Structure of the 53BP1 BRCT region bound to p53 and its comparison to the Brca1 BRCT structure

Developmental and tissue-specific roles of mammalian centrosomes

Centrosomes are dominant microtubule organizing centers in animal cells with a pair of centrioles at their core. They template cilia during interphase and help organize the mitotic spindle for a more efficient cell division. Here, we review the roles of centrosomes in the early developing mouse and during organ formation. Mammalian cells respond to centrosome loss-of-function by activating the mitotic surveillance pathway, a timing mechanism that, when a defined mitotic duration is exceeded, leads to p53-dependent cell death in the descendants. Mouse embryos without centrioles are highly susceptible to this pathway and undergo embryonic arrest at mid-gestation. The complete loss of the centriolar core results in earlier and more severe phenotypes than that of other centrosomal proteins. Finally, different developing tissues possess varying thresholds and mount graded responses to the loss of centrioles that go beyond the germ layer of origin.

Tudor-Containing Methyl-Lysine and Methyl-Arginine Reader Proteins: Disease Implications and Chemical Tool Development

Tudor domains are histone readers that can recognize various methylation marks on lysine and arginine. This recognition event plays a key role in the recruitment of other epigenetic effectors and the control of gene accessibility. The Tudor-containing protein family contains 42 members, many of which are involved in the development and progression of various diseases, especially cancer. The development of chemical tools for this family will not only lead to a deeper understanding of the biological functions of Tudor domains but also lay the foundation for therapeutic discoveries. In this review, we discuss the role of several Tudor domain-containing proteins in a range of relevant diseases and progress toward the development of chemical tools such as peptides, peptidomimetics, or small-molecules that bind Tudor domains. Overall, we highlight how Tudor domains are promising targets for therapeutic development and would benefit from the development of novel chemical tools.

Mechanisms of p53 core tetramer stability mediated by multi-interface interactions: A molecular dynamics study

p53 is a tumor suppressor protein for impeding cancer development and maintaining genetic integrity. The formation of the p53 core tetramer is regulated by multiple cooperative interaction interfaces. To investigate the internal mechanisms of tetramer stability, we performed all-atom molecular dynamics simulations. Our findings indicate that the symmetric interface maintains highly conserved interactions, while the dimer-dimer interface displays notable flexibility. Additionally, we identified a novel salt bridge at the dimer-dimer interface that significantly contributes to the interaction energy. Moreover, the affinity of p53 for DNA is more than twice that of protein-protein interactions, driven primarily by five key residues that form multiple hydrogen bonds. Through independent simulations of the two dimeric models, we provide a theoretical explanation for why only the symmetric dimeric structure has been observed experimentally. The study identifies key regions and residues that contribute to stability at the inter-molecular interaction interfaces within the p53 tetramer, and highlight the important roles of each contact surface in the formation and stability of the tetramer.

53BP1-the 'Pandora's box' of genome integrity

53BP1 has several functions in the maintenance of genome integrity. It functions as a key mediator involved in double-strand break (DSB) repair, which functions to maintain a balance in the repair pathway choices and in preserving genomic stability. While its DSB repair functions are relatively well-characterized, its role in DNA replication and replication fork protection is less understood. In response to replication stress, 53BP1 contributes to fork protection by regulating fork reversal and restart. It helps maintain replication fork stability and speed, with 53BP1 loss leading to defective fork progression and increased sensitivity to replication stress agents. However, 53BP1's precise role in fork protection remains debated, as some studies have not observed protective effects. Therefore, it is critical to determine the role of 53BP1 in replication to better understand when it promotes replication fork protection, and the underlying mechanisms involved. Moreover, 53BP1's function in replication stress extends beyond its activity at active replication forks; it also forms specialized nuclear bodies (NBs) which protect stretches of under-replicated DNA (UR-DNA) transmitted from a previous cell cycle to daughter cells through mitosis. The mechanism of 53BP1 NBs in the coordination of replication and repair events at UR-DNA loci is not fully understood and warrants further investigation. The present review article focuses on elucidating 53BP1's functions in replication stress (RS), its role in replication fork protection, and the significance of 53BP1 NBs in this context to provide a more comprehensive understanding of its less well-established role in DNA replication.

NEAT1 modulates the TIRR/53BP1 complex to maintain genome integrity

Tudor Interacting Repair Regulator (TIRR) is an RNA-binding protein (RBP) that interacts directly with 53BP1, restricting its access to DNA double-strand breaks (DSBs) and its association with p53. We utilized iCLIP to identify RNAs that directly bind to TIRR within cells, identifying the long non-coding RNA NEAT1 as the primary RNA partner. The high affinity of TIRR for NEAT1 is due to prevalent G-rich motifs in the short isoform (NEAT1_1) region of NEAT1. This interaction destabilizes the TIRR/53BP1 complex, promoting 53BP1's function. NEAT1_1 is enriched during the G1 phase of the cell cycle, thereby ensuring that TIRR-dependent inhibition of 53BP1's function is cell cycle-dependent. TDP-43, an RBP that is implicated in neurodegenerative diseases, modulates the TIRR/53BP1 complex by promoting the production of the NEAT1 short isoform, NEAT1_1. Together, we infer that NEAT1_1, and factors regulating NEAT1_1, may impact 53BP1-dependent DNA repair processes, with implications for a spectrum of diseases.

Regulation of p53 by the mitotic surveillance/stopwatch pathway: implications in neurodevelopment and cancer

The transcription factor p53 (encoded by TP53) plays diverse roles in human development and disease. While best known for its role in tumor suppression, p53 signaling also influences mammalian development by triggering cell fate decisions in response to a wide variety of stresses. After over 4 decades of study, a new pathway that triggers p53 activation in response to mitotic delays was recently identified. Termed the mitotic surveillance or mitotic stopwatch pathway, the USP28 and 53BP1 proteins activate p53 in response to delayed mitotic progression to control cell fate and promote genomic stability. In this Minireview, I discuss its identification, potential roles in neurodevelopmental disorders and cancer, as well as explore outstanding questions about its function, regulation and potential use as a biomarker for anti-mitotic therapies.

AIUPred: combining energy estimation with deep learning for the enhanced prediction of protein disorder

Intrinsically disordered proteins and protein regions (IDPs/IDRs) carry out important biological functions without relying on a single well-defined conformation. As these proteins are a challenge to study experimentally, computational methods play important roles in their characterization. One of the commonly used tools is the IUPred web server which provides prediction of disordered regions and their binding sites. IUPred is rooted in a simple biophysical model and uses a limited number of parameters largely derived on globular protein structures only. This enabled an incredibly fast and robust prediction method, however, its limitations have also become apparent in light of recent breakthrough methods using deep learning techniques. Here, we present AIUPred, a novel version of IUPred which incorporates deep learning techniques into the energy estimation framework. It achieves improved performance while keeping the robustness of the original method. Based on the evaluation of recent benchmark datasets, AIUPred scored amongst the top three single sequence based methods. With a new web server we offer fast and reliable visual analysis for users as well as options to analyze whole genomes in mere seconds with the downloadable package. AIUPred is available at https://aiupred.elte.hu.

Control of cell proliferation by memories of mitosis

Mitotic duration is tightly constrained, and extended mitosis is characteristic of problematic cells prone to chromosome missegregation and genomic instability. We show here that mitotic extension leads to the formation of p53-binding protein 1 (53BP1)-ubiquitin-specific protease 28 (USP28)-p53 protein complexes that are transmitted to, and stably retained by, daughter cells. Complexes assembled through a Polo-like kinase 1-dependent mechanism during extended mitosis and elicited a p53 response in G1 that prevented the proliferation of the progeny of cells that experienced an approximately threefold extended mitosis or successive less extended mitoses. The ability to monitor mitotic extension was lost in p53-mutant cancers and some p53-wild-type (p53-WT) cancers, consistent with classification of TP53BP1 and USP28 as tumor suppressors. Cancers retaining the ability to monitor mitotic extension exhibited sensitivity to antimitotic agents.

53BP1: Keeping It under Control, Even at a Distance from DNA Damage

Double-strand breaks (DSBs) are toxic lesions that can be generated by exposure to genotoxic agents or during physiological processes, such as during V(D)J recombination. The repair of these DSBs is crucial to prevent genomic instability and to maintain cellular homeostasis. Two main pathways participate in repairing DSBs, namely, non-homologous end joining (NHEJ) and homologous recombination (HR). The P53-binding protein 1 (53BP1) plays a pivotal role in the choice of DSB repair mechanism, promotes checkpoint activation and preserves genome stability upon DSBs. By preventing DSB end resection, 53BP1 promotes NHEJ over HR. Nonetheless, the balance between DSB repair pathways remains crucial, as unscheduled NHEJ or HR events at different phases of the cell cycle may lead to genomic instability. Therefore, the recruitment of 53BP1 to chromatin is tightly regulated and has been widely studied. However, less is known about the mechanism regulating 53BP1 recruitment at a distance from the DNA damage. The present review focuses on the mechanism of 53BP1 recruitment to damage and on recent studies describing novel mechanisms keeping 53BP1 at a distance from DSBs.

Ser392 phosphorylation modulated a switch between p53 and transcriptional condensates

Human p53 is a transcription factor regulating the transcription of a variety of target genes. Under various stresses, its tumor suppressor function was activated by the phosphorylation of p53. In this study, we found that full-length wild-type p53 could form phase-separated condensates with the aggregation tendency in vitro and in vivo. The LLPS of p53 was regulated by multiple functional domains. Specific DNA could promote the formation of p53 condensates. Fluorescence recovery data after photobleaching revealed that the Ser392 phosphorylation enhanced the fluidity of p53 condensates. Fluorescence analysis suggested that Ser392 phosphorylation increased the p53 concentration in condensates involved in transcription initiation and the stability of p53-mediated transcriptional condensates. The experiments in cells showed that p53 was evenly dispersed in the nucleus, it formed the dynamic condensates under the UV radiation-induced DNA damage, and the Ser392 nonphosphorylatable mutant S392A p53 formed condensates with significantly reduced number and size. These findings revealed that p53 phosphorylation modified its LLPS behavior, and suggested a mechanism that phosphorylation regulated condensate preference.

Hidden electrostatic energy contributions define dynamic allosteric communications within p53 during molecular recognition

Molecular recognition is fundamental to transcription regulation. As a transcription factor, the tumor suppressor p53 has to recognize either specific DNA sequences or repressor protein partners. However, the molecular mechanism underlying the p53 conformational switch from the DNA-bound to repressor-bound states is not fully characterized. The highly charged nature of these interacting molecules prompted us to explore the nonbonded energy contributions behind molecular recognition of either a DNA or the repressor protein iASPP by p53 DNA binding domain (p53DBD), using molecular dynamics simulation followed by rigorous analyses of energy terms. Our results illuminate the allosteric pathway by which iASPP binding to p53 diminishes binding affinity between p53 and DNA. Even though the p53DBD uses a common framework of residues for recognizing both DNA and iASPP, a comparison of the electrostatics in the two p53DBD complexes revealed significant differences in residue-wise contributions to the electrostatic energy. We found that an electrostatic allosteric communication path exists in the presence of both substrates. It consists of evolutionarily conserved residues, from residue K120 of the binding loop L1 to a distal residue R213 of p53DBD. K120 is near the DNA in the p53DBD-DNA complex, whereas iASPP binding moves it away from its DNA binding position in the p53DBD-iASPP complex. The "energy hubs" (the residues show a higher degree of connectivity with other residues in the electrostatic networks) determined from the electrostatic network analysis established that this conformational change in K120 completely rewires the electrostatic network from K120 to R213, thereby impeding DNA binding. Furthermore, we found shifting populations of hydrogen bonds and salt bridges reduce pairwise electrostatic energies within p53DBD in its DNA-bound state.

TIRR inhibits the 53BP1-p53 complex to alter cell-fate programs

53BP1 influences genome stability via two independent mechanisms: (1) regulating DNA double-strand break (DSB) repair and (2) enhancing p53 activity. We discovered a protein, Tudor-interacting repair regulator (TIRR), that associates with the 53BP1 Tudor domain and prevents its recruitment to DSBs. Here, we elucidate how TIRR affects 53BP1 function beyond its recruitment to DSBs and biochemically links the two distinct roles of 53BP1. Loss of TIRR causes an aberrant increase in the gene transactivation function of p53, affecting several p53-mediated cell-fate programs. TIRR inhibits the complex formation between the Tudor domain of 53BP1 and a dimethylated form of p53 (K382me2) that is poised for transcriptional activation of its target genes. TIRR mRNA expression levels negatively correlate with the expression of key p53 target genes in breast and prostate cancers. Further, TIRR loss is selectively not tolerated in p53-proficient tumors. Therefore, we establish that TIRR is an important inhibitor of the 53BP1-p53 complex.

Structural insight into the molecular mechanism of p53-mediated mitochondrial apoptosis

The tumor suppressor p53 is mutated in approximately half of all human cancers. p53 can induce apoptosis through mitochondrial membrane permeabilization by interacting with and antagonizing the anti-apoptotic proteins BCL-xL and BCL-2. However, the mechanisms by which p53 induces mitochondrial apoptosis remain elusive. Here, we report a 2.5 Å crystal structure of human p53/BCL-xL complex. In this structure, two p53 molecules interact as a homodimer, and bind one BCL-xL molecule to form a ternary complex with a 2:1 stoichiometry. Mutations at the p53 dimer interface or p53/BCL-xL interface disrupt p53/BCL-xL interaction and p53-mediated apoptosis. Overall, our current findings of the bona fide structure of p53/BCL-xL complex reveal the molecular basis of the interaction between p53 and BCL-xL, and provide insight into p53-mediated mitochondrial apoptosis.

Significance of abnormal 53BP1 expression as a novel molecular pathologic parameter of follicular-shaped B-cell lymphoid lesions in human digestive tract

The digestive tract is a common site of extranodal malignant lymphomas (MLs) and benign lymphoid lesions (BLs). TP53-binding protein 1 (53BP1) expression has been widely investigated in class switch recombination but rarely in human lymphoid tissues with respect to tumorigenesis. We previously reported that immunofluorescence (IF) analysis of 53BP1 nuclear foci (NF), reflecting DNA double strand breaks, is useful for estimating genomic instability in different tumor types. In this study, we evaluated the potential of IF-based analysis of 53BP1 expression in differentiating MLs from BLs. We examined 231 biopsied tissue samples of primary MLs and BLs in the digestive tract. The 53BP1 immunoreactivity pattern was determined by multicolor IF. Compared to BLs, MLs showed a high frequency of abnormal 53BP1 expression (p < 0.0001). Statistically, abnormal 53BP1 expression is an effective test for distinguishing follicular lymphomas from BLs (specificity 98.6%, sensitivity 86.8%) and for distinguishing small B-cell lymphomas from BLs (specificity 98.3%, sensitivity 77.6%). Furthermore, a high frequency of abnormal 53BP1 expression was associated with "high-risk" MALT lymphomas, which exhibited t(11;18)(q21;21) (p = 0.0145). Collectively, these results suggest that IF-based analysis of 53BP1 expression in biopsy samples is a promising technique for diagnosing MLs in the digestive system.

AMPK-mediated phosphorylation on 53BP1 promotes c-NHEJ

AMP-activated protein kinase (AMPK) is an energy sensor that plays roles in multiple biological processes beyond metabolism. Several studies have suggested that AMPK is involved in the DNA damage response (DDR), but the mechanisms remain unclear. Herein, we demonstrate that AMPK promotes classic non-homologous end joining (c-NHEJ) in double-strand break (DSB) repair through recruiting a key chromatin-based mediator named p53-binding protein 1 (53BP1), which facilitates the end joining of distal DNA ends during DDR. We find that the interaction of AMPK and 53BP1 spatially occurs under DSB stress. In the context of DSBs, AMPK directly phosphorylates 53BP1 at Ser1317 and promotes 53BP1 recruitment during DDR for an efficient c-NHEJ, thus maintaining genomic stability and diversity of the immune repertoire. Taken together, our study demonstrates that AMPK is a regulator of 53BP1 and controls c-NHEJ choice by phospho-regulation.

Structural insights into DNA double-strand break signaling

Genomic integrity is most threatened by double-strand breaks, which, if left unrepaired, lead to carcinogenesis or cell death. The cell generates a network of protein-protein signaling interactions that emanate from the DNA damage which are now recognized as a rich basis for anti-cancer therapy development. Deciphering the structures of signaling proteins has been an uphill task owing to their large size and complex domain organization. Recent advances in mammalian protein expression/purification and cryo-EM-based structure determination have led to significant progress in our understanding of these large multidomain proteins. This review is an overview of the structural principles that underlie some of the key signaling proteins that function at the double-strand break site. We also discuss some plausible ideas that could be considered for future structural approaches to visualize and build a more complete understanding of protein dynamics at the break site.

Predicting Ovarian/Breast Cancer Pathogenic Risks of Human BRCA1 Gene Variants of Unknown Significance

High-throughput sequencing is gaining popularity in clinical diagnoses, but more and more novel gene variants with unknown clinical significance are being found, giving difficulties to interpretations of people's genetic data, precise disease diagnoses, and the making of therapeutic strategies and decisions. In order to solve these issues, it is of critical importance to figure out ways to analyze and interpret such variants. In this work, BRCA1 gene variants with unknown clinical significance were identified from clinical sequencing data, and then, we developed machine learning models so as to predict the pathogenicity for variants with unknown clinical significance. Through performance benchmarking, we found that the optimized random forest model scored 0.85 in area under receiver operating characteristic curve, which outperformed other models. Finally, we applied the best random forest model to predict the pathogenicity of 6321 BRCA1 variants from both sequencing data and ClinVar database. As a result, we obtained the predictive pathogenic risks of BRCA1 variants of unknown significance.

53BP1: a DSB escort

In this review, Mirman et al. summarize the current understanding of the role of 53BP1 in DSB repair at deprotected telomeres, in class switch recombination in the immune system, and in the context of PARPi-treated BRCA1-deficient cells. They argue that the primary function of 53BP1 is not to regulate the choice between c-NHEJ and HDR, but to ensure the fidelity of DSB repair, a function that is corrupted in diseases where DNA repair is rewired.

53BP1 is an enigmatic DNA damage response factor that gained prominence because it determines the efficacy of PARP1 inhibitory drugs (PARPi) in BRCA1-deficient cancers. Recent studies have elevated 53BP1 from its modest status of (yet another) DNA damage factor to master regulator of double-strand break (DSB) repair pathway choice. Our review of the literature suggests an alternative view. We propose that 53BP1 has evolved to avoid mutagenic repair outcomes and does so by controlling the processing of DNA ends and the dynamics of DSBs. The consequences of 53BP1 deficiency, such as diminished PARPi efficacy in BRCA1-deficient cells and altered repair of damaged telomeres, can be explained from this viewpoint. We further propose that some of the fidelity functions of 53BP1 coevolved with class switch recombination (CSR) in the immune system. We speculate that, rather than being deterministic in DSB repair pathway choice, 53BP1 functions as a DSB escort that guards against illegitimate and potentially tumorigenic recombination.

Distinct associations of the Saccharomyces cerevisiae Rad9 protein link Mac1-regulated transcription to DNA repair

While it is known that ScRad9 DNA damage checkpoint protein is recruited to damaged DNA by recognizing specific histone modifications, here we report a different way of Rad9 recruitment on chromatin under non DNA damaging conditions. We found Rad9 to bind directly with the copper-modulated transcriptional activator Mac1, suppressing both its DNA binding and transactivation functions. Rad9 was recruited to active Mac1-target promoters (CTR1, FRE1) and along CTR1 coding region following the association pattern of RNA polymerase (Pol) II. Hir1 histone chaperone also interacted directly with Rad9 and was partly required for its localization throughout CTR1 gene. Moreover, Mac1-dependent transcriptional initiation was necessary and sufficient for Rad9 recruitment to the heterologous ACT1 coding region. In addition to Rad9, Rad53 kinase also localized to CTR1 coding region in a Rad9-dependent manner. Our data provide an example of a yeast DNA-binding transcriptional activator that interacts directly with a DNA damage checkpoint protein in vivo and is functionally restrained by this protein, suggesting a new role for Rad9 in connecting factors of the transcription machinery with the DNA repair pathway under unchallenged conditions.

Calorie Restriction in Adulthood Reduces Hepatic Disorders Induced by Transient Postnatal Overfeeding in Mice

Impaired early nutrition influences the risk of developing metabolic disorders in later life. We observed that transient postnatal overfeeding (OF) in mice induces long-term hepatic alterations, characterized by microsteatosis, fibrosis associated with oxidative stress (OS), and stress-induced premature senescence (SIPS). In this study, we investigated whether such changes can be reversed by moderate calorie restriction (CR). C57BL/6 male mice pups were maintained during lactation in litters adjusted to nine pups in the normal feeding (NF) group and three pups in the transient postnatal OF group. At six months of age, adult mice from the NF and OF groups were randomly assigned to an ad libitum diet or CR (daily energy supply reduced by 20%) for one month. In each group, at the age of seven months, analysis of liver structure, liver markers of OS (superoxide anion, antioxidant defenses), and SIPS (lipofuscin, p53, p21, p16, pRb/Rb, Acp53, sirtuin-1) were performed. CR in the OF group reduced microsteatosis, decreased levels of superoxide anion, and increased protein expression of catalase and superoxide dismutase. Moreover, CR decreased lipofuscin staining, p21, p53, Acp53, and p16 but increased pRb/Rb and sirtuin-1 protein expression. CR did not affect the NF group. These results suggest that CR reduces hepatic disorders induced by OF.

The R882H substitution in the human de novo DNA methyltransferase DNMT3A disrupts allosteric regulation by the tumor supressor p53

A myriad of protein partners modulate the activity of the human DNA methyltransferase 3A (DNMT3A), whose interactions with these other proteins are frequently altered during oncogenesis. We show here that the tumor suppressor p53 decreases DNMT3A activity by forming a heterotetramer complex with DNMT3A. Mutational and modeling experiments suggested that p53 interacts with the same region in DNMT3A as does the structurally characterized DNMT3L. We observed that the p53-mediated repression of DNMT3A activity is blocked by amino acid substitutions within this interface, but surprisingly, also by a distal DNMT3A residue, R882H. DNMT3A R882H occurs frequently in various cancers, including acute myeloid leukemia, and our results suggest that the effects of R882H and other DNMT3A mutations may go beyond changes in DNMT3A methylation activity. To further understand the dynamics of how protein-protein interactions modulate DNMT3A activity, we determined that p53 has a greater affinity for DNMT3A than for DNMT3L and that p53 readily displaces DNMT3L from the DNMT3A:DNMT3L heterotetramer. Interestingly, this occurred even when the preformed DNMT3A:DNMT3L complex was actively methylating DNA. The frequently identified p53 substitutions (R248W and R273H), whereas able to regulate DNMT3A function when forming the DNMT3A:p53 heterotetramer, no longer displaced DNMT3L from the DNMT3A:DNMT3L heterotetramer. The results of our work highlight the complex interplay between DNMT3A, p53, and DNMT3L and how these interactions are further modulated by clinically derived mutations in each of the interacting partners.

iASPP mediates p53 selectivity through a modular mechanism fine-tuning DNA recognition

The most frequently mutated protein in human cancer is p53, a transcription factor (TF) that regulates myriad genes instrumental in diverse cellular outcomes including growth arrest and cell death. Cell context-dependent p53 modulation is critical for this life-or-death balance, yet remains incompletely understood. Here we identify sequence signatures enriched in genomic p53-binding sites modulated by the transcription cofactor iASPP. Moreover, our p53-iASPP crystal structure reveals that iASPP displaces the p53 L1 loop-which mediates sequence-specific interactions with the signature-corresponding base-without perturbing other DNA-recognizing modules of the p53 DNA-binding domain. A TF commonly uses multiple structural modules to recognize its cognate DNA, and thus this mechanism of a cofactor fine-tuning TF-DNA interactions through targeting a particular module is likely widespread. Previously, all tumor suppressors and oncoproteins that associate with the p53 DNA-binding domain-except the oncogenic E6 from human papillomaviruses (HPVs)-structurally cluster at the DNA-binding site of p53, complicating drug design. By contrast, iASPP inhibits p53 through a distinct surface overlapping the E6 footprint, opening prospects for p53-targeting precision medicine to improve cancer therapy.

Significant association between 53 BP1 expression and grade of intraepithelial neoplasia of esophagus: Alteration during esophageal carcinogenesis

BACKGROUND

Abnormal DNA damage response (DDR) leads to genomic instability and carcinogenesis. P53-binding protein 1 (53 BP1), a DDR molecule, is known to accumulate at the sites of DNA double-strand breaks. The aim of this study was to analyze the expression pattern of 53 BP1-nuclear foci (NF) in esophageal neoplasms in order to visualize the state of DDR in esophageal carcinogenesis and to clarify its significance in the molecular pathology of the disease.

METHODS

A total of 61 lesions from 22 surgically resected samples of esophageal cancer, including histologically normal squamous epithelium, low-grade intraepithelial neoplasia (LG-IN), high-grade intraepithelial neoplasia (HG-IN), carcinoma in situ (CIS), and invasive squamous cell carcinoma (SCC), were included in the study. 53 BP1 and Ki-67 expression were analyzed by double-labeled immunofluorescence.

RESULTS

The number of discrete 53 BP1-NF increased as the tumor progressed from normal epithelium through LG-IN, HG-IN, CIS, and SCC. 53 BP1-NF larger than 1 μm in diameter (large foci), indicating intensive DDR, also showed a stepwise increase during the progression of carcinogenesis. Of note, large foci of 53 BP1 were found in significantly higher numbers in HG-IN than in LG-IN. Furthermore, localization of 53 BP1-NF in Ki-67-positive cells, indicating the abnormal timing of DDR, also increased with malignancy progression.

CONCLUSIONS

53 BP1-NF accumulation increases during cancer progression from LG-IN to HG-IN to CIS to SCC. Detection of 53 BP1-NF by immunofluorescence, especially large foci, is a feasible method of estimating DNA instability and the malignant potential of esophageal intraepithelial neoplasia.

Phase separation of 53BP1 determines liquid-like behavior of DNA repair compartments

The DNA damage response (DDR) generates transient repair compartments to concentrate repair proteins and activate signaling factors. The physicochemical properties of these spatially confined compartments and their function remain poorly understood. Here, we establish, based on live cell microscopy and CRISPR/Cas9-mediated endogenous protein tagging, that 53BP1-marked repair compartments are dynamic, show droplet-like behavior, and undergo frequent fusion and fission events. 53BP1 assembly, but not the upstream accumulation of γH2AX and MDC1, is highly sensitive to changes in osmotic pressure, temperature, salt concentration and to disruption of hydrophobic interactions. Phase separation of 53BP1 is substantiated by optoDroplet experiments, which further allowed dissection of the 53BP1 sequence elements that cooperate for light-induced clustering. Moreover, we found the tumor suppressor protein p53 to be enriched within 53BP1 optoDroplets, and conditions that disrupt 53BP1 phase separation impair 53BP1-dependent induction of p53 and diminish p53 target gene expression. We thus suggest that 53BP1 phase separation integrates localized DNA damage recognition and repair factor assembly with global p53-dependent gene activation and cell fate decisions.

Effect of BRCA mutational status on survival outcome in advanced-stage high-grade serous ovarian cancer

Objective

To evaluate impact of germline BRCA mutational status on prognosis in patients with advanced ovarian cancer.

Methods

A total of 128 patients diagnosed with FIGO stage III-IV high-grade serous ovarian cancer (HGSOC) between 2008 and 2017 and underwent BRCA1/2 gene testing at the time of or within two years from cancer diagnosis were included in this study. We compared patients’ clinicopathological characteristics and survival outcomes after primary treatment according to germline BRCA mutational status. Treatment-related factors that might affect patients’ survival outcome were also investigated.

Results

Germline BRCA1/2 mutations were observed in 51 women (39.8%). There were no differences in age and serum CA-125 levels at the time of HGSOC diagnosis, use of neoadjuvant chemotherapy (NAC), extent of debulking surgery, and overall survival (OS) between the BRCA mutation and wild-type BRCA groups. In contrast, the BRCA mutation group displayed longer progression-free survival (PFS) (median, 22.9 vs. 16.9 months, P = 0.001). Multivariate analyses identified germline BRCA1/2 mutation as an independent favorable prognostic factor for PFS (adjusted HR, 0.502; 95% CI, 0.318–0.795; P = 0.003). In the wild-type BRCA group, patients who received NAC as the primary treatment had shorter PFS compared to those who received primary debulking surgery (PDS) (median, 14.2 vs. 16.9 months, P = 0.003). However, in the BRCA mutation group, PFS did not differ between the NAC and PDS groups (P = 0.082).

Conclusions

In advanced-stage HGSOC, patients with germline BRCA1/2 mutations have better prognosis with longer PFS than those lacking BRCA mutations. Prognosis after NAC was different according to the BRCA mutational status.

Electronic supplementary material

The online version of this article (10.1186/s13048-019-0511-7) contains supplementary material, which is available to authorized users.

The effects of BRCA1 expression on the chemosensitivity of gastric cancer cells to platinum agents

Breast cancer type 1 susceptibility protein (BRCA1) is a tumor suppressor gene that encodes a nuclear phosphoprotein, which is involved in homologous recombination to repair DNA double strand breaks and maintain genome stability. When BRCA1 is mutated or altered, DNA damage may not be effectively repaired, which leads to DNA replication errors and cancer growth. Accordingly, people carrying a mutation in the BRCA1 gene possess an increased risk of several types of cancer, including breast and ovarian cancer. Previous clinical studies have reported an association between BRCA1 expression level and the incidence of gastric cancer; however, to the best of our knowledge, an in vitro study has not been performed to support these clinical observations. Therefore, the present study evaluated BRCA1 expression levels in gastric cancer cell lines. In addition, the IC₅₀ values of cisplatin and oxaliplatin in each cell line were determined to investigate a potential correlation between BRCA1 expression level and chemosensitivity to platinum agents. The present results revealed that the BRCA1 expression level in gastric cancer is variable and associated with the treatment response to platinum-based chemotherapy. This suggests that BRCA1 may serve as a therapeutic marker for platinum-based chemotherapy in gastric cancer.

Identification of Novel Interaction Partners of Ets-1: Focus on DNA Repair

The transcription factor Ets-1 (ETS proto-oncogene 1) shows low expression levels except in specific biological processes like haematopoiesis or angiogenesis. Elevated levels of expression are observed in tumor progression, resulting in Ets-1 being named an oncoprotein. It has recently been shown that Ets-1 interacts with two DNA repair enzymes, PARP-1 (poly(ADP-ribose) polymerase 1) and DNA-PK (DNA-dependent protein kinase), through two different domains and that these interactions play a role in cancer. Considering that Ets-1 can bind to distinctly different domains of two DNA repair enzymes, we hypothesized that the interaction can be transposed onto homologs of the respective domains. We have searched for sequence and structure homologs of the interacting ETS(Ets-1), BRCT(PARP-1) and SAP(DNA-PK) domains, and have identified several candidate binding pairs that are currently not annotated as such. Many of the Ets-1 partners are associated to DNA repair mechanisms. We have applied protein-protein docking to establish putative interaction poses and investigated these using centrality analyses at the protein residue level. Most of the identified poses are virtually similar to our recently established interaction model for Ets-1/PARP-1 and Ets-1/DNA-PK. Our work illustrates the potentially high number of interactors of Ets-1, in particular involved in DNA repair mechanisms, which shows the oncoprotein as a potential important regulator of the mechanism.

Allosteric Modulation of Intrinsically Disordered Proteins

The allosteric property of globular proteins is applauded as their intrinsic ability to regulate distant sites, and this property further plays a critical role in a wide variety of cellular regulatory mechanisms. Recent advancements and studies have revealed the manifestation of allostery in intrinsically disordered proteins or regions as allosteric sites present within or mediated by IDP/IDRs facilitates the signaling interactions for various biological mechanisms which would otherwise be impossible for globular proteins to regulate. This thematic review has highlighted the biological outcomes that can be achieved by the mechanism of allosteric regulation of intrinsically disordered proteins or regions. The similar mechanism has been implemented on Adenovirus 5 early region 1A and tumor apoptosis protein p53 in correspondence with other partners in binary and ternary complexes, which are the subject of the current review. Both these proteins regulate once they bind to their partners, consequently, forming either a binary or a ternary complex. Allosteric regulation by IDPs is currently a subject undergoing intense study, and the ongoing research work will ensure a better understanding of precision and efficiency of cellular regulation by them. Allosteric regulation mechanism can also be researched by intrinsically disordered protein-specific force field.

Human papillomavirus type 16 E6 and E7 oncoproteins interact with the nuclear p53-binding protein 1 in an in vitro reconstructed 3D epithelium: new insights for the virus-induced DNA damage response

Background

Despite vaccination and screening measures, anogenital cancer, mainly promoted by HPV16 oncoproteins, still represents the fourth tumor and the second cause of death among women.

Cell replication fidelity is the result of the host DNA damage response (DDR). Unlike many DNA viruses that promote their life cycle through the DDR inactivation, HR-HPVs encourage cells proliferation despite the DDR turned on. Why and how it occurs has been only partially elucidated.

During HPV16 infection, E6 links and degrades p53 via the binding to the E6AP LXXLL sequence; unfortunately, E6 direct role in the DDR response has not clearly identified yet.

Similarly, E7 increases DDR by competing with E2F1-pRb interaction, thus leading to the inactivation of pRb, and promotion, E2F1 mediated, of DDR genes translation, by binding to the pRb-like proteins CBP/p300 and p107, that also harbour LXXLL sequence, and via the interaction and activation of several DDR proteins.

Methods

To gain information regarding E6 and E7 contribution in DDR activation, we produced an in vitro 3D HPV16-E6E7 infected epithelium, already consolidated study model for HPVs, and validated it by assessing H&E staining and BrdU, HPV16 DNA, E6E7 proteins and γH2A.X/53BP1 double-strand break (DSBs) sensors expression; then we made an immuno-colocalization of E6 and E7 with cyclin E2 and B1.

Since 53BP1, like E6 and E7, also binds p53 and pRb, we supposed their possible direct binding. To explore this hypothesis, we performed a double immunofluorescence of E6 and E7 with 53BP1, a sequence analysis of 53BP1 within its BRCT2 domain and then an in situ PLA within CaSki, E6E7HPV16 NHEKs and the 3D model.

Results

The in vitro epithelium resembled the histology and the events typical of in vivo infected tissues. E6E7HPV16 were both expressed in basal and differentiated strata and induced H2A.X phosphorylation and 53BP1 increment into nuclear foci.

After highlighting E6 and E7 co-expression with 53BP1 and a LKVLL sequence within the 53BP1 BRCT2 domain, we demonstrated the bindings via the PLA technique.

Conclusions

Our results reinforce E6 and E7 role in cellular function control providing potentially new insights into the activity of this tumor virus.

Expression pattern of p53-binding protein 1 as a new molecular indicator of genomic instability in bladder urothelial carcinoma

Copy number alterations and loss of heterozygosity are associated with increasing tumor grade and bladder cancer stage. Our previous study suggested that co-expression of Ki-67 and p53-binding protein 1 (53BP1) could provide an indicator of an abnormal DNA damage response (DDR) pathway. The present study investigated 53BP1 expression as a novel molecular marker in urothelial carcinoma (UC) using bladder tissues with in total of 40 cases including a normal urothelium, urothelial papilloma, low-grade UC, or high-grade UC. Double-label immunofluorescence was used to analyze 53BP1 and Ki-67 expression. This was compared with the level of chromosomal instability and with the expression of other DDR molecules catalytic subunit. This study identified clear differences in the 53BP1 expression patterns in urothelial carcinogenesis, and their close association with genomic instability. 53BP1 abnormal immunoreactivity, particularly with co-localization of Ki-67, was restricted to malignant tissues. Our analyses indicated that a cut-off of >4% of nuclei with 53BP1 abnormal expression plus Ki-67 immunoreactivity distinguished high-grade UC from low-grade UC with 80.0% sensitivity and 100% specificity. We therefore propose that double immunofluorescent analysis of 53BP1 and Ki-67 expression could provide a useful tool to estimate the chromosomal instability and malignant potential of urothelial tumors.

Transient postnatal overfeeding causes liver stress-induced premature senescence in adult mice

Unbalanced nutrition early in life is increasingly recognized as an important factor in the development of chronic, non-communicable diseases at adulthood, including metabolic diseases. We aimed to determine whether transient postnatal overfeeding (OF) leads to liver stress-induced premature senescence (SIPS) of hepatocytes in association with liver structure and hepatic function alterations. Litters sizes of male C57BL/6 mice were adjusted to 9 pups (normal feeding, NF) or reduced to 3 pups during the lactation period to induce transient postnatal OF. Compared to the NF group, seven-month-old adult mice transiently overfed during the postnatal period were overweight and developed glucose intolerance and insulin resistance. Their livers showed microsteatosis and fibrosis, while hepatic insulin signaling and glucose transporter protein expressions were altered. Increased hepatic oxidative stress (OS) was observed, with increased superoxide anion production, glucose-6-phosphate dehydrogenase protein expression, oxidative DNA damage and decreased levels of antioxidant defense markers, such as superoxide dismutase and catalase proteins. Hepatocyte senescence was characterized by increased p21^WAF, p53, Acp53, p16^INK4a and decreased pRb/Rb and Sirtuin-1 (SIRT-1) protein expression levels. Transient postnatal OF induces liver OS at adulthood, associated with hepatocyte SIPS and alterations in liver structure and hepatic functions, which could be mediated by a SIRT-1 deficiency.

A USP28-53BP1-p53-p21 signaling axis arrests growth after centrosome loss or prolonged mitosis

Precise regulation of centrosome number is critical for accurate chromosome segregation and the maintenance of genomic integrity. In nontransformed cells, centrosome loss triggers a p53-dependent surveillance pathway that protects against genome instability by blocking cell growth. However, the mechanism by which p53 is activated in response to centrosome loss remains unknown. Here, we have used genome-wide CRISPR/Cas9 knockout screens to identify a USP28-53BP1-p53-p21 signaling axis at the core of the centrosome surveillance pathway. We show that USP28 and 53BP1 act to stabilize p53 after centrosome loss and demonstrate this function to be independent of their previously characterized role in the DNA damage response. Surprisingly, the USP28-53BP1-p53-p21 signaling pathway is also required to arrest cell growth after a prolonged prometaphase. We therefore propose that centrosome loss or a prolonged mitosis activate a common signaling pathway that acts to prevent the growth of cells that have an increased propensity for mitotic errors.

Mutations in the TP53 gene affected recruitment of 53BP1 protein to DNA lesions, but level of 53BP1 was stable after γ-irradiation that depleted MDC1 protein in specific TP53 mutants

53BP1 is a very well-known protein that is recruited to DNA lesions. The focal accumulation of p53 binding protein, 53BP1, is a main feature indicating the repair of spontaneous or irradiation-induced foci (IRIF). Thus, here, we addressed the question of whether mutations in the TP53 gene, which often affect the level of p53 protein, can change the recruitment of 53BP1 to γ- or UVA-irradiated chromatin. In various TP53 mutants, we observed a distinct accumulation of 53BP1 protein to UV-induced DNA lesions: in R273C mutants, 53BP1 appeared transiently at DNA lesions, during 10-30 min after irradiation; the mutation R282W was responsible for accumulation of 53BP1 immediately after UVA-damage; and in L194F mutants, the first appearance of 53BP1 protein at the lesions occurred during 60-70 min. These results showed that specific mutations in the TP53 gene stand behind not only different levels of p53 protein, but also affect the localized kinetics of 53BP1 protein in UVA-damaged chromatin. However, after γ-irradiation, only G245S mutation in TP53 gene was associated with surprisingly decreased level of 53BP1 protein. In other mutant cell lines, levels of 53BP1 were not affected by γ-rays. To these effects, we conversely found a distinct number of 53BP1-positive irradiation-induced foci in various TP53 mutants. The R280K, G245S, L194F mutations, or TP53 deletion were also characterized by radiation-induced depletion in MDC1 protein. Moreover, in mutant cells, an interaction between MDC1 and 53BP1 proteins was abrogated when compared with wild-type counterpart. Together, the kinetics of 53BP1 accumulation at UV-induced DNA lesions is different in various TP53 mutant cells. After γ-irradiation, despite changes in a number and a volume of 53BP1-positive foci, levels of 53BP1 protein were relatively stable. Here, we showed a link between the status of MDC1 protein and TP53 gene, which specific mutations caused radiation-induced MDC1 down-regulation. This observation is significant, especially with regard to radiotherapy of tumors with abrogated function of TP53 gene.

53BP1 contributes to regulation of autophagic clearance of mitochondria

Autophagy, the primary recycling pathway within cells, plays a critical role in mitochondrial quality control under normal growth conditions and in the cellular response to stress. Here we provide evidence that 53BP1, a DNA damage response protein, is involved in regulating mitochondrial clearance from the cell via a type of autophagy termed mitophagy. We found that when either human or mouse cells were 53BP1-deficient, there was an increase in mitochondrial abnormalities, as observed through staining intensity, aggregation, and increased mass. Moreover, a 53BP1-depleted cell population included an increased number of cells with a high mitochondrial membrane potential (ΔΨm) relative to controls, suggesting that the loss of 53BP1 prevents initiation of mitophagy thereby leading to the accumulation of damaged mitochondria. Indeed, both 53BP1 and the mitophagy-associated protein LC3 translocated to mitochondria in response to damage induced by the mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP). The recruitment of parkin, an E3-ubiquitin ligase, to mitochondria in response to CCCP treatment was significantly decreased in 53BP1-deficient cells. And lastly, using p53-deficient H1299 cells, we confirmed that the role of 53BP1 in mitophagy is independent of p53. These data support a model in which 53BP1 plays an important role in modulating mitochondrial homeostasis and in the clearance of damaged mitochondria.

A New Mode of Mitotic Surveillance

Cells have evolved certain precautions to preserve their genomic content during mitosis and avoid potentially oncogenic errors. Besides the well-established DNA damage checkpoint and spindle assembly checkpoint (SAC), recent observations have identified an additional mitotic failsafe referred to as the mitotic surveillance pathway. This pathway triggers a cell cycle arrest to block the growth of potentially unfit daughter cells and is activated by both prolonged mitosis and centrosome loss. Recent genome-wide screens surprisingly revealed that 53BP1 and USP28 act upstream of p53 to mediate signaling through the mitotic surveillance pathway. Here we review advances in our understanding of this failsafe and discuss how 53BP1 and USP28 adopt noncanonical roles to function in this pathway.

TP53 and 53BP1 Reunited

Identified as a TP53-binding protein, 53BP1 is a key regulator of the cellular response to double-strand breaks, a TP53-independent activity. Recent data have established a new TP53-dependent function for 53BP1 in mitotic surveillance after centrosome loss.

p53 Proteoforms and Intrinsic Disorder: An Illustration of the Protein Structure-Function Continuum Concept

Although it is one of the most studied proteins, p53 continues to be an enigma. This protein has numerous biological functions, possesses intrinsically disordered regions crucial for its functionality, can form both homo-tetramers and isoform-based hetero-tetramers, and is able to interact with many binding partners. It contains numerous posttranslational modifications, has several isoforms generated by alternative splicing, alternative promoter usage or alternative initiation of translation, and is commonly mutated in different cancers. Therefore, p53 serves as an important illustration of the protein structure–function continuum concept, where the generation of multiple proteoforms by various mechanisms defines the ability of this protein to have a multitude of structurally and functionally different states. Considering p53 in the light of a proteoform-based structure–function continuum represents a non-canonical and conceptually new contemplation of structure, regulation, and functionality of this important protein.

53BP1 Integrates DNA Repair and p53-Dependent Cell Fate Decisions via Distinct Mechanisms

The tumor suppressor protein 53BP1, a pivotal regulator of DNA double-strand break (DSB) repair, was first identified as a p53-interacting protein over two decades ago. However, its direct contributions to p53-dependent cellular activities remain undefined. Here, we reveal that 53BP1 stimulates genome-wide p53-dependent gene transactivation and repression events in response to ionizing radiation (IR) and synthetic p53 activation. 53BP1-dependent p53 modulation requires both auto-oligomerization and tandem-BRCT domain-mediated bivalent interactions with p53 and the ubiquitin-specific protease USP28. Loss of these activities results in inefficient p53-dependent cell-cycle checkpoint and exit responses. Furthermore, we demonstrate 53BP1-USP28 cooperation to be essential for normal p53-promoter element interactions and gene transactivation-associated events, yet dispensable for 53BP1-dependent DSB repair regulation. Collectively, our data provide a mechanistic explanation for 53BP1-p53 cooperation in controlling anti-tumorigenic cell-fate decisions and reveal these activities to be distinct and separable from 53BP1's regulation of DNA double-strand break repair pathway choice.

A role for the p53 tumour suppressor in regulating the balance between homologous recombination and non-homologous end joining

Loss of p53, a transcription factor activated by cellular stress, is a frequent event in cancer. The role of p53 in tumour suppression is largely attributed to cell fate decisions. Here, we provide evidence supporting a novel role for p53 in the regulation of DNA double-strand break (DSB) repair pathway choice. 53BP1, another tumour suppressor, was initially identified as p53 Binding Protein 1, and has been shown to inhibit DNA end resection, thereby stimulating non-homologous end joining (NHEJ). Yet another tumour suppressor, BRCA1, reciprocally promotes end resection and homologous recombination (HR). Here, we show that in both human and mouse cells, the absence of p53 results in impaired 53BP1 focal recruitment to sites of DNA damage induced by ionizing radiation. This effect is largely independent of cell cycle phase and the extent of DNA damage. In p53-deficient cells, diminished localization of 53BP1 is accompanied by a reciprocal increase in BRCA1 recruitment to DSBs. Consistent with these findings, we demonstrate that DSB repair via NHEJ is abrogated, while repair via homology-directed repair (HDR) is stimulated. Overall, we propose that in addition to its role as an ‘effector’ protein in the DNA damage response, p53 plays a role in the regulation of DSB repair pathway choice.

53BP1 and USP28 mediate p53-dependent cell cycle arrest in response to centrosome loss and prolonged mitosis

Mitosis occurs efficiently, but when it is disturbed or delayed, p53-dependent cell death or senescence is often triggered after mitotic exit. To characterize this process, we conducted CRISPR-mediated loss-of-function screens using a cell-based assay in which mitosis is consistently disturbed by centrosome loss. We identified 53BP1 and USP28 as essential components acting upstream of p53, evoking p21-dependent cell cycle arrest in response not only to centrosome loss, but also to other distinct defects causing prolonged mitosis. Intriguingly, 53BP1 mediates p53 activation independently of its DNA repair activity, but requiring its interacting protein USP28 that can directly deubiquitinate p53 in vitro and ectopically stabilize p53 in vivo. Moreover, 53BP1 can transduce prolonged mitosis to cell cycle arrest independently of the spindle assembly checkpoint (SAC), suggesting that while SAC protects mitotic accuracy by slowing down mitosis, 53BP1 and USP28 function in parallel to select against disturbed or delayed mitosis, promoting mitotic efficiency.

DOI:http://dx.doi.org/10.7554/eLife.16270.001

Association between p53-binding protein 1 expression and genomic instability in oncocytic follicular adenoma of the thyroid

Oncocytic follicular adenomas (FAs) of the thyroid are neoplasms of follicular cell origin that are predominantly composed of large polygonal cells with eosinophilic and granular cytoplasm. However, the pathological characteristics of these tumors are largely unexplored. Both the initiation and progression of cancer can be caused by an accumulation of genetic mutations that can induce genomic instability. Thus, the aim of this study was to evaluate the extent of genomic instability in oncocytic FA. As the presence of p53-binding protein 1 (53BP1) in nuclear foci has been found to reflect DNA double-strand breaks that are triggered by various stresses, the immunofluorescence expression pattern of 53BP-1 was assessed in oncocytic and conventional FA. The association with the degree of DNA copy number aberration (CNA) was also evaluated using array-based comparative genomic hybridization. Data from this study demonstrated increased 53BP1 expression (i.e., "unstable" expression) in nuclear foci of oncocytic FA and a higher incidence of CNAs compared with conventional FA. There was also a particular focus on the amplification of chromosome 1p36 in oncocytic FA, which includes the locus for Tumor protein 73, a member of the p53 family implicated as a factor in the development of malignancies. Further evaluations revealed that unstable 53BP1 expression had a significant positive correlation with the levels of expression of Tumor protein 73. These data suggest a higher level of genomic instability in oncocytic FA compared with conventional FA, and a possible relationship between oncocytic FA and abnormal amplification of Tumor protein 73.

A cell cycle-dependent BRCA1-UHRF1 cascade regulates DNA double-strand break repair pathway choice

BRCA1 is an important mediator of the DNA damage response, which promotes homologous recombination (HR) and antagonizes 53BP1-dependent non-homologous end joining in S/G2 phase. But how this is achieved remains unclear. Here, we report that the E3 ubiquitin ligase UHRF1 (Ubiquitin-like, with PHD and RING finger domains 1) directly participates in the interplay between BRCA1 and 53BP1. Mechanistically, UHRF1 is recruited to DNA double-strand breaks (DSBs) by BRCA1 in S phase, which requires the BRCT domain of BRCA1 and phosphorylated Ser674 of UHRF1. Subsequently, UHRF1 mediates K63-linked polyubiquitination of RIF1, and results in its dissociation from 53BP1 and DSBs thereby facilitating HR initiation. Thus, UHRF1 is a key regulator of DSB repair choice, which is separate from its role in heterochromatin formation and epigenetic regulator.

BRCA1 is a key regulator of DNA double-strand break repair, functioning to promote homologous recombination and repress non-homologous end-joining. Here the authors show that the ubiquitin ligase UHRF1 is recruited to breaks by BRCA1, where it targets RIF1 and thereby facilitates recombination.

Chemical proteomics reveals a γH2AX-53BP1 interaction in the DNA damage response

DNA double-strand break repair involves phosphorylation of histone variant H2AX (‘γH2AX’), which accumulates in foci at sites of damage. In current models, the recruitment of multiple DNA repair proteins to γH2AX foci depends mainly on recognition of this ‘mark’ by a single protein, MDC1. However, DNA repair proteins accumulate at γH2AX sites without MDC1, suggesting that other ‘readers’ exist. Here, we use a quantitative chemical proteomics approach to profile direct, phospho-selective γH2AX binders in native proteomes. We identify γH2AX binders, including the DNA repair mediator, 53BP1, which we show recognizes γH2AX through its BRCT domains. Furthermore, we investigate targeting of wild-type 53BP1 or a mutant form deficient in γH2AX binding, to chromosomal breaks resulting from endogenous and exogenous DNA damage. Our results show how direct recognition of γH2AX modulates protein localization at DNA damage sites, and suggest how specific chromatin ‘mark’-‘reader’ interactions contribute to essential mechanisms ensuring genome stability.

Crystal structure of triple-BRCT-domain of ECT2 and insights into the binding characteristics to CYK-4

Homo sapiens ECT2 is a cell cycle regulator that plays critical roles in cytokinesis. ECT2 activity is restrained during interphase via intra-molecular interactions that involve its N-terminal triple-BRCT-domain and its C-terminal DH-PH domain. At anaphase, this self-inhibitory mechanism is relieved by Plk1-phosphorylated CYK-4, which directly engages the ECT2 BRCT domain. To provide a structural perspective for this auto-inhibitory property, we solved the crystal structure of the ECT2 triple-BRCT-domain. In addition, we systematically analyzed the interaction between the ECT2 BRCT domains with phospho-peptides derived from its binding partner CYK-4, and have identified Ser164 as the major phospho-residue that links CYK-4 to the second ECT2 BRCT domain.

The DNA-binding domain mediates both nuclear and cytosolic functions of p53

Under conditions of genotoxic stress, human p53 activates the apoptotic effectors BAX or BAK to result in mitochondrial outer-membrane permeabilization and apoptosis. Antiapoptotic BCL-2 family member BCL-xL opposes this activity by sequestering cytosolic p53 via association with its DNA-binding domain, an interaction enhanced by p53 tetramerization. Here we characterized the BCL-xL-p53 complex by NMR spectroscopy and modulated it through mutagenesis to determine the relative contributions of BCL-xL's interactions with p53 or other BCL-2 family proteins to the BCL-xL-dependent inhibition of UV irradiation-induced apoptosis. Under our experimental conditions, one-third of the antiapoptotic activity of BCL-xL was mediated by p53 sequestration and the remaining two-thirds through sequestration of proapoptotic BCL-2 family members. Our studies define the contributions of cytosolic p53 to UV irradiation-induced apoptosis and provide opportunities to explore its contributions to other p53-dependent apoptotic signaling pathways.

USP28 is recruited to sites of DNA damage by the tandem BRCT domains of 53BP1 but plays a minor role in double-strand break metabolism

The DNA damage response (DDR) is critical for genome stability and the suppression of a wide variety of human malignancies, including neurodevelopmental disorders, immunodeficiency, and cancer. In addition, the efficacy of many chemotherapeutic strategies is dictated by the status of the DDR. Ubiquitin-specific protease 28 (USP28) was reported to govern the stability of multiple factors that are critical for diverse aspects of the DDR. Here, we examined the effects of USP28 depletion on the DDR in cells and in vivo. We found that USP28 is recruited to double-strand breaks in a manner that requires the tandem BRCT domains of the DDR protein 53BP1. However, we observed only minor DDR defects in USP28-depleted cells, and mice lacking USP28 showed normal longevity, immunological development, and radiation responses. Our results thus indicate that USP28 is not a critical factor in double-strand break metabolism and is unlikely to be an attractive target for therapeutic intervention aimed at chemotherapy sensitization.

Phospho-Ser/Thr-binding domains: navigating the cell cycle and DNA damage response

Coordinated progression through the cell cycle is a complex challenge for eukaryotic cells. Following genotoxic stress, diverse molecular signals must be integrated to establish checkpoints specific for each cell cycle stage, allowing time for various types of DNA repair. Phospho-Ser/Thr-binding domains have emerged as crucial regulators of cell cycle progression and DNA damage signalling. Such domains include 14-3-3 proteins, WW domains, Polo-box domains (in PLK1), WD40 repeats (including those in the E3 ligase SCF(βTrCP)), BRCT domains (including those in BRCA1) and FHA domains (such as in CHK2 and MDC1). Progress has been made in our understanding of the motif (or motifs) that these phospho-Ser/Thr-binding domains connect with on their targets and how these interactions influence the cell cycle and DNA damage response.