DNA repair: counteragent in mutagenesis and carcinogenesis-- accomplice in cancer therapy resistance

Investigating the impact of long term exposure to chemical agents on the chromosomal radiosensitivity using human lymphoblastoid GM1899A cells

This study aimed to investigate the impact of chronic low-level exposure to chemical carcinogens with different modes of action on the cellular response to ionising radiation. Human lymphoblastoid GM1899A cells were cultured in the presence of 4-nitroquinoline N-oxide (4NQO), N-nitroso-N-methylurea (MNU) and hydrogen peroxide (H2O2) for up to 6 months at the highest non-(geno)toxic concentration identified in pilot experiments. Acute challenge doses of 1 Gy X-rays were given and chromosome damage (dicentrics, acentric fragments, micronuclei, chromatid gaps/breaks) was scored. Chronic exposure to 20 ng/ml 4NQO, 0.25 μg/ml MNU or 10 μM H2O2 hardly induced dicentrics and did not significantly alter the yield of X-ray-induced dicentrics. Significant levels of acentric fragments were induced by all chemicals, which did not change during long-term exposure. Fragment data in combined treatment samples compared to single treatments were consistent with an additive effect of chemical and radiation exposure. Low level exposure to 4NQO induced micronuclei, the yields of which did not change throughout the 6 month exposure period. As for fragments, micronuclei yields for combined treatments were consistent with an additive effect of chemical and radiation. These results suggest that cellular radiation responses are not affected by long-term low-level chemical exposure.

Letrozole improves the sensitivity of breast cancer cells overexpressing aromatase to cisplatin via down-regulation of FEN1

PURPOSE

Flap endonuclease 1 (FEN1) is up-regulated by estrogen (17β-estradiol, E2) and related to cisplatin resistance of human breast cancer cells. Letrozole, an aromatase inhibitor, suppresses the change of testosterone into estrogen and is frequently used to treat breast cancer. However, the effects of letrozole on FEN1 expression and cisplatin sensitivity in breast cancer cells overexpressing aromatase have not been revealed.

METHODS

The expression of FEN1 and the proteins in ERK/Elk-1 signaling were evaluated by RT-PCR and Western blot. Cisplatin sensitivity was explored through CCK-8 and flow cytometry analysis, respectively. FEN1 siRNAs and FEN1 expression plasmid were transfected into cells to down-regulate or up-regulate FEN1 expression. The promotor activity of FEN1 was detected using luciferase reporter assay.

RESULTS

FEN1 down-regulation improved cisplatin sensitivity of breast cancer cells overexpressing aromatase. Letrozole down-regulated FEN1 expression and increased cisplatin sensitivity. The sensitizing effect of letrozole to cisplatin was dependent on FEN1 down-regulation. FEN1 overexpression could block the sensitizing effect of letrozole to cisplatin. Testosterone up-regulated the promotor activity, protein expression of FEN1, and phosphorylation of ERK/Elk-1, which could be eliminated by both letrozole and MEK1/2 inhibitor U0126. Letrozole down-regulated FEN1 expression in an ERK/Elk-1-dependent manner.

CONCLUSIONS

Our findings clearly demonstrate that letrozole improves cisplatin sensitivity of breast cancer cells overexpressing aromatase via down-regulation of FEN1 and suggest that a combined use of letrozole and cisplatin may be a potential treatment protocol for relieving cisplatin resistance in human breast cancer.

Differential expression profile analysis of DNA damage repair genes in CD133+/CD133- colorectal cancer cells

The present study examined differential expression levels of DNA damage repair genes in COLO 205 colorectal cancer cells, with the aim of identifying novel biomarkers for the molecular diagnosis and treatment of colorectal cancer. COLO 205-derived cell spheres were cultured in serum-free medium supplemented with cell factors, and CD133⁺/CD133^- cells were subsequently sorted using an indirect CD133 microbead kit. In vitro differentiation and tumorigenicity assays in BABA/c nude mice were performed to determine whether the CD133⁺ cells also possessed stem cell characteristics, in addition to the COLO 205 and CD133^- cells. RNA sequencing was employed for the analysis of differential gene expression levels at the mRNA level, which was determined using reverse transcription-quantitative polymerase chain reaction. The mRNA expression levels of 43 genes varied in all three types of colon cancer cells (false discovery rate ≤0.05; fold change ≥2). Of these 43 genes, 30 were differentially expressed (8 upregulated and 22 downregulated) in the COLO 205 cells, as compared with the CD133^- cells, and 6 genes (all downregulated) were differentially expressed in the COLO 205 cells, as compared with CD133⁺ cells. A total of 18 genes (10 upregulated and 8 downregulated) were differentially expressed in the CD133^- cells, as compared with the CD133⁺ cells. By contrast, 6 genes were downregulated and none were upregulated in the CD133⁺ cells compared with the COLO 205 cells. These findings suggest that CD133⁺ cells may possess the same DNA repair capacity as COLO 205 cells. Heterogeneity in the expression profile of DNA damage repair genes was observed in COLO 205 cells, and COLO 205-derived CD133^- cells and CD133⁺ cells may therefore provide a reference for molecular diagnosis, therapeutic target selection and determination of the treatment and prognosis for colorectal cancer.

Predicting efficacies of anticancer drugs using single cell HaloChip assay

Single cell HaloChip assay can be used to assess DNA repair ability.

Molecular characterization of cancer reveals interactions between ionizing radiation and chemicals on rat mammary carcinogenesis

Although various mechanisms have been inferred for combinatorial actions of multiple carcinogens, these mechanisms have not been well demonstrated in experimental carcinogenesis models. We evaluated mammary carcinogenesis initiated by combined exposure to various doses of radiation and chemical carcinogens. Female rats at 7 weeks of age were γ-irradiated (0.2-2 Gy) and/or exposed to 1-methyl-1-nitrosourea (MNU) (20 or 40 mg/kg, single intraperitoneal injection) or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) (40 mg/kg/day by gavage for 10 days) and were observed until 50 weeks of age. The incidence of mammary carcinoma increased steadily as a function of radiation dose in the absence of chemicals; mathematical analysis supported an additive increase when radiation was combined with a chemical carcinogen, irrespective of the chemical species and its dose. Hras mutations were characteristic of carcinomas that developed after chemical carcinogen treatments and were overrepresented in carcinomas induced by the combination of radiation and MNU (but not PhIP), indicating an interaction of radiation and MNU at the level of initiation. The expression profiles of seven classifier genes, previously shown to distinguish two classes of rat mammary carcinomas, categorized almost all examined carcinomas that developed after individual or combined treatments with radiation (1 Gy) and chemicals as belonging to a single class; more comprehensive screening using microarrays and a separate test sample set failed to identify differences in gene expression profiles among these carcinomas. These results suggest that a complex, multilevel interaction underlies the combinatorial action of radiation and chemical carcinogens in the experimental model.

Fast repair of DNA radicals

This tutorial review highlights the mechanism of a novel non-enzymatic fast repair of DNA damage, which refers exclusively to repair DNA radicals including DNA-OH* adducts, DNA radical cations and anions by various endogenous, natural and synthetic compounds. The repair rate constants are as high as 10(9) M(-1) s(-1). In cells, when the enzymatic repair system was inhibited or before the enzymatic repair mechanism was initiated, DNA oxidative damage was significantly reduced by natural polyphenols. This decrease of DNA damage is assigned to the fast repair. Fast repair takes place through an electron transfer process, and docking of polyphenol into the DNA minor groove could be the essential step.

Polymorphisms of xenobiotic metabolizing enzymes and DNA repair genes and outcome in childhood acute lymphoblastic leukemia

The interindividual variation in the activity of xenobiotic metabolizing enzymes and DNA repair genes could modify an individual's risk of recurrent malignancy and response to therapy. We investigated whether ALL outcome was related to polymorphisms in genes CYP2D6, MPO, EPHX1, NQO1, TS, XPD and XRCC1 in 95 consecutive ALL children by PCR or PCR-FRLP techniques. Polymorphisms in genes NQO1 and TS were associated with a significantly slow response to induction chemotherapy and NQO1 was also associated with a lower five-year event-free survival. This study suggests that polymorphisms of NQO1 and TS could be important for patient response to induction therapy and for treatment outcome.

Pharmacogenomics of acute leukemia

Pharmacogenomics provides knowledge regarding how genetic polymorphisms affect treatment responses. Such an approach is particularly needed in cancer therapy, as most chemotherapeutics drugs affect both tumor and normal cells, are ineffective in many patients and exhibit serious side effects. Leukemia exists in two different forms, myeloid and lymphoid. Acute lymphoblastic leukemia more frequently occurs in children, whereas the risk of acute myeloid leukemia is more common in adults. Despite significant progress in the treatment of these diseases, therapy is still unsuccessful in many patients. Prognosis is particularly poor in adult acute myeloid leukemia. Treatment failure in childhood acute lymphoblastic leukemia due to drug resistance remains the leading cause of cancer-related death in children. Here, we provide an overview of pharmacogenetics studies carried out in children and adults with acute lymphoblastic leukemia and acute myeloid leukemia, attempting to find the associations between treatment responses and polymorphisms in the genes whose products are needed for metabolism, and effects of drugs used in the treatment of leukemia.

Induction and prevention of carcinogenesis in rat skin exposed to space radiation

Quantitative cancer incidence data exist for various laboratory animal models, but little of this information is usable for estimating human risks, primarily because of uncertainties about possible mechanistic differences among species. Acceptance and utilization of animal data for human risk assessment will require a much better understanding of the comparative underlying mechanisms than now exists. A dual-lesion, radiation-track model in rat skin has proven to be consistent with tumor induction data with respect to acute radiation doses ranging from 0.5 up to 10 Gy and higher, and average LETs ranging from 0.34 to 150 keV microm(-1) according to the form neoplastic risk (D,L) = CLD + BD2. A recent result with the 56Fe ion beam showed dose-response consistency for malignant (carcinomas) and benign (fibromas) tumor induction with earlier results utilizing argon and neon ion beams. A discrepancy between the model and experiment was found indicating that proportionality of cancer yield with LET did not occur at 150 versus 125 keV microm(-1), i.e. tumor yield did not increase in spite of a 20% increase of LET, which suggests that a LET response maximum exists at or within this dose range. Concordance between the model and tumor induction data in rat skin implies that potential intervening complexities of carcinogenic progression fail to obscure the basic radiobiological assumptions underpinning the model. Gene expression microarray analysis shows that vitamin A inhibits the expression of about 80% of the inflammation-related genes induced by the radiation and prevents about 46% of the neoplasms associated with 56Fe ion radiation without appearing to interfere with the underlying dose and LET response patterns. Further validation is needed, but the model has the potential to provide quantitative estimates of cancer risk as a function of dose and LET for almost any type of radiation exposure and even for combinations of different radiations provided only three empirical parameters can be established for each type of radiation and organ system.

QSAR of genotoxic active benzazoles

Previously synthesized 2,5-disubstituted benzoxazole and benzimidazole derivatives, were tested for their genotoxic activity in the Bacillus subtilis rec- assay. The results revealed that 5-methyl-2-(p-aminobenzyl)benzoxazole exhibited the highest genotoxic response, which was comparable to 4-nitroquinoline 1-oxide (4-NQO), the reference agent of classical positive mutagen. Among the other tested compounds, four showed a genotoxic activity. A QSAR study revealed that structural parameters IY(C(2)H(4)) and IY(CH(2)O), indicating the bridge elements between the phenyl moiety and the fused ring system at position 2 and the quantum chemical parameter (DeltaE ), showing the difference between HOMO and LUMO energies, were found significant for enhancing the genotoxic activity in these compounds. In addition, the substituent effects on positions R and R(1) were found important for the activity as well as holding a substituent possessing a maximum length with a minimum width property on position R(1) like alkyl groups. On the other hand, substituting position R with an electron donating group instead of electron withdrawing group increased the genotoxic activity.

Genomic polymorphisms provide prognostic information in intermediate-risk acute myeloblastic leukemia

Current prognostic factors for acute myeloblastic leukemia (AML) are not sufficient to accurately predict the group of patients in the intermediate-risk category who will successfully respond to treatment. Distinct patterns of inherited functional genomic polymorphisms might explain part of these heterogeneous prognoses. We used the allelic discrimination method to identify polymorphisms in GSTT1, SULT1C2, CDA, SXR (drug metabolic pathways), XPD, XPA, XPG, ERCC1, TOP2A (DNA repair), VEGF (angiogenesis), and MDR1 (multidrug resistance) genes in 110 adult patients with intermediate-risk AML, enrolled in the CETLAM-99 prospective trial. A multivariate prognostic model adjusted for age, white blood cell (WBC) count, French-American-British group, cytogenetics, MLL rearrangement, internal tandem duplication of FLT3 (FLT3-ITD), induction courses to achieve complete remission, and germline polymorphisms, was used to detect independent risk factors associated with clinical outcome. This analysis showed an increased risk of refractoriness to chemotherapy in the group of patients with XPA variant alleles (RR = 14; P = .02). In the same model, increased relapse risk was associated with SULT1C2 heterozygosity (RR = 4.1; P = .004), FLT3-ITD (RR 3.3; P = .003), and MDR1 variant alleles (RR = 2.4; P = .02). Adverse prognostic variables for overall survival were XPA (RR = 3.4; P = .02) and MDR1 (RR = 2.1; P = .02) variant alleles, and WBC count (RR = 2.1; P = .02). These findings might be useful in selecting risk-adapted treatment strategies in intermediate-risk AML.

Residues of genotoxic alkyl mesylates in mesylate salt drug substances: Real or imaginary problems?

Mesylate esters of short-chain (n = 1-3) alcohols are reactive, direct-acting, genotoxic and possibly carcinogenic alkylating agents. Their chemical and biological properties appear to correlate well with Swain-Scott s constants; for example, high S(N)1 character (low s value) is associated with enhanced carcinogenic potential, but also a rapid hydrolysis rate. Concerns over the possible formation of such esters during the preparation of mesylate salt drug substances, by addition of methane sulfonic acid (MSA) to the free base dissolved in an alcoholic solvent, have led regulatory agencies to require applicants to demonstrate that the synthetic method employed does not lead to the presence of detectable levels of alkyl mesylates. Mechanistic considerations, relating mainly to the extremely low nucleophilicity of the mesylate anion, and experimental data, both indicate that alkyl mesylates should not be formed (except from MSA impurities) during mesylate salt synthesis. Mechanistic arguments also predict that residues of alkyl halides (possibly formed in the preparation of amine hydrochlorides or hydrobromides) could represent a similar or greater potential hazard than alkyl mesylates. The perceived risk of alkyl mesylate formation seems to rely on mistaken assumptions and so the concerns appear unjustified. Further reassurance could be achieved however by applying a variety of strategies during synthesis, including pH control, and use of high-purity MSA or of a non-hydroxylic reaction solvent.

Mammary tumorigenesis in ApcMin/+ mice is enhanced by X irradiation with a characteristic age dependence

The Apc(Min/+) (Min) mouse is genetically predisposed to both intestinal and mammary tumorigenesis. We investigated age-related changes in the susceptibility of mice (before, during and after puberty) to radiation-induced mammary tumorigenesis using this model. Female Min and wild-type mice having the C57BL/6J background were irradiated with 2 Gy of X rays at 2, 5, 7 and 10 weeks and killed humanely at 18 weeks of age. Min mice irradiated at 7-10 weeks of age (after puberty) developed mammary tumors with squamous metaplasia, whereas their wild-type littermates did not. Interestingly, irradiation of Min mice at 2-5 weeks (before and during puberty, respectively) did not induce mammary tumors but rather cystic nodules with metaplasia. The mammary tumors exhibited increased nuclear beta-catenin protein and loss of the wild-type Apc allele. Our results show that susceptibility to radiation-induced mammary tumorigenesis increases after puberty in Min mice, suggesting that the tumorigenic effect of ionizing radiation targets the lobular-alveolar progenitor cells, which increase in number with age and are controlled by beta-catenin signaling.

Polymorphisms in DNA repair gene XRCC1 and increased genetic susceptibility to breast cancer

X-ray repair cross-complementing 1 (XRCC1) gene encodes for a scaffolding protein, which plays an important role in base excision DNA repair by bringing together DNA polymerase beta, DNA ligase III and poly(ADP-Ribose) polymerase (PARP) at the site of DNA damage. Three polymorphisms of the XRCC1 gene at codons 194, 280 and 399 leading to amino acid changes at evolutionary conserved regions are found to alter the efficiency of the resulting protein and may therefore constitute potential breast cancer risk. In the present study we sought to determine whether these genetic variants of the XRCC1 gene was associated with any increased risk of breast cancer among the South Indian women in a hospital based case control study using PCR-RFLP and DNA sequencing techniques. Our data showed a positive association between the polymorphisms of codons 194 (OR = 1.98, 95% CI = 1.13-3.48 for Trp allele) and 399 (OR = 2.14, 95% CI = 1.29-3.58 for Gln allele) and breast cancer risk. However, XRCC1 codon 280 genotype analysis showed no evidence for an association with increased risk of breast cancer. A combined analysis of the effect of XRCC1 codon 194 and 399 revealed the highest risk (OR = 3.64, 95% CI = 1.57-8.46) for carriers of the polymorphic alleles in both these codons. In conclusion, the present study suggested involvement of XRCC1 codon 194 and 399 polymorphisms in the genetic predisposition to breast cancer among South Indian women. Our preliminary results based on the analysis of functionally relevant polymorphisms in XRCC1 low penetrance gene may provide a better model that would exhibit additive effects on individual susceptibility to breast cancer.

Relationship between DNA repair capacity and resistance to genotoxins in four human cell lines

We have developed fast, reliable and simple fluorescent method to assess and compare repair capacity of cells. To this end plasmid pEGFP containing the gene for the enhanced green fluorescent protein was damaged in vitro by genotoxic agents and introduced into cells by transfection. The repair capacity of the cells was determined from the number of fluorescent cells counted with a fluorescent microscope 24 h after transfection. The ability of four human tumor cell lines--HEK293, HeLa, Namalwa and K562 to repair DNA lesions inflicted by cis-diamminedichloroplatinum(II), UV light, 8-methoxypsoralen and 4',5'-8-trimethylpsoralen were determined and compared to the survival rates of the cells after treatment with the same genotoxic agents. In most but not all cases, there was a good correlation between repair capacity and cell survival. This finding indicates that the DNA repair capacity could be used as a biomarker in risk assessment and/or drug resistance assays.

Nucleotide excision repair rates in rat tissues

We have determined and compared nucleotide excision repair capability of several rat tissues by a method based on restoration of the transformation activity of UV-irradiated pBlueScript by incubation in repair-competent protein extracts. After 3 h of incubation, plasmid DNA was isolated and used to transform competent Escherichia coli cells. Damaged plasmids showed low transformation efficiency prior to incubation in repair-competent extracts. After incubation the transformation efficiency was restored to different extents permitting calculation of the repair capacity of the extracts. Our results showed that rapidly proliferating tissues such as liver, kidney and testis showed higher nucleotide excision repair capacity than slowly proliferating tissues such as heart, muscle, lung and spleen. When liver and splenocytes were stimulated to proliferation by partial hepatectomy and mitogen stimulation, their repair capability increased in parallel with the respective proliferative rates.

Correlation between chromosome damage and apoptosis induced by fludarabine and idarubicin in normal human lymphocytes

Fludarabine (FLU, a fluorinated purine analog) and idarubicin (IDA, a DNA-topoisomerase II poison) are frequently used in cancer chemotherapy. The effects of these drugs on cultured normal human lymphocytes were studied to establish the possible involvement of chromosome damage in the apoptotic program. Chromosome aberrations (CA) were evaluated in first division metaphases and the apoptotic process was measured by morphological and electrophoretical techniques. The percentage of abnormal cells was increased from the doses of FLU 1.0 microg/ml and IDA 0.005 microg/ml (P<0.0001) with an important decrease in the mitotic index (MI) for the highest doses assayed. A significant dose-dependent induction of abnormal cells was observed for both drugs. An increase of apoptotic cells was found at 5.0 and 10.0 microg/ml of FLU (P<0.001) while IDA activated apoptosis at 0.05 microg/ml (P<0.01) and markedly from 0.1 microg/ml (P<0.001). These increments were dose dependent. Apoptotic cell morphology was associated with DNA fragmentation at the highest doses. The increased induction of abnormal cells and the decreased MI were in correlation with the apoptotic index for FLU and IDA, suggesting the role of CA in drug-induced cell death.