Defective Repair of Oxidative Base Lesions by the DNA Glycosylase Nth1 Associates with Multiple Telomere Defects

Haritha Vallabhaneni,Julia Sidorova,Nathan O'Callaghan,Yie Liu,Jin-Qiu Zhou

doi:10.1371/journal.pgen.1003639

Abstract

Telomeres are chromosome end structures and are essential for maintenance of genome stability. Highly repetitive telomere sequences appear to be susceptible to oxidative stress-induced damage. Oxidation may therefore have a severe impact on telomere integrity and function. A wide spectrum of oxidative pyrimidine-derivatives has been reported, including thymine glycol (Tg), that are primarily removed by a DNA glycosylase, Endonuclease III-like protein 1 (Nth1). Here, we investigate the effect of Nth1 deficiency on telomere integrity in mice. Nth1 null (Nth1−/−) mouse tissues and primary MEFs harbor higher levels of Endonuclease III-sensitive DNA lesions at telomeric repeats, in comparison to a non-telomeric locus. Furthermore, oxidative DNA damage induced by acute exposure to an oxidant is repaired slowly at telomeres in Nth1−/− MEFs. Although telomere length is not affected in the hematopoietic tissues of Nth1−/− adult mice, telomeres suffer from attrition and increased recombination and DNA damage foci formation in Nth1−/− bone marrow cells that are stimulated ex vivo in the presence of 20% oxygen. Nth1 deficiency also enhances telomere fragility in mice. Lastly, in a telomerase null background, Nth1−/− bone marrow cells undergo severe telomere loss at some chromosome ends and cell apoptosis upon replicative stress. These results suggest that Nth1 plays an important role in telomere maintenance and base repair against oxidative stress-induced base modifications. The fact that telomerase deficiency can exacerbate telomere shortening in Nth1 deficient mouse cells supports that base excision repair cooperates with telomerase to maintain telomere integrity.

Highlights

All eukaryotic linear chromosome ends consist of complex nucleoprotein structures, called telomeres
Oxidative stress causes DNA base damage that is mainly repaired by base excision repair pathway, where a DNA glycosylase initiates the recognition and removal of specific base damage
Though previously we have shown that oxidative damage to a purine base, guanine (G), affects telomere integrity, damage to other telomere bases, e.g. a pyrimidine base, thymine (T), may occur and potentially disrupt telomere maintenance

Summary

Introduction

All eukaryotic linear chromosome ends consist of complex nucleoprotein structures, called telomeres. Telomere DNA is bound by the shelterin complex, including telomere repeat binding proteins TRF1, TRF2, and POT1 [1]. Telomere maintenance involves telomerase extension and telomere recombination, replication, and capping [3]. It is affected by other factors, the most notable being oxidative stress [4]. Telomere length decreases after each cell division, due to the inability of DNA polymerases to completely replicate DNA ends. It has been suggested that the shelterin component, TRF1, might recruit BLM or RTEL helicases to telomeres, thereby helping to resolve G-quadruplex structures that may inhibit telomere replication [7]

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