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Meiosis and DNA repair: researchers decipher the activity of a specific molecular complex

Researchers from I2BC/CEA-Joliot in collaboration with a team from Institut Curie and IRCM/CEA-Jacob, lay the molecular basis of a “molecular scissors” mechanism involved in DNA repair during meiosis – the step leading to the formation of gametes.

They have explained the role of the Mlh1-Mlh3 complex in DNA mismatch repair and, quite uniquely, in one of the key steps of genetic mixing during meiosis whose dysfunctions can explain different types of pathologies. Their study has been published in PNAS.

During meiosis, recombination of genetic material between homologous chromosomes occurs, which contributes to genetic mixing. Recombination can only take place through fine mechanisms of breakage and subsequent repair of DNA molecules. These mechanisms are highly conserved in eukaryotes, from yeast to humans. In particular, cross-shaped structures (Holliday junctions) are formed between homologous chromosomes to make crossovers during recombination. These recombination mechanisms are essential to ensure correct segregation of chromosomes in gametes. Failure of these steps results in the generation of gametes with an abnormal number of chromosomes (trisomy, Turner syndrome, infertility).

Two molecular complexes similar and yet… different!

The “Mlh1-Mlh3” complex is a repair factor for DNA mismatches that are generated as a result of errors in replicative polymerases. Mlh1-Mlh3 has endonuclease activity, i.e. it can cut a DNA molecule between two successive nucleotides and not at its ends. Unlike other similar mismatch repair factors, Mlh1-Mlh3 also exerts its endonuclease activity during meiosis, at the Holliday junctions; it is essential for the exchange of chromosome portions. The Mlh1-Pms1 complex, which is the main factor for repairing DNA mismatches, is not involved in meiosis. However, the two complexes have many structural similarities. For example, they are formed by the interaction between the C-terminal domains of Mlh1 and of its partner Mlh3 or Pms1. What are the structural bases for the differences in specificity between Mlh1-Mlh3 and Mlh1-Pms1? Researchers from CEA-Joliot (I2BC department, Structural Biology Laboratory) and Valérie Borde’s team, “Chromosome dynamics and recombination” (CNRS, Sorbonne Université) at Institute Curie, with the help of CEA-Jacob (IRCM department) and a Swiss team, have solved the three-dimensional structure of a complex formed by the interaction domains of Mlh1 and Mlh3 (from purified recombinant proteins) of the yeast S. cerevisiae by radiocrystallography, and have functionally characterized it. They compared it with the already known equivalent complex formed between Mlh1 and Pms1.

Their study, published in PNAS, reveals differences between the two complexes, including protein folding, surface interaction with DNA, formation of filamentary structures and other DNA recognition specificities.

This first comparison at the structural level of the Mlh1-Mlh3 and Mlh1-Pms1 complexes strongly suggests an evolutionary specialization of each. This work opens perspectives to better understand certain pathologies linked to mutations in these complexes affecting repair of replicative errors and meiotic recombination. To go further, it will be necessary to obtain cryo-electron microscopy structures of the two whole proteins, in interaction with their DNA substrate.

These results are the culmination of a fruitful collaboration that began several years ago with the CEA and our Swiss colleagues. We are delighted to have been able to elucidate the intriguing function of this molecular complex involved in DNA repair during meiosis, when a wave of chromosomal recombination occurs. This is another building block for understanding mechanisms at the origin of pathologies such as certain forms of hereditary cancers.

Said Valérie Borde, CNRS researcher and team leader at the Institut Curie

 Crystallographic structure of the C-terminal region Mlh1-Mlh3 with its endonuclease site (blue) and its interaction site with meiotic recombination or DNA repair factors (brown). Mlh1-Mlh3 forms oligomers, a possible arrangement of which is observed in the crystal. © JB. Charbonnier/CEA

Crystallographic structure of the C-terminal region Mlh1-Mlh3 with its endonuclease site (blue) and its interaction site with meiotic recombination or DNA repair factors (brown). Mlh1-Mlh3 forms oligomers, a possible arrangement of which is observed in the crystal.
© JB. Charbonnier/CEA

Références :

J. Dai, A. Sanchez, C. Adam, L. Ranjha, G. Reginato, P. Chervy, C. Tellier-Lebegue, J. Andreani, R. Guérois, V. Ropars, M-H Le Du, L. Maloisel, E. Martini, P. Legrand, A. Thureau, P. Cejka, V. Borde, J-B Charbonnier. Molecular basis of the dual role of the Mlh1-Mlh3 endonuclease in MMR and crossover formation in meiosis | PNAS, 2021 June 8 118(23)  doi: 10.1073/pnas.2022704118

Source : CEA (Institut des sciences du vivant Frédéric Joliot)

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AURA PRIX AVENBIR RUBAN ROSE

En 2018, l’association Le Cancer du Sein, Parlons-en ! a choisi de mettre à l’honneur la jeune biologiste Aura Carreira et ses travaux sur la fonction biologique du gène de prédisposition au cancer du sein et de l’ovaire : BRCA2.

Aura Carreira, directrice de recherche (CNRS) à l’Institut Curie, s’intéresse au gène BRCA2 depuis son post-doctorat à l’université de Californie (USA). Certains gènes, dès lors qu’ils sont altérés, prédisposent au cancer. Les femmes porteuses d’une altération de BRCA2 ont un risque supérieur au reste de la population de développer un cancer au sein ou de l’ovaire. La protéine BRCA2 produite par ce gène répare l’un des dommages les plus néfastes pour la cellule, les cassures double brin de l’ADN, par un mécanisme appelé recombinaison homologue. L’équipe qu’Aura Carreira dirige sur le site d’Orsay de l’Institut Curie et qui se nomme – sans surprise – Recombinaison homologue et cancer (CNRS/Université Paris-Sud/Institut Curie) décortique les moyens mis en œuvre par BRCA2 pour restaurer au mieux cette cassure de l’ADN. Récemment, son équipe a révélé d’autres fonctions de la protéine lors de la division cellulaire qui pourraient être aussi à l’origine de cancer. Néanmoins, il reste encore beaucoup à élucider sur le rôle de ce gène et de sa mutation dans l’apparition d’un cancer du sein.

Le gène BRCA2 est un gène dit « suppresseur de tumeurs » car il joue un rôle de frein dans la prolifération des cellules. Une mutation sur ce gène peut donc perturber cette fonction. Cependant chaque cellule présente deux copies d’un même gène (un hérité de la mère et l’autre hérité du père) et les personnes porteuses d’une mutation sur BRCA2 possèdent une copie intacte de ce gène. La question qui se pose est donc : comment une mutation sur une seule copie du gène BRCA2 peut donner lieu à l’apparition d’une tumeur ?

Grâce au prix ruban rose, Aura et son équipe ont décidé de s’attaquer à cette question, en collaboration avec le service d’oncogénétique dirigé par le Pr Dominique Stoppa-Lyonnet, le pôle de pathologie dirigé par le Dr Anne Vincent-Salomon et le service bioinformatique de l’Institut Curie. Ainsi ils vont pouvoir combiner les approches fonctionnelles, génomiques, protéomiques, histopathologiques etc. pour éclairer les mécanismes à l’œuvre dans l’apparition d’une tumeur, suite à la présence d’une mutation de BRCA2. Leur objectif est d’apporter de nouvelles pistes en matière de prévention, de suivi et de traitement de toutes les femmes porteuses d’une telle mutation et de leurs familles.

Prix Avenir Ruban Rose 2018

 

 

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BRCA2: an unexpected role for this tumor suppressor gene

Portrait Aura Carreira_0

The BRCA2 protein, produced by the expression of the tumor suppressor gene BRCA2, plays an important role in DNA repair by homologous recombination, which takes place at early phases of the cell cycle. A team from Institut Curie, in collaboration with a group of CEA, revealed an additional role of BRCA2 in the alignment of chromosomes during mitosis (cell division), with significant consequences on chromosome stability. Published in Nature Communications, these results could explain certain chromosomal aberrations observed in BRCA2 mutated tumors.

The “Genome Instability and Cancer Predisposition” team led by Aura Carreira in the “Genome integrity, RNA and Cancer” unit (CNRS/Paris-Saclay university/Institut Curie) is working on the role of BRCA2 in maintaining genome integrity. We know that BRCA2 operates in DNA repair by homologous recombination, the DNA duplication phase of the cell cycle. In addition, we know that a mutation in the BRCA2 gene predisposes to breast and ovarian cancer.

In collaboration with Sophie Zinn-Justin’s group at the CEA, Aura Carreira’s team found an additional and unexpected role of BRCA2 in mitosis, which seems uncoupled to its function in DNA repair. Using a combination of biophysical, biochemistry, cell biology and genetics tools, these researchers showed that the alignment of chromosomes at the metaphase plate depends on the phosphorylation of BRCA2 by the protein kinase PLK1. Importantly, they found that certain BRCA2 variants identified in breast cancer patients this function during mitosis is altered.

“These results reveal a novel function of BRCA2 that could contribute to the numerical chromosomal aberrations that are often observed in BRCA2  mutated tumors,” explains Aura Carreira.

The phosphorylation of BRCA2 by PLK1 at the kinetochore (illustrated as “P”) during mitosis facilitates the alignment of chromosomes at the metaphase plate. Certain BRCA2 variants identified in breast cancer patients impair this phosphorylation, which leads to defects in the alignment and segregation of chromosomes resulting in numerical chromosomal aberrations. (aneuploidy).

Reference:

Proper Chromosome Alignment Depends on BRCA2 Phosphorylation by PLK1. Åsa Ehlén, Charlotte Martin, Simona Miron, Manon Julien, François-Xavier Theillet, Virginie Ropars, Gaetana Sessa, Romane Beaurepere, Virginie Boucherit, Patricia Duchambon, Ahmed El Marjou, Sophie Zinn-Justin, Aura Carreira. Nature Communications. April 14l 2020. doi: 10.1038/s41467-020-15689-9.

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Our tributes to Professor Michael Wakelam, director of the Babraham Institute

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“We have heard with great sadness the tragic news that our dear colleague, Professor Michael Wakelam, director of the Babraham Institute passed away of complications suspected to be due to Covid-19 infection. Professor Wakelam was an active voice on the value of international science. He was always enthusiastic and constructive and he was instrumental for building the  EU-LIFE alliance of research institutes as it is today. As Chair of EU-Life, representing The Institut Curie in the alliance, I could always count on him for input and ideas. Michael’s positive attitude and warm personality were an inspiration for so many of us. His loss is tragic for the scientific community and we will miss him deeply.

Our thoughts go to his family and to our colleagues at Babraham Institute in this difficult moment.

The Babraham Institute has opened a book of remembrance for anyone who would like to share their memories of Michael or tributes to him. Please email these to 

Tribute paid by Dr. Geneviève Almouzni / Research center at the Institut Curie / Head of the Chromatin Dynamics team / Member of the Science Academy in France / Co-Chair of the LifeTime Initiative

Further information: https://www.babraham.ac.uk/news/2020/04/michael-wakelam-1955-2020

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Histone variant H3.3 residue S31 is essential for Xenopus gastrulation regardless of the deposition pathway

Portrait Geneviève Almouzni

During vertebrate development, H3 histone variants H3.1/2 and H3.3 cannot substitute for each other. It has been a longstanding puzzle, whether this is due to unique properties of individual variants or if it simply reflects their different incorporation pathways  (DNA-synthesis dependent or independent). In Xenopus we demonstrated that only the presence of serine at the position 31 (S31) in H3.3 was essential at gastrulation, independent of the incorporation pathway. Moreover, the phosphorylation at S31 promotes acetylation at lysine 27, thus contributing to an open chromatin state. These findings illustrate how a unique residue in H3.3 allows the establishment and the maintenance of particular chromatin states, which are important during cell cycle and development.

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Magnetically focussed proton minibeams : new hopes for radiation therapy!

Yolanda Prezado - PedroLombardi - Science

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that has proven to significantly increase dose tolerances and sparing of normal tissue, while achieving tumor control equivalent to or better than conventional radiotherapy in high-grade gliomas in small animals. pMBRT uses submillimetric proton beams, about an order of magnitude smaller than the beams used in clinical practice. The current implementation of pMBRT with mechanical collimators, although valid,  has shown its limits: rigidity, reduction in efficiency and production of additional secondary neutrons.

One potential solution, explored by the new SIRIC team New approaches in Radiotherapy headed by Yolanda Prezado UMR3347 / U1021 is the generation of proton minibeams by magnetic focusing. With that aim they have designed a new optimized nozzle which could be integrated at existing clinical centers. This design uses conventional beamline elements and has a moderate focal length of approximately 1 m and a shortened air gap of 10 to 30 cm. This new nozzle is capable of providing beam sizes between 0.66 and 1.67 mm FWHM at clinically relevant energies.

It can therefore be considered appropriate for pMBRT, leading to an optimal implementation of pMBRT. It could allow a more effective and flexible treatment, accessible to 3D intensity modulated treatments. This is a major development in the project that approaches pMBRT a step closer to patients’ treatments.  A patent has been filed.

 

Learn more https://www.nature.com/articles/s41598-020-58052-0

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New International PhD Program: Ready? Steady? Apply now!

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Within the frame of Institut Curie’s new PhD Program called EuReCa, 8 positions are offered by Institut Curie’s research teams. The call for applications is now open. The 8 laureates will join their PhD laboratory in September 2020.

Cofounded by the European Commission, EuReCa (Europe, Research & Care) is Institut Curie’s new excellent interdisciplinary, inter-sectorial, and international PhD Program. Candidates wishing to apply must complete an online application form via the EuReCa webpage before January 9th, 2020, 4:00pm CET.

Following the closing date of the call, a Review Board composed of internal and external scientific experts will score and rank the eligible applications.

For each project, 3 to 4 shortlisted candidates will be invited for an interview session on May 27-29 2020. The interview will be led by a Selection committee composed of the future Thesis Director and scientific experts (one internal to Institut Curie, one external).

The 8 laureates will join their labs on September 1st, 2020.

Find the list of projects, the calendar and all the information here! (a link to share without restraint!).

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Invacell: Philippe Chavrier and Harald Stenmark labs received grant to collaborate on cancer invasion and metastases research

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Philippe Chavrier and his colleagues have received a joint grant with Norwegian researchers from the group of Harald Stenmark from The Institute for Cancer Research, Oslo University Hospital, Oslo, Norway. The funds were donated by Trond S. Paulsen and allocated by The Radium Hospital Foundation to combat cancer cell invasiveness through the project Invacell.

The project is a basic research work that facilitates the development of drugs in the long term that can prevent the spread of metastases. The research will run over four years and contribute to strengthening the long-term cooperation between Norway and France. The project has been created simultaneously with a newly established framework agreement for cancer research between Oslo University Hospital and Institut Curie.

Philippe Chavrier and his colleagues at Institut Curie have long-standing interest in understanding the mechanism of tumor invasion in breast cancer. Their studies on MT1-MMP and invadopodia in breast cancer invasion were crucial contributions to the foundation of the collaboration.

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The physicist Pascal Hersen, new director of the Curie Physical Chemistry unit

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Pascal Hersen, CNRS director of research in biophysics, has taken over as director of the Curie Physical Chemistry unit (UMR 168 CNRS/Institut Curie). Following a call for tenders, his application was supported by an international recruitment committee and by the Scientific Advisory Board of Institut Curie, and approved by the director of the Research Center. He replaces Axel Buguin, interim director of the unit following the sudden death of Maxime Dahan in 2018.

Until now, Pascal Hersen (PhD), 42 years old, headed an interdisciplinary team of around fifteen researchers at the Laboratoire matière et systèmes complexes in the Physics Department at Université Paris Diderot. His work is in a new field of research – cybergenetics – which involves determining how cells process information, how they adapt dynamically to changes in their environment and to what extent it is possible to control the cellular processes. This physicist joins the Institut Curie Research Center to head up the Curie Physical Chemistry unit (UMR 168 CNRS/Institut Curie).

The work of the Curie physical chemistry unit, whose aim is to propose a physics-related vision of the fundamental processes at work in living cells, using methods and concepts from experimental and theoretical physics, and the work in the field of cybergenetics conducted by his team joining the unit, will be mutually enhanced. “We are developing computerized feedback loops to control the level of gene expression in real time at the single-cell level. This new technological approach combines microfluidics, microscopy, synthetic biology, optogenetics and theory of control,” he explains, “and we are looking to determine whether or not to apply this approach to a variety of contexts from controlling gene expression in single cells to spatial orchestration of multicellular dynamics.”

“Within the unit, and without prejudice to the interactions with the other research units, a great many collaboration projects are taking shape,” comments Prof. Axel Buguin, interim director of the unit since the death of Maxime Dahan last July. “The research subjects perfectly match those developed in the unit involving quantitative biology. They will benefit from our involvement in the Institut Pierre-Gilles for microfluidics.” For one year, Prof. Buguin, who was deputy director alongside Maxime Dahan, worked tirelessly to manage the transition and prepare for the future of the research unit and the some 120 people working there. It has been decided that Mathieu Coppey, head of the Cellular organization and imaging team (LOCCO), will be the unit’s deputy director.

Encouraged by meetings with team leaders in the unit, as well as with most of the research unit directors on the Paris site of the Research Center, the new unit director was able to observe the “wonderful interdisciplinary and collaborative approach, as well as a rich ecosystem of resources and researchers. Within this exceptional context, the unit will strive to contribute to the Curie research-care continuum model by developing innovative research projects focused on the physics of cancer, from understanding the elementary behavior of cells and tissues, to the medical applications.”

Pascal Hersen has authored over 40 peer-reviewed publications, filed two patents and helped create a start-up. He is the beneficiary of an ERC Consolidator grant (SmartCells). He was involved in founding the Centre de recherche interdisciplinaire (CRI), today attached to the University of Paris, and was auditor for the Institut des hautes études pour la science et la technologie (IHEST).

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A new player in the regulation of epigenetics

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The team of Raphael Margueron, together with that of Deborah Bourc’his and other international teams, highlighted the role of a new key player in the regulation of genes and its dysfunction that can lead to different diseases.

Gene expression, i.e. the control of their use by a given cell at a given time, responds to complex mechanisms. A group of proteins called Polycomb contributes to the orchestration of genes, reducing them to silence when they should be inhibited by the phenomenon called epigenetics. “In mammals, the “Polycomb machinery” is composed of dozens of proteins,” says Raphael Margueron, researcher at Institut Curie (U934 INSERM/CNRS UMR3215). His team is taking a close interest in this group and other players that are moving around to indicate to the Polycomb group proteins which genes should be blocked from expression.

In this context, Raphael Margueron and his colleagues have worked to highlight the role of a previously unknown player in these processes: EZHIP. Its intervention is necessary for the maturation of female gametes (reproductive cells). Their discovery published in Nature Communications comes at the same time that other teams have revealed the role of EZHIP in many types of children’s brain cancer. Highlighting their scientific and medical interests, these different revelations were put into perspective in the Nature journal where the authors note that “these studies lay the groundwork for exploring the role of this protein in triggering cancer and indicate that targeting PRC2 or EZHIP may have therapeutic potential for children with [these cancers]”. With these promising advances, “we will continue to study this co-factor to understand why it is so important,” promises Raphael Margueron.