The conserved LEM-3/Ankle1 nuclease is involved in the combinatorial regulation of meiotic recombination repair and chromosome segregation in Caenorhabditis elegans.

Homologous recombination is essential for crossover (CO) formation and accurate chromosome segregation during meiosis. It is of considerable importance to work out how recombination intermediates are processed, leading to CO and non-crossover (NCO) outcome. Genetic analysis in budding yeast and Caenorhabditis elegans indicates that the processing of meiotic recombination intermediates involves a combination of nucleases and DNA repair enzymes. We previously reported that in C. elegans meiotic joint molecule resolution is mediated by two redundant pathways, conferred by the SLX-1 and MUS-81 nucleases, and by the HIM-6 Bloom helicase in conjunction with the XPF-1 endonuclease, respectively. Both pathways require the scaffold protein SLX-4. However, in the absence of all these enzymes, residual processing of meiotic recombination intermediates still occurs and CO formation is reduced but not abolished. Here we show that the LEM-3 nuclease, mutation of which by itself does not have an overt meiotic phenotype, genetically interacts with slx-1 and mus-81 mutants, the respective double mutants displaying 100% embryonic lethality. The combined loss of LEM-3 and MUS-81 leads to altered processing of recombination intermediates, a delayed disassembly of foci associated with CO designated sites, and the formation of univalents linked by SPO-11 dependent chromatin bridges (dissociated bivalents). However, LEM-3 foci do not colocalize with ZHP-3, a marker that congresses into CO designated sites. In addition, neither CO frequency nor distribution is altered in lem-3 single mutants or in combination with mus-81 or slx-4 mutations. Finally, we found persistent chromatin bridges during meiotic divisions in lem-3; slx-4 double mutants. Supported by the localization of LEM-3 between dividing meiotic nuclei, this data suggest that LEM-3 is able to process erroneous recombination intermediates that persist into the second meiotic division.

Separable Roles for a Caenorhabditis elegans RMI1 Homolog in Promoting and Antagonizing Meiotic Crossovers Ensure Faithful Chromosome Inheritance.

During the first meiotic division, crossovers (COs) between homologous chromosomes ensure their correct segregation. COs are produced by homologous recombination (HR)-mediated repair of programmed DNA double strand breaks (DSBs). As more DSBs are induced than COs, mechanisms are required to establish a regulated number of COs and to repair remaining intermediates as non-crossovers (NCOs). We show that the Caenorhabditis elegans RMI1 homolog-1 (RMH-1) functions during meiosis to promote both CO and NCO HR at appropriate chromosomal sites. RMH-1 accumulates at CO sites, dependent on known pro-CO factors, and acts to promote CO designation and enforce the CO outcome of HR-intermediate resolution. RMH-1 also localizes at NCO sites and functions in parallel with SMC-5 to antagonize excess HR-based connections between chromosomes. Moreover, RMH-1 also has a major role in channeling DSBs into an NCO HR outcome near the centers of chromosomes, thereby ensuring that COs form predominantly at off-center positions.

Model consent clauses for rare disease research.

BACKGROUND: Rare Disease research has seen tremendous advancements over the last decades, with the development of new technologies, various global collaborative efforts and improved data sharing. To maximize the impact of and to further build on these developments, there is a need for model consent clauses for rare diseases research, in order to improve data interoperability, to meet the informational needs of participants, and to ensure proper ethical and legal use of data sources and participants' overall protection. METHODS: A global Task Force was set up to develop model consent clauses specific to rare diseases research, that are comprehensive, harmonized, readily accessible, and internationally applicable, facilitating the recruitment and consent of rare disease research participants around the world. Existing consent forms and notices of consent were analyzed and classified under different consent themes, which were used as background to develop the model consent clauses. RESULTS: The IRDiRC-GA4GH MCC Task Force met in September 2018, to discuss and design model consent clauses. Based on analyzed consent forms, they listed generic core elements and designed the following rare disease research specific core elements; Rare Disease Research Introductory Clause, Familial Participation, Audio/Visual Imaging, Collecting, storing, sharing of rare disease data, Recontact for matching, Data Linkage, Return of Results to Family Members, Incapacity/Death, and Benefits. CONCLUSION: The model consent clauses presented in this article have been drafted to highlight consent elements that bear in mind the trends in rare disease research, while providing a tool to help foster harmonization and collaborative efforts.

Combinatorial regulation of meiotic holliday junction resolution in C. elegans by HIM-6 (BLM) helicase, SLX-4, and the SLX-1, MUS-81 and XPF-1 nucleases.

Holliday junctions (HJs) are cruciform DNA structures that are created during recombination events. It is a matter of considerable importance to determine the resolvase(s) that promote resolution of these structures. We previously reported that C. elegans GEN-1 is a symmetrically cleaving HJ resolving enzyme required for recombinational repair, but we could not find an overt role in meiotic recombination. Here we identify C. elegans proteins involved in resolving meiotic HJs. We found no evidence for a redundant meiotic function of GEN-1. In contrast, we discovered two redundant HJ resolution pathways likely coordinated by the SLX-4 scaffold protein and also involving the HIM-6/BLM helicase. SLX-4 associates with the SLX-1, MUS-81 and XPF-1 nucleases and has been implicated in meiotic recombination in C. elegans. We found that C. elegans [mus-81; xpf-1], [slx-1; xpf-1], [mus-81; him-6] and [slx-1; him-6] double mutants showed a similar reduction in survival rates as slx-4. Analysis of meiotic diakinesis chromosomes revealed a distinct phenotype in these double mutants. Instead of wild-type bivalent chromosomes, pairs of "univalents" linked by chromatin bridges occur. These linkages depend on the conserved meiosis-specific transesterase SPO-11 and can be restored by ionizing radiation, suggesting that they represent unresolved meiotic HJs. This suggests the existence of two major resolvase activities, one provided by XPF-1 and HIM-6, the other by SLX-1 and MUS-81. In all double mutants crossover (CO) recombination is reduced but not abolished, indicative of further redundancy in meiotic HJ resolution. Real time imaging revealed extensive chromatin bridges during the first meiotic division that appear to be eventually resolved in meiosis II, suggesting back-up resolution activities acting at or after anaphase I. We also show that in HJ resolution mutants, the restructuring of chromosome arms distal and proximal to the CO still occurs, suggesting that CO initiation but not resolution is likely to be required for this process.

lethal giant larvae is required with the par genes for the early polarization of the Drosophila oocyte.

Most cell types in an organism show some degree of polarization, which relies on a surprisingly limited number of proteins. The underlying molecular mechanisms depend, however, on the cellular context. Mutual inhibitions between members of the Par genes are proposed to be sufficient to polarize the C. elegans one-cell zygote and the Drosophila oocyte during mid-oogenesis. By contrast, the Par genes interact with cellular junctions and associated complexes to polarize epithelial cells. The Par genes are also required at an early step of Drosophila oogenesis for the maintenance of the oocyte fate and its early polarization. Here we show that the Par genes are not sufficient to polarize the oocyte early and that the activity of the tumor-suppressor gene lethal giant larvae (lgl) is required for the posterior translocation of oocyte-specific proteins, including germline determinants. We also found that Lgl localizes asymmetrically within the oocyte and is excluded from the posterior pole. We further demonstrate that phosphorylation of Par-1, Par-3 (Bazooka) and Lgl is crucial to regulate their activity and localization in vivo and describe, for the first time, adherens junctions located around the ring canals, which link the oocyte to the other cells of the germline cyst. However, null mutations in the DE-cadherin gene, which encodes the main component of the zonula adherens, do not affect the early polarization of the oocyte. We conclude that, despite sharing many similarities with other model systems at the genetic and cellular levels, the polarization of the early oocyte relies on a specific subset of polarity proteins.

Nusinersen treatment significantly improves hand grip strength, hand motor function and MRC sum scores in adult patients with spinal muscular atrophy types 3 and 4.

BACKGROUND: Nusinersen recently became available as the first treatment for Spinal Muscular Atrophy (SMA) and data on its effectiveness and safety in adult SMA patients are still scarce. METHODS: We evaluated the effectiveness and safety of nusinersen treatment during 14 months in 16 adult patients with SMA types 3 and 4 in a prospective study, and retrospectively detailed the natural history of 48 adult SMA patients types 2, 3 and 4. RESULTS: Hand grip strength (p = 0.03), hand motor function (p = 0.04) as assessed by a sub-score of the Revised Upper Limb Module (RULM) and the Medical Research Council (MRC) sum score (p = 0.04) improved significantly at month 14. Importantly, the MRC sum score had declined significantly (p < 0.01) prior to start of treatment in these patients. A minimal clinically important difference (MCID) in the Hammersmith Functional Motor Scale Expanded (HFMSE) and RULM scores was achieved in 31% and 50% of the patients, respectively, but the mean changes from baseline failed to reach significance. Forced Vital Capacity (FVC) transiently increased at month 6 (p = 0.01), whereas the Peak Expiratory Flow (PEF) did not. The Activity Limitations scale declined significantly prior to start of treatment (p < 0.01) and showed an improvement with nusinersen which was not significant. The safety evaluation did not reveal serious adverse events and no signs of nephrotoxicity or antisense oligonucleotide (ASO)-mediated inflammation. CONCLUSIONS: We conclude that hand grip strength and hand motor function, as well as MRC sum scores improved significantly in nusinersen-treated adult patients with SMA types 3 and 4.

[Regulation of germline stem cells: the niche expands in Drosophila]. / Régulation des cellules souches de la lignée germinale: la niche s'agrandit chez la drosophile.

Our fascination for stem cells originates from their ability to divide asymmetrically in order to self-renew and produce daughter cells which can differentiate and replenish tissues. Stem cells could thus represent an unlimited source of differentiated cells that could be used to repair malformed, damaged or ageing tissues. Understanding how their behaviour is regulated is then of paramount medical interest. Specific microenvironments surrounding the stem cells, termed "niches", were proposed to play a major role in the balance between self-renewal and differentiation. However, it is only recently, in the case of the stem cells producing the germline (GSGs) in Drosophila, that the cells and signals creating a niche were identified for the first time. Here, we review how this niche has been defined at the cellular and functional levels in vivo, thanks to the powerful genetic tools available in Drosophila. Such studies have revealed adhesive interactions, cell-cycle modifications and intercellular signals that control the GSC behavior. Extracellular signals from the niche activate the BMP or JAK-STAT pathways in the GSCs and are necessary for their maintenance. Strikingly, both signaling pathways are also sufficient to convert differentiated germ cells into functional GSCs, demonstrating in vivo that a niche has the capacity to regenerate stem cells from differentiated cells. Rapid progresses have further identified direct links between these signaling pathways and the transcriptional regulation of the GSCs, providing a simple paradigm for stem cells regulation. Many of these features and signals are conserved in stem cells niches from Drosophila to mammals. We can thus hope that research on the GSCs in Drosophila will benefit therapeutic approaches to human degenerative diseases.

A mosaic genetic screen for genes involved in the early steps of Drosophila oogenesis.

The first hours of Drosophila embryogenesis rely exclusively on maternal information stored within the egg during oogenesis. The formation of the egg chamber is thus a crucial step for the development of the future adult. It has emerged that many key developmental decisions are made during the very first stages of oogenesis. We performed a clonal genetic screen on the left arm of chromosome 2 for mutations affecting early oogenesis. During the first round of screening, we scored for defects in egg chambers morphology as an easy read-out of early abnormalities. In a second round of screening, we analyzed the localization of centrosomes and Orb protein within the oocyte, the position of the oocyte within the egg chamber, and the progression through meiosis. We have generated a collection of 71 EMS-induced mutants that affect oocyte determination, polarization, or localization. We also recovered mutants affecting the number of germline cyst divisions or the differentiation of follicle cells. Here, we describe the analysis of nine complementation groups and eight single alleles. We mapped several mutations and identified alleles of Bicaudal-D, lethal(2) giant larvae, kuzbanian, GDP-mannose 4,6-dehydratase, tho2, and eiF4A. We further report the molecular identification of two alleles of the Drosophila homolog of Che-1/AATF and demonstrate its antiapoptotic activity in vivo. This collection of mutants will be useful to investigate further the early steps of Drosophila oogenesis at a genetic level.