that our method consistently outperformed the state-of-the-art baseline methods in providing reliable measurements of reproducibility and detecting differential interactions with biological meanings.

Strong interactions between highly dynamic lamina-associated domains and the nuclear envelope stabilize the 3D architecture of Drosophila interphase chromatin

BACKGROUND

Interactions among topologically associating domains (TADs), and between the nuclear envelope (NE) and lamina-associated domains (LADs) are expected to shape various aspects of three-dimensional (3D) chromatin structure and dynamics; however, relevant genome-wide experiments that may provide statistically significant conclusions remain difficult.

RESULTS

We have developed a coarse-grained dynamical model of D. melanogaster nuclei at TAD resolution that explicitly accounts for four distinct epigenetic classes of TADs and LAD-NE interactions. The model is parameterized to reproduce the experimental Hi-C map of the wild type (WT) nuclei; it describes time evolution of the chromatin over the G1 phase of the interphase. The simulations include an ensemble of nuclei, corresponding to the experimentally observed set of several possible mutual arrangements of chromosomal arms. The model is validated against multiple structural features of chromatin from several different experiments not used in model development. Predicted positioning of all LADs at the NE is highly dynamic-the same LAD can attach, detach and move far away from the NE multiple times during interphase. The probabilities of LADs to be in contact with the NE vary by an order of magnitude, despite all having the same affinity to the NE in the model. These probabilities are mostly determined by a highly variable local linear density of LADs along the genome, which also has the same strong effect on the predicted positioning of individual TADs -- higher probability of a TAD to be near NE is largely determined by a higher linear density of LADs surrounding this TAD. The distribution of LADs along the chromosome chains plays a notable role in maintaining a non-random average global structure of chromatin. Relatively high affinity of LADs to the NE in the WT nuclei substantially reduces sensitivity of the global radial chromatin distribution to variations in the strength of TAD-TAD interactions compared to the lamin depleted nuclei, where a small (0.5 kT) increase of cross-type TAD-TAD interactions doubles the chromatin density in the central nucleus region.

CONCLUSIONS

A dynamical model of the entire fruit fly genome makes multiple genome-wide predictions of biological interest. The distribution of LADs along the chromatin chains affects their probabilities to be in contact with the NE and radial positioning of highly mobile TADs, playing a notable role in creating a non-random average global structure of the chromatin. We conjecture that an important role of attractive LAD-NE interactions is to stabilize global chromatin structure against inevitable cell-to-cell variations in TAD-TAD interactions.

Granulosa cell mevalonate pathway abnormalities contribute to oocyte meiotic defects and aneuploidy

With aging, abnormalities during oocyte meiosis become more prevalent. However, the mechanisms of aging-related oocyte aneuploidy are not fully understood. Here we performed Hi-C and SMART-seq of oocytes from young and old mice and reveal decreases in chromosome condensation and disrupted meiosis-associated gene expression in metaphase I oocytes from aged mice. Further transcriptomic analysis showed that meiotic maturation in young oocytes was correlated with robust increases in mevalonate (MVA) pathway gene expression in oocyte-surrounding granulosa cells (GCs), which was largely downregulated in aged GCs. Inhibition of MVA metabolism in GCs by statins resulted in marked meiotic defects and aneuploidy in young cumulus-oocyte complexes. Correspondingly, supplementation with the MVA isoprenoid geranylgeraniol ameliorated oocyte meiotic defects and aneuploidy in aged mice. Mechanically, we showed that geranylgeraniol activated LHR/EGF signaling in aged GCs and enhanced the meiosis-associated gene expression in oocytes. Collectively, we demonstrate that the MVA pathway in GCs is a critical regulator of meiotic maturation and euploidy in oocytes, and age-associated MVA pathway abnormalities contribute to oocyte meiotic defects and aneuploidy.

A-MYB and BRDT-dependent RNA Polymerase II pause release orchestrates transcriptional regulation in mammalian meiosis

During meiotic prophase I, spermatocytes must balance transcriptional activation with homologous recombination and chromosome synapsis, biological processes requiring extensive changes to chromatin state. We explored the interplay between chromatin accessibility and transcription through prophase I of mammalian meiosis by measuring genome-wide patterns of chromatin accessibility, nascent transcription, and processed mRNA. We find that Pol II is loaded on chromatin and maintained in a paused state early during prophase I. In later stages, paused Pol II is released in a coordinated transcriptional burst mediated by the transcription factors A-MYB and BRDT, resulting in ~3-fold increase in transcription. Transcriptional activity is temporally and spatially segregated from key steps of meiotic recombination: double strand breaks show evidence of chromatin accessibility earlier during prophase I and at distinct loci from those undergoing transcriptional activation, despite shared chromatin marks. Our findings reveal mechanisms underlying chromatin specialization in either transcription or recombination in meiotic cells.

Global dissection of the recombination landscape in soybean using a high-density 600K SoySNP array

Recombination is crucial for crop breeding because it can break linkage drag and generate novel allele combinations. However, the high-resolution recombination landscape and its driving forces in soybean are largely unknown. Here, we constructed eight recombinant inbred line (RIL) populations and genotyped individual lines using the high-density 600K SoySNP array, which yielded a high-resolution recombination map with 5636 recombination sites at a resolution of 1.37 kb. The recombination rate was negatively correlated with transposable element density and GC content but positively correlated with gene density. Interestingly, we found that meiotic recombination was enriched at the promoters of active genes. Further investigations revealed that chromatin accessibility and active epigenetic modifications promoted recombination. Our findings provide important insights into the control of homologous recombination and thus will increase our ability to accelerate soybean breeding by manipulating meiotic recombination rate.

Crossover patterning in plants

KEY MESSAGE

Chromatin state, and dynamic loading of pro-crossover protein HEI10 at recombination intermediates shape meiotic chromosome patterning in plants. Meiosis is the basis of sexual reproduction, and its basic progression is conserved across eukaryote kingdoms. A key feature of meiosis is the formation of crossovers which result in the reciprocal exchange of segments of maternal and paternal chromosomes. This exchange generates chromosomes with new combinations of alleles, increasing the efficiency of both natural and artificial selection. Crossovers also form a physical link between homologous chromosomes at metaphase I which is critical for accurate chromosome segregation and fertility. The patterning of crossovers along the length of chromosomes is a highly regulated process, and our current understanding of its regulation forms the focus of this review. At the global scale, crossover patterning in plants is largely governed by the classically observed phenomena of crossover interference, crossover homeostasis and the obligatory crossover which regulate the total number of crossovers and their relative spacing. The molecular actors behind these phenomena have long remained obscure, but recent studies in plants implicate HEI10 and ZYP1 as key players in their coordination. In addition to these broad forces, a wealth of recent studies has highlighted how genomic and epigenomic features shape crossover formation at both chromosomal and local scales, revealing that crossovers are primarily located in open chromatin associated with gene promoters and terminators with low nucleosome occupancy.

Epigenetic markers in the embryonal germ cell development and spermatogenesis

Spermatogenesis is the process of generation of male reproductive cells from spermatogonial stem cells in the seminiferous epithelium of the testis. During spermatogenesis, key spermatogenic events such as stem cell self-renewal and commitment to meiosis, meiotic recombination, meiotic sex chromosome inactivation, followed by cellular and chromatin remodeling of elongating spermatids occur, leading to sperm cell production. All the mentioned events are at least partially controlled by the epigenetic modifications of DNA and histones. Additionally, during embryonal development in primordial germ cells, global epigenetic reprogramming of DNA occurs. In this review, we summarized the most important epigenetic modifications in the particular stages of germ cell development, in DNA and histone proteins, starting from primordial germ cells, during embryonal development, and ending with histone-to-protamine transition during spermiogenesis.

Chromosome architecture and homologous recombination in meiosis

Meiocytes organize higher-order chromosome structures comprising arrays of chromatin loops organized at their bases by linear axes. As meiotic prophase progresses, the axes of homologous chromosomes align and synapse along their lengths to form ladder-like structures called synaptonemal complexes (SCs). The entire process of meiotic recombination, from initiation via programmed DNA double-strand breaks (DSBs) to completion of DSB repair with crossover or non-crossover outcomes, occurs in the context of chromosome axes and SCs. These meiosis-specific chromosome structures provide specialized environments for the regulation of DSB formation and crossing over. In this review, we summarize insights into the importance of chromosome architecture in the regulation of meiotic recombination, focusing on cohesin-mediated axis formation, DSB regulation via tethered loop-axis complexes, inter-homolog template bias facilitated by axial proteins, and crossover regulation in the context of the SCs. We also discuss emerging evidence that the SUMO and the ubiquitin-proteasome system function in the organization of chromosome structure and regulation of meiotic recombination.

A homozygous KASH5 frameshift mutation causes diminished ovarian reserve, recurrent miscarriage, and non-obstructive azoospermia in humans

The meiosis-specific LINC complex, composed of the KASH5 and SUN1 proteins, tethers the moving chromosomes to the nuclear envelope to facilitate homolog pairing and is essential for gametogenesis. Here, we applied whole-exome sequencing for a consanguineous family with five siblings suffering from reproductive failure, and identified a homozygous frameshift mutation in KASH5 (c.1270_1273del, p.Arg424Thrfs*20). This mutation leads to the absence of KASH5 protein expression in testes and non-obstructive azoospermia (NOA) due to meiotic arrest before the pachytene stage in the affected brother. The four sisters displayed diminished ovarian reserve (DOR), with one sister never being pregnant but still having dominant follicle at 35 years old and three sisters suffering from at least 3 miscarriages occurring within the third month of gestation. The truncated KASH5 mutant protein, when expressed in cultured cells, displays a similar localization encircling the nucleus and a weakened interaction with SUN1, as compared with the full-length KASH5 proteins, which provides a potential explanation for the phenotypes in the affected females. This study reported sexual dimorphism for influence of the KASH5 mutation on human germ cell development, and extends the clinical manifestations associated with KASH5 mutations, providing genetic basis for the molecular diagnosis of NOA, DOR, and recurrent miscarriage.

Effects of Dlx2 overexpression on the genes associated with the maxillary process in the early mouse embryo

The transcription factor Dlx2 plays an important role in craniomaxillofacial development. Overexpression or null mutations of Dlx2 can lead to craniomaxillofacial malformation in mice. However, the transcriptional regulatory effects of Dlx2 during craniomaxillofacial development remain to be elucidated. Using a mouse model that stably overexpresses Dlx2 in neural crest cells, we comprehensively characterized the effects of Dlx2 overexpression on the early development of maxillary processes in mice by conducting bulk RNA-Seq, scRNA-Seq and CUT&Tag analyses. Bulk RNA-Seq results showed that the overexpression of Dlx2 resulted in substantial transcriptome changes in E10.5 maxillary prominences, with genes involved in RNA metabolism and neuronal development most significantly affected. The scRNA-Seq analysis suggests that overexpression of Dlx2 did not change the differentiation trajectory of mesenchymal cells during this development process. Rather, it restricted cell proliferation and caused precocious differentiation, which may contribute to the defects in craniomaxillofacial development. Moreover, the CUT&Tag analysis using DLX2 antibody revealed enrichment of MNT and Runx2 motifs at the putative DLX2 binding sites, suggesting they may play critical roles in mediating the transcriptional regulatory effects of Dlx2. Together, these results provide important insights for understanding the transcriptional regulatory network of Dlx2 during craniofacial development.

Nonhomologous Chromosome Interactions in Prophase I: Dynamics of Bizarre Meiotic Contacts in the Alay Mole Vole Ellobius alaicus (Mammalia, Rodentia)

Nonhomologous chromosome interactions take place in both somatic and meiotic cells. Prior to this study, we had discovered special contacts through the SYCP3 (synaptonemal complex protein 3) filament between the short arms of nonhomologous acrocentrics at the pachytene stage in the Alay mole vole, and these contacts demonstrate several patterns from proximity to the complete fusion stage. Here, we investigated the nonhomologous chromosome contacts in meiotic prophase I. It turned out that such contacts do not introduce changes into the classic distribution of DNA double-strand breaks. It is noteworthy that not all meiotic contacts were localized in the H3k9me3-positive heterochromatic environment. Both in the mid zygotene and in the early-mid diplotene, three types of contacts (proximity, touching, and anchoring/tethering) were observed, whereas fusion seems to be characteristic only for pachytene. The number of contacts in the mid pachytene is significantly higher than that in the zygotene, and the distance between centromeres in nonhomologous contacts is also the smallest in mid pachytene for all types of contacts. Thus, this work provides a new insight into the behavior of meiotic contacts during prophase I and points to avenues of further research.

Negative supercoils regulate meiotic crossover patterns in budding yeast

Interference exists ubiquitously in many biological processes. Crossover interference patterns meiotic crossovers, which are required for faithful chromosome segregation and evolutionary adaption. However, what the interference signal is and how it is generated and regulated is unknown. We show that yeast top2 alleles which cannot bind or cleave DNA accumulate a higher level of negative supercoils and show weaker interference. However, top2 alleles which cannot religate the cleaved DNA or release the religated DNA accumulate less negative supercoils and show stronger interference. Moreover, the level of negative supercoils is negatively correlated with crossover interference strength. Furthermore, negative supercoils preferentially enrich at crossover-associated Zip3 regions before the formation of meiotic DNA double-strand breaks, and regions with more negative supercoils tend to have more Zip3. Additionally, the strength of crossover interference and homeostasis change coordinately in mutants. These findings suggest that the accumulation and relief of negative supercoils pattern meiotic crossovers.

Orchestrating recombination initiation in mice and men

Recent discoveries have advanced our understanding of recombination initiation beyond the placement of double-stranded DNA breaks (DSBs) from germline replication timing to the dynamic reorganization of chromatin, and defined critical players of recombination initiation. This article focuses on recombination initiation in mammals utilizing the PRDM9 protein to orchestrate crucial stages of meiotic recombination initiation by interacting with the local DNA environment and several protein complexes. The Pioneer Complex with the SNF2-type chromatin remodeling enzyme HELLS, exposes PRDM9-bound DNA. At the same time, a Compass-Complex containing EWSR1, CXXC1, CDYL, EHMT2 and PRDM9 facilitates the association of putative hotspot sites in DNA loops with the chromosomal axis where DSB-promoting complexes are located, and DSBs are catalyzed by the SPO11/TOPOVIBL complex. Finally, homology search is facilitated at PRDM9-directed sites by ANKRD31. The Reader-Writer system consists of PRDM9 writing characteristic histone methylation signatures, which are read by ZCWPW1, promoting efficient homology engagement.

Cancer and meiotic gene expression: Two sides of the same coin?

Meiosis increases genetic diversity in offspring by generating genetically unique haploid gametes with reshuffled chromosomes. This process requires a specialized set of meiotic proteins, which facilitate chromosome recombination and segregation. However, re-expression of meiotic proteins in mitosis can have catastrophic oncogenic consequences and aberrant expression of meiotic proteins is a common occurrence in human tumors. Mechanistically, re-activation of meiotic genes in cancer promotes oncogenesis likely because cancers-conversely to healthy mitosis-are fueled by genetic instability which promotes tumor evolution, and evasion of immune response and treatment pressure. In this review, we explore similarities between meiotic and cancer cells with a particular focus on the oncogenic activation of meiotic genes in cancer. We emphasize the role of histones and their modifications, DNA methylation, genome organization, R-loops and the availability of distal enhancers.

Meiotic chromosome organization and its role in recombination and cancer

Chromosomes adopt specific conformations to regulate various cellular processes. A well-documented chromosome configuration is the highly compacted chromosome structure during metaphase. More regional chromatin conformations have also been reported, including topologically associated domains encompassing mega-bases of DNA and local chromatin loops formed by kilo-bases of DNA. In this review, we discuss the changes in chromatin conformation taking place between somatic and meiotic cells, with a special focus on the establishment of a proteinaceous structure, called the chromosome axis, at the beginning of meiosis. The chromosome axis is essential to support key meiotic processes such as chromosome pairing, homologous recombination, and balanced chromosome segregation to transition from a diploid to a haploid stage. We review the role of the chromosome axis in meiotic chromatin organization and provide a detailed description of its protein composition. We also review the conserved and distinct roles between species of axis proteins in meiotic recombination, which is a major factor contributing to the creation of genetic diversity and genome evolution. Finally, we discuss situations where the chromosome axis is deregulated and evaluate the effects on genome integrity and the consequences from protein deregulation in meiocytes exposed to heat stress, and aberrant expression of genes encoding axis proteins in mammalian somatic cells associated with certain types of cancers.

Large-scale chromatin reorganization reactivates placenta-specific genes that drive cellular aging

Nuclear deformation, a hallmark frequently observed in senescent cells, is presumed to be associated with the erosion of chromatin organization at the nuclear periphery. However, how such gradual changes in higher-order genome organization impinge on local epigenetic modifications to drive cellular mechanisms of aging has remained enigmatic. Here, through large-scale epigenomic analyses of isogenic young, senescent, and progeroid human mesenchymal progenitor cells (hMPCs), we delineate a hierarchy of integrated structural state changes that manifest as heterochromatin loss in repressive compartments, euchromatin weakening in active compartments, switching in interfacing topological compartments, and increasing epigenetic entropy. We found that the epigenetic de-repression unlocks the expression of pregnancy-specific beta-1 glycoprotein (PSG) genes that exacerbate hMPC aging and serve as potential aging biomarkers. Our analyses provide a rich resource for uncovering the principles of epigenomic landscape organization and its changes in cellular aging and for identifying aging drivers and intervention targets with a genome-topology-based mechanism.

Mouse Chd4-NURD is required for neonatal spermatogonia survival and normal gonad development

Testis development and sustained germ cell production in adults rely on the establishment and maintenance of spermatogonia stem cells and their proper differentiation into spermatocytes. Chromatin remodeling complexes regulate critical processes during gamete development by restricting or promoting accessibility of DNA repair and gene expression machineries to the chromatin. Here, we investigated the role of Chd4 and Chd3 catalytic subunits of the NURD complex during spermatogenesis. Germ cell-specific deletion of chd4 early in gametogenesis, but not chd3, resulted in arrested early gamete development due to failed cell survival of neonate undifferentiated spermatogonia stem cell population. Candidate assessment revealed that Chd4 controls expression of dmrt1 and its downstream target plzf, both described as prominent regulators of spermatogonia stem cell maintenance. Our results show the requirement of Chd4 in mammalian gametogenesis pointing to functions in gene expression early in the process.

Meiotic chromosome organization and crossover patterns

Meiosis is the foundation of sexual reproduction, and crossover recombination is one hallmark of meiosis. Crossovers establish the physical connections between homolog chromosomes (homologs) for their proper segregation and exchange DNA between homologs to promote genetic diversity in gametes and thus progenies. Aberrant crossover patterns, e.g. absence of the obligatory crossover, are the leading cause of infertility, miscarriage, and congenital disease. Therefore, crossover patterns have to be tightly controlled. During meiosis, loop/axis organized chromosomes provide the structural basis and regulatory machinery for crossover patterning. Accumulating evidence shows that chromosome axis length regulates not only the numbers but also the positions of crossovers. In addition, recent studies suggest that alterations in axis length and the resultant alterations in crossover frequency may contribute to evolutionary adaptation. Here, current advances regarding these issues are reviewed, the possible mechanisms for axis length regulating crossover frequency are discussed, and important issues that need further investigations are suggested.

Eco1-dependent cohesin acetylation anchors chromatin loops and cohesion to define functional meiotic chromosome domains

Cohesin organizes the genome by forming intra-chromosomal loops and inter-sister chromatid linkages. During gamete formation by meiosis, chromosomes are reshaped to support crossover recombination and two consecutive rounds of chromosome segregation. Here we show that meiotic chromosomes are organised into functional domains by Eco1 acetyltransferase-dependent positioning of both chromatin loops and sister chromatid cohesion in budding yeast. Eco1 acetylates the Smc3 cohesin subunit in meiotic S phase to establish chromatin boundaries, independently of DNA replication. Boundary formation by Eco1 is critical for prophase exit and for the maintenance of cohesion until meiosis II, but is independent of the ability of Eco1 to antagonize the cohesin-release factor, Wpl1. Conversely, prevention of cohesin release by Wpl1 is essential for centromeric cohesion, kinetochore monoorientation and co-segregation of sister chromatids in meiosis I. Our findings establish Eco1 as a key determinant of chromatin boundaries and cohesion positioning, revealing how local chromosome structuring directs genome transmission into gametes.

Rec8 Cohesin: A Structural Platform for Shaping the Meiotic Chromosomes

Meiosis is critically different from mitosis in that during meiosis, pairing and segregation of homologous chromosomes occur. During meiosis, the morphology of sister chromatids changes drastically, forming a prominent axial structure in the synaptonemal complex. The meiosis-specific cohesin complex plays a central role in the regulation of the processes required for recombination. In particular, the Rec8 subunit of the meiotic cohesin complex, which is conserved in a wide range of eukaryotes, has been analyzed for its function in modulating chromosomal architecture during the pairing and recombination of homologous chromosomes in meiosis. Here, we review the current understanding of Rec8 cohesin as a structural platform for meiotic chromosomes.

Building the genome architecture during the maternal to zygotic transition

Proper higher-order chromatin folding can profoundly impact gene expression. In early animal development, chromatin undergoes dramatic reorganization to convert terminally differentiated gametes to early embryos. The recent rapid development of ultra-sensitive chromatin analysis technologies has revealed a drastic reconstruction of chromatin architecture, which includes a surprising relaxation followed by de novo and slow establishment of 3D genome organization in early embryos. Such progress adds another fascinating dimension to epigenetic reprogramming in early development that also features degradation of maternal RNA and activation of the zygotic genome during the maternal to zygotic transition (MZT). Nevertheless, the role of higher-order chromatin architecture in this critical developmental time window is yet to be understood. In this article, we review the latest findings from a variety of species (with a primary focus on mammals) on the establishment of 3D chromatin structure in gametogenesis and early development. These data shed light on how chromatin organization is regulated, and how it coordinates with MZT and other crucial events in early development. Finally, we discuss the crucial questions that remain to be answered in the future.