BRCA1-BARD1 associate with the synaptonemal complex and pro-crossover factors and influence RAD-51 dynamics during Caenorhabditis elegans meiosis

The conserved ATPase PCH-2 controls the number and distribution of crossovers by antagonizing their formation in Caenorhabditis elegans

Meiotic crossover recombination is essential for both accurate chromosome segregation and the generation of new haplotypes for natural selection to act upon. This requirement is known as crossover assurance and is one example of crossover control. While the conserved role of the ATPase, PCH-2, during meiotic prophase has been enigmatic, a universal phenotype when pch-2 or its orthologs are mutated is a change in the number and distribution of meiotic crossovers. Here, we show that PCH-2 controls the number and distribution of crossovers by antagonizing their formation. This antagonism produces different effects at different stages of meiotic prophase: early in meiotic prophase, PCH-2 prevents double-strand breaks from becoming crossover-eligible intermediates, limiting crossover formation at sites of initial double-strand break formation and homolog interactions. Later in meiotic prophase, PCH-2 winnows the number of crossover-eligible intermediates, contributing to the designation of crossovers and ultimately, crossover assurance. We also demonstrate that PCH-2 accomplishes this regulation through the meiotic HORMAD, HIM-3. Our data strongly support a model in which PCH-2's conserved role is to remodel meiotic HORMADs throughout meiotic prophase to destabilize crossover-eligible precursors and coordinate meiotic recombination with synapsis, ensuring the progressive implementation of meiotic recombination and explaining its function in the pachytene checkpoint and crossover control.

Crossovers are regulated by a conserved and disordered synaptonemal complex domain

During meiosis, the number and distribution of crossovers (COs) must be precisely regulated through CO assurance and interference to prevent chromosome missegregation and genomic instability in the progeny. Here we show that this regulation of COs depends on a disordered and conserved domain within the synaptonemal complex (SC). This domain is located at the C-terminus of the central element protein SYP-4 in Caenorhabditis elegans. While not necessary for synapsis, the C-terminus of SYP-4 is crucial for both CO assurance and interference. Although the SYP-4 C-terminus contains many potential phosphorylation sites, we found that phosphorylation is not the primary regulator of CO events. Instead, we discovered that nine conserved phenylalanines are required to recruit a pro-CO factor predicted to be an E3 ligase and regulate the physical properties of the SC. We propose that this conserved and disordered domain plays a crucial role in maintaining the SC in a state that allows transmitting signals to regulate CO formation. While the underlying mechanisms remain to be fully understood, our findings align with existing models suggesting that the SC plays a critical role in determining the number and distribution of COs along chromosomes, thereby safeguarding the genome for future generations.

The conserved ATPase PCH-2 controls the number and distribution of crossovers by antagonizing their formation in C. elegans

Meiotic crossover recombination is essential for both accurate chromosome segregation and the generation of new haplotypes for natural selection to act upon. This requirement is known as crossover assurance and is one example of crossover control. While the conserved role of the ATPase, PCH-2, during meiotic prophase has been enigmatic, a universal phenotype when pch-2 or its orthologs are mutated is a change in the number and distribution of meiotic crossovers. Here, we show that PCH-2 controls the number and distribution of crossovers by antagonizing their formation. This antagonism produces different effects at different stages of meiotic prophase: early in meiotic prophase, PCH-2 prevents double strand breaks from becoming crossover-eligible intermediates, limiting crossover formation at sites of initial double strand break formation and homolog interactions. Later in meiotic prophase, PCH-2 winnows the number of crossover-eligible intermediates, contributing to the designation of crossovers and ultimately, crossover assurance. We also demonstrate that PCH-2 accomplishes this regulation through the meiotic HORMAD, HIM-3. Our data strongly support a model in which PCH-2's conserved role is to remodel meiotic HORMADs throughout meiotic prophase to destabilize crossover-eligible precursors, coordinate meiotic recombination with synapsis, and contribute to the progressive implementation of meiotic recombination, guaranteeing crossover control.

Primary ovarian insufficiency: update on clinical and genetic findings

Primary ovarian insufficiency (POI) is a disorder of insufficient ovarian follicle function before the age of 40 years with an estimated prevalence of 3.7% worldwide. Its relevance is emerging due to the increasing number of women desiring conception late or beyond the third decade of their lives. POI clinical presentation is extremely heterogeneous with a possible exordium as primary amenorrhea due to ovarian dysgenesis or with a secondary amenorrhea due to different congenital or acquired abnormalities. POI significantly impacts non only on the fertility prospect of the affected women but also on their general, psychological, sexual quality of life, and, furthermore, on their long-term bone, cardiovascular, and cognitive health. In several cases the underlying cause of POI remains unknown and, thus, these forms are still classified as idiopathic. However, we now know the age of menopause is an inheritable trait and POI has a strong genetic background. This is confirmed by the existence of several candidate genes, experimental and natural models. The most common genetic contributors to POI are the X chromosome-linked defects. Moreover, the variable expressivity of POI defect suggests it can be considered as a multifactorial or oligogenic defect. Here, we present an updated review on clinical findings and on the principal X-linked and autosomal genes involved in syndromic and non-syndromic forms of POI. We also provide current information on the management of the premature hypoestrogenic state as well as on fertility preservation in subjects at risk of POI.

BRCA1/BRC-1 and SMC-5/6 regulate DNA repair pathway engagement during Caenorhabditis elegans meiosis

The preservation of genome integrity during sperm and egg development is vital for reproductive success. During meiosis, the tumor suppressor BRCA1/BRC-1 and structural maintenance of chromosomes 5/6 (SMC-5/6) complex genetically interact to promote high fidelity DNA double strand break (DSB) repair, but the specific DSB repair outcomes these proteins regulate remain unknown. Using genetic and cytological methods to monitor resolution of DSBs with different repair partners in Caenorhabditis elegans, we demonstrate that both BRC-1 and SMC-5 repress intersister crossover recombination events. Sequencing analysis of conversion tracts from homolog-independent DSB repair events further indicates that BRC-1 regulates intersister/intrachromatid noncrossover conversion tract length. Moreover, we find that BRC-1 specifically inhibits error prone repair of DSBs induced at mid-pachytene. Finally, we reveal functional interactions of BRC-1 and SMC-5/6 in regulating repair pathway engagement: BRC-1 is required for localization of recombinase proteins to DSBs in smc-5 mutants and enhances DSB repair defects in smc-5 mutants by repressing theta-mediated end joining (TMEJ). These results are consistent with a model in which some functions of BRC-1 act upstream of SMC-5/6 to promote recombination and inhibit error-prone DSB repair, while SMC-5/6 acts downstream of BRC-1 to regulate the formation or resolution of recombination intermediates. Taken together, our study illuminates the coordinated interplay of BRC-1 and SMC-5/6 to regulate DSB repair outcomes in the germline.

Caenorhabditis elegans brc-1 mutation increases the number of COSA-1 foci in him-8 and zim-2 mutants

Crossover designation factors such as COSA-1 are concentrated at the specific DNA double-strand break (DSB) sites to promote crossover formation. zim-1 mutants, which show defects in the homologous chromosome pairing of chromosomes II and III, increase the COSA-1 foci/normal bivalent state compared to the expected value. The excess designation was suppressed by an additional mutation in brc-1 in zim-1 mutants. We demonstrated that the number of COSA-1 foci in him-8 and zim-2 mutants, showing defects in the pairing of the X and V chromosomes, respectively, increased compared to the expected value, and brc-1 mutation accelerated the number of COSA-1 foci in oogenesis.

One of these strains is not like the others: C. elegans DW102 has an altered dependence on brc-1 and brd-1 for regulation of cyp gene transcription

Several strains of Caenorhabditis elegans with mutations in brc-1 or brd-1 are readily available to aid in elucidating the functions of these two genes in DNA damage repair, meiosis, and gene repression. DW102 is the only C. elegans strain to our knowledge with mutations in both brc-1 and brd-1 . However, several groups have reported the DW102 strain is indistinguishable from wild-type when observing levels of embryonic lethality, sensitivity to radiation, and rates of male progeny, while strains with mutations in either brc-1 or brd-1 display increased occurrence of these phenotypes. Here, RT-qPCR analysis of the cyp-13A gene family, reveals distinctive and aberrant expression patterns in DW102 compared to other brc-1 or brd-1 mutant strains underscoring the need for caution in choosing this strain to draw conclusions about brc-1 and brd-1 functions.

COSA-1 mediated pro-crossover complex formation promotes meiotic crossing over in C. elegans

Accurate chromosome segregation during meiosis requires the establishment of at least one crossover (CO) between each pair of homologous chromosomes. CO formation depends on a group of conserved pro-CO proteins, which colocalize at CO-designated sites during late meiotic prophase I. However, it remains unclear whether these pro-CO proteins form a functional complex and how they promote meiotic CO formation in vivo. Here, we show that COSA-1, a key component required for CO formation, interacts with other pro-CO factors, MSH-5 and ZHP-3, via its N-terminal disordered region. Point mutations that impair these interactions do not affect CO designation, but they strongly hinder the accumulation of COSA-1 at CO-designated sites and result in defective CO formation. These defects can be partially bypassed by artificially tethering an interaction-compromised COSA-1 derivate to ZHP-3. Furthermore, we revealed that the accumulation of COSA-1 into distinct foci is required to assemble functional 'recombination nodules'. These prevent early CO-designated recombination intermediates from being dismantled by the RTEL-1 helicase and protect late recombination intermediates, such as Holliday junctions, until they are resolved by CO-specific resolvases. Altogether, our findings provide insight into COSA-1 mediated pro-CO complex assembly and its contribution to CO formation.

Temporal Analysis of DSB Repair Outcome in Caenorhabditis elegans Meiosis

The Caenorhabditis elegans germline is arranged spatiotemporally and is therefore a powerful model system for the interrogation of meiotic molecular dynamics. Coupling this property with the temporal control that the auxin-inducible degron (AID) system allows can unveil new/unappreciated roles for critical meiotic factors in specific germline regions. Here we describe a widely used approach for the introduction of degron tags to specific targets and provide a procedure for applying the AID system to C. elegans meiotic DSB repair dynamics in the germline.

Sexual dimorphic regulation of recombination by the synaptonemal complex in C. elegans

In sexually reproducing organisms, germ cells faithfully transmit the genome to the next generation by forming haploid gametes, such as eggs and sperm. Although most meiotic proteins are conserved between eggs and sperm, many aspects of meiosis are sexually dimorphic, including the regulation of recombination. The synaptonemal complex (SC), a large ladder-like structure that forms between homologous chromosomes, is essential for regulating meiotic chromosome organization and promoting recombination. To assess whether sex-specific differences in the SC underpin sexually dimorphic aspects of meiosis, we examined Caenorhabditis elegans SC central region proteins (known as SYP proteins) in oogenesis and spermatogenesis and uncovered sex-specific roles for the SYPs in regulating meiotic recombination. We find that SC composition, specifically SYP-2, SYP-3, SYP-5, and SYP-6, is regulated by sex-specific mechanisms throughout meiotic prophase I. During pachytene, both oocytes and spermatocytes differentially regulate the stability of SYP-2 and SYP-3 within an assembled SC. Further, we uncover that the relative amount of SYP-2 and SYP-3 within the SC is independently regulated in both a sex-specific and a recombination-dependent manner. Specifically, we find that SYP-2 regulates the early steps of recombination in both sexes, while SYP-3 controls the timing and positioning of crossover recombination events across the genomic landscape in only oocytes. Finally, we find that SYP-2 and SYP-3 dosage can influence the composition of the other SYPs in the SC via sex-specific mechanisms during pachytene. Taken together, we demonstrate dosage-dependent regulation of individual SC components with sex-specific functions in recombination. These sexual dimorphic features of the SC provide insights into how spermatogenesis and oogenesis adapted similar chromosome structures to differentially regulate and execute recombination.

53bp1 mutation enhances brca1 and bard1 embryonic lethality in C. elegans

In mice, mutation of brca1 results in embryonic lethality, which is partially suppressed by 53bp1 mutation. In contrast, mutation of the C. elegans BRCA1 ortholog, brc-1 , or its binding partner, brd-1 , lead to only mild embryonic lethality. We show that in C. elegans , brc-1 and brd-1 embryonic lethality is enhanced when 53bp1 ortholog, hsr-9 , is also mutated. This is not a consequence of activating polq-1 -dependent microhomology-mediated end joining, as polq-1 mutation does not suppress embryonic lethality of hsr-9 ; brc-1 mutants. Together, these results suggest that BRC-1 - BRD-1 and HSR-9 function in parallel pathways and do not act antagonistically as in mammals.

Oogenesis in Caenorhabditis elegans

BACKGROUND

The nematode, Caenorhabditis elegans has proven itself as a valuable model for investigating metazoan biology. C. elegans have a transparent body, an invariant cell lineage, and a high level of genetic conservation which makes it a desirable model organism. Although used to elucidate many aspects of somatic biology, a distinct advantage of C. elegans is its well annotated germline which allows all aspects of oogenesis to be observed in real time within a single animal. C. elegans hermaphrodites have two U-shaped gonad arms which produce their own sperm that is later stored to fertilise their own oocytes. These two germlines take up much of the internal space of each animal and germ cells are therefore the most abundant cell present within each animal. This feature and the genetic phenotypes observed for mutant worm gonads have allowed many novel findings that established our early understanding of germ cell dynamics. The mutant phenotypes also allowed key features of meiosis and germ cell maturation to be unveiled.

SUMMARY

This review will focus on the key aspects that make C. elegans an outstanding model for exploring each feature of oogenesis. This will include the fundamental steps associated with germline function and germ cell maturation and will be of use for those interested in exploring reproductive metazoan biology.

KEY MESSAGES

Since germ cell biology is highly conserved in animals, much can be gained from study of a simple metazoan like C. elegans. Past findings have enhanced understanding on topics that would be more laborious or challenging in more complex animal models.

Meiotic DNA exchanges in C. elegans are promoted by proximity to the synaptonemal complex

During meiosis, programmed double-strand DNA breaks are repaired to form exchanges between the parental chromosomes called crossovers. Chromosomes lacking a crossover fail to segregate accurately into the gametes, leading to aneuploidy. In addition to engaging the homolog, crossover formation requires the promotion of exchanges, rather than non-exchanges, as repair products. However, the mechanism underlying this meiosis-specific preference is not fully understood. Here, we study the regulation of meiotic sister chromatid exchanges in Caenorhabditis elegans by direct visualization. We find that a conserved chromosomal interface that promotes exchanges between the parental chromosomes, the synaptonemal complex, can also promote exchanges between the sister chromatids. In both cases, exchanges depend on the recruitment of the same set of pro-exchange factors to repair sites. Surprisingly, although the synaptonemal complex usually assembles between the two DNA molecules undergoing an exchange, its activity does not rely on a specific chromosome conformation. This suggests that the synaptonemal complex regulates exchanges-both crossovers and sister exchanges-by establishing a nuclear domain conducive to nearby recruitment of exchange-promoting factors.

The DREAM complex functions as conserved master regulator of somatic DNA-repair capacities

The DNA-repair capacity in somatic cells is limited compared with that in germ cells. It has remained unknown whether not only lesion-type-specific, but overall repair capacities could be improved. Here we show that the DREAM repressor complex curbs the DNA-repair capacities in somatic tissues of Caenorhabditis elegans. Mutations in the DREAM complex induce germline-like expression patterns of multiple mechanisms of DNA repair in the soma. Consequently, DREAM mutants confer resistance to a wide range of DNA-damage types during development and aging. Similarly, inhibition of the DREAM complex in human cells boosts DNA-repair gene expression and resistance to distinct DNA-damage types. DREAM inhibition leads to decreased DNA damage and prevents photoreceptor loss in progeroid Ercc1^-/- mice. We show that the DREAM complex transcriptionally represses essentially all DNA-repair systems and thus operates as a highly conserved master regulator of the somatic limitation of DNA-repair capacities.

Conservation of transcriptional regulation by BRCA1 and BARD1 in Caenorhabditis elegans

The tumor-suppressor proteins BRCA1 and BARD1 function as an E3 ubiquitin ligase to facilitate transcriptional repression and DNA damage repair. This is mediated in-part through its ability to mono-ubiquitylate histone H2A in nucleosomes. Studies in Caenorhabditis elegans have been used to elucidate numerous functions of BRCA1 and BARD1; however, it has not been established that the C. elegans orthologs, BRC-1 and BRD-1, retain all the functions of their human counterparts. Here we explore the conservation of enzymatic activity toward nucleosomes which leads to repression of estrogen-metabolizing cytochrome P450 (cyp) genes in humans. Biochemical assays establish that BRC-1 and BRD-1 contribute to ubiquitylation of histone H2A in the nucleosome. Mutational analysis shows that while BRC-1 likely binds the nucleosome using a conserved interface, BRD-1 and BARD1 have evolved different modes of binding, resulting in a difference in the placement of ubiquitin on H2A. Gene expression analysis reveals that in spite of this difference, BRC-1 and BRD-1 also contribute to cyp gene repression in C. elegans. Establishing conservation of these functions in C. elegans allows for use of this powerful model organism to address remaining questions regarding regulation of gene expression by BRCA1 and BARD1.

Epitope tag-specific differences in the detection of COSA-1 marked crossover sites in C. elegans spermatocytes

Nascent crossover sites in C. elegans meiocytes can be cytologically detected using epitope-tagged versions of the pro-crossover protein COSA-1. In spermatocytes, differences exist between cytologically-detected and genetically-detected double crossover rates. Here, we examine nascent crossovers using both GFP- and OLLAS-tagged COSA-1. Similar to previous work, we find that most late pachytene spermatocytes display 5 COSA-1 foci, indicating one crossover per autosome bivalent. However, we detected more nuclei with >5 COSA-1 foci using OLLAS::COSA-1, reflecting some bivalents having 2 COSA-1 foci. These results demonstrate tag-specific differences in the detection of COSA-1 marked nascent crossovers in spermatocytes.

Differential requirement for BRCA1-BARD1 E3 ubiquitin ligase activity in DNA damage repair and meiosis in the Caenorhabditis elegans germ line

The tumor suppressor BRCA1-BARD1 complex regulates many cellular processes; of critical importance to its tumor suppressor function is its role in genome integrity. Although RING E3 ubiquitin ligase activity is the only known enzymatic activity of the complex, the in vivo requirement for BRCA1-BARD1 E3 ubiquitin ligase activity has been controversial. Here we probe the role of BRCA1-BARD1 E3 ubiquitin ligase activity in vivo using C. elegans. Genetic, cell biological, and biochemical analyses of mutants defective for E3 ligase activity suggest there is both E3 ligase-dependent and independent functions of the complex in the context of DNA damage repair and meiosis. We show that E3 ligase activity is important for nuclear accumulation of the complex and specifically to concentrate at meiotic recombination sites but not at DNA damage sites in proliferating germ cells. While BRCA1 alone is capable of monoubiquitylation, BARD1 is required with BRCA1 to promote polyubiquitylation. We find that the requirement for E3 ligase activity and BARD1 in DNA damage signaling and repair can be partially alleviated by driving the nuclear accumulation and self-association of BRCA1. Our data suggest that in addition to E3 ligase activity, BRCA1 may serve a structural role for DNA damage signaling and repair while BARD1 plays an accessory role to enhance BRCA1 function.

PARG and BRCA1-BARD1 cooperative function regulates DNA repair pathway choice during gametogenesis

Meiotic chromosome segregation relies on programmed DNA double-strand break induction. These are in turn repaired by homologous recombination, generating physical attachments between the parental chromosomes called crossovers. A subset of breaks yields recombinant outcomes, while crossover-independent mechanisms repair the majority of lesions. The balance between different repair pathways is crucial to ensure genome integrity. We show that Caenorhabditis elegans BRC-1/BRCA1-BRD-1/BARD1 and PARG-1/PARG form a complex in vivo, essential for accurate DNA repair in the germline. Simultaneous depletion of BRC-1 and PARG-1 causes synthetic lethality due to reduced crossover formation and impaired break repair, evidenced by hindered RPA-1 removal and presence of aberrant chromatin bodies in diakinesis nuclei, whose formation depends on spo-11 function. These factors undergo a similar yet independent loading in developing oocytes, consistent with operating in different pathways. Abrogation of KU- or Theta-mediated end joining elicits opposite effects in brc-1; parg-1 doubles, suggesting a profound impact in influencing DNA repair pathway choice by BRC-1-PARG-1. Importantly, lack of PARG-1 catalytic activity suppresses untimely accumulation of RAD-51 foci in brc-1 mutants but is only partially required for fertility. Our data show that BRC-1/BRD-1-PARG-1 joint function is essential for genome integrity in meiotic cells by regulating multiple DNA repair pathways.

Continuous double-strand break induction and their differential processing sustain chiasma formation during Caenorhabditis elegans meiosis

Faithful chromosome segregation into gametes depends on Spo11-induced DNA double-strand breaks (DSBs). These yield single-stranded 3' tails upon resection to promote crossovers (COs). While early Mre11-dependent end resection is the predominant pathway in most organisms, Exo1 or Dna2/BLM can also contribute to the efficient processing of meiotic DSBs. Although its enzymatic activity has been thoroughly dissected, the temporal dynamics underlying Spo11 activity have remained mostly elusive. We show that, in Caenorhabditis elegans, SPO-11-mediated DSB induction takes place throughout early meiotic prophase I until mid-late pachynema. We find that late DSBs are essential for CO formation and are preferentially processed by EXO-1 and DNA-2 in a redundant fashion. Further, EXO-1-DNA-2-mediated resection ensures completion of conservative DSB repair and discourages activation of KU-dependent end joining. Taken together, our data unveil important temporal aspects of DSB induction and identify previously unknown functional implications for EXO-1-DNA-2-mediated resection activity in C. elegans.

The topoisomerase 3 zinc finger domain cooperates with the RMI1 scaffold to promote stable association of the BTR complex to recombination intermediates in the Caenorhabditis elegans germline

Homologous recombination is the predominant DNA repair pathway used in the gonad. Of the excess DNA double-strand breaks formed in meiosis, only a subset matures into crossovers, with the remainder repaired as non-crossovers. The conserved BTR complex (comprising Bloom helicase, topoisomerase 3 and RMI1/2 scaffold proteins) acts at multiple steps during recombination to dismantle joint DNA molecules, thereby mediating the non-crossover outcome and chromosome integrity. Furthermore, the complex displays a role at the crossover site that is less well understood. Besides catalytic and TOPRIM domains, topoisomerase 3 enzymes contain a variable number of carboxy terminal zinc finger (ZnF) domains. Here, we studied the Caenorhabditis elegans mutant, in which the single ZnF domain is deleted. In contrast to the gene disruption allele, the top-3-ZnF mutant is viable, with no replication defects; the allele appears to be a hypomorph. The TOP-3-ZnF protein is recruited into foci but the mutant has increased numbers of crossovers along its chromosomes, with minor defects in repressing heterologous recombination, and a marked delay in the maturation/processing of recombination intermediates after loading of the RAD-51 recombinase. The ZnF domain cooperates with the RMI1 homolog RMH-2 to stabilize association of the BTR complex with recombination intermediates and to prevent recombination between heterologous DNA sequences.

Inducible degradation of dosage compensation protein DPY-27 facilitates isolation of Caenorhabditis elegans males for molecular and biochemical analyses

Biological sex affects numerous aspects of biology, yet how sex influences different biological processes have not been extensively studied at the molecular level. Caenorhabditis elegans, with both hermaphrodites (functionally females as adults) and males, is an excellent system to uncover how sex influences physiology. Here, we describe a method to isolate large quantities of C. elegans males by conditionally degrading DPY-27, a component of the dosage compensation complex essential for hermaphrodite, but not male, development. We show that germ cells from males isolated following DPY-27 degradation undergo meiosis and spermiogenesis like wild type and these males are competent to mate and sire viable offspring. We further demonstrate the efficacy of this system by analyzing gene expression and performing affinity pull-downs from male worm extracts.

Phosphorylation of HORMA-domain protein HTP-3 at Serine 285 is dispensable for crossover formation

Generation of functional gametes is accomplished through a multilayered and finely orchestrated succession of events during meiotic progression. In the Caenorhabditis elegans germline, the HORMA-domain-containing protein HTP-3 plays pivotal roles for the establishment of chromosome axes and the efficient induction of programmed DNA double-strand breaks, both of which are crucial for crossover formation. Double-strand breaks allow for accurate chromosome segregation during the first meiotic division and therefore are an essential requirement for the production of healthy gametes. Phosphorylation-dependent regulation of HORMAD protein plays important roles in controlling meiotic chromosome behavior. Here, we document a phospho-site in HTP-3 at Serine 285 that is constitutively phosphorylated during meiotic prophase I. pHTP-3S285 localization overlaps with panHTP-3 except in nuclei undergoing physiological apoptosis, in which pHTP-3 is absent. Surprisingly, we observed that phosphorylation of HTP-3 at S285 is independent of the canonical kinases that control meiotic progression in nematodes. During meiosis, the htp-3(S285A) mutant displays accelerated RAD-51 turnover, but no other meiotic abnormalities. Altogether, these data indicate that the Ser285 phosphorylation is independent of canonical meiotic protein kinases and does not regulate HTP-3-dependent meiotic processes. We propose a model wherein phosphorylation of HTP-3 occurs through noncanonical or redundant meiotic kinases and/or is likely redundant with additional phospho-sites for function in vivo.

Proline-specific aminopeptidase P prevents replication-associated genome instability

Genotoxic stress during DNA replication constitutes a serious threat to genome integrity and causes human diseases. Defects at different steps of DNA metabolism are known to induce replication stress, but the contribution of other aspects of cellular metabolism is less understood. We show that aminopeptidase P (APP1), a metalloprotease involved in the catabolism of peptides containing proline residues near their N-terminus, prevents replication-associated genome instability. Functional analysis of C. elegans mutants lacking APP-1 demonstrates that germ cells display replication defects including reduced proliferation, cell cycle arrest, and accumulation of mitotic DSBs. Despite these defects, app-1 mutants are competent in repairing DSBs induced by gamma irradiation, as well as SPO-11-dependent DSBs that initiate meiotic recombination. Moreover, in the absence of SPO-11, spontaneous DSBs arising in app-1 mutants are repaired as inter-homologue crossover events during meiosis, confirming that APP-1 is not required for homologous recombination. Thus, APP-1 prevents replication stress without having an apparent role in DSB repair. Depletion of APP1 (XPNPEP1) also causes DSB accumulation in mitotically-proliferating human cells, suggesting that APP1’s role in genome stability is evolutionarily conserved. Our findings uncover an unexpected role for APP1 in genome stability, suggesting functional connections between aminopeptidase-mediated protein catabolism and DNA replication.

The accurate duplication of DNA that occurs before cells divide is an essential aspect of the cell cycle that is also crucial for the correct development of multicellular organisms. Mutations that compromise the normal function of the DNA replication machinery can lead to the accumulation of replication-related DNA damage, a known cause of human disease and a common feature of cancer and precancerous cells. Therefore, identifying factors that prevent replication-related DNA damage is highly relevant for human health. In this manuscript, we identify aminopeptidase P, an enzyme involved in the breakdown of proteins containing the amino acid Proline at their N-terminus, as a novel factor that prevents replication-related DNA damage. Analysis of C. elegans nematodes lacking aminopeptidase P reveals that this protein is required for normal fertility and development, and that in its absence proliferating germ cells display DNA replication defects, including cell cycle arrest and accumulation of extensive DNA damage. We also show that removal of aminopeptidase P induces DNA damage in proliferating human cells, suggesting that its role in preventing replication defects is evolutionarily conserved. These findings uncover functional connections between aminopeptidase-mediated protein degradation and DNA replication.

Mechanism of mitotic recombination: insights from C. elegans

Homologous recombination (HR) plays a critical role in largely error-free repair of mitotic and meiotic DNA double-strand breaks (DSBs). DSBs are one of the most deleterious DNA lesions, which are repaired by non-homologous end joining (NHEJ), homologous recombination (HR) or, if compromised, micro-homology mediated end joining (MMEJ). If left unrepaired, DSBs can lead to cell death or if repaired incorrectly can result in chromosome rearrangements that drive cancer development. Here, we describe recent advances in the field of mitotic HR made using Caenorhabditis elegans roundworm, as a model system.

Heterogeneity in conservation of multifunctional partner enzymes with meiotic importance, CDK2 kinase and BRCA1 ubiquitin ligase

The evolution of proteins can be accompanied by changes not only to their amino acid sequences, but also their structural and spatial molecular organization. Comparison of the protein conservation within different taxonomic groups (multifunctional, or highly specific) allows to clarify their specificity and the direction of evolution. Two multifunctional enzymes, cyclin-dependent kinase 2 (CDK2) and BRCA1 ubiquitin ligase, that are partners in some mitotic and meiotic processes were investigated in the present work. Two research methods, bioinformatics and immunocytochemical, were combined to examine the conservation levels of the two enzymes. It has been established that CDK2 is a highly conserved protein in different taxonomic lineages of the eukaryotic tree. Immunocytochemically, a conserved CDK2 pattern was revealed in the meiotic autosomes of five rodent species and partially in domestic turkey and clawed frog. Nevertheless, variable CDK2 distribution was detected at the unsynapsed segments of the rodent X chromosomes. BRCA1 was shown to be highly conserved only within certain mammalian taxa. It was also noted that in those rodent nuclei, where BRCA1 specifically binds to antigens, asynaptic regions of sex chromosomes were positive. BRCA1 staining was not always accompanied by specific binding, and a high nonspecificity in the nucleoplasm was observed. Thus, the studies revealed different conservation of the two enzymes at the level of protein structure as well as at the level of chromosome behavior. This suggests variable rates of evolution due to both size and configuration of the protein molecules and their multifunctionality.

The fellowship of the RING: BRCA1, its partner BARD1 and their liaison in DNA repair and cancer

The breast cancer type 1 susceptibility protein (BRCA1) and its partner - the BRCA1-associated RING domain protein 1 (BARD1) - are key players in a plethora of fundamental biological functions including, among others, DNA repair, replication fork protection, cell cycle progression, telomere maintenance, chromatin remodeling, apoptosis and tumor suppression. However, mutations in their encoding genes transform them into dangerous threats, and substantially increase the risk of developing cancer and other malignancies during the lifetime of the affected individuals. Understanding how BRCA1 and BARD1 perform their biological activities therefore not only provides a powerful mean to prevent such fatal occurrences but can also pave the way to the development of new targeted therapeutics. Thus, through this review work we aim at presenting the major efforts focused on the functional characterization and structural insights of BRCA1 and BARD1, per se and in combination with all their principal mediators and regulators, and on the multifaceted roles these proteins play in the maintenance of human genome integrity.

Analysis of representative mutants for key DNA repair pathways on healthspan in Caenorhabditis elegans

Although the link between DNA damage and aging is well accepted, the role of different DNA repair proteins on functional/physiological aging is not well-defined. Here, using Caenorhabditis elegans, we systematically examined the effect of three DNA repair genes involved in key genome stability pathways. We assayed multiple health proxies including molecular, functional and resilience measures to define healthspan. Loss of XPF-1/ERCC-1, a protein involved in nucleotide excision repair (NER), homologous recombination (HR) and interstrand crosslink (ICL) repair, showed the highest impairment of functional and stress resilience measures along with a shortened lifespan. brc-1 mutants, with a well-defined role in HR and ICL are short-lived and highly sensitive to acute stressors, specifically oxidative stress. In contrast, ICL mutant, fcd-2 did not impact lifespan or most healthspan measures. Our efforts also uncover that DNA repair mutants show high sensitivity to oxidative stress with age, suggesting that this measure could act as a primary proxy for healthspan. Together, these data suggest that impairment of multiple DNA repair genes can drive functional/physiological aging. Further studies to examine specific DNA repair genes in a tissue specific manner will help dissect the importance and mechanistic role of these repair systems in biological aging.

The organization, regulation, and biological functions of the synaptonemal complex

The synaptonemal complex (SC) is a meiosis-specific proteinaceous macromolecular structure that assembles between paired homologous chromosomes during meiosis in various eukaryotes. The SC has a highly conserved ultrastructure and plays critical roles in controlling multiple steps in meiotic recombination and crossover formation, ensuring accurate meiotic chromosome segregation. Recent studies in different organisms, facilitated by advances in super-resolution microscopy, have provided insights into the macromolecular structure of the SC, including the internal organization of the meiotic chromosome axis and SC central region, the regulatory pathways that control SC assembly and dynamics, and the biological functions exerted by the SC and its substructures. This review summarizes recent discoveries about how the SC is organized and regulated that help to explain the biological functions associated with this meiosis-specific structure.

Caenorhabditis elegans RMI2 functional homolog-2 (RMIF-2) and RMI1 (RMH-1) have both overlapping and distinct meiotic functions within the BTR complex

Homologous recombination is a high-fidelity repair pathway for DNA double-strand breaks employed during both mitotic and meiotic cell divisions. Such repair can lead to genetic exchange, originating from crossover (CO) generation. In mitosis, COs are suppressed to prevent sister chromatid exchange. Here, the BTR complex, consisting of the Bloom helicase (HIM-6 in worms), topoisomerase 3 (TOP-3), and the RMI1 (RMH-1 and RMH-2) and RMI2 scaffolding proteins, is essential for dismantling joint DNA molecules to form non-crossovers (NCOs) via decatenation. In contrast, in meiosis COs are essential for accurate chromosome segregation and the BTR complex plays distinct roles in CO and NCO generation at different steps in meiotic recombination. RMI2 stabilizes the RMI1 scaffolding protein, and lack of RMI2 in mitosis leads to elevated sister chromatid exchange, as observed upon RMI1 knockdown. However, much less is known about the involvement of RMI2 in meiotic recombination. So far, RMI2 homologs have been found in vertebrates and plants, but not in lower organisms such as Drosophila, yeast, or worms. We report the identification of the Caenorhabditis elegans functional homolog of RMI2, which we named RMIF-2. The protein shows a dynamic localization pattern to recombination foci during meiotic prophase I and concentration into recombination foci is mutually dependent on other BTR complex proteins. Comparative analysis of the rmif-2 and rmh-1 phenotypes revealed numerous commonalities, including in regulating CO formation and directing COs toward chromosome arms. Surprisingly, the prevalence of heterologous recombination was several fold lower in the rmif-2 mutant, suggesting that RMIF-2 may be dispensable or less strictly required for some BTR complex-mediated activities during meiosis.

Bloom syndrome is caused by mutations in proteins of the BTR complex (consisting of the Bloom helicase, topoisomerase 3, and the RMI1 and RMI2 scaffolding proteins) and the clinical characteristics are growth deficiency, short stature, skin photosensitivity, and increased cancer predisposition. At the cellular level, characteristic features are the presence of increased sister chromatid exchange on chromosomes; unresolved DNA recombination intermediates that eventually cause genome instability; and erroneous DNA repair by heterologous recombination (recombination between non-identical sequences, extremely rare in wild type animals), which can trigger translocations and chromosomal rearrangements. Identification of the Caenorhabditis elegans ortholog of RMI2 (called RMIF-2) allowed us to compare heterologous recombination in the germline of mutants of various BTR complex proteins. The heterologous recombination rate was several fold lower in rmif-2 mutants than in mutants of rmh-1 and him-6 (worm homologs of RMI1 and the Bloom helicase, respectively). Nevertheless, many phenotypic features point at RMIF-2 working together with RMH-1. If these germline functions of RMI2/RMIF-2 are conserved in humans, this might mean that individuals with RMI2 mutations have a lower risk of translocations and genome rearrangements than those with mutations in the other BTR complex genes.

BRCA1 and BRCA2 Tumor Suppressor Function in Meiosis

Meiosis is a specialized cell cycle that results in the production of haploid gametes for sexual reproduction. During meiosis, homologous chromosomes are connected by chiasmata, the physical manifestation of crossovers. Crossovers are formed by the repair of intentionally induced double strand breaks by homologous recombination and facilitate chromosome alignment on the meiotic spindle and proper chromosome segregation. While it is well established that the tumor suppressors BRCA1 and BRCA2 function in DNA repair and homologous recombination in somatic cells, the functions of BRCA1 and BRCA2 in meiosis have received less attention. Recent studies in both mice and the nematode Caenorhabditis elegans have provided insight into the roles of these tumor suppressors in a number of meiotic processes, revealing both conserved and organism-specific functions. BRCA1 forms an E3 ubiquitin ligase as a heterodimer with BARD1 and appears to have regulatory roles in a number of key meiotic processes. BRCA2 is a very large protein that plays an intimate role in homologous recombination. As women with no indication of cancer but carrying BRCA mutations show decreased ovarian reserve and accumulated oocyte DNA damage, studies in these systems may provide insight into why BRCA mutations impact reproductive success in addition to their established roles in cancer.

BRCA1-BARD1 Regulates Axon Regeneration in Concert with the Gqα-DAG Signaling Network

The breast cancer susceptibility protein BRCA1 and its partner BRCA1-associated RING domain protein 1 (BARD1) form an E3-ubiquitin (Ub) ligase complex that acts as a tumor suppressor in mitotic cells. However, the roles of BRCA1–BARD1 in postmitotic cells, such as neurons, remain poorly defined. Here, we report that BRC-1 and BRD-1, the Caenorhabditis elegans orthologs of BRCA1 and BARD1, are required for adult-specific axon regeneration, which is positively regulated by the EGL-30 Gqα–diacylglycerol (DAG) signaling pathway. This pathway is downregulated by DAG kinase (DGK), which converts DAG to phosphatidic acid (PA). We demonstrate that inactivation of DGK-3 suppresses the brc-1 brd-1 defect in axon regeneration, suggesting that BRC-1–BRD-1 inhibits DGK-3 function. Indeed, we show that BRC-1–BRD-1 poly-ubiquitylates DGK-3 in a manner dependent on its E3 ligase activity, causing DGK-3 degradation. Furthermore, we find that axon injury causes the translocation of BRC-1 from the nucleus to the cytoplasm, where DGK-3 is localized. These results suggest that the BRC-1–BRD-1 complex regulates axon regeneration in concert with the Gqα–DAG signaling network. Thus, this study describes a new role for breast cancer proteins in fully differentiated neurons and the molecular mechanism underlying the regulation of axon regeneration in response to nerve injury.

SIGNIFICANCE STATEMENT BRCA1–BRCA1-associated RING domain protein 1 (BARD1) is an E3-ubiquitin (Ub) ligase complex acting as a tumor suppressor in mitotic cells. The roles of BRCA1–BARD1 in postmitotic cells, such as neurons, remain poorly defined. We show here that Caenorhabditis elegans BRC-1/BRCA1 and BRD-1/BARD1 are required for adult-specific axon regeneration, a process that requires high diacylglycerol (DAG) levels in injured neurons. The DAG kinase (DGK)-3 inhibits axon regeneration by reducing DAG levels. We find that BRC-1–BRD-1 poly-ubiquitylates and degrades DGK-3, thereby keeping DAG levels elevated and promoting axon regeneration. Furthermore, we demonstrate that axon injury causes the translocation of BRC-1 from the nucleus to the cytoplasm, where DGK-3 is localized. Thus, this study describes a new role for BRCA1–BARD1 in fully-differentiated neurons.

Meiotic Recombination Defects and Premature Ovarian Insufficiency

Premature ovarian insufficiency (POI) is the depletion of ovarian function before 40 years of age due to insufficient oocyte formation or accelerated follicle atresia. Approximately 1–5% of women below 40 years old are affected by POI. The etiology of POI is heterogeneous, including genetic disorders, autoimmune diseases, infection, iatrogenic factors, and environmental toxins. Genetic factors account for 20–25% of patients. However, more than half of the patients were idiopathic. With the widespread application of next-generation sequencing (NGS), the genetic spectrum of POI has been expanded, especially the latest identification in meiosis and DNA repair-related genes. During meiotic prophase I, the key processes include DNA double-strand break (DSB) formation and subsequent homologous recombination (HR), which are essential for chromosome segregation at the first meiotic division and genome diversity of oocytes. Many animal models with defective meiotic recombination present with meiotic arrest, DSB accumulation, and oocyte apoptosis, which are similar to human POI phenotype. In the article, based on different stages of meiotic recombination, including DSB formation, DSB end processing, single-strand invasion, intermediate processing, recombination, and resolution and essential proteins involved in synaptonemal complex (SC), cohesion complex, and fanconi anemia (FA) pathway, we reviewed the individual gene mutations identified in POI patients and the potential candidate genes for POI pathogenesis, which will shed new light on the genetic architecture of POI and facilitate risk prediction, ovarian protection, and early intervention for POI women.

Multivalent weak interactions between assembly units drive synaptonemal complex formation

The synaptonemal complex (SC) is an ordered but highly dynamic structure assembled between homologous chromosomes to control interhomologous crossover formation, ensuring accurate meiotic chromosome segregation. However, the mechanisms regulating SC assembly and dynamics remain unclear. Here, we identified two new SC components, SYP-5 and SYP-6, in Caenorhabditis elegans that have distinct expression patterns and form distinct SC assembly units with other SYPs through stable interactions. SYP-5 and SYP-6 exhibit diverse in vivo SC regulatory functions and distinct phase separation properties in cells. Charge-interacting elements (CIEs) are enriched in SC intrinsically disordered regions (IDRs), and IDR deletion or CIE removal confirmed a requirement for these elements in SC regulation. Our data support the theory that multivalent weak interactions between the SC units drive SC formation and that CIEs confer multivalency to the assembly units.

DNA repair, recombination, and damage signaling

DNA must be accurately copied and propagated from one cell division to the next, and from one generation to the next. To ensure the faithful transmission of the genome, a plethora of distinct as well as overlapping DNA repair and recombination pathways have evolved. These pathways repair a large variety of lesions, including alterations to single nucleotides and DNA single and double-strand breaks, that are generated as a consequence of normal cellular function or by external DNA damaging agents. In addition to the proteins that mediate DNA repair, checkpoint pathways have also evolved to monitor the genome and coordinate the action of various repair pathways. Checkpoints facilitate repair by mediating a transient cell cycle arrest, or through initiation of cell suicide if DNA damage has overwhelmed repair capacity. In this chapter, we describe the attributes of Caenorhabditis elegans that facilitate analyses of DNA repair, recombination, and checkpoint signaling in the context of a whole animal. We review the current knowledge of C. elegans DNA repair, recombination, and DNA damage response pathways, and their role during development, growth, and in the germ line. We also discuss how the analysis of mutational signatures in C. elegans is helping to inform cancer mutational signatures in humans.

Functional characterization of Caenorhabditis elegans cbs-2 gene during meiosis

Cystathionine β-synthase (CBS) is a eukaryotic enzyme that maintains the cellular homocysteine homeostasis and catalyzes the conversion of homocysteine to L-cystathionine and Hydrogen sulfide, via the trans-sulfuration pathway. In Caenorhabditis elegans, two cbs genes are present: cbs-1 functions similarly as to human CBS, and cbs-2, whose roles are instead unknown. In the present study we performed a phenotypic characterization of the cbs-2 mutant. The null cbs-2 mutant is viable, fertile and shows the wild-type complement of six bivalents in most oocyte nuclei, which is indicative of a correct formation of crossover recombination. In absence of synaptonemal complex formation (syp-2 mutant), loss of cbs-2 leads to chromosome fragmentation, suggesting that cbs-2 is essential during inter-sister repair. Interestingly, although proficient in the activation of the DNA damage checkpoint after exposure to genotoxic stress, the cbs-2 mutant is defective in DNA damage-induced apoptosis in meiotic germ cells. These results suggest possible functions for CBS-2 in meiosis, distinct from a role in the trans-sulfuration pathway. We propose that the C. elegans CBS-2 protein is required for both inter-sister repair and execution of DNA damage-induced apoptosis.

Phosphoregulation of HORMA domain protein HIM-3 promotes asymmetric synaptonemal complex disassembly in meiotic prophase in Caenorhabditis elegans

In the two cell divisions of meiosis, diploid genomes are reduced into complementary haploid sets through the discrete, two-step removal of chromosome cohesion, a task carried out in most eukaryotes by protecting cohesion at the centromere until the second division. In eukaryotes without defined centromeres, however, alternative strategies have been innovated. The best-understood of these is found in the nematode Caenorhabditis elegans: after the single off-center crossover divides the chromosome into two segments, or arms, several chromosome-associated proteins or post-translational modifications become specifically partitioned to either the shorter or longer arm, where they promote the correct timing of cohesion loss through as-yet unknown mechanisms. Here, we investigate the meiotic axis HORMA-domain protein HIM-3 and show that it becomes phosphorylated at its C-terminus, within the conserved “closure motif” region bound by the related HORMA-domain proteins HTP-1 and HTP-2. Binding of HTP-2 is abrogated by phosphorylation of the closure motif in in vitro assays, strongly suggesting that in vivo phosphorylation of HIM-3 likely modulates the hierarchical structure of the chromosome axis. Phosphorylation of HIM-3 only occurs on synapsed chromosomes, and similarly to other previously-described phosphorylated proteins of the synaptonemal complex, becomes restricted to the short arm after designation of crossover sites. Regulation of HIM-3 phosphorylation status is required for timely disassembly of synaptonemal complex central elements from the long arm, and is also required for proper timing of HTP-1 and HTP-2 dissociation from the short arm. Phosphorylation of HIM-3 thus plays a role in establishing the identity of short and long arms, thereby contributing to the robustness of the two-step chromosome segregation.

To segregate properly in meiosis, cohesion between replicated chromosomes must remain after the first meiotic cell division, so chromosomes can be held together until they finally separate in the second division. While the majority of organisms use centromeres to protect chromosome cohesion in the first division, the nematode worm C. elegans, which lacks single centromeres, instead protects cohesion only on a segment of the chromosome known as the “long arm”. The long arm (and its complement, the short arm) are known to accumulate specific proteins and protein modifications, but it is not known how the short and long arms are first distinguished, nor how their separate functions are carried out. We report here that the chromosome axis protein HIM-3 and its modification by phosphorylation is important for ensuring the robust establishment of short and long arm functions. We show that phosphorylated HIM-3 partitions to the short arms after crossover recombination sites are designated, and HIM-3 mutants that mimic constitutive phosphorylation delay the normal establishment of the two complementary arm domains. Our findings reveal another layer of regulation to an outstanding mystery in chromosome biology.

Meiotic Double-Strand Break Processing and Crossover Patterning Are Regulated in a Sex-Specific Manner by BRCA1-BARD1 in Caenorhabditis elegans

Meiosis is regulated in a sex-specific manner to produce two distinct gametes, sperm and oocytes, for sexual reproduction. To determine how meiotic recombination is regulated in spermatogenesis, we analyzed the meiotic phenotypes of mutants in the tumor suppressor E3 ubiquitin ligase BRC-1-BRD-1 complex in Caenorhabditis elegans male meiosis. Unlike in mammals, this complex is not required for meiotic sex chromosome inactivation, the process whereby hemizygous sex chromosomes are transcriptionally silenced. Interestingly, brc-1 and brd-1 mutants show meiotic recombination phenotypes that are largely opposing to those previously reported for female meiosis. Fewer meiotic recombination intermediates marked by the recombinase RAD-51 were observed in brc-1 and brd-1 mutants, and the reduction in RAD-51 foci could be suppressed by mutation of nonhomologous-end-joining proteins. Analysis of GFP::RPA-1 revealed fewer foci in the brc-1brd-1 mutant and concentration of BRC-1-BRD-1 to sites of meiotic recombination was dependent on DNA end resection, suggesting that the complex regulates the processing of meiotic double-strand breaks to promote repair by homologous recombination. Further, BRC-1-BRD-1 is important to promote progeny viability when male meiosis is perturbed by mutations that block the pairing and synapsis of different chromosome pairs, although the complex is not required to stabilize the RAD-51 filament as in female meiosis under the same conditions. Analyses of crossover designation and formation revealed that BRC-1-BRD-1 inhibits supernumerary COs when meiosis is perturbed. Together, our findings suggest that BRC-1-BRD-1 regulates different aspects of meiotic recombination in male and female meiosis.

Poly(ADP-ribose) glycohydrolase coordinates meiotic DNA double-strand break induction and repair independent of its catalytic activity

Poly(ADP-ribosyl)ation is a reversible post-translational modification synthetized by ADP-ribose transferases and removed by poly(ADP-ribose) glycohydrolase (PARG), which plays important roles in DNA damage repair. While well-studied in somatic tissues, much less is known about poly(ADP-ribosyl)ation in the germline, where DNA double-strand breaks are introduced by a regulated program and repaired by crossover recombination to establish a tether between homologous chromosomes. The interaction between the parental chromosomes is facilitated by meiotic specific adaptation of the chromosome axes and cohesins, and reinforced by the synaptonemal complex. Here, we uncover an unexpected role for PARG in coordinating the induction of meiotic DNA breaks and their homologous recombination-mediated repair in Caenorhabditis elegans. PARG-1/PARG interacts with both axial and central elements of the synaptonemal complex, REC-8/Rec8 and the MRN/X complex. PARG-1 shapes the recombination landscape and reinforces the tightly regulated control of crossover numbers without requiring its catalytic activity. We unravel roles in regulating meiosis, beyond its enzymatic activity in poly(ADP-ribose) catabolism.

Poly(ADP-ribose) glycohydrolase (PARG) is involved in different cellular processes including DNA repair. Here the authors reveal a role for PARG in regulating meiotic DNA double strand break induction and repair in Caenorhabditis elegans.

BRCA1-associated structural variations are a consequence of polymerase theta-mediated end-joining

Failure to preserve the integrity of the genome is a hallmark of cancer. Recent studies have revealed that loss of the capacity to repair DNA breaks via homologous recombination (HR) results in a mutational profile termed BRCAness. The enzymatic activity that repairs HR substrates in BRCA-deficient conditions to produce this profile is currently unknown. We here show that the mutational landscape of BRCA1 deficiency in C. elegans closely resembles that of BRCA1-deficient tumours. We identify polymerase theta-mediated end-joining (TMEJ) to be responsible: knocking out polq-1 suppresses the accumulation of deletions and tandem duplications in brc-1 and brd-1 animals. We find no additional back-up repair in HR and TMEJ compromised animals; non-homologous end-joining does not affect BRCAness. The notion that TMEJ acts as an alternative to HR, promoting the genome alteration of HR-deficient cells, supports the idea that polymerase theta is a promising therapeutic target for HR-deficient tumours.

In vivo analysis of FANCD2 recruitment at meiotic DNA breaks in Caenorhabditis elegans

Fanconi Anemia is a rare genetic disease associated with DNA repair defects, congenital abnormalities and infertility. Most of FA pathway is evolutionary conserved, allowing dissection and mechanistic studies in simpler model systems such as Caenorhabditis elegans. In the present study, we employed C. elegans to better understand the role of FA group D2 (FANCD2) protein in vivo, a key player in promoting genome stability. We report that localization of FCD-2/FANCD2 is dynamic during meiotic prophase I and requires its heterodimeric partner FNCI-1/FANCI. Strikingly, we found that FCD-2 recruitment depends on SPO-11-induced double-strand breaks (DSBs) but not RAD-51-mediated strand invasion. Furthermore, exposure to DNA damage-inducing agents boosts FCD-2 recruitment on the chromatin. Finally, analysis of genetic interaction between FCD-2 and BRC-1 (the C. elegans orthologue of mammalian BRCA1) supports a role for these proteins in different DSB repair pathways. Collectively, we showed a direct involvement of FCD-2 at DSBs and speculate on its function in driving meiotic DNA repair.

Aneuploidy and recombination in the human preimplantation embryo. Copy number variation analysis and genome-wide polymorphism genotyping

RESEARCH QUESTION

What are the incidence and patterns of meiotic trisomies and recombination separately and in relation to each other at the blastocyst stage via single nucleotide polymorphism genotyping combined with array comparative genomic hybridization.

DESIGN

Single nucleotide polymorphism microarrays were carried out on a total of 1442 blastocyst stage embryos derived from 268 fertile couples undergoing preimplantation genetic diagnosis for the purposes of avoiding transmittance of known single gene disorders to their offspring; 24-chromosome aneuploidy screening via array comparative genomic hybridization was carried out in parallel.

RESULTS

One hundred per cent of meiotic trisomies identified in these embryos were of maternal origin and their incidence increased significantly with advancing maternal age (P < 0.0001). A total of 55.8% of meiotic trisomies were meiosis I-type and 44.2% were meiosis II-type. Certain chromosomes were affected more by meiosis I-type errors, whereas others experienced more meiosis II-type errors. A detailed recombination analysis was carried out for 11,476 chromosomes and 17,763 recombination events were recorded. The average number of recombination sites was 24.0 ± 0.3 for male meiosis and 41.2 ± 0.6 for female meiosis (autosomes only). Sex-specific differences were observed in the locations of recombination sites. Comparative analysis conducted between 190 euploid embryos and 69 embryos presenting maternal meiotic trisomies showed similar recombination rates (P = 0.425) and non-recombinant chromatid rates (P = 0.435) between the two categories; differences, however, were observed when analysing embryos affected with specific maternal meiotic trisomies.

CONCLUSIONS

This study yielded unique data concerning recombination and the origin of aneuploidies observed during the first few days of life and provides a novel insight into these important biological processes.

Spatial Regulation of Polo-Like Kinase Activity During Caenorhabditis elegans Meiosis by the Nucleoplasmic HAL-2/HAL-3 Complex

Proper partitioning of homologous chromosomes during meiosis relies on the coordinated execution of multiple interconnected events: Homologs must locate, recognize, and align with their correct pairing partners. Further, homolog pairing must be coupled to assembly of the synaptonemal complex (SC), a meiosis-specific tripartite structure that maintains stable associations between the axes of aligned homologs and regulates formation of crossovers between their DNA molecules to create linkages that enable their segregation. Here, we identify HAL-3 (Homolog Alignment 3) as an important player in coordinating these key events during Caenorhabditis elegans meiosis. HAL-3, and the previously identified HAL-2, are interacting and interdependent components of a protein complex that localizes to the nucleoplasm of germ cells. hal-3 (or hal-2) mutants exhibit multiple meiotic prophase defects including failure to establish homolog pairing, inappropriate loading of SC subunits onto unpaired chromosome axes, and premature loss of synapsis checkpoint protein PCH-2. Further, loss of hal function results in misregulation of the subcellular localization and activity of Polo-like kinases (PLK-1 and PLK-2), which dynamically localize to different defined subnuclear sites during wild-type prophase progression to regulate distinct cellular events. Moreover, loss of PLK-2 activity partially restores tripartite SC structure in a hal mutant background, suggesting that the defect in pairwise SC assembly in hal mutants reflects inappropriate PLK activity. Together, our data support a model in which the nucleoplasmic HAL-2/HAL-3 protein complex constrains both localization and activity of meiotic Polo-like kinases, thereby preventing premature interaction with stage-inappropriate targets.

Synergistic lethality between BRCA1 and H3K9me2 loss reflects satellite derepression

Padeken et al. performed a genome-wide synthetic lethality screen and show that the BRCA1/BARD1 complex is necessary for germline viability in worms lacking MET-2 but not SET-25. The synthetic sterility upon BRCA1/BARD1 and H3K9me2 loss is directly linked to the DNA damage provoked by unscheduled satellite repeat transcription.

Caenorhabditis elegans has two histone H3 Lys9 methyltransferases, MET-2 (SETDB1 homolog) and SET-25 (G9a/SUV39H1 related). In worms, we found simple repeat sequences primarily marked by H3K9me2, while transposable elements and silent tissue-specific genes bear H3K9me3. RNA sequencing (RNA-seq) in histone methyltransferase (HMT) mutants shows that MET-2-mediated H3K9me2 is necessary for satellite repeat repression, while SET-25 silences a subset of transposable elements and tissue-specific genes through H3K9me3. A genome-wide synthetic lethality screen showed that RNA processing, nuclear RNA degradation, the BRCA1/BARD1 complex, and factors mediating replication stress survival are necessary for germline viability in worms lacking MET-2 but not SET-25. Unlike set-25 mutants, met-2-null worms accumulated satellite repeat transcripts, which form RNA:DNA hybrids on repetitive sequences, additively with the loss of BRCA1 or BARD1. BRCA1/BARD1-mediated H2A ubiquitination and MET-2 deposited H3K9me2 on satellite repeats are partially interdependent, suggesting both that the loss of silencing generates BRCA-recruiting DNA damage and that BRCA1 recruitment by damage helps silence repeats. The artificial induction of MSAT1 transcripts can itself trigger damage-induced germline lethality in a wild-type background, arguing that the synthetic sterility upon BRCA1/BARD1 and H3K9me2 loss is directly linked to the DNA damage provoked by unscheduled satellite repeat transcription.