Telomere shortening is associated with poor prognosis and telomerase activity correlates with DNA repair impairment in non-small cell lung cancer

Summary

Background and purpose

Telomere function and DNA damage response pathways are frequently inactivated in cancer. Moreover, some telomere-binding proteins have been implicated in DNA repair. The purpose of this work consists of evaluating the prognostic impact of telomere dysfunction and its relationship with DNA repair systems in non-small cell lung cancer (NSCLC).

Patients and methods

We analysed 83 NSCLCs and their corresponding control samples obtained from patients submitted to surgery. Telomere function was evaluated by determining telomerase activity and telomere length. DNA repair expression assays were established by using cDNA arrays containing 96 DNA-repair genes and by Real Time Quantitative PCR.

Results

Our data indicated that telomere attrition was significantly associated with poor clinical outcome of patients (P = 0.02), being this parameter a significant prognostic factor independent of tumour stage (P = 0.012; relative risk = 1.887; 95% CI: 1.147–3.102). DNA-repair gene expression studies showed down regulation of DCLRE1C and GTF2H1 and a clear FLJ10858 up regulation in tumour tissues, as compared to controls. In addition, a number of genes related to DNA-repair were significantly down regulated in tumours that reactivated telomerase (DCLRE1C, GTF2H1, PARP-3, MLH1, and TRF2).

Conclusions

Telomere shortening emerged as a poor clinical evolution parameter in NSCLC. Moreover, results from this work suggest a relationship between the loss of several DNA repair genes and telomerase activity, which may be of relevance in the pathogenesis of non-small lung cancer.

Introduction

Lung cancer accounts for 12% of all cancers diagnosed worldwide, making it one of the most common cancer malignancy in our society. Among men in Spain, lung cancer is the leading form of cancer, and squamous cell carcinoma (SCC) is the main histologic subtype. Spanish women have one of the lowest average incidences of the European cancer registers [1]. Lung cancer is unique among human solid cancers in that a single environmental factor, tobacco smoke, is believed to promote sequential changes in target cells that lead to carcinogenesis. Recently, progress has been made in elucidating changes involved in the transformation of a normal bronchial epithelium to frank malignancy [2].

Among the different molecular factors implicated in carcinogenesis, telomere dysfunction emerges as an early event associated to genetic instability. In fact, genetic integrity is maintained, in part, by the architecture of telomeres [3]. This genetic integrity, however, is gradually lost as telomeres progressively shorten with each cell replication cycle. This telomere shortening is a result of end-replication problems caused by DNA polymerase having difficulty in replicating the very ends of linear DNA [4], [5]. Telomere shortening may induce cells to undergo apoptosis or may induce chromosomal instability [6]. In fact, telomere dysfunction (short telomeres) has been associated with the initiation and progression of mouse and human intestinal neoplasia [7].

At date, it is thus somewhat surprising that research on telomeres in lung cancer specimens is relatively rare. While telomerase expression and activity have been proposed to be of potential prognostic value [8], [9], reports on the use of telomeres to predict lung cancer progression are scarce [10].

Although the mechanism of how telomere dysfunction accelerates tumour onset is unclear, there is some evidence that has linked telomeres and DNA damage signalling or repair pathways in cells. The DNA damage response, upon sensing an uncapped telomere or another broken DNA end, recruits repair enzymes to the DNA ends [11], [12]. As well, different DNA-repair factors have been found at telomeres. This fact has led to believe that telomeres are recognized by these factors as damaged or abnormal DNA, but that telomere-specific proteins somehow organize or modify them so that the net result is a complex that protects the end from degradation and from eliciting a DNA damage response [13]. Even though, it has been suggested that telomere-binding factors may be ancient and general DNA-repair factors that have been exploited by the cell to protect telomeres [14].

In this work, our first aim consists of evaluating clinical implications of telomere dysfunction in the major histological subtype of lung cancer, non-small cell lung cancer (NSCLC). Following, we investigate a total of 96 DNA-repair genes from the different DNA-repair pathways, to assess both the implication of impairment concerning to these systems in non-small cell lung carcinogenesis, as well as a possible relationship between telomere function and abnormalities in the expression of these repair factors.