USP3 controls BRCA1 "foci".

Cell Cycle News & Views News & Views Cell Cycle 13:2, 183–183; January 15, 2014; © 2014 Landes Bioscience USP3 controls BRCA1 “foci” Comment on: Sharma N, et al. Cell Cycle 2014; 13: 106–14; PMID:24196443; http://dx.doi.org/10.4161/cc.26814 Pradip Raychaudhuri; Department of Biochemistry and Molecular Genetics (M/C 669); University of Illinois at Chicago; College of Medicine; Chicago, IL USA; Email: ; http://dx.doi.org/10.4161/cc.27303 BRCA1, which is mutated in the familial forms of breast and ovarian cancer, plays important roles in genome stability through its participation in DNA damage response (DDR) following double-stranded breaks (DSBs). BRCA1 activates the checkpoint pathway to retard cell cycle progression and stimulates repair of the DSBs (reviewed in ref. 1). It is recruited to the damaged chromatin through an interaction with RAP80 that binds to K63 ubiquitin chain on histones H2A and γH2AX.1 Recruitment of BRCA1 at the damaged-sites on chromatin is detected by co-immunostaining with γH2AX that forms microscopically visible aggregates, also known as “foci”. Studies on double-stranded break repair (DSBR) provided evidence that BRCA1 is important for driving repair through homologous recombination (HR), mainly by antagonizing the activities of 53BP1 (reviewed in ref. 2). 53BP1 promotes DSBR through the non-homologous end-joining (NHEJ) pathway. In the absence of functional BRCA1 error-prone NHEJ pathway prevails, whereas recruitment of BRCA1 opposes the activity of 53BP1 and directs repair through the more reliable mechanism of HR. That concept was supported also by studies in mouse models in which it was shown that the embryonic defects and chromosomal abnormality in BRCA1-mutant mice were reversed by deletion of the 53BP1 alleles (ref. 2 and references therein). Therefore, we have gained some insights into how BRCA1 maintains genome integrity. The mechanism by which BRCA1 is recruited to the damaged chromatin has been studied extensively, because the components in that mechanism are likely to be involved in the tumor suppression pathway of BRCA1. Those studies led to the identification of 2 E3 ligases, RNF8 and RNF168, which are recruited by MDC1 at the damaged chromatin (ref. 1 www.landesbioscience.com and references therein). Once recruited, these ligases introduce K63 ubiquitin chains on K13/K15 residues in H2A/γH2AX.3 The K63 ubiquitin chains on H2A/γH2AX are recognized by RAP80 that leads to the recruitment of BRCA1.4 The recruitment of 53BP1 also requires ubiquitylation; however, the mechanism is indirect. It is thought that ubiquitylation unmasks chromatin, exposing methylated histones recognized by the Tudor domain of 53BP1.5 Given the critical roles of ubiquitylation in recruiting BRCA1 and 53BP1 onto the damaged chromatin, the deubiquitylating enzymes that reverse the effects of RNF8/ RNF168 have drawn a lot of attention, at least partly because of their obvious implications in oncogenesis. In that regard one study6 demonstrated USP44 to be involved in inhibiting recruitment of 53BP1 and RAP80 to the damaged chromatin. USP44 promotes deubiquitylation of H2A and antagonizes the recruitment of RNF168. In an equally elegant study, Dr Wani’s group, in the January 1, 2014 issue of Cell Cycle, shows that USP3 counteracts the effects of RNF168 by inducing deubiquitylation on K13/K15 residues of H2A/ γH2AX.7 USP3 was shown to be involved in deubiquitylation of H2A at K119, which is the ubquitylation target of the polycomb group proteins (ref. 7 and references therein). The Wani lab clearly demonstrates USP3 catalyzes deubiquitylation of K63 ubiquitin chain on K13/K15 residues in H2A and γH2AX. That new observation on USP3 is highly significant, as the authors show that USP3 regulates recruitment of BRCA1 and 53BP1. Cells overexpressing USP3 exhibited attenuated levels of BRCA1 and 53BP1 “foci”. Interestingly, the authors did not see a stable interaction of USP3 with the damaged chromatin, suggesting the possibility that the deubiquitylation process by USP3 is highly catalytic. Cell Cycle It is possible that the deubiquitylation of H2A/γH2AX is important for restoring normal chromatin structure after repair. A potential role of the deubiquitylation for some later steps of DSBR has not been ruled out. Previous studies demonstrated that USP3 is required for genome stability and progression through S phase.8 Although overexpression of USP3 removed both BRCA1 and 53BP1 from the damaged chromatin, the observations by the Wani group are significant, because overexpression of USP3 could have a negative effect on DSBR. In that regard it is noteworthy that, while BRCA1 mutations are found in only small percentages of breast and ovarian cancers, genome instability is a common feature in those cancers. It will be important to determine whether USP3 or USP44 that removes BRCA1/RAP80 is overexpressed in breast/ ovarian cancers. Tumor microarray analyses for these proteins might generate valuable insights into the basis of genome instability phenotype of breast/ovarian cancers. References Ciccia A, et al. Mol Cell 2010; 40:179-204; PMID:20965415; http://dx.doi.org/10.1016/j. molcel.2010.09.019 2. Chapman JR, et al. Mol Cell 2012; 47:497-510; PMID:22920291; http://dx.doi.org/10.1016/j. molcel.2012.07.029 3. Mattiroli F, et al. Cell 2012; 150:1182-95; PMID:22980979; http://dx.doi.org/10.1016/j. cell.2012.08.005 4. Sobhian B, et al. Science 2007; 316:1198-202; PMID:17525341; http://dx.doi.org/10.1126/ science.1139516 5. Mallette FA, et al. EMBO J 2012; 31:1865-78; PMID:22373579; http://dx.doi.org/10.1038/ emboj.2012.47 6. Mosbech A, et al. J Biol Chem 2013; 288:16579-87; PMID:23615962; http://dx.doi.org/10.1074/jbc. M113.459917 7. Sharma N, et al. Cell Cycle 2014; 13: In press; http:// dx.doi.org/10.4161/cc.26814 8. Nicassio F, et al. Curr Biol 2007; 17:1972-7; PMID:17980597; http://dx.doi.org/10.1016/j. cub.2007.10.034 1. 183  (...truncated)