Anti-cancer effect of disrupting the transcription coupled repair gene, CSB

Abstract

Certain DNA lesions when present in the transcribed strand of active genes are repaired faster than those located elsewhere. This phenomenon, transcription-coupled repair (TCR), is defective in cells from individuals with Cockayne syndrome (CS), which have two complementation groups, CSA and CSB. CSB is a DNA-dependent ATPase in the SNF2 family that remodels chromatin and stimulates transcript elongation by RNAP II CSB protein might function in rearranging interactions between RNAP II and damaged DNA, thus allowing the DNA repair apparatus to gain access to DNA lesions.;Despite their DNA repair defect, CS patients do not demonstrate an elevated cancer incidence, in drastic contrast with individuals defective with other kinds of DNA repair. To study the effect of CSB disruption on carcinogenesis, we crossed CSB-/- mice with cancer prone mice lacking the p16 INK4a/p19ARF tumor suppressor locus. CSB-/- Ink4a/ARF-/- mice developed 60% fewer tumors than Ink4a/ARF-/- mice and demonstrated a longer tumor-free latency time (260 versus 150 days). Moreover, CSB-/- Ink4a/ARF-/- mouse embryo fibroblast (MEFs) exhibited a lower colony formation rate, a lower rate of H-Ras-induced transformation, slower proliferation, a lower mRNA synthesis rate, increased UV-induced p53 and apoptosis than Ink4a/ARF-/- MEFs. These results indicate that the antineoplastic effect of CSB gene disruption may result from impaired transcription or from apoptosis secondary to environmental or endogenous DNA damage.;To investigate the potential use of CSB disruption in cancer chemotherapy, we designed and characterized phosphorothioate antisense oligodeoxynucleotides (AOs) that reduced CSB mRNA level in A2780/CP70 ovarian carcinoma cells. The AOs caused the cells to proliferate more slowly and made them more sensitive to cisplatin, oxaliplatin, hydrogen peroxide and gamma-irradiation, all of which induce lesions that are subject to TCR. Chemically modified AOs (MBOs) targeting CSB were able to potentiate the anti-tumor effect of cisplatin against A2780/CP70 tumor xenografts formed in nude mice. The MBOs enabled a non-toxic (3 mg/kg) dose of cisplatin to have the same degree of anti-tumor efficacy as a more toxic (5 mg/kg) cisplatin dose.;Collectively, CSB gene disruption increased DNA-damage induced apoptosis, decreased spontaneous tumorigenesis in vivo, and might be a novel target for cancer chemotherapy.