Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: EC:5.99.1.2 (topoisomerase)
9,166 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Induced cell cycle delays were among the first described cellular responses to ionizing radiation (IR). To understand the sensitivity and the molecular events involved in the response to low doses of IR and to examine the role of p53 and its downstream effector p21Waf1, we measured changes in expression of genes postulated to be involved in the cellular response to IR. Expression levels were examined in normal human diploid fibroblasts irradiated and maintained in quiescent density-inhibited growth up to 24-48 h after exposure to X-ray doses as low as 0.1-0.3 Gy, which have negligible effects on cell survival. Among 31 genes analyzed, we observed down-regulation in response to IR of the mRNA levels of CDC2, cyclin A, cyclin B, thymidine kinase, topoisomerase IIalpha, and RAD51. A similar reduction in the expression levels of these genes occurred when irradiated cells were released from confluence and allowed to proliferate. This was not observed in cells in which p53 function was defective and up-regulation of p21Waf1 levels either did not occur (E6 transfected normal human fibroblasts and Li-Fraumeni fibroblasts) or was delayed (ataxia telangiectasia fibroblasts) after irradiation. Down-regulation was also absent in p21Waf1-null mouse embryo fibroblasts (MEFs) but occurred at a lower level in p53-null MEFs, due to slight increases in p21Waf1 levels by a p53-independent pathway. These findings indicate that the down-regulation of these cell cycle regulated genes in irradiated cells is p53-dependent and involves its effector p21Waf1. Although no down-regulation in the expression of genes involved in G2-M was observed in p53 or in p21Waf1-null MEFs, these cells showed a G2-M delay after irradiation, indicating that the expression levels of these genes does not regulate the G2-M delay.
...
PMID:Regulation by ionizing radiation of CDC2, cyclin A, cyclin B, thymidine kinase, topoisomerase IIalpha, and RAD51 expression in normal human diploid fibroblasts is dependent on p53/p21Waf1. 983 Dec 41

Unlike other chemicals that have been tested in mammalian germ cells, the type-II topoisomerase inhibitor etoposide exhibits significant mutagenicity in primary spermatocytes. Because this is the cell stage during which meiotic recombination normally occurs, and because topoisomerases play a role in recombination, we studied the effect of etoposide on crossing-over in male mice. Exposure to those meiotic prophase stages (probably early to mid-pachytene) during which specific-locus deletion mutations can be induced resulted in decreased crossing-over in the p-Tyr(c) interval of mouse chromosome 7. Accompanying cytological studies with fluorescent antibodies indicated that while there was no detectable effect on the number of recombination nodules (MLH1 foci), there were marked changes in the stage of appearance and localization of RAD51 and RPA proteins. These temporal and spatial protein patterns suggest the formation of multiple lesions in the DNA after MLH1 has already disappeared from spermatocytes. Since etoposide blocks religation of the cut made by type II topoisomerases, repair of DNA damage may result in rejoining of the original DNA strands, undoing the reciprocal exchange that had already occurred and resulting in reduced crossing-over despite a normal frequency of MLH1 foci. Crossing-over could conceivably be affected differentially in different chromosomal regions. If, however, the predominant action of etoposide is to decrease homologous meiotic recombination, the chemical could be expected to increase nondisjunction, an event associated with human genetic risk. Three periods in spermatogenesis respond to etoposide in different ways. Exposure of (a) late differentiating spermatogonia (and, possibly, preleptotene spermatocytes) results in cell death; (b) early- to mid-pachytene induces specific-locus deletions and crossover reduction; and, (c) late pachytene-through-diakinesis leads to genetically unbalanced conceptuses as a result of clastogenic damage.
...
PMID:Effect of the topoisomerase-II inhibitor etoposide on meiotic recombination in male mice. 1064 7

The RAD51 protein has been shown to participate in homologous recombination by promoting ATP-dependent homologous pairing and strand transfer reactions. In the present study, we have investigated the possible involvement of RAD51 in non-homologous recombination. We demonstrate that overexpression of CgRAD51 enhances the frequency of spontaneous non-homologous recombination in the hprt gene of Chinese hamster cells. However, the rate of non-homologous recombination induced by the topoisomerase inhibitors campothecin and etoposide was not altered by overexpression of RAD51. These results indicate that the RAD51 protein may perform a function in connection with spontaneous non-homologous recombination that is not essential to or not rate-limiting for non-homologous recombination induced by camptothecin or etoposide. We discuss the possibility that the role played by RAD51 in non-homologous recombination observed here may not be linked to non-homologous end-joining.
...
PMID:RAD51 supports spontaneous non-homologous recombination in mammalian cells, but not the corresponding process induced by topoisomerase inhibitors. 1116 Aug 87

During mouse meiosis, the early prophase RAD51/DMC1 recombination protein sites, which are associated with the chromosome cores and which serve as markers for ongoing DNA-DNA interactions, are in ten-fold excess of the eventual reciprocal recombinant events. Most, if not all, of these early interactions are eliminated as prophase progresses. The manner in which these sites are eliminated is the focus of this investigation. We report that these sites acquire replication protein A, RPA and the Escherichia coli MUTS homologue, MSH4p, and somewhat later the Bloom helicase, BLM, while simultaneously losing the RAD51/DMC1 component. Eventually the RPA component is also lost and BLM sites remain. At that time, the MUTL homologue, MLH1p, which is essential for reciprocal recombination in the mouse, appears in numbers and locations that correspond to the distribution of reciprocal recombination events. However, the MLH1 foci do not appear to coincide with the remaining BLM sites. The MLH1p is specifically localized to electron-microscope-defined recombination nodules. We consider the possibility that the homology-search RAD51/DMC1 complexes are involved in homologous chromosome synapsis but that most of these early DNA-DNA interactions are later resolved by the anti-recombination RPA/MSH4/BLM-topoisomerase complex, thereby preventing the formation of superfluous reciprocal recombinant events.
...
PMID:The time course and chromosomal localization of recombination-related proteins at meiosis in the mouse are compatible with models that can resolve the early DNA-DNA interactions without reciprocal recombination. 1195 Aug 80

In budding yeast, loss of topoisomerase III, encoded by the TOP3 gene, leads to a genomic instability phenotype that includes slow growth, hyper-sensitivity to genotoxic agents, mitotic hyper-recombination, increased chromosome missegregation, and meiotic failure. Slow growth and other defects of top3 mutants are suppressed by mutation of SGS1, which encodes the only RecQ helicase in S. cerevisiae. sgs1 is epistatic to top3, suggesting that the two proteins act in the same pathway. To identify other factors that function in the Sgs1-Top3 pathway, we undertook a genetic screen for non-sgs1 suppressors of top3 defects. We found that slow growth and DNA damage sensitivity of top3 mutants are suppressed by mutations in RAD51, RAD54, RAD55, and RAD57. In contrast, top3 mutants show extreme synergistic growth defects with mutations in RAD50, MRE11, XRS2, RDH54, and RAD1. We also analyzed recombination at the SUP4-o region, showing that in a rad51, rad54, rad55, or rad57 background top3Delta does not increase recombination to the same degree as in a wild-type strain. These results suggest that the presence of the Rad51 homologous recombination complex in a top3 background facilitates creation of detrimental intermediates by Sgs1. We present a model wherein Rad51 helps recruit Sgs1-Top3 to sites of replicative damage.
...
PMID:Mutations in homologous recombination genes rescue top3 slow growth in Saccharomyces cerevisiae. 1239 78

Two human small cell lung cancer (SCLC) subpopulations, CPH 54A, and CPH 54B, established from the same patient tumor by in vitro cloning, were investigated. The tumor was classified as intermediate-type SCLC. The cellular sensitivity to ionizing radiation (IR) was previously determined in the two sublines both in vivo and in vitro. Here we measured the etoposide (VP16) sensitivity together with the induction and repair of VP16- and IR-induced DNA double-strand breaks (DSBs). The two subpopulations were found to differ significantly in sensitivity to VP16, with the radioresistant 54B subline also being VP16 resistant. In order to explain the VP16 resistant phenotype several mechanisms where considered. The p53 status, P-glycoprotein, MRP, topoisomerase IIalpha, and Mre11 protein levels, as well as growth kinetics, provided no explanations of the observed VP16 resistance. In contrast, a significant difference in repair of both VP16- and IR-induced DSBs, together with a difference in the levels of the DSB repair proteins DNA-dependent protein kinase (DNA-PK(cs)) and RAD51 was observed. The VP16- and radioresistant 54B subline exhibited a pronounced higher repair rate of DSBs and higher protein levels of both DNA-PK(cs) and RAD51 compared with the sensitive 54A subline. We suggest, that different DSB repair rates among tumor cell subpopulations of individual SCLC tumors may be a major determinant for the variation in clinical treatment effect observed in human SCLC tumors of identical histological subtype.
...
PMID:DNA repair rate and etoposide (VP16) resistance of tumor cell subpopulations derived from a single human small cell lung cancer. 1271 Nov 16

Etoposide (VP16) is a potent inducer of DNA double-strand breaks (DSBs) and is efficiently used in small cell lung cancer (SCLC) therapy. However, acquired VP16 resistance remains an important barrier to effective treatment. To understand the underlying mechanisms for VP16 resistance in SCLC, we investigated DSB repair and cellular VP16 sensitivity of SCLC cells. VP16 sensitivity and RAD51, DNA-PK(cs), topoisomerase IIalpha and P-glycoprotein protein levels were determined in 17 SCLC cell lines. In order to unravel the role of RAD51 in VP16 resistance, we cloned the human RAD51 gene, transfected SCLC cells with RAD51 sense or antisense constructs and measured the VP16 resistance. Finally, we measured VP16-induced DSBs in the 17 SCLC cell lines. Two cell lines exhibited a multidrug-resistant phenotype. In the other SCLC cell lines, the cellular VP16 resistance was positively correlated with the RAD51 protein level. In addition, downregulation or overexpression of the RAD51 gene altered the VP16 sensitivity. Furthermore, the levels of the RAD51 and DNA-PK(cs) proteins were related to VP16-induced DSBs. The results suggest that repair of VP16-induced DSBs is mediated through both RAD51-dependent homologous recombination and DNA-PK(cs)-dependent nonhomologous end-joining and may be a determinant of the variation in clinical treatment effect observed in human SCLC tumors of identical histologic subtype. Finally, we propose RAD51 as a potential target to improve VP16 efficacy and predict tumor resistance in the treatment of SCLC patients.
...
PMID:The role of RAD51 in etoposide (VP16) resistance in small cell lung cancer. 1271 36

Very few gene conversions in mitotic cells are associated with crossovers, suggesting that these events are regulated. This may be important for the maintenance of genetic stability. We have analyzed the relationship between homologous recombination and crossing-over in haploid budding yeast and identified factors involved in the regulation of crossover outcomes. Gene conversions unaccompanied by a crossover appear 30 min before conversions accompanied by exchange, indicating that there are two different repair mechanisms in mitotic cells. Crossovers are rare (5%), but deleting the BLM/WRN homolog, SGS1, or the SRS2 helicase increases crossovers 2- to 3-fold. Overexpressing SRS2 nearly eliminates crossovers, whereas overexpression of RAD51 in srs2Delta cells almost completely eliminates the noncrossover recombination pathway. We suggest Sgs1 and its associated topoisomerase Top3 remove double Holliday junction intermediates from a crossover-producing repair pathway, thereby reducing crossovers. Srs2 promotes the noncrossover synthesis-dependent strand-annealing (SDSA) pathway, apparently by regulating Rad51 binding during strand exchange.
...
PMID:Srs2 and Sgs1-Top3 suppress crossovers during double-strand break repair in yeast. 1462 95

Spermatocytes normally sustain many meiotically induced double-strand DNA breaks (DSBs) early in meiotic prophase; in autosomal chromatin, these are repaired by initiation of meiotic homologous-recombination processes. Little is known about how spermatocytes respond to environmentally induced DNA damage after recombination-related DSBs have been repaired. The experiments described here tested the hypothesis that, even though actively completing meiotic recombination, pachytene spermatocytes cultured in the absence of testicular somatic cells initiate appropriate chromatin remodeling and cell-cycle responses to environmentally induced DNA damage. Two DNA-damaging agents were employed for in vitro treatment of pachytene spermatocytes: gamma-irradiation and etoposide, a topoisomerase II (TOP2) inhibitor that results in persistent unligated DSBs. Chromatin modifications associated with DSBs were monitored after exposure by labeling surface-spread chromatin with antibodies against RAD51 (which recognizes DSBs) and the phosphorylated variant of histone H2AFX (herein designated by its commonly used symbol, H2AX), gammaH2AX (which modifies chromatin associated with DSBs). Both gammaH2AX and RAD51 were rapidly recruited to irradiation- or etoposide-damaged chromatin. These chromatin modifications imply that spermatocytes recruit active DNA damage responses, even after recombination is substantially completed. Furthermore, irradiation-induced DNA damage inhibited okadaic acid-induced progression of spermatocytes from meiotic prophase to metaphase I (MI), implying efficacy of DNA damage checkpoint mechanisms. Apoptotic responses of spermatocytes with DNA damage differed, with an increase in frequency of early apoptotic spermatocytes after etoposide treatment, but not following irradiation. Taken together, these results demonstrate modification of pachytene spermatocyte chromatin and inhibition of meiotic progress after DNA damage by mechanisms that may ensure gametic genetic integrity.
...
PMID:Spermatocyte responses in vitro to induced DNA damage. 1670 71

Fanconi anemia (FA) is a rare chromosomal-instability disorder associated with a variety of developmental abnormalities, bone marrow failure and predisposition to leukemia and other cancers. We have identified a homozygous missense mutation in the RAD51C gene in a consanguineous family with multiple severe congenital abnormalities characteristic of FA. RAD51C is a member of the RAD51-like gene family involved in homologous recombination-mediated DNA repair. The mutation results in loss of RAD51 focus formation in response to DNA damage and in increased cellular sensitivity to the DNA interstrand cross-linking agent mitomycin C and the topoisomerase-1 inhibitor camptothecin. Thus, biallelic germline mutations in a RAD51 paralog are associated with an FA-like syndrome.
...
PMID:Mutation of the RAD51C gene in a Fanconi anemia-like disorder. 2042 93


1 2 3 Next >>

---