Project description:DNA replication stress is an established driver of cancer-associated chromosomal rearrangements. Replication stress perturbs the duplication of late-replicating loci and activates a mitotic DNA repair pathway (termed MiDAS) for completion of replication. We here investigated RAD51-independent MiDAS.
Project description:Multiple replication abnormalities cause cells lacking BRCA2 to enter mitosis with under-replicated DNA and to activate mitotic DNA synthesis (MiDAS). However, the precise position of these MiDAS sites, as well as their origin, remains unknown. Here we labelled mitotic nascent DNA and performed high-throughput sequencing to identify at high-resolution the sites where MiDAS occurs in the absence of BRCA2. This approach revealed 150 genomic loci affected by MiDAS, which map within regions replicating during early S-phase and are therefore distinct from the aphidicolin-induced common fragile sites. Moreover, these sites largely localise near early firing origins and within genes transcribed in early S, suggesting that they stem from transcription-replication conflicts (TCRs). Inhibiting transcription with 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole (DRB) during early S-phase abrogates MiDAS. Strikingly, MiDAS sites co-localise with genomic loci where R-loops form in unchallenged conditions, suggesting that R-loop accumulation caused by BRCA2 inactivation leads to DNA lesion which are repaired by MiDAS. RAD52 is required in this process, as its abrogation in BRCA2-deficient cells reduces the rate of MiDAS and causes DNA damage accumulation in G1. Furthermore, MiDAS sites triggered by BRCA2 inactivation are hotspots for genomic rearrangement in BRCA2-mutated breast tumours. These results indicate that BRCA2 acts in early S-phase to protect TRC- and R-loop-induced DNA lesions, thereby preventing them from becoming a source of genomic instability and tumorigenesis.
Project description:RAD51 is a highly conserved DNA repair protein and is indispensable for the execution of homologous recombination, thereby participating in maintaining genomic stability. Since constitutive Rad51 knockout mice exhibit embryonic lethality, the physiological functions of RAD51, and the consequences of lacking it, are largely unknown. We herein demonstrated a critical role of RAD51 in postnatal liver development and regeneration. RAD51 is highly expressed during liver development and during regeneration following hepatectomy and hepatic injury, and is also elevated in liver-related diseases. We generated hepatocyte-specific Rad51 deletion mouse model (Rad51-CKO) to evaluate the function of RAD51 in liver development and regeneration.RAD51 deletion in postnatal hepatocytes results in aborted mitosis, global polyploidization, oxidative stress and cellular senescence. Remarkable liver fibrosis occurs as early as in 3-month-old Rad51fl/fl; Alb-Cre+/+ mice.The senescence-associated secretory phenotype in the livers of Rad51-CKO mice creates a niche that favors the activation and propagation of hepatic progenitor cells (HPCs) in which Rad51 is spared due to lack of Alb-Cre expression. The Rad51 functional HPCs and immature hepatocytes can thus proliferate vigorously, acquire increased malignancy, and eventually give rise to HCC. Our results thus demonstrate a novel function of RAD51 in liver development, homeostasis and tumorigenesis. The RAD51-CKO mice represent a unique genetic model for premature liver senescence, fibrosis, impaired regeneration and hepatocellular carcinogenesis.
Project description:RAD51 is a highly conserved DNA repair protein and is indispensable for the execution of homologous recombination, thereby participating in maintaining genomic stability. Since constitutive Rad51 knockout mice exhibit embryonic lethality, the physiological functions of RAD51, and the consequences of lacking it, are largely unknown. We herein demonstrated a critical role of RAD51 in postnatal liver development and regeneration. RAD51 is highly expressed during liver development and during regeneration following hepatectomy and hepatic injury, and is also elevated in liver-related diseases. We generated hepatocyte-specific Rad51 deletion mouse model (Rad51-CKO) to evaluate the function of RAD51 in liver development and regeneration.RAD51 deletion in postnatal hepatocytes results in aborted mitosis, global polyploidization, oxidative stress and cellular senescence. Remarkable liver fibrosis occurs as early as in 3-month-old Rad51fl/fl; Alb-Cre+/+ mice.The senescence-associated secretory phenotype in the livers of Rad51-CKO mice creates a niche that favors the activation and propagation of hepatic progenitor cells (HPCs) in which Rad51 is spared due to lack of Alb-Cre expression. The Rad51 functional HPCs and immature hepatocytes can thus proliferate vigorously, acquire increased malignancy, and eventually give rise to HCC. Our results thus demonstrate a novel function of RAD51 in liver development, homeostasis and tumorigenesis. The RAD51-CKO mice represent a unique genetic model for premature liver senescence, fibrosis, impaired regeneration and hepatocellular carcinogenesis.
Project description:The proper maintenance of genetic material is essential for the survival of living organisms. One of the main safeguards of genome stability is homologous recombination involved in the faithful repair of DNA double-strand breaks, the restoration of collapsed replication forks, and the bypass of replication barriers. Homologous recombination relies on the formation of Rad51 nucleofilaments which are responsible for the homology-based interactions between DNA strands. Here we demonstrate that without the regulation of these filaments by Srs2 and Rad54, which are known to remove Rad51 from single-stranded and double-stranded DNA respectively, the filaments strongly inhibit damage-associated DNA synthesis during DNA repair. Furthermore, this regulation is essential for cell survival under normal growth conditions as in the srs2Δ rad54Δ mutants unregulated Rad51 nucleofilaments cause activation of the DNA damage checkpoint, formation of mitotic bridges and loss of genetic material. These genome instability features may stem from the problems at stalled replication forks as the lack of Srs2 and Rad54 in the presence of Rad51 nucleofilaments impedes cell recovery from replication stress. This study demonstrates that the timely and efficient disassembly of recombination machinery is essential for genome maintenance and cell survival.