Accumulation of mitochondrial DNA damage and bioenergetic dysfunction in CSB defective cells

Abstract

Cockayne syndrome (CS) is a severe, complex, progressive and autosomal recessive disease characterized by developmental, skeletal, neurological and muscular failure, as well as a premature aging phenotype [1]. Patients with CS can be assigned to two different complementation groups, with the majority carrying mutations in the CSB gene [2]. A clinically identified CSB truncation (CS1AN) has been genetically introduced into a mouse, which displays similar but milder CS phenotypes [3]. The CSB protein plays distinct roles in different DNA repair pathways. It is involved in the transcriptioncoupled repair of UV-induced DNA damage, which is reflected by the sensitivity of human and murine csb m ⁄ m fibroblasts to UV radiation. Furthermore, CSB is involved in the global repair of different types of endogenously formed DNA damage and also participates in regulation of the major nuclear and mitochondrial 8-oxoguanine (8oxoG) Cockayne syndrome (CS) is a complex, progressive disease that involves neurological and developmental impairment and premature aging. The majority of CS patients have mutations in the CSB gene. The CSB protein is involved in multiple DNA repair pathways and CSB mutated cells are sensitive to a broad spectrum of genotoxic agents. We tested the hypothesis that sensitivity to such genotoxins could be mediated by mitochondrial dysfunction as a consequence of the CSB mutation. mtDNA from csb m ⁄ m mice accumulates oxidative damage including 8-oxoguanine, and cells from this mouse are hypersensitive to the mitochondrial oxidant menadione. Inhibitors of mitochondrial complexes and the glycolysis inhibitor 2-deoxyglucose kill csb m ⁄ m cells more efficiently than wild-type cells, via a mechanism that does not correlate with mtDNA damage formation. Menadione depletes cellular ATP, and recovery after depletion is slower in csb m ⁄ m cells. The bioenergetic alteration in csb m ⁄ m cells parallels the simpler organization of supercomplexes consisting of complexes I, III and IV in addition to partially disassembled complex V in the inner mitochondrial membrane. Exposing wild-type cells to DNA intercalating agents induces complex alterations, suggesting a link between mtDNA integrity, respiratory complexes and mitochondrial function. Thus, mitochondrial dysfunction may play a role in the pathology of CS.Abbreviations

Cockayne syndrome group B protein prevents the accumulation of damaged mitochondria by promoting mitochondrial autophagy

“…Oxidative stress has been observed in several neurodegenerative diseases (Trushina and McMurray, 2007), providing a link between our findings of increased ROS production and decreased autophagy to the neuropathology seen in CS. These findings could also explain the accumulation of lesions found in mitochondria from CSB deficient cells (Muftuoglu et al, 2009;Osenbroch et al, 2009;Aamann et al, 2010;Kamenisch et al, 2010). The in creased oxygen and glucose consumption we observe indicates a state of cellular energy deprivation.…”

Oxidative DNA Damage and Nucleotide Excision Repair

“…Results of Kamenisch and colleagues indicated that the proteins directly interact with mtDNA and OGG1 (43), where they stabilize repair complexes at the mitochondrial membrane (1). In addition, mtDNA damage has been shown to accumulate in CSB-defective cells (61).…”

Regulation of Human Endonuclease V Activity and Relocalization to Cytoplasmic Stress Granules

“…Our data suggest that under normal intracellular ATP concentrations, hEndoV is inactive. Previous studies have demonstrated that in response to cellular insults such as methylnitronitrosoguanidine, arsenite, hydrogen peroxide, or menadione, intracellular ATP levels drop significantly, probably due to activation of poly(ADPribose) polymerase, which depletes energy stores in the cells (35)(36)(37). Dependent on dose and exposure time, ATP stores may be reduced with 70 -80%, which gives an ATP concentration that alleviates hEndoV inhibition in vitro.…”

“…Among the other metabolites in the cells, ATP is abundant reaching millimolar concentrations in most cell types (33). Furthermore, the intracellular ATP concentration is known to fluctuate and for example, under certain stress conditions, like oxidation and arsenite exposure, a large reduction is seen (34,35). Therefore, ATP was included in activity assays for hEndoV and intriguingly, 1 mM ATP almost completely inhibited hEndoV activity (Fig.…”