Targeting DNA repair proteins: a promising avenue for cancer gene therapy.

Enhanced DNA repair in many cancer cells can be correlated to the resistance to cancer treatment, and thus contributes to a poor prognosis. Ionizing radiation and many anti-cancer drugs induce DNA double-strand breaks (DSBs), which are usually regarded as the most toxic types of DNA damages. Repair of DNA DSBs is vital for maintaining genomic stability and hence crucial for survival and propagation of all cellular organisms. Therefore, reducing the capacity of cancer cells to repair DSBs could sensitize tumors to radio/chemotherapy. Many investigators have used gene therapy strategies to down-regulate or inactivate proteins involved in the repair of DSBs in order to reduce the survival of cancer cells. Herein, are reviewed several protein candidates that have been targeted by different gene therapy approaches. Results obtained from in vitro and in vivo experiments are presented and discussed in the perspective of potential gene therapy clinical trials.

Avascular necrosis of the femoral head: vascular hypotheses.

Vascular hypotheses provide compelling pathogenic mechanisms for the etiology of avascular necrosis of the femoral head (ANFH). A decrease in local blood flow of the femoral head has been postulated to be the cause of the disease. Several studies in human and animal models of ANFH have shown microvascular thrombosis. Endothelial cell damage could be followed by abnormal blood coagulation and thrombus formation with any resulting degeneration distal to the site of vascular occlusion. Other studies suggest that thrombophilia, particularly impaired fibrinolysis, plays a potential role in thrombus formation in ANFH. Reduction in shear stress due to decreased blood flow could lead to apoptosis of endothelial cells, which can ultimately contribute to plaque erosion and thrombus formation. Dysregulation of endothelial cell activating factors and stimulators of angiogenesis or repair processes could also affect the progression and outcome of ANFH. Likewise, regional endothelium dysfunction (RED), referred to as a potential defect in endothelial cells located in the feeding vessels of the femoral head itself, may also have a crucial role in the pathogenesis of ANFH. Molecular gene analysis of regional endothelial cells could also help to determine potential pathways important in the pathogenesis of ANFH.

An antisense oligonucleotide targeted to human Ku86 messenger RNA sensitizes M059K malignant glioma cells to ionizing radiation, bleomycin, and etoposide but not DNA cross-linking agents.

Ku is a heterodimer of M(r) 70,000 and M(r) 86,000 subunits. It binds with strong affinity to DNA ends and is indispensable for nonhomologous DNA end joining (NHEJ) and V(D)J recombination. In this study, we investigated whether down-regulation of the Ku86 gene, by 2'-O-methoxyethyl/uniform phosphorothioate chimeric antisense oligonucleotides (ASOs), increases the sensitivity of the DNA-protein kinase catalytic subunit (PKcs)-proficient human glioma cell line (M059K), and its isogenic DNA-PKcs-deficient counterpart (M059J), to ionizing radiation and anticancer drugs. Transfection of these cell lines with 200 nM Ku86 antisense ASOs was associated with a specific decrease in Ku86 mRNA levels (IC(50) <25 nM; n = 3) and a concomitant rapid decrease (<10% of control) in Ku86 protein expression. Moreover, transfection of M059K cells with Ku86 antisense ASOs markedly increased cell death after treatment with ionizing radiation, bleomycin, and etoposide. However, no sensitization to the DNA cross-linking agents chlorambucil and cisplatin was observed after Ku86 antisense transfection. As expected, transfection of M059J cells with Ku86 antisense ASOs did not result in any sensitization to ionizing radiation, bleomycin, or DNA cross-linking agents, but there was a 2-fold increase in sensitivity to etoposide. Thus, our results indicate that antisense ASOs targeted against Ku86 may increase the efficacy of radiotherapy and DNA-damaging agents in tumor treatment. Furthermore, Ku86 antisense ASOs may be used to create a temporal knockout in different human cell lines to further investigate the biological roles of Ku86.

The DNA double-stranded break repair protein endo-exonuclease as a therapeutic target for cancer.

DNA repair mechanisms are crucial for the maintenance of genomic stability and are emerging as potential therapeutic targets for cancer. In this study, we report that the endo-exonuclease, a protein involved in the recombination repair process of the DNA double-stranded break pathway, is overexpressed in a variety of cancer cells and could represent an effective target for developing anticancer drugs. We identify a dicationic diarylfuran, pentamidine, which has been used clinically to treat opportunistic infections and is an inhibitor of the endo-exonuclease as determined by enzyme kinetic assay. In clonogenic and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays as well as in the in vivo Lewis lung carcinoma mouse tumor model, pentamidine is shown to possess the ability to selectively kill cancer cells. The LD50 of pentamidine on cancer cells maintained in vitro is correlated with the endo-exonuclease enzyme activity. Tumor cell that has been treated with pentamidine is reduced in the endo-exonuclease as compared with the untreated control. Furthermore, pentamidine synergistically potentiates the cytotoxic effect of DNA strand break and cross-link-inducing agents such as mitomycin C, etoposide, and cisplatin. In addition, we used the small interfering RNA for the mouse homologue of the endo-exonuclease to down-regulate the level of endo-exonuclease in the mouse myeloma cell line B16F10. Down-regulation of the endo-exonuclease sensitizes the cell to 5-fluorouracil. These studies suggested the endo-exonuclease enzyme as a novel potential therapeutic target for cancer.

Erythropoietin induces cancer cell resistance to ionizing radiation and to cisplatin.

Recent studies suggest that erythropoietin plays an important role in the process of neoplastic transformation and malignant phenotype progression observed in malignancy. To study the role of erythropoietin and its receptor (EPOR) on the response of cancer cells in vitro, we used two solid tumor cell lines, namely the human malignant glioma cell line U87 and the primary cervical cancer cell line HT100. All experiments were done with heat-inactivated fetal bovine serum in order to inactivate any endogenous bovine erythropoietin. The expression of the EPOR in these cells was confirmed with immunoblot techniques. The addition of exogenous recombinant human erythropoietin (rhEPO) induces the cancer cells to become more resistant to ionizing radiation and to cisplatin. Furthermore, this rhEPO-induced resistance to ionizing radiation and to cisplatin was reversed by the addition of tyrphostin (AG490), an inhibitor of JAK2. Our findings indicate that rhEPO result in a significant, JAK2-dependent, in vitro resistance to ionizing radiation and to cisplatin in the human cancer cells lines studied in this report.

Synergistic effect of TRM2/RNC1 and EXO1 in DNA double-strand break repair in Saccharomyces cerevisiae.

In our recently published study, we provided in vitro as well as in vivo data demonstrating the involvement of TRM2/RNC1 in homologous recombination based repair (HRR) of DNA double strand breaks (DSBs), in support of such claims reported earlier. To further validate its role in DNA DSB processing, our present study revealed that the trm2 single mutant displays higher sensitivity to persistent induction of specific DSBs at the MAT locus by HO-endonuclease with higher sterility rate among the survivors compared to wild type (wt) or exo1 single mutants. Intriguingly, both sensitivity and sterility rate increased dramatically in trm2exo1 double mutants lacking both endo-exonucleases with a progressively increased sterility rate in trm2exo1 double mutants with short-induction periods, reaching a very high level of sterility with persistent DSB inductions. Mutation analysis of the mating type (MAT) locus among the sterile survivors with persistent HO-induction in trm2 and exo1 single mutants as well as in trm2exo1 double mutants revealed a similar small insertions and deletions events, characteristic of non-homologous end joining (NHEJ) that might have occurred due to the lack of proper processing function in these mutants. In addition, trm2ku80 and trm2rad52 double mutants also displayed significantly higher sterility with persistent DSB induction compared to ku80 and rad52 single mutants, respectively, exhibiting a mutation spectra that shifted from base substitution (in ku80 and rad52 single mutants) to small insertions and deletions in the double mutants (in trm2ku80 and trm2rad52 mutants). These data indicate a defective processing in absence of TRM2, with a synergistic effect of TRM2, and EXO1 in such processing.

Silencing of endo-exonuclease expression sensitizes mouse B16F10 melanoma cells to DNA damaging agents.

We previously identified an endo-exonuclease that is highly expressed in cancer cells and plays an important role in DSB repair mechanisms. A small molecular compound pentamidine, which specifically inhibited nuclease activity of the isolated endo-exonuclease from yeast as well as from mammalian cells, was capable of sensitizing tumor cells to DNA damaging agents. In this study, we investigated the effect of precisely silencing the endo-exonuclease expression by small interfering RNA (siRNA) upon treatment with a variety of DNA damaging agents in mouse B16F10 melanoma cells. A maximum of 3.6 to approximately 4-fold reduction in endo-exonuclease mRNA expression was achieved, over a period of 48-72 h of post transfection with a concomitant reduction in protein expression (approximately 4-5 fold), resulting in a substantial reduction (approximately 45-50%) of the corresponding nuclease activity. Suppressed endo-exonuclease expression conferred significant decrease in cell survival, ranging from approximately 30 to approximately 50% cell killing, in presence of DNA damaging drugs methyl methane sulfonate (MMS), cisplatin, 5-fluoro uracil (5-FU) and gamma-irradiation but not at varying dosages of ultra violet (UV) radiation. The data strongly support a role for the endo-exonuclease in repairing DNA damages, induced by MMS, cisplatin, 5-FU and gamma irradiation but not by UV radiation. The results presented in this study suggest that the endo-exonuclease siRNA could be useful as a therapeutic tool in targeting the endo-exonuclease in cancer therapy.

Functional and genetic analysis of the Saccharomyces cerevisiae RNC1/TRM2: evidences for its involvement in DNA double-strand break repair.

We previously isolated the RNC1/TRM2 gene and provided evidence that it encodes a protein with a possible role in DNA double strand break repair. RNC1 was independently re-isolated as the TRM2 gene encoding a methyl transferase involved in tRNA maturation. Here we show that Trm2p purified as a fusion protein displayed 5' --> 3' exonuclease activity on double-strand (ds) DNA, and endonuclease activity on single-strand (ss) DNA, properties characteristic of previously isolated endo-exonucleases. A variant of Trm2p, Trm2p(ctDelta76aa) lacking 76 amino acids at the C-terminus retained nuclease activities but not the methyl transferase activity. Both the native and the variant exhibited sensitivity to the endo-exonuclease inhibitor pentamidine. The Saccharomyces cerevisiae trm2(Delta232-1920nt) mutant (containing only the first 231 nucleotides of the TRM2 gene) displayed low sensitivity to methyl methane sulfonate (MMS) and suppressed the MMS sensitivity of rad52 mutants in trm2(Delta232-1920nt)rad52 double mutants. The deletion of KU80, in trm2(Delta232-1920nt) mutant background displayed higher MMS sensitivity supporting the view of the possible role of Trm2p in a competing repair pathway separate from NHEJ. In addition, trm2 exo1 double mutants were synergistically more sensitive to MMS and ionizing radiation than either of the single mutant suggesting that TRM2 and EXO1 can functionally complement each other. However, the C-terminal portion, required for its methyl transferase activity was found not important for DNA repair. These results propose an important role for TRM2 in DNA repair with a potential involvement of its nuclease function in homologous recombination based repair of DNA DSBs.

DNA repair protein: endo-exonuclease as a new frontier in cancer therapy.

DNA repair mechanisms are essential for cellular survival in mammals. A rapid repair of DNA breaks ensures faster growth of normal cells as well as cancer cells, making DNA repair machinery, a potential therapeutic target. Although efficiency of these repair processes substantially decrease the efficacy of cancer chemotherapies that target DNA, compromised DNA repair contributes to mutagenesis and genomic instability leading to carcinogenesis. Thus, an ideal target in DNA repair mechanisms would be one that specifically kills the rapidly dividing cancer cells without further mutagenesis and does not affect normal cells. Endo-exonucleases play a pivotal role in nucleolytic processing of DNA ends in different DNA repair mechanisms especially in homologous recombination repair (HRR) which mainly repairs damaged DNA in S and G2 phases of the cell cycle in rapidly dividing cells. HRR machinery has also been implicated in cell signaling and regulatory functions in response to DNA damage that is essential for cell viability in mammalian cells where as the predominant nonhomologous end-joining pathway is constitutive. Although HRR is thought to be involved at other stages of the cell cycle, it is predominant in growing phases (S and G2) of the cell cycle. The faster growing cells are believed to carryout more HRR in replicative stages of the cell cycle where homologous DNA is available for HRR. Targeting endo-exonucleases specifically involved in HRR will make the normal cells less prone to mutagenesis, rendering the fast growing tumor cells more susceptible to DNA-damaging agents, used in cancer chemotherapy.

1,5-isoquinolinediol increases the frequency of gene targeting by homologous recombination in mouse fibroblasts.

Gene targeting is a technique that allows the introduction of predefined alterations into chromosomal DNA. It involves a homologous recombination reaction between the targeted genomic sequence and an exogenous targeting vector. In theory, gene targeting constitutes the ideal method of gene therapy for single gene disorders. In practice, gene targeting remains extremely inefficient for at least two reasons: very low frequency of homologous recombination in mammalian cells and high proficiency of the mammalian cells to randomly integrate the targeting vector by illegitimate recombination. One known method to improve the efficiency of gene targeting is inhibition of poly(ADP-ribose)polymerase (PARP). It has been shown that PARP inhibitors, such as 3-methoxybenzamide, could lower illegitimate recombination, thus increasing the ratio of gene targeting to random integration. However, the above inhibitors were reported to decrease the absolute frequency of gene targeting. Here we show that treatment of mouse Ltk cells with 1,5-isoquinolinediol, a recent generation PARP inhibitor, leads to an increase up to 8-fold in the absolute frequency of gene targeting in the correction of the mutation at the stable integrated HSV tk gene.