DNA damage by nitrogen mustard in a gene containing multiple Sp1-binding sites

Development and validation of a PCR-based assay for the selection of patients more likely to benefit from therapeutic treatment with alkylating drugs

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Previous studies have indicated that the levels of DNA damage induced in peripheral blood mononuclear cells by the alkylating drugs melphalan, cisplatin and carboplatin can serve as useful biomarkers predictive of the therapeutic response of cancer patients to these drugs.

WHAT THIS STUDY ADDS

In the present study we developed a quantitative PCR-based assay, for the measurement of DNA damage. The advantages of this methodology are based on: its far greater sensitivity (about 250 times) than the traditional Southern blot-based method (the detection limit is ~10-20 lesions/10(6) nucleotides from the equivalent DNA of ~8000 cells); its simplicity and speed (results obtained within ~8h); its excellent reproducibility, with a coefficient of variance of 10-15% for different DNA preparations from similarly treated cells; its requirement for only minute amounts of material, and; the avoidance of radioisotope labeling. Moreover, emphasis was given to translate basic research findings into clinical practice through the validation of this assay for prediction of clinical outcome in multiple myeloma patients.

AIM

In order to develop and validate a simple, sensitive and rapid method for the quantitation of alkylating drug-induced DNA damage.

METHODS

HepG2 cells and blood samples were treated with alkylating drugs (melphalan, cisplatin, carboplatin). Gene-specific damage was examined using Southern blot and a multiplex long quantitative PCR (QPCR) carried out in a 7 kb fragment (part of the p53 gene) and a 0.5 kb fragment (part of the IFN-β1 sequence; internal standard).

RESULTS

The extent of PCR amplification of a p53 fragment was inversely proportional to the treatment concentrations of all anticancer drugs examined, indicating a dose-related inhibition by the DNA adducts formed. Parallel analysis of the same samples using both Southern blot and QPCR showed that the DNA adducts measured by QPCR corresponded to the interstrand cross-links in the case of melphalan, and to total drug-induced lesions in the case of the platinum drugs. The detection limit was ~10-20 lesions/10(6) nucleotides using DNA from ~8000 cells. The method is about 250 times more sensitive than the Southern blot-based method and the reproducibility is excellent, with an intraday coefficient of variance (CV) of 5-9% and an interday CV of 4-12%. Application of the QPCR assay to ex vivo melphalan-treated peripheral blood mononuclear cells from multiple myeloma patients, showed that the positive predictive value of this assay for clinical response to melphalan therapy was 92.9%.

CONCLUSION

The PCR-based assay developed in this study can be used for the selection of cancer patients more likely to benefit from therapeutic treatment with alkylating drugs.

Quantitative PCR analysis of diepoxybutane and epihalohydrin damage to nuclear versus mitochondrial DNA

The bifunctional alkylating agents diepoxybutane (DEB) and epichlorohydrin (ECH) are linked to the elevated incidence of certain cancers among workers in the synthetic polymer industry. Both compounds form interstrand cross-links within duplex DNA, an activity suggested to contribute to their cytotoxicity. To assess the DNA targeting of these compounds in vivo, we assayed for damage within chicken erythro-progenitor cells at three different sites: one within mitochondrial DNA, one within expressed nuclear DNA, and one within unexpressed nuclear DNA. We determined the degree of damage at each site via a quantitative polymerase chain reaction, which compares amplification of control, untreated DNA to that from cells exposed to the agent in question. We found that ECH and the related compound epibromohydrin preferentially target nuclear DNA relative to mitochondrial DNA, whereas DEB reacts similarly with the two genomes. Decreased reactivity of the mitochondrial genome could contribute to the reduced apoptotic potential of ECH relative to DEB. Additionally, formation of lesions by all agents occurred at comparable levels for unexpressed and expressed nuclear loci, suggesting that alkylation is unaffected by the degree of chromatin condensation.

Eruptive cherry angiomas associated with vitiligo: Provoked by topical nitrogen mustard?

We report a 27-year-old man who had suffered with vitiligo for 7 years and with eruptive cherry angiomas within or around the repigmented vitiliginous skin for 2 years. After continual therapy for vitiligo with topical nitrogen mustard in a concentration of 0.001% for 5 years, multiple cherry angiomas erupted within or around the repigmented vitiliginous plaques. The discontinue therapy with nitrogen mustard stopped the appearance of new cherry angiomas. The presence of eruptive cherry angiomas was evident and was confirmed by histopathology. We suggest that the chronic chemical stimuli caused by topical nitrogen mustard might result in the formation of eruptive cherry angiomas.

Quantitation of gene-specific DNA damage by competitive PCR

A sensitive assay for quantitating DNA damage within individual genes would be a valuable tool for identifying the molecular mechanisms of disease and the sites of action of various carcinogens and anticancer drugs. This report describes a competitive PCR assay that was used to quantitate DNA damage induced by anticancer drugs within a 683-bp region of the c-myc gene in human CEM leukemia cells. Absolute quantitation of gene-specific DNA damage (attomoles or molecules of damaged DNA sequences) was achieved by coamplification of a homologous internal standard that has the same primer binding sites and PCR amplification efficiency as c-myc. The variability (standard error) associated with four separate determinations of the amount of c-myc sequence in 300 ng of DNA from untreated cells (6.80 +/- 0.05 SE amol) was less than 1% of the mean. The assay was capable of quantitating direct DNA damage that was induced by therapeutic concentrations of VM-26 and cisplatin prior to the onset of cellular apoptosis or necrosis. Both VM-26 (1-10 microM) and cisplatin (25-100 microM) induced a dose-dependent decrease in the amount of intact c-myc sequence. This assay should be readily adaptable to current real-time PCR protocols.

Chronic cyclophosphamide treatment alters the expression of stress response genes in rat male germ cells

Increases in the survival rate of men treated with chemotherapeutic drugs and their desire to have children precipitate concerns about the effects of these drugs on germ cells. Azoospermia, oligospermia, and infertility are common outcomes resulting from treatment with cyclophosphamide, an alkylating agent. Exposure of male rats to cyclophosphamide results in dose-dependent and time-specific adverse effects on progeny outcome. Elucidation of the effects of chronic low-dose cyclophosphamide treatment on the expression of stress response genes in male germ cells may provide insight into the mechanisms underlying such adverse effects. Male rats were gavaged with saline or cyclophosphamide (6 mg/kg) for 4-5 wk; pachytene spermatocytes, round spermatids, and elongating spermatids were isolated; RNA was extracted and probed on cDNA arrays containing 216 cDNAs. After saline treatment, 125 stress response genes were expressed in pachytene spermatocytes (57% of genes studied), 122 in round spermatids (56%), and 83 in elongating spermatids (38%). Cyclophosphamide treatment reduced the number of genes detected in all germ cell types. The predominant effect of chronic cyclophosphamide exposure was to decrease the expression level of genes in pachytene spermatocytes (34% of genes studied), round spermatids (29%), and elongating spermatids (4%). In elongating spermatids only, drug treatment increased the expression of 8% of the genes studied. The expression profiles of genes involved in DNA repair, posttranslational modification, and antioxidant defense in male germ cells were altered by chronic cyclophosphamide treatment. We hypothesize that the effects of cyclophosphamide exposure on germ cell gene expression during spermatogenesis may have adverse consequences on male fertility and progeny outcome.

Expression of base excision, mismatch, and recombination repair genes in the organogenesis-stage rat conceptus and effects of exposure to a genotoxic teratogen, 4-hydroperoxycyclophosphamide

BACKGROUND

DNA repair capability may influence the outcome of genotoxic teratogen exposure. The goals of this study were to assess the expression of base excision repair (BER), mismatch repair (MMR), and recombination repair (RCR) genes in the mid-organogenesis rat conceptus and to determine the effects on expression of exposure to the genotoxic teratogen, 4-hydroperoxycyclophosphamide (4-OOHCPA).

METHODS

The expression of 17 BER, MMR, and RCR genes was examined in gestational day (GD) 10-12 rat conceptuses using the antisense RNA (aRNA) technique. Embryos were cultured with 10 microM 4-OOHCPA to examine effects on gene expression.

RESULTS

Yolk sacs and embryos had similar gene expression patterns for all three DNA repair pathways from GD10-12. Transcripts for APNG, PMS1, and RAD54 were present at high concentrations in both tissues. The remainder of the genes were expressed at low levels in yolk sac, with a few not detected on GD10 and 11. In the embryo, transcripts for most genes were low on GD10 and 11; several increased by GD12. After exposure to 4-OOHCPA for 24 hr, XRCC1 and RAD57 expression decreased in yolk sac, whereas only RAD51 transcripts decreased in the embryo. By 44 hr, transcripts for all BER genes decreased in yolk sac; in the embryo, most BER, MMR, and RCR genes decreased, many below the level of detection.

CONCLUSIONS

The expression of DNA repair genes in the mid-organogenesis rat conceptus is varied and subject to down-regulation by 4-OOHCPA. DNA repair gene expression may determine the consequences of genotoxicant exposure during development.