Cancer Research Cancer Epigenetics Protein Translation and Cancer
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Cancer Research Clinical Cancer Research
Cancer Epidemiology Biomarkers & Prevention Molecular Cancer Therapeutics
Molecular Cancer Research Cancer Prevention Research
Cancer Prevention Journals Portal Cancer Reviews Online
Annual Meeting Education Book Meeting Abstracts Online
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Christensen, L. A.
Right arrow Articles by Vasquez, K. M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Christensen, L. A.
Right arrow Articles by Vasquez, K. M.
[Cancer Research 66, 4089-4094, April 15, 2006]
© 2006 American Association for Cancer Research

Molecular Biology, Pathobiology, and Genetics

Targeting Oncogenes to Improve Breast Cancer Chemotherapy

Laura A. Christensen1, Rick A. Finch2, Adam J. Booker1 and Karen M. Vasquez1

1 Department of Carcinogenesis, University of Texas M.D. Anderson Cancer Center, Science Park-Research Division, Smithville, Texas and 2 Lexicon Genetics, Inc., The Woodlands, Texas

Requests for reprints: Karen M. Vasquez, Department of Carcinogenesis, University of Texas M.D. Anderson Cancer Center, 1808 Park Road 1-C, P.O. Box 389, Smithville, TX 78957. Phone: 512-237-9324; Fax: 512-237-2475; E-mail: kvasquez{at}mdanderson.org.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 References
 
Despite recent advances in treatment, breast cancer remains a serious health threat for women. Traditional chemotherapies are limited by a lack of specificity for tumor cells and the cell cycle dependence of many chemotherapeutic agents. Here we report a novel strategy to help overcome these limitations. Using triplex-forming oligonucleotides (TFOs) to direct DNA damage site-specifically to oncogenes overexpressed in human breast cancer cells, we show that the effectiveness of the anticancer nucleoside analogue gemcitabine can be improved significantly. TFOs targeted to the promoter region of c-myc directly inhibited gene expression by ~40%. When used in combination, specific TFOs increased the incorporation of gemcitabine at the targeted site ~4-fold, presumably due to induction of replication-independent DNA synthesis. Cells treated with TFOs and gemcitabine in combination showed a reduction in both cell survival and capacity for anchorage-independent growth (~19% of untreated cells). This combination affected the tumorigenic potential of these cancer cells to a significantly greater extent than either treatment alone. This novel strategy may be used to increase the range of effectiveness of antitumor nucleosides in any tumor which overexpresses a targetable oncogene. Multifaceted chemotherapeutic approaches such as this, coupled with triplex-directed gene targeting, may lead to more than incremental improvements in nonsurgical treatment of breast tumors. (Cancer Res 2006; 66(8): 4089-94)


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 References
 
Breast cancer is a major cause of death for women ages 35 to 50 years. Available treatments include radiation therapy, surgery, HER2/neu antibody–based therapy (Herceptin), and chemotherapy. Whereas significant advances in treatment have been made, breast cancer remains the second leading cause of cancer-related deaths in women. Thus, improvements, particularly in chemotherapies, are sorely needed. Currently used chemotherapies often produce only marginal benefits due to their lack of specificity for tumor cells, and many cells in solid breast tumors are refractory to cell cycle–dependent anticancer agents.

One drug that has shown promise in the treatment of solid tumors is the antimetabolite gemcitabine. Gemcitabine, once incorporated into the DNA, is a potent inhibitor of DNA synthesis and inducer of apoptosis (1, 2). Gemcitabine has significant activity against a broad spectrum of tumors when used as a single agent and shows synergistic activity in combination with DNA-damaging agents (3–5). Clinical trials with combinations of nucleoside analogues and DNA-damaging agents have shown promise in low growth fraction tumors (6–9). However, in general, the effectiveness of the antimetabolites is restricted to actively cycling tumor cells and is limited by a lack of specificity for tumor cells. Triplex technology offers an approach to target DNA damage specifically to oncogenes in tumors cells.

Single-stranded triplex-forming oligonucleotides (TFOs) bind specifically to duplex DNA forming triple-helical DNA structures (10, 11). The formation of triplex DNA can inhibit the binding of transcription factors (12, 13) and physically block the progression of polymerases (14–17), resulting in specific transcription inhibition of the targeted gene (18). Further, triplex formation can be used to direct site-specific DNA damage and thereby induce replication-independent (i.e., unscheduled) DNA repair synthesis (19–21).

The c-myc oncogene provides a useful target for such TFO-based targeting strategies. Genetic abnormalities in human breast tumors include amplification of the c-myc gene and it is estimated that >50% of breast tumors show increased levels of the c-Myc protein (22), correlating with a poor prognosis for these patients (23, 24).

In this study, TFOs targeted to the c-myc oncogene were administered in combination with gemcitabine to investigate their effects on tumor cell growth. Our results suggest that a beneficial antitumor effect can be gained when gemcitabine and TFOs are administered in combination. We propose two mechanisms to explain these results: First, TFOs have been shown to induce unscheduled DNA repair synthesis (19), thereby increasing the incorporation of nucleosides into the DNA of these cells irrespective of their proliferative status. Second, we show that the c-myc-targeted TFOs inhibit oncogene expression, providing an additional mechanism of inhibition of tumor cell proliferation.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 References
 
Oligonucleotides and plasmid DNA. Oligonucleotides were synthesized and high-performance liquid chromatography purified by Midland Certified Reagent Co. (Midland, TX). 5'-Psoralen-modified oligonucleotides were synthesized with the derivative 2-[4'-(hydroxymethyl)-4,5',8-trimethylpsoralen]-hexyl-1-O-(2-cyanoethyl)-(N,N-diisopropyl)-phosphoramidite and a 3' C7 amine derivative to inhibit degradation in mammalian cells (25). Oligonucleotides were dissolved in water and purified over NAP-5 columns (Amersham Pharmacia Biotech AB, Uppsala, Sweden). DNA concentrations were determined by UV absorbance at 260 nm. c-myc-specific TFO sequences used in this study are shown in Fig. 1A . Myc2T and CSPEC bind a triplex-forming site in the promoter 2 region of the human c-myc gene (26, 27), identical to the one targeted in this study. The psoralen moiety was situated to align with a 5'-AT-3' cross-linking site at the triplex-duplex junction. Control oligonucleotides were designed with a scrambled sequence so they do not bind the target site.


Figure 1
View larger version (29K):
[in this window]
[in a new window]
  		Figure 1. c-myc-specific TFO sequences and binding affinities. A, c-myc P2 promoter target site and TFO sequences. The c-myc target duplex is depicted with the TFO binding site shown in bold. The c-myc-specific TFO sequences (CSPEC and Myc2T) are shown under their duplex target site in an antiparallel orientation. B, duplex DNA containing the c-myc P2 promoter site was end-labeled with [{gamma}-32P]ATP, incubated with varying concentrations of CSPEC as indicated, and run on a 12% Tris-borate-magnesium native gel. C, end-labeled Myc2T was incubated with varying concentrations of c-myc target duplex and subjected to electrophoresis on a 12% Tris-borate-magnesium native gel. Samples were visualized by autoradiography. D, cross-linking of psoralen-modified TFOs to the c-myc target DNA. To determine efficiency of psoralen photocross-linking to the c-myc target duplex, 10–10 to 10–4 mol/L psoralen–modified CSPEC (pCSPEC) and Myc2T (pMyc2T) were incubated with 10–10 mol/L [{gamma}-32P] end-labeled duplex DNA. Samples were irradiated with 1.8 J/cm2 of UVA light (365 nm) to covalently cross-link the psoralen-modified TFO to the c-myc target duplex. Samples were run on a 12% denaturing polyacrylamide gel and visualized by autoradiography.

 
The 3-kb target plasmid, pTA, was constructed by inserting a triplex target site into pGEM-T (Promega Corporation, Madison, WI) and a 5'-AT-3' psoralen cross-linking site at the triplex-duplex junction.

Electrophoretic mobility shift assays. Complementary oligonucleotides corresponding to the c-myc target site duplex were annealed in a 1:1 molar ratio at a final concentration of 10–6 mol/L in 25 mmol/L NaCl. Duplex DNA was 5'-end-labeled using [{gamma}-32P]ATP and T4 polynucleotide kinase and gel purified. To determine binding affinities of TFOs to the target duplex, 10–10 to 10–4 mol/L TFO was preheated to 65°C for 10 minutes, chilled on ice for 10 minutes, and then incubated with 10–10 mol/L duplex DNA for 16 hours in triplex binding buffer (10 mmol/L MgCl2, 10 mmol/L Tris, pH 7.6) at 37°C. Samples containing psoralen-modified TFOs were irradiated with 1.8 J/cm2 of UVA light to covalently cross-link the psoralen-modified TFO to the c-myc target duplex. Samples were run on either a 12% denaturing polyacrylamide gel (psoralen-modified TFOs) or a 12% Tris-borate-magnesium native gel (unmodified TFOs) to determine the amount of triplex substrate formed.

Cell culture and cell-free extract preparation. Human SK-BR-3 mammary epithelial adenocarcinoma cells were cultured in DMEM with 10% fetal bovine serum (FBS) at 37°C in a humidified atmosphere. It has been estimated that these cells contain ~10 copies of the c-myc gene via amplification (28).

Cell-free extracts were prepared using the NucBuster Protein Extraction Kit (Novagen, Madison, WI) according to the instructions of the manufacturer; cytoplasmic components were collected and recombined with the nuclear extracts to make whole-cell extracts. Cell pellets used for the preparation of cell-free extracts were obtained from the National Cell Culture Center (Minneapolis, MN).

Western blot analysis of c-Myc protein levels. SK-BR-3 cells were seeded at 1 x 106 per 10-cm dish 24 hours before treatment. SK-BR-3 cells were transfected separately with 10 µmol/L c-myc specific TFO or control oligonucleotide for 3 hours in serum-free media with Geneporter transfection reagent according to the recommendations of the manufacturer (Gene Therapy Systems, Inc., San Diego, CA). Following addition of media with 10% FBS, cells were incubated for various times as indicated in the figure legends. c-Myc protein levels were then quantified by Western blot analysis using anti-Myc primary antibody (Upstate Biotechnology, Charlottesville, VA) and a Kodak Image Station 440CF (Eastman Kodak, Rochester, NY). c-Myc protein levels were normalized to either glyceraldehyde-3-phosphate dehydrogenase (GAPDH) protein levels or to total protein loaded determined by Coomassie staining.

Gemcitabine incorporation assay. DNA synthesis assays to measure the incorporation of [3H]gemcitabine (Moravek Biochemicals, La Brea, CA) into TFO-damaged plasmid were done with a 3-kb plasmid (pTA) containing a TFO-binding site. TFO-damaged plasmid was created by incubating 10 µg of pTA with 10–5 mol/L psoralen-conjugated TFO for 16 hours, followed by irradiation with 1.8 J/cm2 of UVA light (365 nm). Undamaged and damaged pTA were incubated separately with (150 µg) HeLa cell-free extracts and [3H]gemcitabine in buffer for 3 hours, followed by EcoRI digestion, gel purification, and extraction (QIAquick gel extraction kit, Qiagen, Valencia, CA) as previously described (19). Total recovered plasmid was measured by UV absorption. Incorporation of [3H]gemcitabine was measured as counts per minute (cpm) using a scintillation counter. Results are reported as cpm per microgram of plasmid.

Cell proliferation assays. SK-BR-3 cells were seeded in 96-well plates at 5 x 103 per well. Twenty-four hours after plating, SK-BR-3 cells were transfected in triplicate with preheated TFO (0.1, 1, and 10 µmol/L) using Geneporter transfection reagent. Following a 3-hour incubation, medium alone or medium with gemcitabine (1, 10, and 100 nmol/L), formulated as GEMZAR (Eli Lilly and Company, Indianapolis, IN), was added to the appropriate wells. Cells treated with psoralen-modified TFOs were irradiated after 2 hours of incubation with gemcitabine. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays were done 72 hours post-treatment using Promega CellTitre 96 Nonradioactive Cell Proliferation Assay kit.

Anchorage-independent growth assays. SK-BR-3 cells were plated in 10-cm plates at 1 x 106 per plate. After 24 hours, SK-BR-3 cells were transfected with preheated 10 µmol/L specific TFO, 100 nmol/L gemcitabine, or 10 µmol/L TFO and 100 nmol/L gemcitabine in combination using FuGENE 6 transfection reagent (Roche Applied Science, Indianapolis, IN). Following a 4-hour incubation, 1 x 104 treated cells were plated in triplicate in six-well plates in 0.3% agar over a 0.8% bottom layer of agar and incubated at 37°C in a humidified atmosphere. Medium was removed after 72 hours and cells were refed weekly with medium in 0.3% agar. After 32 to 34 days, colonies >100 µm in diameter were counted using a dissecting microscope and ImagePro software.

Statistical analysis. Results from the cell survival (MTT) assays and the anchorage-independent growth assays were normalized to cells treated with Geneporter or FuGENE 6 only, and statistical analyses of the data were done using a two-tailed Student's t test.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
References
 
TFO-directed psoralen cross-linking of the human c-myc target gene. Two TFOs that bind the same target sequence in promoter 2 of c-myc were used in this study (Fig. 1A; refs. 26, 27). We determined the binding affinities of psoralen-modified and unmodified TFOs to their target duplex in the c-myc gene using electrophoretic mobility shift assays. The AG-rich unmodified TFO, CSPEC, showed a greater binding affinity (Kd ~10–8 mol/L) than its GT-rich counterpart, Myc2T (Kd ~10–6 mol/L; Fig. 1, compare B with C). Psoralen-modified TFOs were incubated with radiolabeled c-myc target duplex, irradiated with 365-nm UVA (1.8 J/cm2) to activate psoralen, and subjected to denaturing PAGE to determine the amount of substrate photomodified by the psoralen-conjugated TFOs. As shown in Fig. 1D, ~80% and ~50% (pCSPEC and pMyc2T, respectively) of the target duplex were covalently cross-linked by psoralen.

TFO-induced transcription inhibition of the c-myc gene in SK-BR-3 cells. CSPEC and Myc2T have previously been shown to inhibit transcription of the c-myc gene in cancer cells, including HeLa and CEM cells (26, 27). To verify that these TFOs are similarly functional in SK-BR-3 human breast cancer cells, which contain multiple copies of c-myc via amplification, c-Myc protein levels were measured at various times following treatment with 10 µmol/L TFO. Western blot analyses revealed that both TFOs caused a significant reduction in c-Myc protein levels by 24 hours, showing TFO-targeted transcription inhibition of the amplified c-myc gene in the SK-BR-3 cells (Fig. 2 ). The inhibitory effect of CSPEC was highest at 4 hours, reducing c-Myc levels by >40%. Myc2T treatment reduced c-Myc protein to ~71% of untreated levels by 24 hours (Fig. 2B). The increased time required for maximal expression inhibition by Myc2T may be a result of its lower binding affinity for the target duplex relative to CSPEC. The scrambled sequence control oligonucleotide showed minimal effects on gene expression as expected. These data provide evidence that the c-myc-directed TFOs specifically inhibit its expression.


Figure 2
View larger version (14K):
[in this window]
[in a new window]
  		Figure 2. TFO-induced expression inhibition and gemcitabine incorporation. A, inhibition of c-myc expression by TFOs. SK-BR-3 cells were transfected separately with 10 µmol/L CSPEC, Myc2T, or control oligonucleotide. Cells were incubated for either 4 or 24 hours. Protein levels of c-Myc were analyzed by Western blot analysis. B, c-Myc protein levels were quantified using a Kodak Image Station 440CF and normalized to either GAPDH protein levels or total protein loaded determined by Coomassie staining. c-Myc levels in untreated cells were labeled as 100%. C, increased incorporation of [3H]gemcitabine into TFO-damaged DNA in human cell-free extracts. Undamaged and psoralen-TFO cross-linked plasmids were incubated with HeLa cell-free extracts and [3H]gemcitabine in repair buffer followed by gel purification and extraction. Total recovered plasmid was measured by UV absorption; total [3H]gemcitabine incorporation was measured as cpm using a scintillation counter. Gemcitabine incorporation was determined by dividing cpm by total DNA. Columns, mean of three independent experiments. Bars, SD.

 
Enhanced incorporation of gemcitabine into DNA damaged by TFOs in human cell-free extracts. To verify whether TFO-induced DNA damage would increase the incorporation of gemcitabine into the damaged DNA, a nucleoside incorporation assay was done using [3H]gemcitabine. The results shown in Fig. 2C show that TFO-directed DNA damage does substantially increase (~4-fold) the incorporation of [3H]gemcitabine into the damaged DNA. These results provide evidence to support our hypothesis that TFO-directed induction of DNA repair synthesis specifically increases the incorporation of gemcitabine into the DNA.

Increased cytotoxicity in human breast cancer cell lines with combination TFO + gemcitabine treatment. MTT assays were done to determine the percentage of SK-BR-3 cells that remained viable following 72 hours of exposure to gemcitabine and TFOs independently or in combination. The median concentration of both CSPEC and Myc2T that reduced cell viability by 50% (IC50) was determined to be ~1 µmol/L. Treatment with gemcitabine alone (at concentrations 10-100 nmol/L) reduced cell survival to ~60%, with no further reduction at higher concentrations (data not shown). When SK-BR-3 cells were treated with a range of concentrations of TFO (0.1-10 µmol/L), gemcitabine (1-100 nmol/L), or both in combination for 72 hours, both CSPEC and Myc2T affected cell survival at a concentration of 0.1 µmol/L. When used in combination with gemcitabine, the effect of specific TFO treatment on cell survival was greater than that of TFO or gemcitabine alone, reducing survival to ~34% (Myc2T) and ~50% (CSPEC) at 10 µmol/L TFO/100 nmol/L gemcitabine (Fig. 3 , compare A with C). As expected, the control oligonucleotide showed no effect on cell survival (~109% survival at 10 µmol/L), and when used together with gemcitabine, the effect was similar to that produced by gemcitabine alone (~68% survival). Our results show that treatment with gemcitabine and TFOs in combination reduces cancer cell survival to a greater extent than either agent alone (Fig. 3).


Figure 3
View larger version (26K):
[in this window]
[in a new window]
  		Figure 3. Effect of combination treatment (gemcitabine + TFO) on cell proliferation. Percent survival of SK-BR-3 cells transfected in triplicate with gemcitabine (Gem) and/or CSPEC (A), pCSPEC (B), Myc2T (C), and pMyc2T (D) at the concentrations indicated using Geneporter transfection reagent. Cells treated with psoralen-modified TFOs were irradiated to cross-link the TFO to the c-myc target site. MTT assays were done following a 72-hour incubation. Geneporter-treated-only cells are set at 100%. Columns, representative results of three separate experiments performed in triplicate. Bars, SD.

 
To test the effects of covalently cross-linked TFOs and gemcitabine on cell survival, SK-BR-3 cells were treated with specific psoralen-modified TFOs (pCSPEC and pMyc2T), gemcitabine, or the combination, followed by UVA irradiation to form TFO-directed, site-specific psoralen DNA interstrand cross-links. Surprisingly, neither psoralen-modified TFO reduced cell survival significantly at 0.1 µmol/L concentrations (Fig. 3B and D). However, cell viability was substantially reduced by treatment with TFOs at a concentration of 1 µmol/L (~68% pCSPEC and ~58% pMyc2T). As with unmodified TFOs, pMyc2T consistently showed greater antiproliferative effects than pCSPEC at 10 µmol/L concentrations, reducing cell survival to ~33% on average compared with ~48% for pCSPEC. The control oligonucleotide showed no effect on cell survival at a concentration of 10 µmol/L (~108% survival). When used in combination with gemcitabine, the effect of the psoralen-modified TFO treatment on cell survival was greater than that produced by either agent alone. As with unmodified TFOs, the effects of the combination treatment were more pronounced with pMyc2T (19% survival) than with pCSPEC (40% survival) at 10 µmol/L TFO. Our results show that treatment with gemcitabine and psoralen-modified TFOs in combination reduces cancer cell survival greater than unmodified TFOs or gemcitabine alone. The additional decrease in cell survival using psoralen-modified TFOs, which covalently cross-link the TFO to the DNA, is consistent with our hypothesis that unscheduled DNA repair synthesis induced by DNA damage increases the incorporation of nucleoside analogues into the damaged oncogene.

Anchorage-independent cell growth. To assess the tumorigenic potential of the SK-BR-3 cells following treatment with either c-myc-targeted TFOs and/or gemcitabine, we measured the anchorage-independent growth capacity of the cells using soft agar assays. Because Myc2T showed greater antiproliferative effects in the cell proliferation assays (Fig. 3), this TFO was chosen for use in the soft agar assays. Treatment with the combination of TFOs + gemcitabine resulted in significantly increased inhibition of colony formation (19% of untreated cells) compared with treatment with either agent alone (78% and 58% of untreated cells for Myc2T and gemcitabine, respectively; Fig. 4 ). Statistical analyses confirmed that the reduction in cell proliferation induced by the combination therapy was significantly greater than either gemcitabine (P = 0.0072) or Myc2T (P = 0.00029) alone.


Figure 4
View larger version (43K):
[in this window]
[in a new window]
  		Figure 4. Effect of combination treatment (gemcitabine + TFO) on anchorage-independent cell growth. Colony formation of SK-BR-3 cells in soft agar following transfection with FuGENE 6 only (A), or FuGENE 6 with Myc2T (B), gemcitabine (C), or Myc2T plus gemcitabine (D). Colonies >100 µm in diameter were counted. E, mean percent colony formation compared with FuGENE 6-treated-only cells for three independent experiments (n = 8). Bars, SE.

 
Our results support a dual mode of antitumor action of TFOs in conjunction with gemcitabine. First, TFOs inhibit the expression of target genes, as shown here and previously in other cell lines (26, 27). Second, TFOs induce unscheduled DNA repair synthesis (19–21), perhaps triggering the incorporation of gemcitabine and increasing the potential effectiveness of gemcitabine in tumor cells regardless of proliferative status. This multifaceted chemotherapeutic approach using triplex-directed gene targeting may lead to more than incremental improvements in nonsurgical treatment of solid tumors.


   Acknowledgments
 
Grant support: Commonwealth Cancer Foundation for Research, and a National Cancer Institute grant CA93729 (K.M. Vasquez).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

We thank Dr. William Plunkett for useful discussion, and Sarah Henninger and Shawna Johnson for manuscript preparation.

Received 11/30/05. Revised 1/18/06. Accepted 2/ 1/06.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 References
 

  1. Crowther PJ, Cooper IA, Woodcock DM. Biology of cell killing by 1-ß-D-arabinofuranosylcytosine and its relevance to molecular mechanisms of cytotoxicity. Cancer Res 1985;45:4291–300.[Abstract/Free Full Text]
  2. Huang P, Plunkett W. Fludarabine- and gemcitabine-induced apoptosis: incorporation of analogs into DNA is a critical event. Cancer Chemother Pharmacol 1995;36:181–8.[Medline]
  3. Sandler A, Ettinger DS. Gemcitabine: single-agent and combination therapy in non-small cell lung cancer. Oncologist 1999;4:241–51.[Abstract/Free Full Text]
  4. Storniolo AM, Enas NH, Brown CA, Voi M, Rothenberg ML, Schilsky R. An investigational new drug treatment program for patients with gemcitabine: results for over 3000 patients with pancreatic carcinoma. Cancer 1999;85:1261–8.[CrossRef][Medline]
  5. Yang LY, Li L, Jiang H, Shen Y, Plunkett W. Expression of ERCC1 antisense RNA abrogates gemcitabine-mediated cytotoxic synergism with cisplatin in human colon tumor cells defective in mismatch repair but proficient in nucleotide excision repair. Clin Cancer Res 2000;6:773–81.[Abstract/Free Full Text]
  6. Robertson LE, Kantarjian H, O'Brien S, et al. Cisplatin, fludarabine, and ara-C(PFA): a regimen for advanced fluarabine-refractory chronic lymphocytic leukemia (CLL). Pro Am Soc Clin Oncol 1993;12:308.
  7. Robertson LE, O'Brien S, Kantarjian H, et al. Fludarabine plus doxorubicin in previously treated chronic lymphocytic leukemia. Leukemia 1995;9:943–5.[Medline]
  8. Keating MJ, O'Brien S, McLaughlin P, Kantarjian H, Cabanillas F. Fludarabine in combinations in the management of chronic lymphocytic leukemia and low grade lymphoma. Ann Oncol 1996;7:1996.
  9. McLaughlin P, Hagemeister FB, Romaguera JE, et al. Fludarabine, mitoxantrone, and dexamethasone: an effective new regimen for indolent lymphoma. J Clin Oncol 1996;14:1262–8.[Abstract/Free Full Text]
  10. Cooney M, Czernuszewicz G, Postel EH, Flint SJ, Hogan ME. Site-specific oligonucleotide binding represses transcription of the human c-myc gene in vitro. Science 1988;241:456–9.[Abstract/Free Full Text]
  11. Beal PA, Dervan PB. Second structural motif for recognition of DNA by oligonucleotide-directed triple-helix formation. Science 1991;251:1360–3.[Abstract/Free Full Text]
  12. Maher LJD, Wold B, Dervan PB. Inhibition of DNA binding proteins by oligonucleotide-directed triple helix formation. Science 1989;245:725–30.[Abstract/Free Full Text]
  13. Mayfield C, Squibb M, Miller D. Inhibition of nuclear protein binding to the human Ki-ras promoter by triplex-forming oligonucleotides. Biochemistry 1994;33:3358–63.[CrossRef][Medline]
  14. Samadashwily GM, Mirkin SM. Trapping DNA polymerases using triplex-forming oligodeoxyribonucleotides. Gene 1994;149:127–36.[CrossRef][Medline]
  15. Giovannangeli C, Perrouault L, Escude C, Gryaznov S, Helene C. Efficient inhibition of transcription elongation in vitro by oligonucleotide phosphoramidates targeted to proviral HIV DNA. J Mol Biol 1996;261:386–98.[CrossRef][Medline]
  16. Krasilnikov AS, Panyutin IG, Samadashwily GM, Cox R, Lazurkin YS, Mirkin SM. Mechanisms of triplex-caused polymerization arrest. Nucleic Acids Res 1997;25:1339–46.[Abstract/Free Full Text]
  17. Ebbinghaus SW, Fortinberry H, Gamper HBJ. Inhibition of transcription elongation in the HER-2/neu coding sequence by triplex-directed covalent modification of the template strand. Biochemistry 1999;38:619–28.[CrossRef][Medline]
  18. Vasquez KM, Wilson JH. Triplex-directed modification of genes and gene activity. Trends Biochem Sci 1998;23:4–9.[CrossRef][Medline]
  19. Wu Q, Christensen LA, Legerski RJ, Vasquez KM. Mismatch repair participates in error-free processing of DNA interstrand cross-links in human cells. EMBO J 2005;6:551–7.[CrossRef]
  20. Vasquez KM, Christensen J, Li L, Finch RA, Glazer PM. Human XPA and RPA DNA repair proteins participate in specific recognition of triplex-induced helical distortions. Proc Natl Acad Sci U S A 2002;99:5848–53.[Abstract/Free Full Text]
  21. Wang G, Seidman MM, Glazer PM. Mutagenesis in mammalian cells induced by triple helix formation and transcription-coupled repair. Science 1996;271:802–5.[Abstract]
  22. Liao DJ, Dickson RB. c-Myc in breast cancer. Endocr Relat Cancer 2000;7:143–64.[Abstract]
  23. Berns EM, Foekens JA, van Staveren IL, et al. Oncogene amplification and prognosis in breast cancer: relationship with systemic treatment. Gene 1995;159:11–8.[CrossRef][Medline]
  24. Berns EM, Klijn JG, Smid M, et al. TP53 and MYC gene alterations independently predict poor prognosis in breast cancer patients. Genes Chromosomes Cancer 1996;16:170–9.[CrossRef][Medline]
  25. Zendegui JG, Vasquez KM, Tinsley JH, Kessler DJ, Hogan ME. In vivo stability and kinetics of absorption and disposition of 3' phosphopropyl amine oligonucleotides. Nucleic Acids Res 1992;20:307–14.[Abstract/Free Full Text]
  26. Kim H-G, Reddoch JF, Mayfield C, et al. Inhibition of transcription of the human c-myc proto-oncogene by intermolecular triplex. Biochemistry 1998;37:2299–304.[CrossRef][Medline]
  27. Catapano CV, McGuffie EM, Pacheco D, Carbone GMR. Inhibition of gene expression and cell proliferation by triple helix-forming oligonucleotides directed to the c-myc gene. Biochemistry 2000;39:5126–38.[CrossRef][Medline]
  28. Kozbor D, Croce CM. Amplification of the c-myc oncogene in one of five human breast carcinoma cell lines. Cancer Res 1984;44:438–41.[Abstract/Free Full Text]



This article has been cited by other articles:


Home page
 Nucleic Acids ResHome page
J. Zhao, A. Jain, R. R. Iyer, P. L. Modrich, and K. M. Vasquez
Mismatch repair and nucleotide excision repair proteins cooperate in the recognition of DNA interstrand crosslinks
Nucleic Acids Res., July 1, 2009; 37(13): 4420 - 4429.
[Abstract] [Full Text] [PDF]

Home page
 Nucleic Acids ResHome page
M. Duca, P. Vekhoff, K. Oussedik, L. Halby, and P. B. Arimondo
The triple helix: 50 years later, the outcome
Nucleic Acids Res., September 1, 2008; 36(16): 5123 - 5138.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Christensen, L. A.
Right arrow Articles by Vasquez, K. M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Christensen, L. A.
Right arrow Articles by Vasquez, K. M.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Cancer Research Clinical Cancer Research
Cancer Epidemiology Biomarkers & Prevention Molecular Cancer Therapeutics
Molecular Cancer Research Cancer Prevention Research
Cancer Prevention Journals Portal Cancer Reviews Online
Annual Meeting Education Book Meeting Abstracts Online