A map of human PRDM9 binding provides evidence for novel behaviors of PRDM9 and other zinc-finger proteins in meiosis

YY1 is involved in homologous recombination inhibition at guanine quadruplex sites in human cells

Homologous recombination (HR) is a key process for repairing DNA double strand breaks and for promoting genetic diversity. However, HR occurs unevenly across the genome, and certain genomic features can influence its activity. One such feature is the presence of guanine quadruplexes (G4s), stable secondary structures widely distributed throughout the genome. These G4s play essential roles in gene transcription and genome stability regulation. Especially, elevated G4 levels in cells deficient in the Bloom syndrome helicase (BLM) significantly enhance HR at G4 sites, potentially threatening genome stability. Here, we investigated the role of G4-binding protein Yin Yang-1 (YY1) in modulating HR at G4 sites in human cells. Our results show that YY1's binding to G4 structures suppresses sister chromatid exchange after BLM knockdown, and YY1's chromatin occupancy negatively correlates with the overall HR rate observed across the genome. By limiting RAD51 homolog 1 (RAD51) access, YY1 preferentially binds to essential genomic regions, shielding them from excessive HR. Our findings unveil a novel role of YY1-G4 interaction, revealing novel insights into cellular mechanisms involved in HR regulation.

Exploration of Genome-Wide Recombination Rate Variation Patterns at Different Scales in Pigs

Meiotic recombination is a prevalent process in eukaryotic sexual reproduction organisms that plays key roles in genetic diversity, breed selection, and species evolution. However, the recombination events differ across breeds and even within breeds. In this study, we initially computed large-scale population recombination rates of both sexes using approximately 52 K SNP genotypes in a total of 3279 pigs from four different Chinese and Western breeds. We then constructed a high-resolution historical recombination map using approximately 16 million SNPs from a sample of unrelated individuals. Comparative analysis of porcine recombination events from different breeds and at different resolutions revealed the following observations: Firstly, the 1Mb-scale pig recombination maps of the same sex are moderately conserved among different breeds, with the similarity of recombination events between Western pigs and Chinese indigenous pigs being lower than within their respective groups. Secondly, we identified 3861 recombination hotspots in the genome and observed medium- to high-level correlation between historical recombination rates (0.542~0.683) and estimates of meiotic recombination rates. Third, we observed that recombination hotspots are significantly far from the transcription start sites of pig genes, and the silico-predicted PRDM9 zinc finger domain DNA recognition motif is significantly enriched in the regions of recombination hotspots compared to recombination coldspots, highlighting the potential role of PRDM9 in regulating recombination hotspots in pigs. Our study analyzed the variation patterns of the pig recombination map at broad and fine scales, providing a valuable reference for genomic selection breeding and laying a crucial foundation for further understanding the molecular mechanisms of pig genome recombination.

A working model for the formation of Robertsonian chromosomes

Robertsonian chromosomes form by fusion of two chromosomes that have centromeres located near their ends, known as acrocentric or telocentric chromosomes. This fusion creates a new metacentric chromosome and is a major mechanism of karyotype evolution and speciation. Robertsonian chromosomes are common in nature and were first described in grasshoppers by the zoologist W. R. B. Robertson more than 100 years ago. They have since been observed in many species, including catfish, sheep, butterflies, bats, bovids, rodents and humans, and are the most common chromosomal change in mammals. Robertsonian translocations are particularly rampant in the house mouse, Mus musculus domesticus, where they exhibit meiotic drive and create reproductive isolation. Recent progress has been made in understanding how Robertsonian chromosomes form in the human genome, highlighting some of the fundamental principles of how and why these types of fusion events occur so frequently. Consequences of these fusions include infertility and Down's syndrome. In this Hypothesis, I postulate that the conditions that allow these fusions to form are threefold: (1) sequence homology on non-homologous chromosomes, often in the form of repetitive DNA; (2) recombination initiation during meiosis; and (3) physical proximity of the homologous sequences in three-dimensional space. This Hypothesis highlights the latest progress in understanding human Robertsonian translocations within the context of the broader literature on Robertsonian chromosomes.

2023 ASHG Scientific Achievement Award

This article is based on the address given by the author at the 2023 meeting of The American Society of Human Genetics (ASHG) in Washington, D.C. A video of the original address can be found at the ASHG website.

Natural variation in the zinc-finger-encoding exon of Prdm9 affects hybrid sterility phenotypes in mice

PRDM9-mediated reproductive isolation was first described in the progeny of Mus musculus musculus (MUS) PWD/Ph and Mus musculus domesticus (DOM) C57BL/6J inbred strains. These male F1 hybrids fail to complete chromosome synapsis and arrest meiosis at prophase I, due to incompatibilities between the Prdm9 gene and hybrid sterility locus Hstx2. We identified 14 alleles of Prdm9 in exon 12, encoding the DNA-binding domain of the PRDM9 protein in outcrossed wild mouse populations from Europe, Asia, and the Middle East, 8 of which are novel. The same allele was found in all mice bearing introgressed t-haplotypes encompassing Prdm9. We asked whether 7 novel Prdm9 alleles in MUS populations and the t-haplotype allele in 1 MUS and 3 DOM populations induce Prdm9-mediated reproductive isolation. The results show that only combinations of the dom2 allele of DOM origin and the MUS msc1 allele ensure complete infertility of intersubspecific hybrids in outcrossed wild populations and inbred mouse strains examined so far. The results further indicate that MUS mice may share the erasure of PRDM9msc1 binding motifs in populations with different Prdm9 alleles, which implies that erased PRDM9 binding motifs may be uncoupled from their corresponding Prdm9 alleles at the population level. Our data corroborate the model of Prdm9-mediated hybrid sterility beyond inbred strains of mice and suggest that sterility alleles of Prdm9 may be rare.

Patterns of recombination in snakes reveal a tug-of-war between PRDM9 and promoter-like features

In some mammals, notably humans, recombination occurs almost exclusively where the protein PRDM9 binds, whereas in vertebrates lacking an intact PRDM9, such as birds and canids, recombination rates are elevated near promoter-like features. To determine whether PRDM9 directs recombination in nonmammalian vertebrates, we focused on an exemplar species with a single, intact PRDM9 ortholog, the corn snake (Pantherophis guttatus). Analyzing historical recombination rates along the genome and crossovers in pedigrees, we found evidence that PRDM9 specifies the location of recombination events, but we also detected a separable effect of promoter-like features. These findings reveal that the uses of PRDM9 and promoter-like features need not be mutually exclusive and instead reflect a tug-of-war that is more even in some species than others.

Transposable elements may enhance antiviral resistance in HIV-1 elite controllers

Less than 0.5% of people living with HIV-1 are elite controllers (ECs) - individuals who have a replication-competent viral reservoir in their CD4+ T cells but maintain undetectable plasma viremia without the help of antiretroviral therapy. While the EC CD4+ T cell transcriptome has been investigated for gene expression signatures associated with disease progression (or, in this case, a lack thereof), the expression and regulatory activity of transposable elements (TEs) in ECs has not been explored. Yet previous studies have established that TEs can directly impact the immune response to pathogens, including HIV-1. Thus, we hypothesize that the regulatory activities of TEs could contribute to the natural resistance of ECs against HIV-1. We perform a TE-centric analysis of previously published multi-omics data derived from EC individuals and other populations. We find that the CD4+ T cell transcriptome and retrotranscriptome of ECs are distinct from healthy controls, treated patients, and viremic progressors. However, there is a substantial level of transcriptomic heterogeneity among ECs. We categorize individuals with distinct chromatin accessibility and expression profiles into four clusters within the EC group, each possessing unique repertoires of TEs and antiviral factors. Notably, several TE families with known immuno-regulatory activity are differentially expressed among ECs. Their transcript levels in ECs positively correlate with their chromatin accessibility and negatively correlate with the expression of their KRAB zinc-finger (KZNF) repressors. This coordinated variation is seen at the level of individual TE loci likely acting or, in some cases, known to act as cis-regulatory elements for nearby genes involved in the immune response and HIV-1 restriction. Based on these results, we propose that the EC phenotype is driven in part by the reduced availability of specific KZNF proteins to repress TE-derived cis-regulatory elements for antiviral genes, thereby heightening their basal level of resistance to HIV-1 infection. Our study reveals considerable heterogeneity in the CD4+ T cell transcriptome of ECs, including variable expression of TEs and their KZNF controllers, that must be taken into consideration to decipher the mechanisms enabling HIV-1 control.

Meiotic DNA breaks drive multifaceted mutagenesis in the human germ line

Meiotic recombination commences with hundreds of programmed DNA breaks; however, the degree to which they are accurately repaired remains poorly understood. We report that meiotic break repair is eightfold more mutagenic for single-base substitutions than was previously understood, leading to de novo mutation in one in four sperm and one in 12 eggs. Its impact on indels and structural variants is even higher, with 100- to 1300-fold increases in rates per break. We uncovered new mutational signatures and footprints relative to break sites, which implicate unexpected biochemical processes and error-prone DNA repair mechanisms, including translesion synthesis and end joining in meiotic break repair. We provide evidence that these mechanisms drive mutagenesis in human germ lines and lead to disruption of hundreds of genes genome wide.

Take a walk on the KRAB side

Canonical Krüppel-associated box (KRAB)-containing zinc finger proteins (KZFPs) act as major repressors of transposable elements (TEs) via the KRAB-mediated recruitment of the heterochromatin scaffold KRAB-associated protein (KAP)1. KZFP genes emerged some 420 million years ago in the last common ancestor of coelacanth, lungfish, and tetrapods, and dramatically expanded to give rise to lineage-specific repertoires in contemporary species paralleling their TE load and turnover. However, the KRAB domain displays sequence and function variations that reveal repeated diversions from a linear TE-KZFP trajectory. This Review summarizes current knowledge on the evolution of KZFPs and discusses how ancestral noncanonical KZFPs endowed with variant KRAB, SCAN or DUF3669 domains have been utilized to achieve KAP1-independent functions.

Down the Penrose stairs, or how selection for fewer recombination hotspots maintains their existence

In many species, meiotic recombination events tend to occur in narrow intervals of the genome, known as hotspots. In humans and mice, double strand break (DSB) hotspot locations are determined by the DNA-binding specificity of the zinc finger array of the PRDM9 protein, which is rapidly evolving at residues in contact with DNA. Previous models explained this rapid evolution in terms of the need to restore PRDM9 binding sites lost to gene conversion over time, under the assumption that more PRDM9 binding always leads to more DSBs. This assumption, however, does not align with current evidence. Recent experimental work indicates that PRDM9 binding on both homologs facilitates DSB repair, and that the absence of sufficient symmetric binding disrupts meiosis. We therefore consider an alternative hypothesis: that rapid PRDM9 evolution is driven by the need to restore symmetric binding because of its role in coupling DSB formation and efficient repair. To this end, we model the evolution of PRDM9 from first principles: from its binding dynamics to the population genetic processes that govern the evolution of the zinc finger array and its binding sites. We show that the loss of a small number of strong binding sites leads to the use of a greater number of weaker ones, resulting in a sharp reduction in symmetric binding and favoring new PRDM9 alleles that restore the use of a smaller set of strong binding sites. This decrease, in turn, drives rapid PRDM9 evolutionary turnover. Our results therefore suggest that the advantage of new PRDM9 alleles is in limiting the number of binding sites used effectively, rather than in increasing net PRDM9 binding. By extension, our model suggests that the evolutionary advantage of hotspots may have been to increase the efficiency of DSB repair and/or homolog pairing.

Single-cell multi-omics sequencing of human spermatogenesis reveals a DNA demethylation event associated with male meiotic recombination

Human spermatogenesis is a highly ordered process; however, the roles of DNA methylation and chromatin accessibility in this process remain largely unknown. Here by simultaneously investigating the chromatin accessibility, DNA methylome and transcriptome landscapes using the modified single-cell chromatin overall omic-scale landscape sequencing approach, we revealed that the transcriptional changes throughout human spermatogenesis were correlated with chromatin accessibility changes. In particular, we identified a set of transcription factors and cis elements with potential functions. A round of DNA demethylation was uncovered upon meiosis initiation in human spermatogenesis, which was associated with male meiotic recombination and conserved between human and mouse. Aberrant DNA hypermethylation could be detected in leptotene spermatocytes of certain nonobstructive azoospermia patients. Functionally, the intervention of DNA demethylation affected male meiotic recombination and fertility. Our work provides multi-omics landscapes of human spermatogenesis at single-cell resolution and offers insights into the association between DNA demethylation and male meiotic recombination.

Emergence and influence of sequence bias in evolutionarily malleable, mammalian tandem arrays

BACKGROUND

The radiation of mammals at the extinction of the dinosaurs produced a plethora of new forms-as diverse as bats, dolphins, and elephants-in only 10-20 million years. Behind the scenes, adaptation to new niches is accompanied by extensive innovation in large families of genes that allow animals to contact the environment, including chemosensors, xenobiotic enzymes, and immune and barrier proteins. Genes in these "outward-looking" families are allelically diverse among humans and exhibit tissue-specific and sometimes stochastic expression.

RESULTS

Here, we show that these tandem arrays of outward-looking genes occupy AT-biased isochores and comprise the "tissue-specific" gene class that lack CpG islands in their promoters. Models of mammalian genome evolution have not incorporated the sharply different functions and transcriptional patterns of genes in AT- versus GC-biased regions. To examine the relationship between gene family expansion, sequence content, and allelic diversity, we use population genetic data and comparative analysis. First, we find that AT bias can emerge during evolutionary expansion of gene families in cis. Second, human genes in AT-biased isochores or with GC-poor promoters experience relatively low rates of de novo point mutation today but are enriched for non-synonymous variants. Finally, we find that isochores containing gene clusters exhibit low rates of recombination.

CONCLUSIONS

Our analyses suggest that tolerance of non-synonymous variation and low recombination are two forces that have produced the depletion of GC bases in outward-facing gene arrays. In turn, high AT content exerts a profound effect on their chromatin organization and transcriptional regulation.

Transposon waves at the water-to-land transition

The major transitions in vertebrate evolution are associated with significant genomic reorganizations. In contrast to the evolutionary processes that occurred at the origin of vertebrates or prior to the radiation of teleost fishes, no whole-genome duplication events occurred during the water-to-land transition, and it remains an open question how did genome dynamics contribute to this prominent evolutionary event. Indeed, the recent sequencing of sarcopterygian and amphibian genomes has revealed that the extant lineages immediately preceding and succeeding this transition harbor an exceptional number of transposable elements and it is tempting to speculate that these sequences might have catalyzed the adaptations that enabled vertebrates to venture into land. Here, we review the genome dynamics associated with the major transitions in vertebrate evolution and discuss how the highly repetitive genomic landscapes revealed by recent efforts to characterize the genomes of amphibians and sarcopterygians argue for turbulent genome dynamics occurring before the water-to-land transition and possibly enabling it.

Patterns of recombination in snakes reveal a tug of war between PRDM9 and promoter-like features

In vertebrates, there are two known mechanisms by which meiotic recombination is directed to the genome: in humans, mice, and other mammals, recombination occurs almost exclusively where the protein PRDM9 binds, while in species lacking an intact PRDM9, such as birds and canids, recombination rates are elevated near promoter-like features. To test if PRDM9 also directs recombination in non-mammalian vertebrates, we focused on an exemplar species, the corn snake (Pantherophis guttatus). Unlike birds, this species possesses a single, intact PRDM9 ortholog. By inferring historical recombination rates along the genome from patterns of linkage disequilibrium and identifying crossovers in pedigrees, we found that PRDM9 specifies the location of recombination events outside of mammals. However, we also detected an independent effect of promoter-like features on recombination, which is more pronounced on macro- than microchromosomes. Thus, our findings reveal that the uses of PRDM9 and promoter-like features are not mutually-exclusive, and instead reflect a tug of war, which varies in strength along the genome and is more lopsided in some species than others.

On the dependent recognition of some long zinc finger proteins

The human genome contains about 800 C2H2 zinc finger proteins (ZFPs), and most of them are composed of long arrays of zinc fingers. Standard ZFP recognition model asserts longer finger arrays should recognize longer DNA-binding sites. However, recent experimental efforts to identify in vivo ZFP binding sites contradict this assumption, with many exhibiting short motifs. Here we use ZFY, CTCF, ZIM3, and ZNF343 as examples to address three closely related questions: What are the reasons that impede current motif discovery methods? What are the functions of those seemingly unused fingers and how can we improve the motif discovery algorithms based on long ZFPs' biophysical properties? Using ZFY, we employed a variety of methods and find evidence for 'dependent recognition' where downstream fingers can recognize some previously undiscovered motifs only in the presence of an intact core site. For CTCF, high-throughput measurements revealed its upstream specificity profile depends on the strength of its core. Moreover, the binding strength of the upstream site modulates CTCF's sensitivity to different epigenetic modifications within the core, providing new insight into how the previously identified intellectual disability-causing and cancer-related mutant R567W disrupts upstream recognition and deregulates the epigenetic control by CTCF. Our results establish that, because of irregular motif structures, variable spacing and dependent recognition between sub-motifs, the specificities of long ZFPs are significantly underestimated, so we developed an algorithm, ModeMap, to infer the motifs and recognition models of ZIM3 and ZNF343, which facilitates high-confidence identification of specific binding sites, including repeats-derived elements. With revised concept, technique, and algorithm, we can discover the overlooked specificities and functions of those 'extra' fingers, and therefore decipher their broader roles in human biology and diseases.

Recombination between heterologous human acrocentric chromosomes

The short arms of the human acrocentric chromosomes 13, 14, 15, 21 and 22 (SAACs) share large homologous regions, including ribosomal DNA repeats and extended segmental duplications1,2. Although the resolution of these regions in the first complete assembly of a human genome-the Telomere-to-Telomere Consortium's CHM13 assembly (T2T-CHM13)-provided a model of their homology3, it remained unclear whether these patterns were ancestral or maintained by ongoing recombination exchange. Here we show that acrocentric chromosomes contain pseudo-homologous regions (PHRs) indicative of recombination between non-homologous sequences. Utilizing an all-to-all comparison of the human pangenome from the Human Pangenome Reference Consortium4 (HPRC), we find that contigs from all of the SAACs form a community. A variation graph5 constructed from centromere-spanning acrocentric contigs indicates the presence of regions in which most contigs appear nearly identical between heterologous acrocentric chromosomes in T2T-CHM13. Except on chromosome 15, we observe faster decay of linkage disequilibrium in the pseudo-homologous regions than in the corresponding short and long arms, indicating higher rates of recombination6,7. The pseudo-homologous regions include sequences that have previously been shown to lie at the breakpoint of Robertsonian translocations8, and their arrangement is compatible with crossover in inverted duplications on chromosomes 13, 14 and 21. The ubiquity of signals of recombination between heterologous acrocentric chromosomes seen in the HPRC draft pangenome suggests that these shared sequences form the basis for recurrent Robertsonian translocations, providing sequence and population-based confirmation of hypotheses first developed from cytogenetic studies 50 years ago9.

A novel hypervariable variable number tandem repeat in the dopamine transporter gene (SLC6A3)

The dopamine transporter gene, SLC6A3, has received substantial attention in genetic association studies of various phenotypes. Although some variable number tandem repeats (VNTRs) present in SLC6A3 have been tested in genetic association studies, results have not been consistent. VNTRs in SLC6A3 that have not been examined genetically were characterized. The Tandem Repeat Annotation Library was used to characterize the VNTRs of 64 unrelated long-read haplotype-phased SLC6A3 sequences. Sequence similarity of each repeat unit of the five VNTRs is reported, along with the correlations of SNP-SNP, SNP-VNTR, and VNTR-VNTR alleles across the gene. One of these VNTRs is a novel hyper-VNTR (hyVNTR) in intron 8 of SLC6A3, which contains a range of 3.4-133.4 repeat copies and has a consensus sequence length of 38 bp, with 82% G+C content. The 38-base repeat was predicted to form G-quadruplexes in silico and was confirmed by circular dichroism spectroscopy. In addition, this hyVNTR contains multiple putative binding sites for PRDM9, which, in combination with low levels of linkage disequilibrium around the hyVNTR, suggests it might be a recombination hotspot.

The RNA-binding protein FUS/TLS interacts with SPO11 and PRDM9 and localize at meiotic recombination hotspots

In mammals, meiotic recombination is initiated by the introduction of DNA double strand breaks (DSBs) into narrow segments of the genome, defined as hotspots, which is carried out by the SPO11/TOPOVIBL complex. A major player in the specification of hotspots is PRDM9, a histone methyltransferase that, following sequence-specific DNA binding, generates trimethylation on lysine 4 (H3K4me3) and lysine 36 (H3K36me3) of histone H3, thus defining the hotspots. PRDM9 activity is key to successful meiosis, since in its absence DSBs are redirected to functional sites and synapsis between homologous chromosomes fails. One protein factor recently implicated in guiding PRDM9 activity at hotspots is EWS, a member of the FET family of proteins that also includes TAF15 and FUS/TLS. Here, we demonstrate that FUS/TLS partially colocalizes with PRDM9 on the meiotic chromosome axes, marked by the synaptonemal complex component SYCP3, and physically interacts with PRDM9. Furthermore, we show that FUS/TLS also interacts with REC114, one of the axis-bound SPO11-auxiliary factors essential for DSB formation. This finding suggests that FUS/TLS is a component of the protein complex that promotes the initiation of meiotic recombination. Accordingly, we document that FUS/TLS coimmunoprecipitates with SPO11 in vitro and in vivo. The interaction occurs with both SPO11β and SPO11α splice isoforms, which are believed to play distinct functions in the formation of DSBs in autosomes and male sex chromosomes, respectively. Finally, using chromatin immunoprecipitation experiments, we show that FUS/TLS is localized at H3K4me3-marked hotspots in autosomes and in the pseudo-autosomal region, the site of genetic exchange between the XY chromosomes.

Decorating chromatin for enhanced genome editing using CRISPR-Cas9

CRISPR-associated (Cas) enzymes have revolutionized biology by enabling RNA-guided genome editing. Homology-directed repair (HDR) in the presence of donor templates is currently the most versatile method to introduce precise edits following CRISPR-Cas-induced double-stranded DNA cuts, but HDR efficiency is generally low relative to end-joining pathways that lead to insertions and deletions (indels). We tested the hypothesis that HDR could be increased using a Cas9 construct fused to PRDM9, a chromatin remodeling factor that deposits histone methylations H3K36me3 and H3K4me3 to mediate homologous recombination in human cells. Our results show that the fusion protein contacts chromatin specifically at the Cas9 cut site in the genome to increase the observed HDR efficiency by threefold and HDR:indel ratio by fivefold compared with that induced by unmodified Cas9. HDR enhancement occurred in multiple cell lines with no increase in off-target genome editing. These findings underscore the importance of chromatin features for the balance between DNA repair mechanisms during CRISPR-Cas genome editing and provide a strategy to increase HDR efficiency.

Nuclear-embedded mitochondrial DNA sequences in 66,083 human genomes

DNA transfer from cytoplasmic organelles to the cell nucleus is a legacy of the endosymbiotic event-the majority of nuclear-mitochondrial segments (NUMTs) are thought to be ancient, preceding human speciation^1-3. Here we analyse whole-genome sequences from 66,083 people-including 12,509 people with cancer-and demonstrate the ongoing transfer of mitochondrial DNA into the nucleus, contributing to a complex NUMT landscape. More than 99% of individuals had at least one of 1,637 different NUMTs, with 1 in 8 individuals having an ultra-rare NUMT that is present in less than 0.1% of the population. More than 90% of the extant NUMTs that we evaluated inserted into the nuclear genome after humans diverged from apes. Once embedded, the sequences were no longer under the evolutionary constraint seen within the mitochondrion, and NUMT-specific mutations had a different mutational signature to mitochondrial DNA. De novo NUMTs were observed in the germline once in every 10⁴ births and once in every 10³ cancers. NUMTs preferentially involved non-coding mitochondrial DNA, linking transcription and replication to their origin, with nuclear insertion involving multiple mechanisms including double-strand break repair associated with PR domain zinc-finger protein 9 (PRDM9) binding. The frequency of tumour-specific NUMTs differed between cancers, including a probably causal insertion in a myxoid liposarcoma. We found evidence of selection against NUMTs on the basis of size and genomic location, shaping a highly heterogenous and dynamic human NUMT landscape.

Evolutionary dynamics of pseudoautosomal region 1 in humans and great apes

BACKGROUND

The pseudoautosomal region 1 (PAR1) is a 2.7 Mb telomeric region of human sex chromosomes. PAR1 has a crucial role in ensuring proper segregation of sex chromosomes during male meiosis, exposing it to extreme recombination and mutation processes. We investigate PAR1 evolution using population genomic datasets of extant humans, eight populations of great apes, and two archaic human genome sequences.

RESULTS

We find that PAR1 is fast evolving and closer to evolutionary nucleotide equilibrium than autosomal telomeres. We detect a difference between substitution patterns and extant diversity in PAR1, mainly driven by the conflict between strong mutation and recombination-associated fixation bias at CpG sites. We detect excess C-to-G mutations in PAR1 of all great apes, specific to the mutagenic effect of male recombination. Despite recent evidence for Y chromosome introgression from humans into Neanderthals, we find that the Neanderthal PAR1 retained similarity to the Denisovan sequence. We find differences between substitution spectra of these archaics suggesting rapid evolution of PAR1 in recent hominin history. Frequency analysis of alleles segregating in females and males provided no evidence for recent sexual antagonism in this region. We study repeat content and double-strand break hotspot regions in PAR1 and find that they may play roles in ensuring the obligate X-Y recombination event during male meiosis.

CONCLUSIONS

Our study provides an unprecedented quantification of population genetic forces governing PAR1 biology across extant and extinct hominids. PAR1 evolutionary dynamics are predominantly governed by recombination processes with a strong impact on mutation patterns across all species.

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.

scPCOR-seq enables co-profiling of chromatin occupancy and RNAs in single cells

Cell-to-cell variation in gene expression is a widespread phenomenon, which may play important roles in cellular differentiation, function, and disease development^1–9. Chromatin is implicated in contributing to the cellular heterogeneity in gene expression^10–16. Fully understanding the mechanisms of cellular heterogeneity requires simultaneous measurement of RNA and occupancy of histone modifications and transcription factors on chromatin due to their critical roles in transcriptional regulation^17,18. We generally term the occupancy of histone modifications and transcription factors as Chromatin occupancy. Here, we report a technique, termed scPCOR-seq (single-cell Profiling of Chromatin Occupancy and RNAs Sequencing), for simultaneously profiling genome-wide chromatin protein binding or histone modification marks and RNA expression in the same cell. We demonstrated that scPCOR-seq can profile either H3K4me3 or RNAPII and RNAs in a mixture of human H1, GM12878 and 293 T cells at a single-cell resolution and either H3K4me3, RNAPII, or RNA profile can correctly separate the cells. Application of scPCOR-seq to the in vitro differentiation of the erythrocyte precursor CD36 cells from human CD34 stem or progenitor cells revealed that H3K4me3 and RNA exhibit distinct properties in clustering cells during differentiation. Overall, our work provides a promising approach to understand the relationships among different omics layers.

scPCOR-seq is a single-cell sequencing technique that enables simultaneous profiling of genome-wide chromatin protein binding or histone modification marks and RNA expression in the same cell.

PRDM9 losses in vertebrates are coupled to those of paralogs ZCWPW1 and ZCWPW2

We take a phylogenetic approach to search for molecular partners of PRDM9, a key meiotic recombination gene, by leveraging the fact that the complete PRDM9 gene has been lost at least 13 times independently in vertebrates. We identify two genes, ZCWPW1 and its paralog ZCWPW2, whose presence or absence across vertebrates is coupled to that of PRDM9. ZCWPW1 was recently shown to be recruited to sites of PRDM9 binding and to aid in the repair of double strand breaks. ZCWPW2 is likely recruited to sites of PRDM9 binding as well; its tight coevolution with PRDM9 across vertebrates suggests that it too plays an important role in mammals and beyond, either in double strand break formation or repair.

In most mammals and likely throughout vertebrates, the gene PRDM9 specifies the locations of meiotic double strand breaks; in mice and humans at least, it also aids in their repair. For both roles, many of the molecular partners remain unknown. Here, we take a phylogenetic approach to identify genes that may be interacting with PRDM9 by leveraging the fact that PRDM9 arose before the origin of vertebrates but was lost many times, either partially or entirely—and with it, its role in recombination. As a first step, we characterize PRDM9 domain composition across 446 vertebrate species, inferring at least 13 independent losses. We then use the interdigitation of PRDM9 orthologs across vertebrates to test whether it coevolved with any of 241 candidate genes coexpressed with PRDM9 in mice or associated with recombination phenotypes in mammals. Accounting for the phylogenetic relationship among a subsample of 189 species, we find two genes whose presence and absence is unexpectedly coincident with that of PRDM9: ZCWPW1, which was recently shown to facilitate double strand break repair, and its paralog ZCWPW2, as well as, more tentatively, TEX15 and FBXO47. ZCWPW2 is expected to be recruited to sites of PRDM9 binding; its tight coevolution with PRDM9 across vertebrates suggests that it is a key interactor within mammals and beyond, with a role either in recruiting the recombination machinery or in double strand break repair.

Cataloging Human PRDM9 Allelic Variation Using Long-Read Sequencing Reveals PRDM9 Population Specificity and Two Distinct Groupings of Related Alleles

The PRDM9 protein determines sites of meiotic recombination in humans by directing meiotic DNA double-strand breaks to specific loci. Targeting specificity is encoded by a long array of C₂H₂ zinc fingers that bind to DNA. This zinc finger array is hypervariable, and the resulting alleles each have a potentially different DNA binding preference. The assessment of PRDM9 diversity is important for understanding the complexity of human population genetics, inheritance linkage patterns, and predisposition to genetic disease. Due to the repetitive nature of the PRDM9 zinc finger array, the large-scale sequencing of human PRDM9 is challenging. We, therefore, developed a long-read sequencing strategy to infer the diploid PRDM9 zinc finger array genotype in a high-throughput manner. From an unbiased study of PRDM9 allelic diversity in 720 individuals from seven human populations, we detected 69 PRDM9 alleles. Several alleles differ in frequency among human populations, and 32 alleles had not been identified by previous studies, which were heavily biased to European populations. PRDM9 alleles are distinguished by their DNA binding site preferences and fall into two major categories related to the most common PRDM9-A and PRDM9-C alleles. We also found that it is likely that inter-conversion between allele types is rare. By mapping meiotic double-strand breaks (DSBs) in the testis, we found that small variations in PRDM9 can substantially alter the meiotic recombination landscape, demonstrating that minor PRDM9 variants may play an under-appreciated role in shaping patterns of human recombination. In summary, our data greatly expands knowledge of PRDM9 diversity in humans.

Mutational bias in spermatogonia impacts the anatomy of regulatory sites in the human genome

Mutation in the germline is the ultimate source of genetic variation, but little is known about the influence of germline chromatin structure on mutational processes. Using ATAC-seq, we profile the open chromatin landscape of human spermatogonia, the most proliferative cell type of the germline, identifying transcription factor binding sites (TFBSs) and PRDM9 binding sites, a subset of which will initiate meiotic recombination. We observe an increase in rare structural variant (SV) breakpoints at PRDM9-bound sites, implicating meiotic recombination in the generation of structural variation. Many germline TFBSs, such as NRF1, are also associated with increased rates of SV breakpoints, apparently independent of recombination. Singleton short insertions (≥5 bp) are highly enriched at TFBSs, particularly at sites bound by testis active TFs, and their rates correlate with those of structural variant breakpoints. Short insertions often duplicate the TFBS motif, leading to clustering of motif sites near regulatory regions in this male-driven evolutionary process. Increased mutation loads at germline TFBSs disproportionately affect neural enhancers with activity in spermatogonia, potentially altering neurodevelopmental regulatory architecture. Local chromatin structure in spermatogonia is thus pervasive in shaping both evolution and disease.

Altering the binding properties of PRDM9 partially restores fertility across the species boundary

Sterility or subfertility of male hybrid offspring is commonly observed. This phenomenon contributes to reproductive barriers between the parental populations, an early step in the process of speciation. One frequent cause of such infertility is a failure of proper chromosome pairing during male meiosis. In subspecies of the house mouse, the likelihood of successful chromosome synapsis is improved by the binding of the histone methyltransferase PRDM9 to both chromosome homologues at matching positions. Using genetic manipulation, we altered PRDM9 binding to occur more often at matched sites, and find that chromosome pairing defects can be rescued, not only in an inter-subspecific cross, but also between distinct species. Using different engineered variants, we demonstrate a quantitative link between the degree of matched homologue binding, chromosome synapsis and rescue of fertility in hybrids between Mus musculus and Mus spretus. The resulting partial restoration of fertility reveals additional mechanisms at play that act to lock-in the reproductive isolation between these two species.

Haplotype Shuffling and Dimorphic Transposable Elements in the Human Extended Major Histocompatibility Complex Class II Region

The major histocompatibility complex (MHC) on chromosome 6p21 is one of the most single-nucleotide polymorphism (SNP)-dense regions of the human genome and a prime model for the study and understanding of conserved sequence polymorphisms and structural diversity of ancestral haplotypes/conserved extended haplotypes. This study aimed to follow up on a previous analysis of the MHC class I region by using the same set of 95 MHC haplotype sequences downloaded from a publicly available BioProject database at the National Center for Biotechnology Information to identify and characterize the polymorphic human leukocyte antigen (HLA)-class II genes, the MTCO3P1 pseudogene alleles, the indels of transposable elements as haplotypic lineage markers, and SNP-density crossover (XO) loci at haplotype junctions in DNA sequence alignments of different haplotypes across the extended class II region (∼1 Mb) from the telomeric PRRT1 gene in class III to the COL11A2 gene at the centromeric end of class II. We identified 42 haplotypic indels (20 Alu, 7 SVA, 13 LTR or MERs, and 2 indels composed of a mosaic of different transposable elements) linked to particular HLA-class II alleles. Comparative sequence analyses of 136 haplotype pairs revealed 98 unique XO sites between SNP-poor and SNP-rich genomic segments with considerable haplotype shuffling located in the proximity of putative recombination hotspots. The majority of XO sites occurred across various regions including in the vicinity of MTCO3P1 between HLA-DQB1 and HLA-DQB3, between HLA-DQB2 and HLA-DOB, between DOB and TAP2, and between HLA-DOA and HLA-DPA1, where most XOs were within a HERVK22 sequence. We also determined the genomic positions of the PRDM9-recombination suppression sequence motif ATCCATG/CATGGAT and the PRDM9 recombination activation partial binding motif CCTCCCCT/AGGGGAG in the class II region of the human reference genome (NC_ 000006) relative to published meiotic recombination positions. Both the recombination and anti-recombination PRDM9 binding motifs were widely distributed throughout the class II genomic regions with 50% or more found within repeat elements; the anti-recombination motifs were found mostly in L1 fragmented repeats. This study shows substantial haplotype shuffling between different polymorphic blocks and confirms the presence of numerous putative ancestral recombination sites across the class II region between various HLA class II genes.

The duality of PRDM proteins: epigenetic and structural perspectives

PRDF1 and RIZ1 homology domain containing (PRDMs) are a subfamily of Krüppel-like zinc finger proteins controlling key processes in metazoan development and in cancer. PRDMs exhibit unique dualities: (a) PR domain/ZNF arrays-their structure combines a SET-like domain known as a PR domain, typically found in methyltransferases, with a variable array of C2H2 zinc fingers (ZNF) characteristic of DNA-binding transcription factors; (b) transcriptional activators/repressors-their physiological function is context- and cell-dependent; mechanistically, some PRDMs have a PKMT activity and directly catalyze histone lysine methylation, while others are rather pseudomethyltransferases and act by recruiting transcriptional cofactors; (c) oncogenes/tumor suppressors-their pathological function depends on the specific PRDM isoform expressed during tumorigenesis. This duality is well known as the 'Yin and Yang' of PRDMs and involves a complex regulation of alternative splicing or alternative promoter usage, to generate full-length or PR-deficient isoforms with opposing functions in cancer. In conclusion, once their dualities are fully appreciated, PRDMs represent a promising class of targets in oncology by virtue of their widespread upregulation across multiple tumor types and their somatic dispensability, conferring a broad therapeutic window and limited toxic side effects. The recent discovery of a first-in-class compound able to inhibit PRDM9 activity has paved the way for the identification of further small molecular inhibitors able to counteract PRDM oncogenic activity.

Snake Recombination Landscapes Are Concentrated in Functional Regions despite PRDM9

Meiotic recombination in vertebrates is concentrated in hotspots throughout the genome. The location and stability of hotspots have been linked to the presence or absence of PRDM9, leading to two primary models for hotspot evolution derived from mammals and birds. Species with PRDM9-directed recombination have rapid turnover of hotspots concentrated in intergenic regions (i.e., mammals), whereas hotspots in species lacking PRDM9 are concentrated in functional regions and have greater stability over time (i.e., birds). Snakes possess PRDM9, yet virtually nothing is known about snake recombination. Here, we examine the recombination landscape and test hypotheses about the roles of PRDM9 in rattlesnakes. We find substantial variation in recombination rate within and among snake chromosomes, and positive correlations between recombination rate and gene density, GC content, and genetic diversity. Like mammals, snakes appear to have a functional and active PRDM9, but rather than being directed away from genes, snake hotspots are concentrated in promoters and functional regions-a pattern previously associated only with species that lack a functional PRDM9. Snakes therefore provide a unique example of recombination landscapes in which PRDM9 is functional, yet recombination hotspots are associated with functional genic regions-a combination of features that defy existing paradigms for recombination landscapes in vertebrates. Our findings also provide evidence that high recombination rates are a shared feature of vertebrate microchromosomes. Our results challenge previous assumptions about the adaptive role of PRDM9 and highlight the diversity of recombination landscape features among vertebrate lineages.

SNP-Density Crossover Maps of Polymorphic Transposable Elements and HLA Genes Within MHC Class I Haplotype Blocks and Junction

The genomic region (~4 Mb) of the human major histocompatibility complex (MHC) on chromosome 6p21 is a prime model for the study and understanding of conserved polymorphic sequences (CPSs) and structural diversity of ancestral haplotypes (AHs)/conserved extended haplotypes (CEHs). The aim of this study was to use a set of 95 MHC genomic sequences downloaded from a publicly available BioProject database at NCBI to identify and characterise polymorphic human leukocyte antigen (HLA) class I genes and pseudogenes, MICA and MICB, and retroelement indels as haplotypic lineage markers, and single-nucleotide polymorphism (SNP) crossover loci in DNA sequence alignments of different haplotypes across the Olfactory Receptor (OR) gene region (~1.2 Mb) and the MHC class I region (~1.8 Mb) from the GPX5 to the MICB gene. Our comparative sequence analyses confirmed the identity of 12 haplotypic retroelement markers and revealed that they partitioned the HLA-A/B/C haplotypes into distinct evolutionary lineages. Crossovers between SNP-poor and SNP-rich regions defined the sequence range of haplotype blocks, and many of these crossover junctions occurred within particular transposable elements, lncRNA, OR12D2, MUC21, MUC22, PSORS1A3, HLA-C, HLA-B, and MICA. In a comparison of more than 250 paired sequence alignments, at least 38 SNP-density crossover sites were mapped across various regions from GPX5 to MICB. In a homology comparison of 16 different haplotypes, seven CEH/AH (7.1, 8.1, 18.2, 51.x, 57.1, 62.x, and 62.1) had no detectable SNP-density crossover junctions and were SNP poor across the entire ~2.8 Mb of sequence alignments. Of the analyses between different recombinant haplotypes, more than half of them had SNP crossovers within 10 kb of LTR16B/ERV3-16A3_I, MLT1, Charlie, and/or THE1 sequences and were in close vicinity to structurally polymorphic Alu and SVA insertion sites. These studies demonstrate that (1) SNP-density crossovers are associated with putative ancestral recombination sites that are widely spread across the MHC class I genomic region from at least the telomeric OR12D2 gene to the centromeric MICB gene and (2) the genomic sequences of MHC homozygous cell lines are useful for analysing haplotype blocks, ancestral haplotypic landscapes and markers, CPSs, and SNP-density crossover junctions.

EWSR1 affects PRDM9-dependent histone 3 methylation and provides a link between recombination hotspots and the chromosome axis protein REC8

Meiotic recombination in most mammals requires recombination hotspot activation through the action of the histone 3 Lys-4 and Lys-36 methyltransferase PRDM9 to ensure successful double-strand-break initiation and repair. Here we show that EWSR1, a protein whose role in meiosis was not previously clarified in detail, binds to both PRDM9 and pREC8, a phosphorylated meiosis-specific cohesin, in male meiotic cells. We created a Ewsr1 conditional knockout mouse model to deplete EWSR1 before the onset of meiosis and found that absence of EWSR1 causes meiotic arrest with decreased histone trimethylation at meiotic hotspots, impaired DNA double-strand-break repair, and reduced crossover number. Our results demonstrate that EWSR1 is essential for promoting PRDM9-dependent histone methylation and normal meiotic progress, possibly by facilitating the linking between PRDM9-bound hotspots and the nascent chromosome axis through its component cohesin pREC8.

The Configuration of RPA, RAD51, and DMC1 Binding in Meiosis Reveals the Nature of Critical Recombination Intermediates

Meiotic recombination proceeds via binding of RPA, RAD51, and DMC1 to single-stranded DNA (ssDNA) substrates created after formation of programmed DNA double-strand breaks. Here we report high-resolution in vivo maps of RPA and RAD51 in meiosis, mapping their binding locations and lifespans to individual homologous chromosomes using a genetically engineered hybrid mouse. Together with high-resolution microscopy and DMC1 binding maps, we show that DMC1 and RAD51 have distinct spatial localization on ssDNA: DMC1 binds near the break site, and RAD51 binds away from it. We characterize inter-homolog recombination intermediates bound by RPA in vivo, with properties expected for the critical displacement loop (D-loop) intermediates. These data support the hypothesis that DMC1, not RAD51, performs strand exchange in mammalian meiosis. RPA-bound D-loops can be resolved as crossovers or non-crossovers, but crossover-destined D-loops may have longer lifespans. D-loops resemble crossover gene conversions in size, but their extent is similar in both repair pathways.

ZCWPW1 is recruited to recombination hotspots by PRDM9 and is essential for meiotic double strand break repair

During meiosis, homologous chromosomes pair and recombine, enabling balanced segregation and generating genetic diversity. In many vertebrates, double-strand breaks (DSBs) initiate recombination within hotspots where PRDM9 binds, and deposits H3K4me3 and H3K36me3. However, no protein(s) recognising this unique combination of histone marks have been identified. We identified Zcwpw1, containing H3K4me3 and H3K36me3 recognition domains, as having highly correlated expression with Prdm9. Here, we show that ZCWPW1 has co-evolved with PRDM9 and, in human cells, is strongly and specifically recruited to PRDM9 binding sites, with higher affinity than sites possessing H3K4me3 alone. Surprisingly, ZCWPW1 also recognises CpG dinucleotides. Male Zcwpw1 knockout mice show completely normal DSB positioning, but persistent DMC1 foci, severe DSB repair and synapsis defects, and downstream sterility. Our findings suggest ZCWPW1 recognition of PRDM9-bound sites at DSB hotspots is critical for synapsis, and hence fertility.

Sexual reproduction – that is, the combination of sex cells from two different individuals to produce an embryo – is one of the many mechanisms that have evolved to maintain genetic diversity. Most human cells contain 23 pairs of chromosomes, with each chromosome in a pair carrying either a paternal or maternal copy of the same gene. To form an embryo with the right number of chromosomes, each sex cell (the egg or sperm cell) must only contain one chromosome from each pair.

Sex cells are produced from parent cells containing two sets of paternal and maternal chromosomes: these cells then divide twice to form four sex cells which contain only one chromosome from each pair. Before the parent cell divides, a process known as ‘recombination’ takes place, which allows chromosomes in a pair to exchange bits of genetic information. This reshuffling ensures that each chromosome in a sex cell is unique. A protein called PRDM9 helps control which sections of genetic information are recombined by modifying proteins attached to the chromosomes, marking them as locations for exchange. The DNA at each of these sites is then broken and repaired using the genetic sequence of the chromosome it is paired with as a template, thus causing the two chromosomes to swap genes.

In 2019, a group of researchers found a set of genes in the testis of mice that are expressed at the same time as the gene for PRDM9. This suggested that another protein called ZCWPW1 is likely involved in recombination, but the precise role of this protein was unclear. To answer this question, Wells, Bitoun et al. – including many of the researchers involved in the 2019 study – examined human cells grown in the laboratory to determine where ZCWPW1 binds to in the chromosome.

This revealed that ZCWPW1 can be found at the same sites as PRDM9, which is responsible for bringing it there. Furthermore, cells from male mice lacking the gene for ZCWPW1 cannot complete the exchange of genetic information between chromosomes, meaning that the mice are infertile. As such, ZCWPW1 seems to connect location selection by PRDM9 to the DNA repair mechanisms needed for gene exchange between chromosomes.

Infertility is a significant issue for humans affecting as many as one in every six couples. Fertility is complex and many of the biological mechanisms involved are not fully understood. This work suggests that both PRDM9 and ZCWPW1 are key to the production of sex cells and may be worth investigating as factors that affect fertility in humans.

Magnesium, Calcium, Potassium, Sodium, Phosphorus, Selenium, Zinc, and Chromium Levels in Alcohol Use Disorder: A Review

Macronutrients and trace elements are important components of living tissues that have different metabolic properties and functions. Trace elements participate in the regulation of immunity through humoral and cellular mechanisms, nerve conduction, muscle spasms, membrane potential regulation as well as mitochondrial activity and enzymatic reactions. Excessive alcohol consumption disrupts the concentrations of crucial trace elements, also increasing the risk of enhanced oxidative stress and alcohol-related liver diseases. In this review, we present the status of selected macroelements and trace elements in the serum and plasma of people chronically consuming alcohol. Such knowledge helps to understand the mechanisms of chronic alcohol-use disorder and to progress and prevent withdrawal effects, also improving treatment strategies.

An equivariant Bayesian convolutional network predicts recombination hotspots and accurately resolves binding motifs

MOTIVATION

Convolutional neural networks (CNNs) have been tremendously successful in many contexts, particularly where training data are abundant and signal-to-noise ratios are large. However, when predicting noisily observed phenotypes from DNA sequence, each training instance is only weakly informative, and the amount of training data is often fundamentally limited, emphasizing the need for methods that make optimal use of training data and any structure inherent in the process.

RESULTS

Here we show how to combine equivariant networks, a general mathematical framework for handling exact symmetries in CNNs, with Bayesian dropout, a version of Monte Carlo dropout suggested by a reinterpretation of dropout as a variational Bayesian approximation, to develop a model that exhibits exact reverse-complement symmetry and is more resistant to overtraining. We find that this model combines improved prediction consistency with better predictive accuracy compared to standard CNN implementations and state-of-art motif finders. We use our network to predict recombination hotspots from sequence, and identify binding motifs for the recombination-initiation protein PRDM9 previously unobserved in this data, which were recently validated by high-resolution assays. The network achieves a predictive accuracy comparable to that attainable by a direct assay of the H3K4me3 histone mark, a proxy for PRDM9 binding.

AVAILABILITY AND IMPLEMENTATION

https://github.com/luntergroup/EquivariantNetworks.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Multifaceted Role of PRDM Proteins in Human Cancer

The PR/SET domain family (PRDM) comprise a family of genes whose protein products share a conserved N-terminal PR [PRDI-BF1 (positive regulatory domain I-binding factor 1) and RIZ1 (retinoblastoma protein-interacting zinc finger gene 1)] homologous domain structurally and functionally similar to the catalytic SET [Su(var)3-9, enhancer-of-zeste and trithorax] domain of histone methyltransferases (HMTs). These genes are involved in epigenetic regulation of gene expression through their intrinsic HMTase activity or via interactions with other chromatin modifying enzymes. In this way they control a broad spectrum of biological processes, including proliferation and differentiation control, cell cycle progression, and maintenance of immune cell homeostasis. In cancer, tumor-specific dysfunctions of PRDM genes alter their expression by genetic and/or epigenetic modifications. A common characteristic of most PRDM genes is to encode for two main molecular variants with or without the PR domain. They are generated by either alternative splicing or alternative use of different promoters and play opposite roles, particularly in cancer where their imbalance can be often observed. In this scenario, PRDM proteins are involved in cancer onset, invasion, and metastasis and their altered expression is related to poor prognosis and clinical outcome. These functions strongly suggest their potential use in cancer management as diagnostic or prognostic tools and as new targets of therapeutic intervention.

Discovery of a chemical probe for PRDM9

PRDM9 is a PR domain containing protein which trimethylates histone 3 on lysine 4 and 36. Its normal expression is restricted to germ cells and attenuation of its activity results in altered meiotic gene transcription, impairment of double-stranded breaks and pairing between homologous chromosomes. There is growing evidence for a role of aberrant expression of PRDM9 in oncogenesis and genome instability. Here we report the discovery of MRK-740, a potent (IC₅₀: 80 ± 16 nM), selective and cell-active PRDM9 inhibitor (Chemical Probe). MRK-740 binds in the substrate-binding pocket, with unusually extensive interactions with the cofactor S-adenosylmethionine (SAM), conferring SAM-dependent substrate-competitive inhibition. In cells, MRK-740 specifically and directly inhibits H3K4 methylation at endogenous PRDM9 target loci, whereas the closely related inactive control compound, MRK-740-NC, does not. The discovery of MRK-740 as a chemical probe for the PRDM subfamily of methyltransferases highlights the potential for exploiting SAM in targeting SAM-dependent methyltransferases.

The Arms Race Between KRAB–Zinc Finger Proteins and Endogenous Retroelements and Its Impact on Mammals

Nearly half of the human genome consists of endogenous retroelements (EREs) and their genetic remnants, a small fraction of which carry the potential to propagate in the host genome, posing a threat to genome integrity and cell/organismal survival. The largest family of transcription factors in tetrapods, the Krüppel-associated box domain zinc finger proteins (KRAB-ZFPs), binds to specific EREs and represses their transcription. Since their first appearance over 400 million years ago, KRAB-ZFPs have undergone dramatic expansion and diversification in mammals, correlating with the invasions of new EREs. In this article we review our current understanding of the structure, function, and evolution of KRAB-ZFPs and discuss growing evidence that the arms race between KRAB-ZFPs and the EREs they target is a major driving force for the evolution of new traits in mammals, often accompanied by domestication of EREs themselves.

Inference and analysis of population-specific fine-scale recombination maps across 26 diverse human populations

Fine-scale rates of meiotic recombination vary by orders of magnitude across the genome and differ between species and even populations. Studying cross-population differences has been stymied by the confounding effects of demographic history. To address this problem, we developed a demography-aware method to infer fine-scale recombination rates and applied it to 26 diverse human populations, inferring population-specific recombination maps. These maps recapitulate many aspects of the history of these populations including signatures of the trans-Atlantic slave trade and the Iberian colonization of the Americas. We also investigated modulators of the local recombination rate, finding further evidence that Polycomb group proteins and the trimethylation of H3K27 elevate recombination rates. Further differences in the recombination landscape across the genome and between populations are driven by variation in the gene that encodes the DNA binding protein PRDM9, and we quantify the weak effect of meiotic drive acting to remove its binding sites.

A high-resolution map of non-crossover events reveals impacts of genetic diversity on mammalian meiotic recombination

During meiotic recombination, homologue-templated repair of programmed DNA double-strand breaks (DSBs) produces relatively few crossovers and many difficult-to-detect non-crossovers. By intercrossing two diverged mouse subspecies over five generations and deep-sequencing 119 offspring, we detect thousands of crossover and non-crossover events genome-wide with unprecedented power and spatial resolution. We find that both crossovers and non-crossovers are strongly depleted at DSB hotspots where the DSB-positioning protein PRDM9 fails to bind to the unbroken homologous chromosome, revealing that PRDM9 also functions to promote homologue-templated repair. Our results show that complex non-crossovers are much rarer in mice than humans, consistent with complex events arising from accumulated non-programmed DNA damage. Unexpectedly, we also find that GC-biased gene conversion is restricted to non-crossover tracts containing only one mismatch. These results demonstrate that local genetic diversity profoundly alters meiotic repair pathway decisions via at least two distinct mechanisms, impacting genome evolution and Prdm9-related hybrid infertility.

During meiotic recombination, genetic information is transferred or exchanged between parental chromosome copies. Using a large hybrid mouse pedigree, the authors generated high-resolution maps of these transfer/exchange events and discovered new properties governing their processing and resolution.

Heterogeneous transposable elements as silencers, enhancers and targets of meiotic recombination

During meiosis, DNA double-strand breaks are initiated by the topoisomerase-like enzyme SPO11 and are repaired by inter-sister chromatid and inter-homologue DNA repair pathways. Genome-wide maps of initiating DNA double-strand breaks and inter-homologue repair events are now available for a number of mammalian, fungal and plant species. In mammals, PRDM9 specifies the location of meiotic recombination initiation via recognition of specific DNA sequence motifs by its C2H2 zinc finger array. In fungi and plants, meiotic recombination appears to be initiated less discriminately in accessible chromatin, including at gene promoters. Generally, meiotic crossover is suppressed in highly repetitive genomic regions that are made up of transposable elements (TEs), to prevent deleterious non-allelic homologous recombination events. However, recent and older studies have revealed intriguing relationships between meiotic recombination initiation and repair, and transposable elements. For instance, gene conversion events have been detected in maize centromeric retroelements, mouse MULE-MuDR DNA transposons undergo substantial meiotic recombination initiation, Arabidopsis Helitron TEs are among the hottest of recombination initiation hotspots, and human TE sequences can modify the crossover rate at adjacent PRDM9 motifs in cis. Here, we summarize the relationship between meiotic recombination and TEs, discuss recent insights from highly divergent eukaryotes and highlight outstanding questions in the field.

PRDM9 forms a trimer by interactions within the zinc finger array

PRDM9 forms a trimer as a soluble protein and in complex with DNA mediated by the ZnF domain. Five ZnFs are already sufficient for multimerization and only one DNA molecule is complexed in the trimer.

PRDM9 is a trans-acting factor directing meiotic recombination to specific DNA-binding sites by its zinc finger (ZnF) array. It was suggested that PRDM9 is a multimer; however, we do not know the stoichiometry or the components inducing PRDM9 multimerization. In this work, we used in vitro binding studies and characterized with electrophoretic mobility shift assays, mass spectrometry, and fluorescence correlation spectroscopy the stoichiometry of the PRDM9 multimer of two different murine PRDM9 alleles carrying different tags and domains produced with different expression systems. Based on the migration distance of the PRDM9–DNA complex, we show that PRDM9 forms a trimer. Moreover, this stoichiometry is adapted already by the free, soluble protein with little exchange between protein monomers. The variable ZnF array of PRDM9 is sufficient for multimerization, and at least five ZnFs form already a functional trimer. Finally, we also show that only one ZnF array within the PRDM9 oligomer binds to the DNA, whereas the remaining two ZnF arrays likely maintain the trimer by ZnF–ZnF interactions.

Unified single-cell analysis of testis gene regulation and pathology in five mouse strains

To fully exploit the potential of single-cell functional genomics in the study of development and disease, robust methods are needed to simplify the analysis of data across samples, time-points and individuals. Here we introduce a model-based factor analysis method, SDA, to analyze a novel 57,600 cell dataset from the testes of wild-type mice and mice with gonadal defects due to disruption of the genes Mlh3, Hormad1, Cul4a or Cnp. By jointly analyzing mutant and wild-type cells we decomposed our data into 46 components that identify novel meiotic gene-regulatory programs, mutant-specific pathological processes, and technical effects, and provide a framework for imputation. We identify, de novo, DNA sequence motifs associated with individual components that define temporally varying modes of gene expression control. Analysis of SDA components also led us to identify a rare population of macrophages within the seminiferous tubules of Mlh3-/- and Hormad1-/- mice, an area typically associated with immune privilege.

Meiotic gene activation in somatic and germ cell tumours

Background

Germ cell tumours are uniquely associated with the gametogenic tissues of males and females. A feature of these cancers is that they can express genes that are normally tightly restricted to meiotic cells. This aberrant gene expression has been used as an indicator that these cancer cells are attempting a programmed germ line event, meiotic entry. However, work in non‐germ cell cancers has also indicated that meiotic genes can become aberrantly activated in a wide range of cancer types and indeed provide functions that serve as oncogenic drivers. Here, we review the activation of meiotic factors in cancers and explore commonalities between meiotic gene activation in germ cell and non‐germ cell cancers.

Objectives

The objectives of this review are to highlight key questions relating to meiotic gene activation in germ cell tumours and to offer possible interpretations as to the biological relevance in this unique cancer type.

Materials and Methods

PubMed and the GEPIA database were searched for papers in English and for cancer gene expression data, respectively.

Results

We provide a brief overview of meiotic progression, with a focus on the unique mechanisms of reductional chromosome segregation in meiosis I. We then offer detailed insight into the role of meiotic chromosome regulators in non‐germ cell cancers and extend this to provide an overview of how this might relate to germ cell tumours.

Conclusions

We propose that meiotic gene activation in germ cell tumours might not indicate an unscheduled attempt to enter a full meiotic programme. Rather, it might simply reflect either aberrant activation of a subset of meiotic genes, with little or no biological relevance, or aberrant activation of a subset of meiotic genes as positive tumour evolutionary/oncogenic drivers. These postulates provide the provocation for further studies in this emerging field.

The impact of poly-A microsatellite heterologies in meiotic recombination

Meiosis strongly influences the transmission and evolution of heterozygous poly-A repeats as measured experimentally in a large collection of single recombination products in a human hotspot.

Meiotic recombination has strong, but poorly understood effects on short tandem repeat (STR) instability. Here, we screened thousands of single recombinant products with sperm typing to characterize the role of polymorphic poly-A repeats at a human recombination hotspot in terms of hotspot activity and STR evolution. We show that the length asymmetry between heterozygous poly-A’s strongly influences the recombination outcome: a heterology of 10 A’s (9A/19A) reduces the number of crossovers and elevates the frequency of non-crossovers, complex recombination products, and long conversion tracts. Moreover, the length of the heterology also influences the STR transmission during meiotic repair with a strong and significant insertion bias for the short heterology (6A/7A) and a deletion bias for the long heterology (9A/19A). In spite of this opposing insertion-/deletion-biased gene conversion, we find that poly-A’s are enriched at human recombination hotspots that could have important consequences in hotspot activation.

Factors influencing meiotic recombination revealed by whole-genome sequencing of single sperm

Recombination is critical to meiosis and evolution, yet many aspects of the physical exchange of DNA via crossovers remain poorly understood. We report an approach for single-cell whole-genome DNA sequencing by which we sequenced 217 individual hybrid mouse sperm, providing a kilobase-resolution genome-wide map of crossovers. Combining this map with molecular assays measuring stages of recombination, we identified factors that affect crossover probability, including PRDM9 binding on the non-initiating template homolog and telomere proximity. These factors also influence the time for sites of recombination-initiating DNA double-strand breaks to find and engage their homologs, with rapidly engaging sites more likely to form crossovers. We show that chromatin environment on the template homolog affects positioning of crossover breakpoints. Our results also offer insights into recombination in the pseudoautosomal region.