Tetracycline-dependent regulation of formamidopyrimidine DNA glycosylase in transgenic mice conditionally reduces oxidative DNA damage in vivo

Sensitive homogeneous fluorescent detection of DNA glycosylase by target-triggering ligation-dependent tricyclic cascade amplification

Abnormal DNA glycosylases are concerned with the aging process as well as numerous pathologies in humans. Herein, a sensitive fluorescence method utilizing target-induced ligation-dependent tricyclic cascade amplification reaction was developed for the detecting DNA glycosylase activity. The presence of DNA glycosylase triggered the cleavage of damaged base in hairpin substrate, successively activating ligation-dependent strand displacement amplification (SDA) and exponential amplification reaction (EXPAR) for the generation of large amount of reporter probes. The resultant reporter probes bound with the signal probes to form stable dsDNA duplexes. And then the signal probes could be digested circularly in the dsDNA duplexes by T7 exonuclease, leading to the generation of an enhanced fluorescence signal. Due to the high efficiency of tricyclic cascade amplification and the low background signal deriving from the inhibition of nonspecific amplification, this method exhibited a detection limit of 0.14 U/mL and a dynamic range from 0.16 to 8.0 U/mL. Moreover, it could be applied for detecting DNA glycosylase activity in human serum with good selectivity and high sensitivity, and even quantifying other types of enzyme with 5'-PO₄ residue cleavage product by rationally designing the corresponding substrate. Importantly, this method could be performed in homogenous solution without any complicated separation steps, providing a new strategy for DNA glycosylase-related biomedical research.

Base excision repair of reactive oxygen species-initiated 7,8-dihydro-8-oxo-2'-deoxyguanosine inhibits the cytotoxicity of platinum anticancer drugs

Anticancer therapy with cisplatin and oxaliplatin is limited by toxicity and onset of tumor resistance. Both drugs form platinum-DNA cross-linked adducts, and cisplatin causes oxidative DNA damage including the 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG) lesion. To assess oxidative DNA damage as a mechanism of cisplatin and oxaliplatin cytotoxicity, 8-oxodG-directed base excision repair was stably enhanced in human embryonic kidney cells by FLAG-tagged expression of human oxoguanine glycosylase 1 (alpha-OGG1) or its functional homologue, Escherichia coli formamidopyrimidine glycosylase (fpg). Both drugs increased reactive oxygen species and 8-oxodG levels, and cytotoxicity was decreased by antioxidant pretreatment. Ectopic expression of alpha-OGG1 or fpg in cell clones increased nuclear and mitochondrial 8-oxodG repair, and reduced death by reactive oxygen species initiators (H(2)O(2), menadione) and both platinum drugs. Exposure to oxaliplatin caused a more marked and sustained block of cell proliferation than exposure to cisplatin. We conclude that the 8-oxodG lesion is cytotoxic, and base excision repair a likely determinant of risk. The greater antitumor efficacy of oxaliplatin seems unrelated to oxidative DNA damage, suggesting a novel strategy for improving the therapeutic index in cancer therapy.

Increased apoptosis, p53 up-regulation, and cerebellar neuronal degeneration in repair-deficient Cockayne syndrome mice

Cockayne syndrome (CS) is a rare recessive childhood-onset neurodegenerative disease, characterized by a deficiency in the DNA repair pathway of transcription-coupled nucleotide excision repair. Mice with a targeted deletion of the CSB gene (Csb-/-) exhibit a much milder ataxic phenotype than human patients. Csb-/- mice that are also deficient in global genomic repair [Csb-/-/xeroderma pigmentosum C (Xpc)-/-] are more profoundly affected, exhibiting whole-body wasting, ataxia, and neural loss by postnatal day 21. Cerebellar granule cells demonstrated high TUNEL staining indicative of apoptosis. Purkinje cells, identified by the marker calbindin, were severely depleted and, although not TUNEL-positive, displayed strong immunoreactivity for p53, indicating cellular stress. A subset of animals heterozygous for Csb and Xpc deficiencies was more mildly affected, demonstrating ataxia and Purkinje cell loss at 3 months of age. Mouse, Csb-/-, and Xpc-/- embryonic fibroblasts each exhibited increased sensitivity to UV light, which generates bulky DNA damage that is a substrate for excision repair. Whereas Csb-/-/Xpc-/- fibroblasts were more UV-sensitive than either single knockout, double-heterozygote fibroblasts had normal UV sensitivity. Csb-/- mice crossed with a strain defective in base excision repair (Ogg1) demonstrated no enhanced neurodegenerative phenotype. Complete deficiency in nucleotide excision repair therefore renders the brain profoundly sensitive to neurodegeneration in specific cell types of the cerebellum, possibly because of unrepaired endogenous DNA damage that is a substrate for nucleotide but not base excision repair.

Retrovirus-mediated tk gene therapy of implanted human breast cancer in nude mice under the regulation of Tet-On

Tight regulation of the therapeutic gene expression is critical in gene therapy. In this report, a doxycycline (Dox)-regulated retrovirus-mediated gene expression system was used to study the effects of suicide gene therapy on human breast cancer cell line MCF-7 and the nude mice model of implanted human breast cancer. To render the expression of suicide gene under control, we used two pseudoviruses simultaneously, RevTRE/HSVtk and RevTet-On, to infect MCF-7 cells or xenografts of nude mice. When infected by the pseudoviruses and followed by Dox and Ganciclovir (GCV) treatment, MCF-7 cells were arrested at S phase and the growth was suppressed. We then evaluated the antitumor efficiency of this system in vivo through studying the mice bearing human breast cancer xenografts. Compared with control groups, the HSVtk mRNA level increased significantly in tumor tissues, mass of the tumors shrank remarkably, and tumor necrosis features occurred after treatment with Dox and GCV. These data suggest that suicide gene therapy using the Dox-induced Tet-On-controlled HSVtk gene expression system is a feasible method to treat human breast cancer.