Homologous Recombination, NHEJ and Maintenance of Genomic Integrity

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Defects of the mechanisms of detection and repair of DNA double-strand breaks can lead to cancer. In order to better characterize this process, our team investigates the molecular mechanisms of two repair pathways of genotoxic lesions: homologous recombination and non-homologous end joining (NHEJ). These repair pathways also allow cancer cells to resist radiotherapy- or chemotherapy-based treatment involving molecules such as mitomycin C or cisplatin. These treatments are indeed based on the capacity of these drugs to induce DNA double-strand breaks in cancer cells in order to kill them. Targeting factors involved in homologous recombination and NHEJ is thus a sensible strategy to improve cancer therapy.

Our team focuses on understanding the mechanisms of DNA double-strand break repair systems at the molecular level, including repair by Homologous Recombination and Non-Homologous End Joining. We exploit classical ensemble biochemical assays and cell biology to investigate these mechanisms. In addition, we also use “single-molecule” methods that allow visualization and monitoring of the dynamic behavior of repair proteins acting on single DNA molecules. To this purpose we are using optical tweezers to tether and manipulate single DNA molecules, and fluorescence microscopy to observe in real-time fluorescently labeled proteins interacting with the DNA substrate.

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DNA molecules are subject to damage. If these lesions are left unrepaired or ill-repaired, mutations can appear and lead to genomic instability. Cells presenting such a situation can either start proliferating in an uncontrolled manner and lead to the formation of tumors, or die. The efficient repair of such lesions is thus essential for living organisms. Amongst such lesions, DNA double-strand breaks are particularly dangerous as they can lead to chromosomal rearrangements such as translocations or loss of chromosome fragments.

In order to tackle the potential hazards of DNA double-strand breaks, our cells have developed two repair systems. The first one uses ‘Homologous Recombination’ which involves an exchange of strands between two identical or quasi-identical DNA molecules catalyzed by the RAD51 recombinase, the human ortholog of the bacterial recA protein. This system is a powerful means to maintain genome integrity, but when it operates incorrectly, or when it is defective or deregulated, the consequences can lead to the onset of cancer as is the case in breast cancer when the regulatory protein ErbB2 is defective. The second DNA double-strand break repair pathway is Non-Homologous End Joining (NHEJ). This pathway involves a distinct a group of effector proteins, in particular DNA ligase 4 and its cofactors XRCC4 and XLF which we study in detail in the team. This NHEJ system is essential for the repair of DNA double-strand breaks induced by ionizing radiations and protects our genome possible translocations. 

Our work aims to better characterize these two repair pathways at the molecular level. We study DNA repair proteins in vitro using biochemistry techniques, but also in vivo using cell biology approaches. We also use “single-molecule” methods that allow visualization and monitoring of the dynamic behavior of repair proteins acting on single DNA molecules.

Our ultimate goal is to provide a better understanding of these pathways in order to identify new therapeutic strategies for cancer.