Showing papers by "Yves Pommier published in 2002"
Journal Article•
TL;DR: It is reported that TC-NER-deficient cells are hypersensitive to cisplatin irrespective of their GG-NER status, and that gene complementation with XPA and XPD increases resistance to cisPlatin.
Abstract: The resistance of tumor cells to chemotherapeutic agents, such as cisplatin,is an important problem to be solved in cancer chemotherapy. One of the mechanisms associated with cisplatin resistance is nucleotide excision repair (NER). There are two pathways in NER, transcription-coupled NER (TC-NER) and global genome NER (GG-NER). Here, we report that TC-NER-deficient cells [xeroderma pigmentosum group A (XP-A), XP-D, XP-F, XP-G, Cockayne syndrome group A (CS-A), and CS-B] are hypersensitive to cisplatin irrespective of their GG-NER status, and that gene complementation with XPA and XPD increases resistance to cisplatin. By contrast, XP-C cells with selective defect in GG-NER but with normal TC-NER have normal resistance to cisplatin. XPC complementation had no effect on cisplatin antiproliferative activity. We propose that one of the pathways related to cisplatin response is TC-NER, not GG-NER.
355 citations
TL;DR: Interestingly, several analogues of L-708,906 with varied substituents on the left side aryl ring, while having good inhibitory potencies against IN in extracellular assays, are not antiviral in whole-cell systems.
Abstract: The 4-aryl-2-hydroxy-4-oxo-2-butenoic acids and their isosteric tetrazoles are among an emerging class of aryl β-diketo (ADK)-based agents which exhibit potent inhibition of HIV-1 integrase (IN)-catalyzed strand transfer (ST) processes, while having much reduced potencies against 3‘-processing (3‘-P) reactions. In the current study, L-708,906 (10e) and 5CITEP (13b), which are two examples of ADK inhibitors that have been reported by Merck and Shionogi pharmaceutical companies, served as model ADK leads. Structural variations to both the “left” and “right” sides of these molecules were made in order to examine effects on HIV-1 integrase inhibitory potencies. It was found that a variety of groups could be introduced onto the left side aryl ring with maintenance of good ST inhibitory potency. However, introduction of carboxylic acid-containing substituents onto the left side aryl ring enhanced 3‘-P inhibitory potency and reduced selectivity toward ST reactions. Although both L-708,906 and 5CITEP show potent ...
180 citations
Journal Article•
TL;DR: UCN-01 prevented IR-induced p53 up-regulation and p53 phosphorylation on serine 20, a site previously identified for Chk2 (or/and Chk1) kinase, and efficiently abrogated both the initiation and maintenance ofIR-induced G(2) arrest in HCT116 cells and their isogenic p53 (-/-) derivative, indicating that G( 2) checkpoint abrogation by UCN- 01 is p53 independent.
Abstract: UCN-01 (7-hydroxystaurosporine) is a cell-cycle checkpoint abrogator that sensitizes cells to ionizing radiation (IR) and chemotherapeutic agents. It has been shown previously that UCN-01 abrogates DNA-damage-induced G2 checkpoint most selectively in p53-defective cells, by primarily targeting Chk1. Here we show that UCN-01 prevented IR-induced p53 up-regulation and p53 phosphorylation on serine 20, a site previously identified for Chk2 (or/and Chk1) kinase. We found that in human colon carcinoma HCT116 cells, IR treatment enhanced Chk2 kinase activity, whereas Chk1 activity remained unchanged, which suggested that UCN-01 may interrupt IR-induced p53 response by inhibiting Chk2 kinase. This conclusion is supported by in vitro kinase assays, showing that UCN–01 inhibits Chk2 immunoprecipitated from HCT116 cells (IC50, ∼10 nm). In addition, UCN-01 efficiently abrogated both the initiation and maintenance of IR-induced G2 arrest in HCT116 cells and their isogenic p53 (−/−) derivative, indicating that G2 checkpoint abrogation by UCN-01 is p53 independent. In the p53 (−/−) cells, there was no p21Waf1/Cip1 induction nor UCN-01-induced apoptosis. Taken together, these observations indicate that UCN-01 can modulate both Chk1 and Chk2 in intact cells and enhance IR-induced apoptosis in p53-deficient, and consequently p21-deficient, cells.
147 citations
TL;DR: Et743 is a selective transcription inhibitor, which has the unique characteristic of poisoning transcription-coupled nucleotide excision repair, which should help in deciding which patients should receive Et743 treatments and which agents should be most useful in association.
Abstract: Ecteinascidin 743 (Et743) is an interesting compound in phase II/III clinical trials. Its chemistry is complex, its mechanism of action is original and it is active in human cancers, such as sarcomas refractory to conventional chemotherapy. The present review describes the discovery of the drug, its specific interactions with DNA and its reversible alkylation mechanism with guanine N2 in the DNA minor groove. Et743 is a selective transcription inhibitor, which has the unique characteristic of poisoning transcription-coupled nucleotide excision repair. Understanding the molecular pharmacology of Et743 should help in deciding which patients should receive Et743 treatments and which agents should be most useful in association.
125 citations
TL;DR: It is found that TDP1 can process nucleopeptides with up to 13 amino acid residues but is poorly active with a 70 kDa fragment of topoisomerase I covalently linked to a suicide DNA substrate, and that Tdp1 would be most effective after topoisomersase I has been proteolyzed in vivo.
Abstract: Tyrosyl-DNA phosphodiesterase-1 (Tdp1) is the only known enzyme to remove tyrosine from complexes in which the amino acid is linked to the 3'-end of DNA fragments Such complexes can be produced following DNA processing by topoisomerase I, and recent studies in yeast have demonstrated the importance of TDP1 for cell survival following topoisomerase I-mediated DNA damage In the present study, we used synthetic oligodeoxynucleotide-peptide conjugates (nucleopeptides) and recombinant yeast Tdp1 to investigate the molecular determinants for Tdp1 activity We find that Tdp1 can process nucleopeptides with up to 13 amino acid residues but is poorly active with a 70 kDa fragment of topoisomerase I covalently linked to a suicide DNA substrate Furthermore, Tdp1 was more effective with nucleopeptides with one to four amino acids than 15 amino acids Tdp1 was also more effective with nucleopeptides containing 15 nt than with homolog nucleopeptides containing 4 nt These results suggest that DNA binding contributes to the activity of Tdp1 and that Tdp1 would be most effective after topoisomerase I has been proteolyzed in vivo
125 citations
Journal Article•
TL;DR: A fundamental difference at the gene expression level is suggested between the molecular mechanism of reversible G(2) delay that follows mild DNA damage and the mechanism of permanent G( 2) arrest that follows more extensive DNA damage.
Abstract: cDNA microarray technology can be used to establish associations betweencharacteristic gene expression patterns and molecular responses to drug therapy. In this study, we used cDNA microarrays of 1694 cancer-related genes to monitor the gene expression consequences of the treatment of HCT116 colon cancer cells with the topoisomerase I inhibitor camptothecin (CPT). To obtain a more homogeneous cellular response, we synchronized the cells in S-phase using aphidicolin (APH) before CPT treatment. Brief incubation with 20 and 1000 nm CPT caused reversible and irreversible G2 arrest, respectively, and the patterns of gene expression change (with reference to untreated controls) were strikingly different at the two concentrations. Thirty-three genes, mainly divided into three groups, showed characteristic changes in the first 20 h as a consequence of treatment. Northern blots performed for five of these genes (each under eight experimental conditions) were quite consistent with the microarray results (average correlation coefficient, 0.86). Several p53-activated stress response genes were up-regulated after treatment with 1000 nm CPT or prolonged exposure to APH, but it seemed that the up-regulation did not directly cause cell cycle arrest because the up-regulation induced by prolonged treatment with APH did not prevent cell cycle progression after removal of APH. In contrast, cell cycle-dependent up-regulation of a group of mitosis-related genes was delayed or blocked after CPT treatments. The interrupted up-regulation of this group of genes was directly associated with G2 arrest. In addition, we observed down-regulation of gene expression in cells that were recovering from cell cycle delay. The observations reported here suggest a fundamental difference at the gene expression level between the molecular mechanism of reversible G2 delay that follows mild DNA damage and the mechanism of permanent G2 arrest that follows more extensive DNA damage.
115 citations
TL;DR: The top1-mediated DNA cleavage patterns of the indenoisoquinolines examined were found to be similar to each other but different from that of camptothecin, which indicated promising anticancer activities in hollow fiber animal models.
Abstract: A number of novel dihydroindenoisoquinolines and indenoisoquinolinium salts were synthesized and examined for cytotoxicity in cancer cell cultures and for inhibition of topoisomerase I (top1). The top1-mediated DNA cleavage patterns produced in the presence of several of the new analogues were also investigated, and a few of the more potent compounds were examined for activity in hollow fiber animal models. Very cytotoxic dihydroindenoisoquinoline and isoquinolinium salts were obtained with mean graph midpoints (MGMs) for growth inhibition in the low submicromolar range. Two of the new dihydroindenoisoquinolines were found to be weaker top1 inhibitors than the lead compound 1, while two of the indenoisoquinolinium salts were more potent. The top1-mediated DNA cleavage patterns of the indenoisoquinolines examined were found to be similar to each other but different from that of camptothecin. Several of the more potent indenoisoquinolines displayed promising anticancer activities in hollow fiber animal models.
107 citations
Journal Article•
TL;DR: The enhancement of camptothecin-induced top1 cleavage complexes may, at least in part, contribute to the synergistic or additive effects of gemcitabine in combination with topotecan and irinotecans in human breast or lung cancer cells.
Abstract: Purpose: Gemcitabine-containing regimens are among standard therapies for the treatment of advanced non-small cell lung,pancreatic, or bladder cancers. Gemcitabine is a nucleoside analogue and its cytotoxicity is correlated with incorporation into genomic DNA and concomitant inhibition of DNA synthesis. However, it is still unclear by which mechanism(s) gemcitabine incorporation leads to cell death. Experimental Design: We used purified oligodeoxynucleotides to study the effects of gemcitabine incorporation on topoisomerase I (top1) activity and tested the role of top1 poisoning in gemcitabine-induced cytotoxicity in cancer cells. Results: We found that top1-mediated DNA cleavage was enhanced when gemcitabine was incorporated immediately 3′ from a top1 cleavage site on the nonscissile strand. This position-specific enhancement was attributable to an increased DNA cleavage by top1 and was likely to have resulted from a combination of gemcitabine-induced conformational and electrostatic effects. Gemcitabine also enhanced camptothecin-induced cleavage complexes. We also detected top1 cleavage complexes in human leukemia CEM cells treated with gemcitabine and a 5-fold resistance of P388/CPT45 top1-deficient cells to gemcitabine, indicating that poisoning of top1 can contribute to the antitumor activity of gemcitabine. Conclusions: The present results extend our recent finding that incorporation of 1-β-d-arabinofuranosylcytosine into DNA can induce top1 cleavage complexes [P. Pourquier et al . Proc. Natl. Acad. Sci. USA, 97: 1885–1890, 2000]. The enhancement of camptothecin-induced top1 cleavage complexes may, at least in part, contribute to the synergistic or additive effects of gemcitabine in combination with topotecan and irinotecan in human breast or lung cancer cells.
101 citations
TL;DR: Findings suggest that altered mitochondrial‐mediated apoptosis could play a role in oxaliplatin resistance.
85 citations
TL;DR: It is experimentally determined that the lactone form of CPT was stabilized by an irreversible top1/DNA covalent complex, consistent with the known structure-activity relationships of C PT and derivatives.
Abstract: Human topoisomerase I (top1) is an important target for anti-cancer drugs, which include camptothecin (CPT) and its derivatives. To elucidate top1 inhibition in vitro, we made a series of duplex DNA substrates containing a deoxyadenosine stereospecifically modified by a covalent adduct of benzo[a]pyrene (BaP) diol epoxide [Pommier, Y., et al. (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 10739−10744]. The known orientation of the hydrocarbon adduct in the DNA duplex relative to the top1 cleavage site, in combination with a top1/DNA crystal structure [Redinbo, M. R., et al. (1998) Science 279, 1504−1513], was used to construct a structure-based model to explain the in vitro top1 inhibition results obtained with adducted DNA duplexes. Here we experimentally determined that the lactone form of CPT was stabilized by an irreversible top1/DNA covalent complex. We removed the BaP moiety from the DNA in the published model, and docked the lactone forms of CPT and derivatives into the top1/DNA active site cavity. The d...
81 citations
TL;DR: The crystal structure of human topoisomerase I in noncovalent complex with a DNA oligonucleotide containing 8-oxoG at the +1 position in the scissile strand shows the docked, inactive conformation of Tyr-723–Phe is reminiscent of the related tyrosine recombinase family of integrases and recombinases, suggesting a common regulatory mechanism.
Abstract: 7,8-Dihydro-8-oxoguanine (8-oxoG) is the most common form of oxidative DNA damage in human cells. Biochemical studies have shown that 8-oxoG decreases the DNA cleavage activity of human topoisomerase I, an enzyme vital to DNA metabolism and stability. We present the 3.1-A crystal structure of human topoisomerase I in noncovalent complex with a DNA oligonucleotide containing 8-oxoG at the +1 position in the scissile strand. We find that 8-oxoG reorganizes the active site of human topoisomerase I into an inactive conformation relative to the structures of topoisomerase I-DNA complexes elucidated previously. The catalytic Tyr-723-Phe rotates away from the DNA cleavage site and packs into the body of the molecule. A second active-site residue, Arg-590, becomes disordered and is not observed in the structure. The docked, inactive conformation of Tyr-723-Phe is reminiscent of the related tyrosine recombinase family of integrases and recombinases, suggesting a common regulatory mechanism. We propose that human topoisomerase I binds to DNA first in an inactive conformation and then rearranges its active site for catalysis. 8-OxoG appears to impact topoisomerase I by stabilizing the inactive, DNA-bound state.
TL;DR: BTBD1 and BTBD2 belong to a small family of uncharacterized proteins that appear to be specific to animals and their interaction with topoisomerase I is underway.
Abstract: Two-hybrid screening for proteins that interact with the core domain of human topoisomerase I identified two novel proteins, BTBD1 and BTBD2, which share 80% amino acid identities. The interactions were confirmed by co-precipitation assays demonstrating the physical interaction of BTBD1 and BTBD2 with 100 kDa topoisomerase I from HeLa cells. Deletion mapping using two-hybrid and GST-pulldown assays demonstrated that less than the C-terminal half of BTBD1 is sufficient for binding topoisomerase I. The topoisomerase I sequences sufficient to bind BTBD2 were mapped to residues 215 to 329. BTBD2 with an epitope tag localized to cytoplasmic bodies. Using truncated versions that direct BTBD2 and TOP1 to the same cellular compartment, either the nucleus or the cytoplasm, co-localization was demonstrated in co-transfected Hela cells. The supercoil relaxation and DNA cleavage activities of topoisomerase I in vitro were affected little or none by co-incubation with BTBD2. Northern analysis revealed only a single sized mRNA for each BTBD1 and BTBD2 in all human tissues tested. Characterization of BTBD2 mRNA revealed a 255 nucleotide 90% GC-rich region predicted to encode the N-terminus. BTBD1 and BTBD2 are widely if not ubiquitously expressed in human tissues, and have two paralogs as well as putative orthologs in C. elegans and D. melanogaster. BTBD1 and BTBD2 belong to a small family of uncharacterized proteins that appear to be specific to animals. Epitope-tagged BTBD2 localized to cytoplasmic bodies. The characterization of BTBD1 and BTBD2 and their interaction with TOP1 is underway.
Journal Article•
TL;DR: Ecteinascidin 743 can potentiate the activity of other chemotherapeutic agents by down-regulating P-gp/MDR1, suggesting that the combination of Et-743 and chemotherAPEutic agents that are substrates for P-glycoprotein/mDR1 may be valuable in the clinic.
Abstract: Ecteinascidin 743 (Et-743) is a novel anticancer agent forming covalent guanine adducts at specific sites in the DNA minor groove. Et-743 has a unique mechanism of action because it kills cancer cells by poisoning transcription-coupled nucleotide excision repair. Recent studies suggested a complex relationship between P-glycoprotein (P-gp)/MDR1 and Et-743. On one hand, Et-743 was reported to down-regulate the MDR1 promoter in vitro. On the other hand, P-gp overexpression was hypothesized to contribute to Et-743 resistance in an ovarian cell line. The present study was performed to further investigate the relationship between P-gp/MDR1 and the activity of Et-743. First, we found no P-gp/MDR1 overexpression (mRNA and protein levels) in two independently generated Et-743-resistant human colon carcinoma cell lines (HCT116/ER5 and SW480/ER0.5). Secondly, we found no cross-resistance to Et-743 in two well-characterized P-gp/MDR1-overexpressing cell lines (KB-8-5 and KB-C-2). Third, Et-743 pretreatment enhanced the cytotoxicity and the cellular accumulation of doxorubicin and vincristine in P-gp/MDR1-overexpressing KB-8-5/KB-C-2 cell lines. Fourth, we observed P-gp/MDR1 down-regulation by Et-743 in KB-C-2 cells. These results indicate that Et-743 does not select for the emergence of a P-gp phenotype in all cell lines made resistant to Et-743 and that P-gp overexpression is not sufficient to confer resistance to Et-743. Furthermore, Et-743 is an effective agent in P-gp-overexpressing cells. Et-743 can potentiate the activity of other chemotherapeutic agents by down-regulating P-gp/MDR1, suggesting that the combination of Et-743 and chemotherapeutic agents that are substrates for P-gp/MDR1 may be valuable in the clinic.
TL;DR: It is demonstrated that either DNA or RNA G-rich single-stranded and G-quartet-forming oligonucleotides can bind to top1 and prevent cleavage of duplex DNA.
TL;DR: The results further underscore the similarities between RAG1/2 and integrase and suggest that certain integrase inhibitors may have the potential to interfere with aspects of B and T cell development.
Abstract: Assembly of functional Ig and T cell receptor genes by V(D)J recombination depends on site-specific cleavage of chromosomal DNA by the RAG1/2 recombinase. As RAG1/2 action has mechanistic similarities to DNA transposases and integrases such as HIV-1 integrase, we sought to determine how integrase inhibitors of the diketo acid type would affect the various activities of RAG1/2. Both of the inhibitors we tested interfered with DNA cleavage and disintegration activities of RAG1/2, apparently by disrupting interaction with the DNA motifs bound specifically by the recombinase. The inhibitors did not ablate RAG1/2's transposition activity or capture of nonspecific transpositional target DNA, suggesting this DNA occupies a site on the recombinase different from that used for specific binding. These results further underscore the similarities between RAG1/2 and integrase and suggest that certain integrase inhibitors may have the potential to interfere with aspects of B and T cell development.
TL;DR: It is demonstrated that covalent diol epoxide adducts provide a novel set of tools to study DNA topoisomerases and emphasize the importance of contacts between the minor groove and top1′s catalytic site.
TL;DR: It is shown for the first time that topoisomerase IIβ is a substrate for PKCζ, and that PKC ζ may significantly influence topoisomease II inhibitor-induced cytotoxicity by altering topoisomersase II ββ activity through its kinase function.
TL;DR: Attempts to verify inhibition of virus attachment and fusion as antiviral targets using time-of-addition experiments failed to confirm these observations, and instead identified an antiviral target occurring after completion of reverse transcription.
TL;DR: The α4-helix within the conserved CAP homology domain of type II topoisomerases is critical for selecting the sites of DNA cleavage, and it is demonstrated that selective amino acid residues in the α2+- and drug-stimulated DNA cleaved sites are important in determining the activity and possibly the binding of quinolones to the topoisomersase II-DNA complexes.
Abstract: We report that point mutations causing alteration of the fourth alpha-helix (alpha4-helix) of the CAP homology domain of eukaryotic (Saccharomyces cerevisiae) type II topoisomerases (Ser(740)Trp, Gln(743)Pro, and Thr(744)Pro) change the selection of type II topoisomerase-mediated DNA cleavage sites promoted by Ca(2+) or produced by etoposide, the fluoroquinolone CP-115,953, or mitoxantrone. By contrast, Thr(744)Ala substitution had minimal effect on Ca(2+)- and drug-stimulated DNA cleavage sites, indicating the selectivity of single amino acid substitutions within the alpha4-helix on type II topoisomerase-mediated DNA cleavage. The equivalent mutation in the gene for Escherichia coli gyrase causing Ser(83)Trp also changed the DNA cleavage pattern generated by Ca(2+) or quinolones. Finally, Thr(744)Pro substitution in the yeast type II topoisomerase rendered the enzyme sensitive to antibacterial quinolones. This study shows that the alpha4-helix within the conserved CAP homology domain of type II topoisomerases is critical for selecting the sites of DNA cleavage. It also demonstrates that selective amino acid residues in the alpha4-helix are important in determining the activity and possibly the binding of quinolones to the topoisomerase II-DNA complexes.
01 Jan 2002
TL;DR: A review of the pharmacology of agents that are currently known to act on the cell cycle control and checkpoint pathways can be found in this article, where the authors discuss the rationale for targeting these pathways.
Abstract: The chapter focuses on cell cycle and checkpoint control, discusses the rationale for targeting these pathways, and reviews the pharmacology of the agents that are currently known to act on the cell cycle control and checkpoint pathways. Cell cycle checkpoints are mechanisms that monitor cell regulatory pathways and DNA structure before the cells enter the next phase of the cell cycle. Their activation in response to DNA damage either leads to cell cycle arrest to allow repair of DNA damage, or leads to cell death by apoptosis or terminal growth arrest. Alterations of the cell cycle control and checkpoint pathways are generally present in human cancers and such deficiencies can be identified in patient tumors. The dependence of tumors on activated cyclin-dependent kinases and the apparent deficiencies of tumor cells with respect to redundant pathways provide a rationale for the use of cyclin-dependent kinase inhibitors as anticancer agents. The key proteins in these pathways have been identified, and these can be used to study the interactions of drugs at the molecular level and for the discovery of novel inhibitors. Various small molecule inhibitors have also been discovered, some of which are in early clinical trials. Gene therapy has also been applied successfully for p53. An important challenge is to integrate the cell cycle and checkpoint pathways in a logical and comprehensive framework, to increase the selectivity of current therapies against specific targets, and to adapt these therapies to individual patients based on the molecular dissection of their tumors.
TL;DR: A number of novel dihydroindenoisoquinolines and indenois-oquinolinium salts were synthesized and examined for cytotoxicity in cancer cell cultures and for inhibition of topoisomerase I (top1) as mentioned in this paper.
Abstract: A number of novel dihydroindenoisoquinolines and indenoisoquinolinium salts were synthesized and examined for cytotoxicity in cancer cell cultures and for inhibition of topoisomerase I (top1). The top1-mediated DNA cleavage patterns produced in the presence of several of the new analogues were also investigated, and a few of the more potent compounds were examined for activity in hollow fiber animal models. Very cytotoxic dihydroindenoisoquinoline and isoquinolinium salts were obtained with mean graph midpoints (MGMs) for growth inhibition in the low submicromolar range. Two of the new dihydroindenoisoquinolines were found to be weaker top1 inhibitors than the lead compound 1, while two of the indenoisoquinolinium salts were more potent. The top1-mediated DNA cleavage patterns of the indenoisoquinolines examined were found to be similar to each other but different from that of camptothecin. Several of the more potent indenoisoquinolines displayed promising anticancer activities in hollow fiber animal models.
01 Jan 2002
TL;DR: Further structure activity studies are carried out with the aim of developing physiologically more stable analogs, to evaluate their specificities, and to develop a peptide based pharmacophore model for enzyme inhibition.
Abstract: The HIV virus encoded integrase (IN) enzyme is required for the integration of the viral genome into infected mammalian cells, and thus for the replication of the virus. Because the virus can not replicate without it, IN is a logical therapeutic target [1,2]. IN does not have an obvious cellular counterpart, therefore drugs that specifically inhibit integration may not be toxic for the cell. The mechanism of integration is well documented. After reverse transcription the IN enzyme removes two nucleotides from the 3' terminal end of the viral DNA, and the enzyme also functions in inserting the viral DNA through its 3'-end into the genomic DNA of the cell. Several years ago Plasterk and coworkers identified a hexapeptide, 1, with sequence, H-C-K-F-W-W-amide, using synthetic peptide combinatorial library methods [3], that inhibited IN function at low micromolar levels. To capitalize on this finding we have carried out further structure activity studies with the aim of developing physiologically more stable analogs, to evaluate their specificities, and to develop a peptide based pharmacophore model for enzyme inhibition.
Patent•
15 Feb 2002
TL;DR: An isolated or purified nucleic acid molecule consisting of a nucleotide sequence encoding mt topoisomerase I (mt topo I), a 5' fragment thereof, or a variant of mt topoI, is defined in this article.
Abstract: An isolated or purified nucleic acid molecule consisting essentially of a nucleotide sequence encoding mitochondrial topoisomerase I (mt topo I), a 5' fragment thereof, or a variant mt topo I, an isolated or purified nucleic acid molecule consisting essentially of a nucleotide sequence that is complementary to a nucleotide sequence encoding mt topo I, a 5' fragment thereof, or a variant mt topo I, a vector comprising such an isolated or purified nucleic acid molecule, a cell comprising such a vector, an isolated or purified polypeptide molecule consisting essentially of an amino acid sequence encoding mt topo I or a variant mt topo I, a conjugate comprising such an isolated or purified polypeptide molecule and a cell-surface targeting moiety, a hybridoma cell line that produces a monoclonal antibody that is specific for an aforementioned isolated or purified polypeptide molecule, the monoclonal antibody produced by the hybridoma cell line, a polyclonal antiserum raised against an aforementioned isolated or purified polypeptide molecule, a method of altering the level of mt topo I in a cell, and a method of identifying an inhibitor or an activator of mt topo I.