http://repub.eur.nl/res/aut/30913/ List of Publications en http://repub.eur.nl/static-eur/img/logo.png http://repub.eur.nl http://repub.eur.nl/res/pub/34429/ 2011-11-10T00:00:00Z Ultraviolet (UV) radiation-induced DNA lesions can be efficiently repaired by nucleotide excision repair (NER). However, NER is less effective during replication of UV-damaged chromosomes. In contrast, translesion DNA synthesis (TLS) and homologous recombination (HR) are capable of dealing with lesions in replicating DNA. The core HR protein in mammalian cells is the strand exchange protein RAD51, which is aided by numerous proteins, including RAD54. We used RAD54 as a cellular marker for HR to study the response of mammalian embryonic stem (ES) cells to UV irradiation. In contrast to yeast, ES cells lacking RAD54 are not UV sensitive. Here we show that the requirement for mammalian RAD54 is masked by active NER. By genetically inactivating NER and HR through disruption of the Xpa and Rad54 genes, respectively, we demonstrate the contribution of HR to chromosomal integrity upon UV irradiation. We demonstrate using chromosome fiber analysis at the individual replication fork level, that HR activity is important for the restart of DNA replication after induction of DNA damage by UV-light in NER-deficient cells. Furthermore, our data reveal RAD54-dependent and -independent contributions of HR to the cellular sensitivity to UV-light, and they uncover that RAD54 can compensate for the loss of TLS polymerase η with regard to UV-light sensitivity. In conclusion, we show that HR is important for the progression of UV-stalled replication forks in ES cells, and that protection of the fork is an interplay between HR and TLS. http://repub.eur.nl/res/pub/33400/ 2011-06-14T00:00:00Z Defective homologous recombination (HR) DNA repair imposed by BRCA1 or BRCA2 deficiency sensitizes cells to poly (ADP-ribose) polymerase (PARP)-1 inhibition and is currently exploited in clinical treatment of HR-deficient tumors. Here we show that mild hyperthermia (41-42.5°C) induces degradation of BRCA2 and inhibits HR. We demonstrate that hyperthermia can be used to sensitize innately HR-proficient tumor cells to PARP-1 inhibitors and that this effect can be enhanced by heat shock protein inhibition. Our results, obtained from cell lines and in vivo tumor models, enable the design of unique therapeutic strategies involving localized ondemand induction of HR deficiency, an approach that we term induced synthetic lethality. http://repub.eur.nl/res/pub/34223/ 2011-04-08T00:00:00Z http://repub.eur.nl/res/pub/33502/ 2011-03-07T00:00:00Z Rad54, a member of the SWI/SNF protein family of DNA-dependent ATPases, repairs DNA double-strand breaks (DSBs) through homologous recombination. Here we demonstrate that Rad54 is required for the timely accumulation of the homologous recombination proteins Rad51 and Brca2 at DSBs. Because replication protein A and Nbs1 accumulation is not affected by Rad54 depletion, Rad54 is downstream of DSB resection. Rad54-mediated Rad51 accumulation does not require Rad54's ATPase activity. Thus, our experiments demonstrate that SWI/SNF proteins may have functions independent of their ATPase activity. However, quantitative real-time analysis of Rad54 focus formation indicates that Rad54's ATPase activity is required for the disassociation of Rad54 from DNA and Rad54 turnover at DSBs. Although the non-DNA-bound fraction of Rad54 reversibly interacts with a focus, independent of its ATPase status, the DNA-bound fraction is immobilized in the absence of ATP hydrolysis by Rad54. Finally, we show that ATP hydrolysis by Rad54 is required for the redistribution of DSB repair sites within the nucleus. http://repub.eur.nl/res/pub/34232/ 2011-03-04T00:00:00Z The cellular response to DNA double-strand breaks (DSBs) is mobilized by the protein kinase ATM, which phosphorylates key players in the DNA damage response (DDR) network. A major question is how ATM controls DSB repair. Optimal repair requires chromatin relaxation at damaged sites. Chromatin reorganization is coupled to dynamic alterations in histone posttranslational modifications. Here, we show that in human cells, DSBs induce monoubiquitylation of histone H2B, a modification that is associated in undamaged cells with transcription elongation. We find that this process relies on recruitment to DSB sites and ATM-dependent phosphorylation of the responsible E3 ubiquitin ligase: the RNF20-RNF40 heterodimer. H2B monoubiquitylation is required for timely recruitment of players in the two major DSB repair pathways-nonhomologous end-joining and homologous recombination repair-and optimal repair via both pathways. Our data and previous data suggest a two-stage model for chromatin decondensation that facilitates DSB repair. http://repub.eur.nl/res/pub/26722/ 2010-06-11T00:00:00Z The structure of DNA and subsequently the integrity of the genetic code are constantly threatened by numerous endogenous and exogenous DNA damaging agents. Restoration of proper DNA structure occurs through distinct and overlapping DNA damage repair pathways, depending on the specific lesion. Here, we review the classical DNA repair pathways as they were established from genetic and biochemical experiments. DNA repair pathways are often pictured to act in a precise machine-like manner. We discuss an alternative view that envisions DNA repair to arise from more unstable stochastic interactions between proteins and DNA substrates. Paradoxically, this more messy process makes DNA repair more robust and simultaneously provides the flexibility for quality control and pathway cross talk. Finally, we discuss how basic mechanistic insight in DNA repair mechanisms is currently being translated into anti-cancer therapies. We conclude with the scope of this thesis. http://repub.eur.nl/res/pub/24472/ 2009-07-10T00:00:00Z Budding yeast Slx4 interacts with the structure-specific endonuclease Slx1 to ensure completion of ribosomal DNA replication. Slx4 also interacts with the Rad1-Rad10 endonuclease to control cleavage of 3′ flaps during repair of double-strand breaks (DSBs). Here we describe the identification of human SLX4, a scaffold for DNA repair nucleases XPF-ERCC1, MUS81-EME1, and SLX1. SLX4 immunoprecipitates show SLX1-dependent nuclease activity toward Holliday junctions and MUS81-dependent activity toward other branched DNA structures. Furthermore, SLX4 enhances the nuclease activity of SLX1, MUS81, and XPF. Consistent with a role in processing recombination intermediates, cells depleted of SLX4 are hypersensitive to genotoxins that cause DSBs and show defects in the resolution of interstrand crosslink-induced DSBs. Depletion of SLX4 causes a decrease in DSB-induced homologous recombination. These data show that SLX4 is a regulator of structure-specific nucleases and that SLX4 and SLX1 are important regulators of genome stability in human cells.