DNA double-strand break repair from head to tail

Lysine-specific demethylase 1 (LSD1) participate in porcine early embryonic development by regulating cell autophagy and apoptosis through the mTOR signaling pathway

Lysine-specific demethylase 1 (LSD1) stands as the pioneering histone demethylase uncovered, proficient in demethylating H3K4me1/2 and H3K9me1/2, thereby governing transcription and participating in cell apoptosis, proliferation, or differentiation. Nevertheless, the complete understanding of LSD1 during porcine early embryonic development and the underlying molecular mechanism remains unclear. Thus, we investigated the mechanism by which LSD1 plays a regulatory role in porcine early embryos. This study revealed that LSD1 inhibition resulted in parthenogenetic activation (PA) and in vitro fertilization (IVF) embryo arrested the development, and decreased blastocyst quality. Meanwhile, H3K4me1/2 and H3K9me1/2 methylase activity was increased at the 4-cell embryo stage. RNA-seq results revealed that autophagy related biological processes were highly enriched through GO and KEGG pathway analyses when LSD1 inhibition. Further studies showed that LSD1 depletion in porcine early embryos resulted in low mTOR and p-mTOR levels and high autophagy and apoptosis levels. The LSD1 deletion-induced increases in autophagy and apoptosis could be reversed by addition of mTOR activators. We further demonstrated that LSD1 inhibition induced mitochondrial dysfunction and mitophagy. In summary, our research results indicate that LSD1 may regulate autophagy and apoptosis through the mTOR pathway and affect early embryonic development of pigs.

Biochemical characterization and mutational analysis of the NurA protein from the hyperthermophilic euryarchaeon Thermococcus barophilus Ch5

Archaeal NurA protein plays a key role in producing 3'-single stranded DNA used for homologous recombination repair, together with HerA, Mre11, and Rad50. Herein, we describe biochemical characteristics and roles of key amino acid residues of the NurA protein from the hyperthermophilic euryarchaeon Thermococcus barophilus Ch5 (Tba-NurA). Tba-NurA possesses 5'-3' exonuclease activity for degrading DNA, displaying maximum efficiency at 45 °C-65 °C and at pH 8.0 in the presence of Mn2+. The thermostable Tba-NurA also possesses endonuclease activity capable of nicking plasmid DNA and circular ssDNA. Mutational data demonstrate that residue D49 of Tba-NurA is essential for exonuclease activity and is involved in binding ssDNA since the D49A mutant lacked exonuclease activity and reduced ssDNA binding. The R96A and R129A mutants had no detectable dsDNA binding, suggesting that residues R96 and R129 are important for binding dsDNA. The abolished degradation activity and reduced dsDNA binding of the D120A mutant suggest that residue D120 is essential for degradation activity and dsDNA binding. Additionally, residues Y392 and H400 are important for exonuclease activity since these mutations resulted in exonuclease activity loss. To our knowledge, it is the first report on biochemical characterization and mutational analysis of the NurA protein from Thermococcus.

SNM1A is crucial for efficient repair of complex DNA breaks in human cells

DNA double-strand breaks (DSBs), such as those produced by radiation and radiomimetics, are amongst the most toxic forms of cellular damage, in part because they involve extensive oxidative modifications at the break termini. Prior to completion of DSB repair, the chemically modified termini must be removed. Various DNA processing enzymes have been implicated in the processing of these dirty ends, but molecular knowledge of this process is limited. Here, we demonstrate a role for the metallo-β-lactamase fold 5'-3' exonuclease SNM1A in this vital process. Cells disrupted for SNM1A manifest increased sensitivity to radiation and radiomimetic agents and show defects in DSB damage repair. SNM1A is recruited and is retained at the sites of DSB damage via the concerted action of its three highly conserved PBZ, PIP box and UBZ interaction domains, which mediate interactions with poly-ADP-ribose chains, PCNA and the ubiquitinated form of PCNA, respectively. SNM1A can resect DNA containing oxidative lesions induced by radiation damage at break termini. The combined results reveal a crucial role for SNM1A to digest chemically modified DNA during the repair of DSBs and imply that the catalytic domain of SNM1A is an attractive target for potentiation of radiotherapy.

Encoding and Decoding of p53 Dynamics in Cellular Response to Stresses

In the cellular response to stresses, the tumor suppressor p53 is activated to maintain genomic integrity and fidelity. As a transcription factor, p53 exhibits rich dynamics to allow for discrimination of the type and intensity of stresses and to direct the selective activation of target genes involved in different processes including cell cycle arrest and apoptosis. In this review, we focused on how stresses are encoded into p53 dynamics and how the dynamics are decoded into cellular outcomes. Theoretical modeling may provide a global view of signaling in the p53 network by coupling the encoding and decoding processes. We discussed the significance of modeling in revealing the mechanisms of the transition between p53 dynamic modes. Moreover, we shed light on the crosstalk between the p53 network and other signaling networks. This review may advance the understanding of operating principles of the p53 signaling network comprehensively and provide insights into p53 dynamics-based cancer therapy.

ATPase Activity of Bacillus subtilis RecA Affects the Dynamic Formation of RecA Filaments at DNA Double Strand Breaks

RecA plays a central role in DNA repair and is a main actor involved in homologous recombination (HR). In vivo, RecA forms filamentous structures termed "threads," which are essential for HR, but whose nature is still ill defined. We show that RecA from Bacillus subtilis having lower ATP binding activity can still form nucleoprotein filaments in vitro, features lower dsDNA binding activity, but still retains most of wild type RecA activity in vivo. Contrarily, loss of ATPase activity strongly reduced formation of nucleoprotein filaments in vitro, and effectivity to repair double strand breaks (DSBs) in vivo. In the presence of wild type RecA protein, additionally expressed RecA with lowered ATPbinding activity only moderately affected RecA dynamics, while loss of ATPase activity leads to a large reduction of the formation of threads, as well as of their dynamic changes observed in a seconds-scale. Single molecule tracking of RecA revealed incorporation of freely diffusing and nonspecifically DNA-bound molecules into threads upon induction of a single DSB. This change of dynamics was highly perturbed in the absence of ATPase activity, revealing that filamentous forms of RecA as well as their dynamics depend on ATPase activity. Based on the idea that ATPase activity of RecA is most important for DNA strand exchange activity, our data suggest that extension and retraction of threads due is to many local strand invasion events during the search for sequences homologous to the induced DNA break site. IMPORTANCE Single-strand (ss) DNA binding ATPase RecA is the central recombinase in homologous recombination, and therefore essential for DNA repair pathways involving DNA strand exchange reactions. In several bacterial, RecA forms filamentous structures along the long axis of cells after induction of double strand breaks (DSBs) in the chromosome. These striking assemblies likely reflect RecA/ssDNA nucleoprotein filaments, which can extend and remodel within a time frame of few minutes. We show that ATPase activity of RecA is pivotal for these dynamic rearrangements, which include recruitment of freely diffusing molecules into low-mobile molecules within filaments. Our data suggest that ssDNA binding- and unbinding reactions are at the heart of RecA dynamics that power the dynamics of subcellular filamentous assemblies, leading to strand exchange reactions over a distance of several micrometers.

Modes of action of the archaeal Mre11/Rad50 DNA-repair complex revealed by fast-scan atomic force microscopy

The Mre11/Rad50 (M/R) complex forms the core of an essential DNA-repair complex, conserved in all divisions of life. Here we investigate this complex from the thermophilic archaeon Sulfolobus acidocaldarius using real-time atomic force microscopy. We demonstrate that the coiled-coil regions of Rad50 facilitate M/R interaction with DNA and permit substrate translocation until a free end is encountered. We also observe that the M/R complex drives unprecedented unwinding of the DNA duplexes. Taking these findings together, we provide a model for how the M/R complex can identify DNA double-strand breaks and orchestrate repair events.

Mre11 and Rad50 (M/R) proteins are part of an evolutionarily conserved macromolecular apparatus that maintains genomic integrity through repair pathways. Prior structural studies have revealed that this apparatus is extremely dynamic, displaying flexibility in the long coiled-coil regions of Rad50, a member of the structural maintenance of chromosome (SMC) superfamily of ATPases. However, many details of the mechanics of M/R chromosomal manipulation during DNA-repair events remain unclear. Here, we investigate the properties of the thermostable M/R complex from the archaeon Sulfolobus acidocaldarius using atomic force microscopy (AFM) to understand how this macromolecular machinery orchestrates DNA repair. While previous studies have observed canonical interactions between the globular domains of M/R and DNA, we observe transient interactions between DNA substrates and the Rad50 coiled coils. Fast-scan AFM videos (at 1–2 frames per second) of M/R complexes reveal that these interactions result in manipulation and translocation of the DNA substrates. Our study also shows dramatic and unprecedented ATP-dependent DNA unwinding events by the M/R complex, which extend hundreds of base pairs in length. Supported by molecular dynamic simulations, we propose a model for M/R recognition at DNA breaks in which the Rad50 coiled coils aid movement along DNA substrates until a DNA end is encountered, after which the DNA unwinding activity potentiates the downstream homologous recombination (HR)-mediated DNA repair.

Genome sequencing of evolved aspergilli populations reveals robust genomes, transversions in A. flavus, and sexual aberrancy in non-homologous end-joining mutants

Background

Aspergillus spp. comprises a very diverse group of lower eukaryotes with a high relevance for industrial applications and clinical implications. These multinucleate species are often cultured for many generations in the laboratory, which can unknowingly propagate hidden genetic mutations. To assess the likelihood of such events, we studied the genome stability of aspergilli by using a combination of mutation accumulation (MA) lines and whole genome sequencing.

Results

We sequenced the whole genomes of 30 asexual and 10 sexual MA lines of three Aspergillus species (A. flavus, A. fumigatus and A. nidulans) and estimated that each MA line accumulated mutations for over 4000 mitoses during asexual cycles. We estimated mutation rates of 4.2 × 10⁻¹¹ (A. flavus), 1.1 × 10⁻¹¹ (A. fumigatus) and 4.1 × 10⁻¹¹ (A. nidulans) per site per mitosis, suggesting that the genomes are very robust. Unexpectedly, we found a very high rate of GC → TA transversions only in A. flavus. In parallel, 30 asexual lines of the non-homologous end-joining (NHEJ) mutants of the three species were also allowed to accumulate mutations for the same number of mitoses. Sequencing of these NHEJ MA lines gave an estimated mutation rate of 5.1 × 10⁻¹¹ (A. flavus), 2.2 × 10⁻¹¹ (A. fumigatus) and 4.5 × 10⁻¹¹ (A. nidulans) per base per mitosis, which is slightly higher than in the wild-type strains and some ~ 5–6 times lower than in the yeasts. Additionally, in A. nidulans, we found a NHEJ-dependent interference of the sexual cycle that is independent of the accumulation of mutations.

Conclusions

We present for the first time direct counts of the mutation rate of filamentous fungal species and find that Aspergillus genomes are very robust. Deletion of the NHEJ machinery results in a slight increase in the mutation rate, but at a rate we suggest is still safe to use for biotechnology purposes. Unexpectedly, we found GC→TA transversions predominated only in the species A. flavus, which could be generated by the hepatocarcinogen secondary metabolite aflatoxin. Lastly, a strong effect of the NHEJ mutation in self-crossing was observed and an increase in the mutations of the asexual lines was quantified.

Association between selenium, cadmium, and arsenic levels and genetic polymorphisms in DNA repair genes (XRCC5, XRCC6) in gastric cancerous and non-cancerous tissue

Gastric cancer is one of the most prevalent cancers in northern Iran. The DNA repair genes X-ray repair cross-complementing (XRCC) group 5, XRCC6, which are important members of non-homologous end-joining repair system, play an important role in repairing the DNA double-strand breaks. Chronic exposure to heavy metals has long been recognized as being capable of augmenting gastric cancer incidence among exposed human populations. Since trace elements could directly or indirectly damage DNA, and polymorphism in DNA DSBs-repair genes can alter the capacity of system repair, we assumed that XRCC5 VNTR and XRCC6-61C >G polymorphism also impress the DSBs-repair system ability and contribute to gastric cancer. Therefore, the objective of this research was to evaluate the tissue accumulation of Selenium (Se), Cadmium (Cd) and Arsenic (As), and XRCC5 VNTR, XRCC6-61C >G polymorphisms in cancerous and non-cancerous tissues in Golestan province. The study population included 46 gastric cancer patients and 43 cancer-free controls. Two polymorphisms of XRCC5, XRCC6 were genotyped using polymerase chain reaction (PCR) or polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Further employed was atomic absorption spectroscopy so as to determine the levels of Se, Cd and As. Finally, the data were analyzed by SPSS (version 16) statistical software. The Se level was significantly higher in tumors as compared to non-tumor tissues, but there was no significant correlation between As and Cd in cancerous and noncancerous tissues. Allele frequencies of the selected genes were not statistically different between groups regarding XRCC6 (-61C>G). XRCC5 0R/0R, 0R/1R, 1R/1R, and 0R/2R genotypes were more common in cancerous group. High levels of Se in cancerous tissues vs. non-cancerous tissues may be one of the carcinogenic factors; in Golestan province, unlike other regions of Iran and the world, the level of Se is high, hence the higher risks of gastric cancer.

Role of the Mre11 Complex in Preserving Genome Integrity

DNA double-strand breaks (DSBs) are hazardous lesions that threaten genome integrity and cell survival. The DNA damage response (DDR) safeguards the genome by sensing DSBs, halting cell cycle progression and promoting repair through either non-homologous end joining (NHEJ) or homologous recombination (HR). The Mre11-Rad50-Xrs2/Nbs1 (MRX/N) complex is central to the DDR through its structural, enzymatic, and signaling roles. The complex tethers DNA ends, activates the Tel1/ATM kinase, resolves protein-bound or hairpin-capped DNA ends, and maintains telomere homeostasis. In addition to its role at DSBs, MRX/N associates with unperturbed replication forks, as well as stalled replication forks, to ensure complete DNA synthesis and to prevent chromosome rearrangements. Here, we summarize the significant progress made in characterizing the MRX/N complex and its various activities in chromosome metabolism.

Transactivation domain of p53 regulates DNA repair and integrity in human iPS cells

The role of p53 transactivation domain (p53-TAD), a multifunctional and dynamic domain, on DNA repair and retaining DNA integrity in human induced pluripotent stem cells (hiPSCs) has never been studied. p53-TAD was knocked out in iPSCs using CRISPR/Cas9 and was confirmed by DNA sequencing. p53-TAD knockout (KO) cells were characterized by accelerated proliferation, decreased population doubling time, and unaltered Bcl-2, Bcl-2-binding component 3, insulin-like growth factor 1 receptor, and Bax and altered Mdm2, p21, and p53-induced death domain transcript expression. In p53-TAD KO cells, the p53-regulated DNA repair proteins xeroderma pigmentosum group A, DNA polymerase H, and DNA-binding protein 2 expression were found to be reduced compared with p53 wild-type cells. Exposure to a low dose of doxorubicin (Doxo) induced similar DNA damage and DNA damage response (DDR) as measured by RAD50 and MRE11 expression, checkpoint kinase 2 activation, and γH2A.X recruitment at DNA strand breaks in both cell groups, indicating that silence of p53-TAD does not affect the DDR mechanism upstream of p53. After removal of Doxo, p53 wild-type hiPSCs underwent DNA repair, corrected their damaged DNA, and restored DNA integrity. Conversely, p53-TAD KO hiPSCs did not undergo complete DNA repair and failed to restore DNA integrity. More importantly, continuous culture of p53-TAD KO hiPSCs underwent G2/M cell cycle arrest and expressed the cellular senescent marker p16INK4a. Our data clearly show that silence of the TAD of p53 did not affect DDR but affected the DNA repair process, implying the crucial role of p53-TAD in maintaining DNA integrity. Therefore, activating p53-TAD domain using small molecules may promote DNA repair and integrity of cells and prevent cellular senescence.

ZmCom1 Is Required for Both Mitotic and Meiotic Recombination in Maize

CtIP/Ctp1/Sae2/Com1, a highly conserved protein from yeast to higher eukaryotes, is required for DNA double-strand break repair through homologous recombination (HR). In this study, we identified and characterized the COM1 homolog in maize. The ZmCom1 gene is abundantly expressed in reproductive tissues at meiosis stages. In ZmCom1-deficient plants, meiotic chromosomes are constantly entangled as a formation of multivalents and accompanied with chromosome fragmentation at anaphase I. In addition, the formation of telomere bouquet, homologous pairing and synapsis were disturbed. The immunostaining assay showed that the localization of ASY1 and DSY2 was normal, while ZYP1 signals were severely disrupted in Zmcom1 meiocytes, indicating that ZmCom1 is critically required for the proper SC assembly. Moreover, RAD51 signals were almost completely absent in Zmcom1 meiocytes, implying that COM1 is required for RAD51 loading. Surprisingly, in contrast to the Atcom1 and Oscom1 mutants, Zmcom1 mutant plants exhibited a number of vegetative phenotypes under normal growth condition, which may be partly attributed to mitotic aberrations including chromosomal fragmentation and anaphase bridges. Taken together, our results suggest that although the roles of COM1 in HR process seem to be primarily conserved, the COM1 dysfunction can result in the marked dissimilarity in mitotic and meiotic outcomes in maize compared to Arabidopsis and rice. We suggest that this character may be related to the discrete genome context.

The role of poly ADP-ribosylation in the first wave of DNA damage response

Poly ADP-ribose polymerases (PARPs) catalyze massive protein poly ADP-ribosylation (PARylation) within seconds after the induction of DNA single- or double-strand breaks. PARylation occurs at or near the sites of DNA damage and promotes the recruitment of DNA repair factors via their poly ADP-ribose (PAR) binding domains. Several novel PAR-binding domains have been recently identified. Here, we summarize these and other recent findings suggesting that PARylation may be the critical event that mediates the first wave of the DNA damage response. We also discuss the potential for functional crosstalk with other DNA damage-induced post-translational modifications.

γ-H2AX responds to DNA damage induced by long-term exposure to combined low-dose-rate neutron and γ-ray radiation

Risk estimates for low-dose radiation (LDR) remain controversial. The possible involvement of DNA repair-related genes in long-term low-dose-rate neutron-gamma radiation exposure is poorly understood. In this study, 60 rats were divided into control groups and irradiated groups, which were exposed to low-dose-rate n-γ combined radiation (LDCR) for 15, 30, or 60 days. The effects of different cumulative radiation doses on peripheral blood cell (PBC), subsets of T cells of peripheral blood lymphocytes (PBL) and DNA damage repair were investigated. Real-time PCR and immunoblot analyses were used to detect expression of DNA DSB-repair-related genes involved in the NHEJ pathway, such as Ku70 and Ku80, in PBL. The mRNA level of H2AX and the expression level of γ-H2AX were detected by real-time PCR, immunoblot, and flow cytometry. White blood cells (WBC) and platelets (PLT) of all ionizing radiation (IR) groups decreased significantly, while no difference was seen between the 30 day and 60 day exposure groups. The numbers of CD3(+), CD4(+) T cells and CD4(+)/CD8(+) in the PBL of IR groups were lower than in the control group. In the 30 day and 60 day exposure groups, CD8(+) T cells decreased significantly. Real-time PCR and immunoblot results showed no significant difference in the mRNA and protein expression of Ku70 and Ku80 between the control groups and IR groups. However, the mRNA of H2AX increased significantly, and there was a positive correlation with dose. There was no difference in the protein expression of γ-H2AX between 30 day and 60 day groups, which may help to explain the damage to PBL. In conclusion, PBL damage increased with cumulative dose, suggesting that γ-H2AX, but neither Ku70 nor Ku80, plays an important role in PBL impairment induced by LDCR.

DNA End Resection: Nucleases Team Up with the Right Partners to Initiate Homologous Recombination

The repair of DNA double-strand breaks by homologous recombination commences by nucleolytic degradation of the 5'-terminated strand of the DNA break. This leads to the formation of 3'-tailed DNA, which serves as a substrate for the strand exchange protein Rad51. The nucleoprotein filament then invades homologous DNA to drive template-directed repair. In this review, I discuss mainly the mechanisms of DNA end resection in Saccharomyces cerevisiae, which includes short-range resection by Mre11-Rad50-Xrs2 and Sae2, as well as processive long-range resection by Sgs1-Dna2 or Exo1 pathways. Resection mechanisms are highly conserved between yeast and humans, and analogous machineries are found in prokaryotes as well.

Disruption of lig4 improves gene targeting efficiency in the oleaginous fungus Mortierella alpina 1S-4

The oil-producing zygomycete Mortierella alpina 1S-4 is known to accumulate beneficial polyunsaturated fatty acids. We identified the lig4 gene that encodes for a DNA ligase 4 homolog, which functions to repair double strand breaks by non-homologous end joining. We disrupted the lig4 gene to improve the gene targeting efficiency in M. alpina. The M. alpina 1S-4 Δlig4 strains showed no defect in vegetative growth, formation of spores, and fatty acid production, but exhibited high sensitivity to methyl methansulfonate, an agent that causes DNA double-strand breaks. Importantly, gene replacement of ura5 marker by CBXB marker occurred in 67% of Δlig4 strains and the gene targeting efficiency was 21-fold greater than that observed in disruption of the lig4 gene in the M. alpina 1S-4 host strain. Further metabolic engineering of the Δlig4 strains is expected to result in strains that produce higher levels of rare and beneficial polyunsaturated fatty acids and contribute to basic research on the zygomycete.

Gene targeting in the oil-producing fungus Mortierella alpina 1S-4 and construction of a strain producing a valuable polyunsaturated fatty acid

To develop an efficient gene-targeting system in Mortierella alpina 1S-4, we identified the ku80 gene encoding the Ku80 protein, which is involved in the nonhomologous end-joining pathway in genomic double-strand break (DSB) repair, and constructed ku80 gene-disrupted strains via single-crossover homologous recombination. The Δku80 strain from M. alpina 1S-4 showed no negative effects on vegetative growth, formation of spores, and fatty acid productivity, and exhibited high sensitivity to methyl methanesulfonate, which causes DSBs. Dihomo-γ-linolenic acid (DGLA)-producing strains were constructed by disruption of the Δ5-desaturase gene, encoding a key enzyme of bioconversion of DGLA to ARA, using the Δku80 strain as a host strain. The significant improvement of gene-targeting efficiency was not observed by disruption of the ku80 gene, but the construction of DGLA-producing strain by disruption of the Δ5-desaturase gene was succeeded using the Δku80 strain as a host strain. This report describes the first study on the identification and disruption of the ku80 gene in zygomycetes and construction of a DGLA-producing transformant using a gene-targeting system in M. alpina 1S-4.

Envisioning the dynamics and flexibility of Mre11-Rad50-Nbs1 complex to decipher its roles in DNA replication and repair

The Mre11-Rad50-Nbs1 (MRN) complex is a dynamic macromolecular machine that acts in the first steps of DNA double strand break repair, and each of its components has intrinsic dynamics and flexibility properties that are directly linked with their functions. As a result, deciphering the functional structural biology of the MRN complex is driving novel and integrated technologies to define the dynamic structural biology of protein machinery interacting with DNA. Rad50 promotes dramatic long-range allostery through its coiled-coil and zinc-hook domains. Its ATPase activity drives dynamic transitions between monomeric and dimeric forms that can be modulated with mutants modifying the ATPase rate to control end joining versus resection activities. The biological functions of Mre11's dual endo- and exonuclease activities in repair pathway choice were enigmatic until recently, when they were unveiled by the development of specific nuclease inhibitors. Mre11 dimer flexibility, which may be regulated in cells to control MRN function, suggests new inhibitor design strategies for cancer intervention. Nbs1 has FHA and BRCT domains to bind multiple interaction partners that further regulate MRN. One of them, CtIP, modulates the Mre11 excision activity for homologous recombination repair. Overall, these combined properties suggest novel therapeutic strategies. Furthermore, they collectively help to explain how MRN regulates DNA repair pathway choice with implications for improving the design and analysis of cancer clinical trials that employ DNA damaging agents or target the DNA damage response.

Molecular architecture of the HerA-NurA DNA double-strand break resection complex

DNA double-strand breaks can be repaired by homologous recombination, during which the DNA ends are long-range resected by helicase-nuclease systems to generate 3' single strand tails. In archaea, this requires the Mre11-Rad50 complex and the ATP-dependent helicase-nuclease complex HerA-NurA. We report the cryo-EM structure of Sulfolobus solfataricus HerA-NurA at 7.4Å resolution and present the pseudo-atomic model of the complex. HerA forms an ASCE hexamer that tightly interacts with a NurA dimer, with each NurA protomer binding three adjacent HerA HAS domains. Entry to NurA's nuclease active sites requires dsDNA to pass through a 23Å wide channel in the HerA hexamer. The structure suggests that HerA is a dsDNA translocase that feeds DNA into the NurA nuclease sites.

Impact of amino acid substitutions in two functional domains of Ku80: DNA-damage-sensing ability of Ku80 and survival after irradiation

Various chemotherapeutic drugs, such as etoposide, and ionizing radiation (IR) have been clinically applied for the treatment of many types of animal and human malignancies. IR and chemotheraputic drugs kill tumor cells mainly by inducing DNA double-strand breaks (DSBs). On the other hand, unrepaired or incorrectly repaired DSBs can lead to chromosomal truncations and translocations, which can contribute to the development of cancer in humans and animals. Thus, it is important to clarify the molecular mechanisms underlying the chemosensitivity or radiosensitivity of mammalian cells in order to develop medical treatments and next-generation chemotherapeutic drugs for cancer. Previously, we established and analyzed cell lines stably expressing chimeric constructs of EGFP and the wild-type Ku80 (XRCC5) protein or its mutant protein to which mutations were introduced by the site-directed mutagenesis. We found that the Ku70 (XRCC6)-binding-site mutations (A453H/V454H) of Ku80 and nuclear localization signal (NLS)-dysfunctional mutations (K565A/K566A/K568A) affected the ability to complement etoposide sensitivity. In this study, we examined the radiosensitivity of these cell lines. We found that either or both amino acid substitutions in two functional domains of Ku80, i.e., Ku70-binding-site mutations (A453H/V454H) and NLS-dysfunctional mutations (K565A/K566A/K568A), affect the ability to complement radiosensitivity. Moreover, these mutations in the two domains of Ku80 affect the DSB-sensing ability of Ku80. These information and Ku80 mutant cell lines used might be useful for the study of not only the dynamics and function of Ku80, but also the molecular mechanism underlying the cellular response to IR and chemotherapeutic drugs in mammalian cells.

The neural crest transcription factor Brn3a is expressed in melanoma and required for cell cycle progression and survival

Pigment cells and neuronal cells both are derived from the neural crest. Here, we describe the Pit-Oct-Unc (POU) domain transcription factor Brn3a, normally involved in neuronal development, to be frequently expressed in melanoma, but not in melanocytes and nevi. RNAi-mediated silencing of Brn3a strongly reduced the viability of melanoma cell lines and decreased tumour growth in vivo. In melanoma cell lines, inhibition of Brn3a caused DNA double-strand breaks as evidenced by Mre11/Rad50-containing nuclear foci. Activated DNA damage signalling caused stabilization of the tumour suppressor p53, which resulted in cell cycle arrest and apoptosis. When Brn3a was ectopically expressed in primary melanocytes and fibroblasts, anchorage-independent growth was increased. In tumourigenic melanocytes and fibroblasts, Brn3a accelerated tumour growth in vivo. Furthermore, Brn3a cooperated with proliferation pathways such as oncogenic BRAF, by reducing oncogene-induced senescence in non-malignant melanocytes. Together, these results identify Brn3a as a new factor in melanoma that is essential for melanoma cell survival and that promotes melanocytic transformation and tumourigenesis.

Ku80 attentuates cytotoxicity induced by green fluorescent protein transduction independently of non-homologous end joining

The green fluorescent protein (GFP) is the most commonly used reporter protein for monitoring gene expression and protein localization in a variety of living and fixed cells, including not only prokaryotes, but also eukaryotes, e.g., yeasts, mammals, plants and fish. In general, it is thought that GFP is nontoxic to cells, although there are some reports on the side effect of GFP. Further, details of the molecular mechanism concerning the side effect of GFP remain unclear. Here we show that Ku80, but not XRCC4, plays an important role in the mechanism of the resistance to cytotoxicity induced by enhanced GFP (EGFP). EGFP inhibited both cell proliferation and colony formation, and induced cell death in Ku80-deficient hamster cells, i.e., xrs-6 cells. In addition, Ku80 attenuated EGFP-induced cytotoxicity in the xrs-6 cells. No EGFP-induced cytotoxicity was observed in the NHEJ core protein XRCC4-deficient hamster cells, i.e., XR-1 cells. Furthermore, EGFP markedly enhanced X-ray-induced cytotoxicity in the xrs-6 cells. These results suggest that Ku80 plays a key role in the novel NHEJ-independent defense mechanism against EGFP-induced cytotoxicity. Caution should be taken in considering of the potential influence by the stress response mechanism, namely, the Ku80-dependent elimination mechanism of EGFP-induced cytotoxicity, being activated, even when using EGFP-expressing cells in which Ku80 functions normally.

The defect of Ku70 affects sensitivity to x-ray and radiation-induced caspase-dependent apoptosis in lung cells

The DNA repair protein Ku70 is a key player in chemoresistance to anticancer agents (e.g., etoposide) or radioresistance. The responses of different organs to radiation vary widely and likely depend on the cell population in the organs. Previously, we established and characterized Ku70-deficient murine lung epithelial (Ku70 -/- MLE) cells and found that these cells are more sensitive than Ku70 +/- MLE cells (control cells) to X-irradiation, as determined by clonogenic survival assay; however, the mechanism underlying this sensitivity remains unclear. In this study, we examined the mechanism by which X-irradiation triggers the death of Ku70 -/- MLE cells. Our results showed that Ku70 -/- MLE cells were more sensitive to radiation-induced apoptosis than control cells, although X-irradiation activated caspase-3 and caspase-7, and cleaved PARP in both cell lines. We also examined the expression level of phosphorylated H2AX (γH2AX), which is a marker of DSB, and observed the phosphorylation of H2AX and the elimination of γH2AX in both cell lines after X-irradiation. The elimination in Ku70 -/- MLE cells was slower than that in control cells, suggesting that DSB repair activity in the Ku70 -/- MLE cells is lower than that in control cells. These findings suggest that Ku70 might play a key role in the inhibition of apoptosis through the DSB repair pathway in lung epithelial cells. Our findings also suggest that these cell lines might be useful for the study of Ku70 functions and the Ku70-dependent DSB repair pathway in lung epithelial cells.

Current evidence on the relationship between three polymorphisms in the XRCC7 gene and cancer risk

Inconsistency of the association of polymorphisms of XRCC7 with cancer is noted. Three commonly studied XRCC7 polymorphisms including rs7003908 (T>G), rs7830743 (A>G), and rs10109984 (T>C) were selected to explore their association with risk of development of cancer by meta-analysis of published case-control studies. The results showed that no significant associations with cancer risk were found in any model in terms of rs7003908, rs7830743 and rs10109984 when all studies were pooled into the meta-analysis. But when stratified by cancer type, statistically significantly elevated cancer risk was only found in prostate cancer for rs7003908 (GG vs. TT: OR = 1.845, 95 % CI = 1.178-2.888; dominant model: OR = 1.423, 95 % CI = 1.050-1.929; recessive model: OR = 1.677, 95 % CI = 1.133-2.482). In the subgroup analysis by ethnicity or study design, no significantly increased risks were found for all three polymorphisms. This meta-analysis suggests that XRCC7 rs7003908 polymorphism may contribute to cancer susceptibility for prostate cancer, which is recommended to be included in future large-sample studies and functional assays.

Enhanced genotoxicity of silver nanoparticles in DNA repair deficient Mammalian cells

Silver nanoparticles (Ag-np) have been used in medicine and commercially due to their anti-microbial properties. Therapeutic potentials of these nanoparticles are being explored extensively despite the lack of information on their mechanism of action at molecular and cellular level. Here, we have investigated the DNA damage response and repair following Ag-np treatment in mammalian cells. Studies have shown that Ag-np exerts genotoxicity through double-strand breaks (DSBs). DNA-PKcs, the catalytic subunit of DNA dependent protein kinase, is an important caretaker of the genome which is known to be the main player mediating Non-homologous End-Joining (NHEJ) repair pathway. We hypothesize that DNA-PKcs is responsible for the repair of Ag-np induced DNA damage. In vitro studies have been carried out to investigate both cytotoxicity and genotoxicity induced by Ag-np in normal human cells, DNA-PKcs proficient, and deficient mammalian cells. Chemical inhibition of DNA-PKcs activity with NU7026, an ATP-competitive inhibitor of DNA-PKcs, has been performed to further validate the role of DNA-PKcs in this model. Our results suggest that Ag-np induced more prominent dose-dependent decrease in cell viability in DNA-PKcs deficient or inhibited cells. The deficiency or inhibition of DNA-PKcs renders the cells with higher susceptibility to DNA damage and genome instability which in turn contributed to greater cell cycle arrest/cell death. These findings support the fact that DNA-PKcs is involved in the repair of Ag-np induced genotoxicity and NHEJ repair pathway and DNA-PKcs particularly is activated to safeguard the genome upon Ag-np exposure.

The Rad50 coiled-coil domain is indispensable for Mre11 complex functions

The Mre11 complex (Mre11, Rad50 and Xrs2 in Saccharomyces cerevisiae) influences diverse functions in the DNA damage response. The complex comprises the globular DNA-binding domain and the Rad50 hook domain, which are linked by a long and extended Rad50 coiled-coil domain. In this study, we constructed rad50 alleles encoding truncations of the coiled-coil domain to determine which Mre11 complex functions required the full length of the coils. These mutations abolished telomere maintenance and meiotic double-strand break (DSB) formation, and severely impaired homologous recombination, indicating a requirement for long-range action. Nonhomologous end joining, which is probably mediated by the globular domain of the Mre11 complex, was also severely impaired by alteration of the coiled-coil and hook domains, providing the first evidence of their influence on this process. These data show that functions of Mre11 complex are integrated by the coiled coils of Rad50.

Accumulation of Ku70 at DNA double-strand breaks in living epithelial cells

Ku70 and Ku80 play an essential role in the DNA double-strand break (DSB) repair pathway, i.e., nonhomologous DNA-end-joining (NHEJ). No accumulation mechanisms of Ku70 at DSBs have been clarified in detail, although the accumulation mechanism of Ku70 at DSBs plays key roles in regulating the NHEJ activity. Here, we show the essential domains for the accumulation and function of Ku70 at DSBs in living lung epithelial cells. Our results showed that EGFP-Ku70 accumulation at DSBs began immediately after irradiation. Our findings demonstrate that three domains of Ku70, i.e., the α/β, DNA-binding, and Ku80-binding domains, but not the SAP domain, are necessary for the accumulation at or recognition of DSBs in the early stage after irradiation. Moreover, our findings demonstrate that the leucine at amino acid 385 of Ku70 in the Ku80-binding domain, but not the three target amino acids for acetylation in the DNA-binding domain, is involved in the localization and accumulation of Ku70 at DSBs. Furthermore, accumulations of XRCC4 and XLF, but not that of Artemis, at DSBs are dependent on the presence of Ku70. These findings suggest that Artemis can work in not only the Ku-dependent repair process, but also the Ku-independent process at DSBs in living epithelial cells.

The MRE11 complex: starting from the ends

The maintenance of genome stability depends on the DNA damage response (DDR), which is a functional network comprising signal transduction, cell cycle regulation and DNA repair. The metabolism of DNA double-strand breaks governed by the DDR is important for preventing genomic alterations and sporadic cancers, and hereditary defects in this response cause debilitating human pathologies, including developmental defects and cancer. The MRE11 complex, composed of the meiotic recombination 11 (MRE11), RAD50 and Nijmegen breakage syndrome 1 (NBS1; also known as nibrin) proteins is central to the DDR, and recent insights into its structure and function have been gained from in vitro structural analysis and studies of animal models in which the DDR response is deficient.

Ribosome recycling depends on a mechanistic link between the FeS cluster domain and a conformational switch of the twin-ATPase ABCE1

Despite some appealing similarities of protein synthesis across all phyla of life, the final phase of mRNA translation has yet to be captured. Here, we reveal the ancestral role and mechanistic principles of the newly identified twin-ATPase ABCE1 in ribosome recycling. We demonstrate that the unique iron-sulfur cluster domain and an ATP-dependent conformational switch of ABCE1 are essential both for ribosome binding and recycling. By direct (11) interaction, the peptide release factor aRF1 is shown to synergistically promote ABCE1 function in posttermination ribosome recycling. Upon ATP binding, ABCE1 undergoes a conformational switch from an open to a closed ATP-occluded state, which drives ribosome dissociation as well as the disengagement of aRF1. ATP hydrolysis is not required for a single round of ribosome splitting but for ABCE1 release from the 30S subunit to reenter a new cycle. These results provide a mechanistic understanding of final phases in mRNA translation.

The carboxyl terminal of the archaeal nuclease NurA is involved in the interaction with single-stranded DNA-binding protein and dimer formation

The nuclease NurA is present in all known thermophilic archaea and has been implicated to facilitate efficient DNA double-strand break end processing in Mre11/Rad50-mediated homologous recombinational repair. To understand the structural and functional relationship of this enzyme, we constructed five site-directed mutants of NurA from Sulfolobus tokodaii (StoNurA), D56A, E114A, D131A, Y291A, and H299A, at the conserved motifs, and four terminal deletion mutants, StoNurAΔN (19-331), StoNurAΔNΔC (19-303), StoNurAΔC (1-281), and StoNurAΔC (1-303), and characterized the proteins biochemically. We found that mutation at the acidic residue, D56, E114, D131, or at the basic residue, H299, abolishes the nuclease activity, while mutation at the aromatic residue Y291 only impairs the activity. Interestingly, by chemical cross-linking assay, we found that the mutant Y291A is unable to form stable dimer. Additionally, we demonstrated that deletion of the C-terminal amino acid residues 304-331 of StoNurA results in loss of the physical and functional interaction with the single-stranded DNA-binding protein (StoSSB). These results established that the C-terminal conserved aromatic residue Y291 is involved in dimer formation and the C-terminal residues 304-331 of NurA are involved in the interaction with single-stranded DNA-binding protein.

Mre11-Rad50-Nbs1 conformations and the control of sensing, signaling, and effector responses at DNA double-strand breaks

Repair and integrity of DNA ends at breaks, replication forks and telomeres are essential for life; yet, paradoxically, these responses are, in many cases, controlled by a single protein complex, Mre11-Rad50-Nbs1 (MRN). The MRN complex consists of dimers of each subunit and this heterohexamer controls key sensing, signaling, regulation, and effector responses to DNA double-strand breaks including ATM activation, homologous recombinational repair, microhomology-mediated end joining and, in some organisms, non-homologous end joining. We propose that this is possible because each MRN subunit can exist in three or more distinct states; thus, the trimer of MRN dimers can exist in a stunning 6(3) or 216 states, a number that can be expanded further when post-translational modifications are taken into account. MRN can therefore be considered as a molecular computer that effectively assesses optimal responses and pathway choice based upon its states as set by cell status and the nature of the DNA damage. This extreme multi-state concept demands a paradigm shift from striving to understand DNA damage responses in separate terms of signaling, checkpoint, and effector proteins: we must now endeavor to characterize conformational and assembly states of MRN and other DNA repair machines that couple, coordinate, and control biological outcomes. Addressing the emerging challenge of gaining a detailed molecular understanding of MRN and other multi-state dynamic DNA repair machines promises to provide opportunities to develop master keys for controlling cell biology with probable impacts on therapeutic interventions.

Polymorphic variants of DNA repair gene XRCC3 and XRCC7 and risk of prostate cancer: a study from North Indian population

DNA repair gene alterations may cause a reduction in DNA repair capacity and influence an individual's susceptibility to carcinogenesis. We hypothesized that single nucleotide polymorphisms of DNA repair genes may be a risk factor for prostate cancer (PCa) susceptibility, influencing expression of homologous recombination (XRCC3) and nonhomologous end-joining (XRCC7) genes and conferring predisposition to PCa. In a case-control study, genotyping was done in 192 patients with PCa and 224 age matched unrelated healthy controls of similar ethnicity to determine variants in XRCC3 Exon 7 (C18067T, rs861539), IVS5-14 (A17893G, rs1799796), and XRCC7 Intron 8 (G6721T, rs7003908) by polymerase chain reaction-restriction fragment-length polymorphism methods. Variant genotype GG (odds ratio [OR], 2.23; p=0.003) and combined genotype TG+GG (OR, 1.541; p=0.049), G allele of XRCC7 Intron 8 (G>T), demonstrated significant risk for PCa (OR, 1.529; p=0.002). Stratification on bases of Gleason grade and bone metastasis, significant risk with high Gleason grade for CT genotype of XRCC3 Exon 7, and variant genotype GG of XRCC7 Intron 8 were observed. Our results strongly support that common sequence variants (GG) genotype of XRCC7 may increase risk of PCa. G allele being a risk allele in our study also suggests that this polymorphism be used as a marker for the PCa susceptibility.

Crystal structure of the first eubacterial Mre11 nuclease reveals novel features that may discriminate substrates during DNA repair

Mre11 nuclease plays a central role in the repair of cytotoxic and mutagenic DNA double-strand breaks. As X-ray structural information has been available only for the Pyrococcus furiosus enzyme (PfMre11), the conserved and variable features of this nuclease across the domains of life have not been experimentally defined. Our crystal structure and biochemical studies demonstrate that TM1635 from Thermotoga maritima, originally annotated as a putative nuclease, is an Mre11 endo/exonuclease (TmMre11) and the first such structure from eubacteria. TmMre11 and PfMre11 display similar overall structures, despite sequence identity in the twilight zone of only approximately 20%. However, they differ substantially in their DNA-specificity domains and in their dimeric organization. Residues in the nuclease domain are highly conserved, but those in the DNA-specificity domain are not. The structural differences likely affect how Mre11 from different organisms recognize and interact with single-stranded DNA, double-stranded DNA and DNA hairpin structures during DNA repair. The TmMre11 nuclease active site has no bound metal ions, but is conserved in sequence and structure with the exception of a histidine that is important in PfMre11 nuclease activity. Nevertheless, biochemical characterization confirms that TmMre11 possesses both endonuclease and exonuclease activities on single-stranded and double-stranded DNA substrates, respectively.

Cellular radiosensitivity: how much better do we understand it?

PURPOSE

Ionising radiation exposure gives rise to a variety of lesions in DNA that result in genetic instability and potentially tumourigenesis or cell death. Radiation extends its effects on DNA by direct interaction or by radiolysis of H(2)O that generates free radicals or aqueous electrons capable of interacting with and causing indirect damage to DNA. While the various lesions arising in DNA after radiation exposure can contribute to the mutagenising effects of this agent, the potentially most damaging lesion is the DNA double strand break (DSB) that contributes to genome instability and/or cell death. Thus in many cases failure to recognise and/or repair this lesion determines the radiosensitivity status of the cell. DNA repair mechanisms including homologous recombination (HR) and non-homologous end-joining (NHEJ) have evolved to protect cells against DNA DSB. Mutations in proteins that constitute these repair pathways are characterised by radiosensitivity and genome instability. Defects in a number of these proteins also give rise to genetic disorders that feature not only genetic instability but also immunodeficiency, cancer predisposition, neurodegeneration and other pathologies.

CONCLUSIONS

In the past 50 years our understanding of the cellular response to radiation damage has advanced enormously with insight being gained from a wide range of approaches extending from more basic early studies to the sophisticated approaches used today. In this review we discuss our current understanding of the impact of radiation on the cell and the organism gained from the array of past and present studies and attempt to provide an explanation for what it is that determines the response to radiation.

A novel SMC-like protein, SbcE (YhaN), is involved in DNA double-strand break repair and competence in Bacillus subtilis

Bacillus subtilis and most Gram positive bacteria possess four SMC like proteins: SMC, SbcC, RecN and the product of the yhaN gene, termed SbcE. SbcE is most similar to SbcC but contains a unique central domain. We show that SbcE plays a role during transformation in competent cells and in DNA double-strand break (DSB) repair. The phenotypes were strongly exacerbated by the additional deletion of recN or of sbcC, suggesting that all three proteins act upstream of RecA and provide distinct avenues for presynapsis. SbcE accumulated at the cell poles in competent cells, and localized as a discrete focus on the nucleoids in 10% of growing cells. This number moderately increased after treatment with DNA damaging agents and in the absence of RecN or of SbcC. Damage-induced foci of SbcE arose early after induction of DNA damage and rarely colocalized with the replication machinery. Our work shows that SMC-like proteins in B. subtilis play roles at different subcellular sites during DNA repair. SbcC operates at breaks occurring at the replication machinery, whereas RecN and SbcE function mainly, but not exclusively, at DSBs arising elsewhere on the chromosome. In agreement with this idea, we found that RecN-YFP damage-induced assemblies also arise in the absence of ongoing replication.

Ku and DNA-dependent protein kinase dynamic conformations and assembly regulate DNA binding and the initial non-homologous end joining complex

DNA double strand break (DSB) repair by non-homologous end joining (NHEJ) is initiated by DSB detection by Ku70/80 (Ku) and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) recruitment, which promotes pathway progression through poorly defined mechanisms. Here, Ku and DNA-PKcs solution structures alone and in complex with DNA, defined by x-ray scattering, reveal major structural reorganizations that choreograph NHEJ initiation. The Ku80 C-terminal region forms a flexible arm that extends from the DNA-binding core to recruit and retain DNA-PKcs at DSBs. Furthermore, Ku- and DNA-promoted assembly of a DNA-PKcs dimer facilitates trans-autophosphorylation at the DSB. The resulting site-specific autophosphorylation induces a large conformational change that opens DNA-PKcs and promotes its release from DNA ends. These results show how protein and DNA interactions initiate large Ku and DNA-PKcs rearrangements to control DNA-PK biological functions as a macromolecular machine orchestrating assembly and disassembly of the initial NHEJ complex on DNA.

Do DNA repair genes OGG1, XRCC3 and XRCC7 have an impact on susceptibility to bladder cancer in the North Indian population?

OBJECTIVE

Polymorphisms in DNA repair genes may be associated with altered DNA repair capacity, thereby influencing an individual's susceptibility to smoking-related cancers such as bladder cancer. Therefore, we sought to examine the correlation between single nucleotide polymorphisms in DNA repair genes and bladder cancer.

METHODOLOGY

We undertook a case-control study of 212 urothelial bladder cancer (UBC) cases and 250 controls to investigate the association between OGG1 (C1245G rs1052133), XRCC3 (C18067T, rs861539) and XRCC7 (G6721T, rs7003908) polymorphisms and bladder cancer susceptibility by PCR-RFLP and the ARMS method. We also investigated gene-environment interactions.

RESULTS

The OGG1 GG genotype was associated with an elevated risk of urothelial bladder cancer (UBC) (OR, 2.10; p, 0.028). XRCC7 + 6721 GG was also associated with increased susceptibility to UBC (OR, 4.45; p, 0.001). In a recessive model, the OGG1 GG genotype showed an increased risk of TaG(2,3) + T1G(1-3) tumors. Additionally, the OGG1 GG genotype in non-smokers represented a 2.46-fold greater risk (OR, 2.46; p, 0.035) in bladder cancer patients. Subsequent analysis demonstrated more pronounced association of XRCC7 with smokers (OR, 4.39; p, 0.001). XRCC7 also showed increased association with TaG(2,3) + T1G(1-3) tumors and muscle invasive tumors (OR, 3.16; p, 0.001 and OR, 4.24; p, 0.001, respectively). Multiple Cox regression analysis in non-muscle invasive bladder tumor (NMIBT) patients demonstrated an association of the OGG1 GG polymorphism with a high risk of recurrence in patients on cystoscopic surveillance (HR, 4.04; p, 0.013). Subsequently, shorter recurrence-free survival (log rank p, 0.024; CC/GG, 42/24) was observed.

CONCLUSION

Our data suggest association of a variant (GG) genotype of OGG1 with increased UBC susceptibility and a high risk of tumor recurrence in NMIBT patients on cystoscopic surveillance. XRCC7 G allele carriers (TG+GG) are also at an elevated risk for susceptibility to UBC as evidenced by a high odds ratio throughout the analysis.

p38alpha MAP kinase C-terminal domain binding pocket characterized by crystallographic and computational analyses

The mitogen-activated protein (MAP) kinase protein family has a critical role in cellular signaling events, with MAP kinase p38alpha acting in inflammatory processes and being an important drug discovery target. MAP kinase drug design efforts have focused on small-molecule inhibitors of the ATP catalytic site, which exhibit dose-limiting adverse effects. Therefore, characterizing other potential sites that bind substrates, inhibitors, or allosteric effectors is of great interest. Here, we present the crystal structure of human p38alpha MAP kinase, which has a lead compound bound both in the active site and in the lipid-binding site of the C-terminal cap. This C-terminal cap is formed from an extension to the kinase fold, unique to the MAP kinase and cyclin-dependent kinase families and glycogen synthase kinase 3. Binding of this lead, 4-[3-(4-fluorophenyl)-1H-pyrazol-4-yl]pyridine, to wild-type p38alpha induces movement of the C-terminal cap region, creating a hydrophobic pocket centered around residue Trp197. Computational analysis of this C-terminal domain pocket indicates notable flexibility for potentially binding different-shaped compounds, including lipids, oxidized arachidonic acid species such as leukotrienes, and small-molecule effectors. Furthermore, our structural results defining the open p38alpha C-lobe pocket provide a detailed framework for the design of novel small molecules with affinities comparable to active-site binders: to bind and potentially modulate the shape and activity of p38alpha in predetermined ways. Moreover, these results and analyses of p38alpha suggest strategies for designing specific binding compounds applicable to other MAP kinases, as well as the cyclin-dependent kinase family and glycogen synthase kinase 3beta that also utilize the C-terminal insert in their interactions.

Head and neck squamous cell carcinoma targeted chemosensitization

OBJECTIVE

The current treatment for advanced head and neck squamous cell carcinoma continues to result in poor outcomes. The purpose of this study is to investigate the benefit of fibroblast growth factor 2-targeted adenovirus-mediated mutant-Rad50 (FGF2-Ad-Rad50) gene transfer in enhancing chemosensitization for head and neck squamous cell carcinoma and reducing chemotoxicity.

STUDY DESIGN

Randomized controlled laboratory study.

SETTING

University of Pennsylvania, Philadelphia, PA.

SUBJECTS AND METHODS

Human head and neck squamous cell carcinoma tumor cells and a mouse model with human head and neck squamous cell carcinoma were used for this study. There were five mice in each study group. FGF2-fab' molecule was conjugated with an adenoviral mutant-Rad50 construct. FGF2-targeted transgene expression efficiency was evaluated in vitro. Tumor cytotoxicity and growth inhibition were examined after combined FGF2-Ad-Rad50 with cisplatin treatment in vitro and in vivo. Anti-tumor mechanisms were investigated.

RESULTS

FGF2-targeted gene transfer approach significantly improved transgene expression in head and neck squamous cell carcinoma tumor cells over a nontargeted approach (207.51+/-33.62 vs 51.44+/-8.28, respectively). FGF2-Ad-Rad50 with cisplatin demonstrated a superior tumor suppression effect (264.5+/-124.1 mm3 vs 567.1+/-267.6 mm3), increased DNA double-strand breaks (1349+/-51.67 vs 774+/-28.56), and anti-angiogenesis (%ROI: 0.76%+/-0.38% vs 2.10%+/-1.66%) in tumor cells over nontargeted adenovirus.

CONCLUSION

Combination of FGF2-Ad-Rad50 with cisplatin significantly improves anti-tumor effect by targeting DNA repair systems and tumor angiogenesis. The great benefit of this strategy supports clinical trial for novel treatment of head and neck squamous cell carcinoma.

Molecular disruption of RAD50 sensitizes human tumor cells to cisplatin-based chemotherapy

Platinum-based drugs that induce DNA damage are commonly used first-line chemotherapy agents for testicular, bladder, head and neck, lung, esophageal, stomach, and ovarian cancers. The inherent resistance of tumors to DNA damage often limits the therapeutic efficacy of these agents, such as cisplatin. An enhanced DNA repair and telomere maintenance response by the Mre11/Rad50/Nbs1 (MRN) complex is critical in driving this chemoresistance. We hypothesized therefore that the targeted impairment of native cellular MRN function could sensitize tumor cells to cisplatin. To test this, we designed what we believe to be a novel dominant-negative adenoviral vector containing a mutant RAD50 gene that significantly downregulated MRN expression and markedly disrupted MRN function in human squamous cell carcinoma cells. A combination of cisplatin and mutant RAD50 therapy produced significant tumor cytotoxicity in vitro, with a corresponding increase in DNA damage and telomere shortening. In cisplatin-resistant human squamous cell cancer xenografts in nude mice, this combination therapy caused dramatic tumor regression with increased apoptosis. Our findings suggest the use of targeted RAD50 disruption as what we believe to be a novel chemosensitizing approach for cancer therapy in the context of chemoresistance. This strategy is potentially applicable to several types of malignant tumors that demonstrate chemoresistance and may positively impact the treatment of these patients.

On the path to seeking novel radiosensitizers

Radiation therapy is a highly effective cancer treatment modality, and extensive investigations have been undertaken over the years to augment its efficacy in the clinic. This review summarizes the current understanding of the biologic bases underpinning many of the clinically used radiosensitizers. In addition, this review illustrates how the advent of innovative, high-throughput technologies with integration of different disciplines could be harnessed for an expeditious discovery process for novel radiosensitizers, providing an exciting future for such pursuits in radiation biology and oncology.

Coprinus cinereus rad50 mutants reveal an essential structural role for Rad50 in axial element and synaptonemal complex formation, homolog pairing and meiotic recombination

The Mre11/Rad50/Nbs1 (MRN) complex is required for eukaryotic DNA double-strand break (DSB) repair and meiotic recombination. We cloned the Coprinus cinereus rad50 gene and showed that it corresponds to the complementation group previously named rad12, identified mutations in 15 rad50 alleles, and mapped two of the mutations onto molecular models of Rad50 structure. We found that C. cinereus rad50 and mre11 mutants arrest in meiosis and that this arrest is Spo11 dependent. In addition, some rad50 alleles form inducible, Spo11-dependent Rad51 foci and therefore must be forming meiotic DSBs. Thus, we think it likely that arrest in both mre11-1 and the collection of rad50 mutants is the result of unrepaired or improperly processed DSBs in the genome and that Rad50 and Mre11 are dispensable in C. cinereus for DSB formation, but required for appropriate DSB processing. We found that the ability of rad50 mutant strains to form Rad51 foci correlates with their ability to promote synaptonemal complex formation and with levels of stable meiotic pairing and that partial pairing, recombination initiation, and synapsis occur in the absence of wild-type Rad50 catalytic domains. Examination of single- and double-mutant strains showed that a spo11 mutation that prevents DSB formation enhances axial element (AE) formation for rad50-4, an allele predicted to encode a protein with intact hook region and hook-proximal coiled coils, but not for rad50-1, an allele predicted to encode a severely truncated protein, or for rad50-5, which encodes a protein whose hook-proximal coiled-coil region is disrupted. Therefore, Rad50 has an essential structural role in the formation of AEs, separate from the DSB-processing activity of the MRN complex.