The effect of chemotherapeutic agents on telomere length maintenance in breast cancer cell lines

Exposure to hypomethylating 5-aza-2'-deoxycytidine (decitabine) causes rapid, severe DNA damage, telomere elongation and mitotic dysfunction in human WIL2-NS cells

BACKGROUND

5-aza-2'-deoxycytidine (5azadC, decitabine) is a DNA hypomethylating agent used in the treatment of myelodysplastic syndromes. Due to cytotoxic side effects dose optimization is essential. The aim of this study was to define and quantify the effects of 5azadC on biomarkers of chromosomal stability, and telomere length, in human lymphoblastoid cell line, WIL2-NS, at clinically relevant dosages.

METHODS

Human WIL2-NS cells were maintained in complete medium containing 0, 0.2 or 1.0 μM 5azadC for four days, and analysed daily for telomere length (flow cytometry), chromosomal stability (cytokinesis-block micronucleus cytome (CBMN-cyt) assay), and global methylation (%5me-C).

RESULTS

DNA methylation decreased significantly in 1.0 μM 5azadC, relative to control (p < 0.0001). Exposure to 1.0 μM 5azadC resulted in 1.7-fold increase in telomere length (p < 0.0001), in parallel with rapid increase in biomarkers of DNA damage; (micronuclei (MN, 6-fold increase), nucleoplasmic bridges (NPB, a 12-fold increase), and nuclear buds (NBud, a 13-fold increase) (all p < 0.0001). Fused nuclei (FUS), indicative of mitotic dysfunction, showed a 5- and 13-fold increase in the 0.2 μM and 1.0 μM conditions, respectively (p = 0.001) after 4 days.

CONCLUSIONS

These data show that (i) clinically relevant concentrations of 5azadC are highly genotoxic; (ii) hypomethylation was associated with increased TL and DNA damage; and (iii) longer TL was associated with chromosomal instability. These findings suggest that lower doses of 5azdC may be effective as a hypomethylating agent, while potentially reducing DNA damage and risk for secondary disease.

Mutagen-induced telomere instability in human cells

Telomere instability is one of the main sources of genome instability and may result from chromosome end loss (due to chromosome breakage at one or both ends) or, more frequently, telomere dysfunction. Dysfunctional telomeres arise when they lose their end-capping function or become critically short, which causes chromosomal termini to behave like a DNA double-strand break. Telomere instability may occur at the chromosomal or at the molecular level, giving rise, respectively, to telomere-related chromosomal aberrations or the loss or modification of any of the components of the telomere (telomere DNA, telomere-associated proteins, or telomere RNA). Since telomeres play a fundamental role in maintaining genome stability, the study of telomere instability in cells exposed to mutagens is of great importance to understand the telomere-driven genomic instability present in those cells. In the present review, we will focus on the current knowledge about telomere instability induced by physical, chemical, and biological mutagens in human cells.

Non-canonical roles of canonical telomere binding proteins in cancers

Reactivation of telomerase is a major hallmark observed in 90% of all cancers. Yet paradoxically, enhanced telomerase activity does not correlate with telomere length and cancers often possess short telomeres; suggestive of supplementary non-canonical roles that telomerase might play in the development of cancer. Moreover, studies have shown that aberrant expression of shelterin proteins coupled with their release from shortening telomeres can further promote cancer by mechanisms independent of their telomeric role. While targeting telomerase activity appears to be an attractive therapeutic option, this approach has failed in clinical trials due to undesirable cytotoxic effects on stem cells. To circumvent this concern, an alternative strategy could be to target the molecules involved in the non-canonical functions of telomeric proteins. In this review, we will focus on emerging evidence that has demonstrated the non-canonical roles of telomeric proteins and their impact on tumorigenesis. Furthermore, we aim to address current knowledge gaps in telomeric protein functions and propose future research approaches that can be undertaken to achieve this.

Integrated analysis of promoter methylation and expression of telomere related genes in breast cancer

Telomeres at the ends of eukaryotic chromosomes play a critical role in tumorgenesis. Using microfluidic PCR and next-generation bisulfite sequencing technology, we investigated the promoter methylation of 29 telomere related genes in paired tumor and normal tissues from 184 breast cancer patients. The expression of significantly differentially methylated genes was quantified using qPCR method.We observed that the average methylation level of the 29 telomere related genes was significant higher in tumor than that in normal tissues (P = 4.30E-21). A total of 4 genes (RAD50, RTEL, TERC and TRF1) showed significant hyper-methylation in breast tumor tissues. RAD51D showed significant methylation difference among the four breast cancer subtypes. The methylation of TERC showed significant association with ER status of breast cancer. The expression profiles of the 4 hyper-methylated genes showed significantly reduced expression in tumor tissues. The integration analysis of methylation and expression of these 4 genes showed a good performance in breast cancer prediction (AUC = 0.947).Our results revealed the methylation pattern of telomere related genes in breast cancer and suggested a novel 4-gene panel might be a valuable biomarker for breast cancer diagnosis.

Functional role of SETD2, BAP1, PARP-3 and PBRM1 candidate genes on the regulation of hTERT gene expression

Narrowing the search for the critical hTERT repressor sequence(s) has identified three regions on chromosome 3p (3p12-p21.1, 3p21.2 and 3p21.3-p22). However, the precise location and identity of the sequence(s) responsible for hTERT transcriptional repression remains elusive. In order to identify critical hTERT repressor sequences located within human chromosome 3p12-p22, we investigated hTERT transcriptional activity within 21NT microcell hybrid clones containing chromosome 3 fragments. Mapping of chromosome 3 structure in a single hTERT-repressed 21NT-#3fragment hybrid clone, revealed a 490kb region of deletion localised to 3p21.3 and encompassing the histone H3, lysine 36 (H3K36) trimethyltransferase enzyme SETD2; a putative tumour suppressor gene in breast cancer. Three additional genes, BAP1, PARP-3 and PBRM1, were also selected for further investigation based on their location within the 3p21.1-p21.3 region, together with their documented role in the epigenetic regulation of target gene expression or hTERT regulation. All four genes (SETD2, BAP1, PARP-3 and PBRM1) were found to be expressed at low levels in 21NT. Gene copy number variation (CNV) analysis of SETD2, BAP1, PARP-3 and PBRM1 within a panel of nine breast cancer cell lines demonstrated single copy number loss of all candidate genes within five (56%) cell lines (including 21NT cells). Stable, forced overexpression of BAP1, but not PARP2, SETD2 or PBRM1, within 21NT cells was associated with a significant reduction in hTERT expression levels relative to wild-type controls. We propose that at least two sequences exist on human chromosome 3p, that function to regulate hTERT transcription within human breast cancer cells.

Epigenetic perturbation driving asleep telomerase reverse transcriptase: Possible therapeutic avenues in carcinoma

In the last decade, implications of human telomerase reverse transcriptase (hTERT), a component of ribonucleoprotein telomerase in aging, senescence, and stem cell are highly evident. Besides, the activation of hTERT is also being documented several cancer types including carcinoma. The awakening of telomerase during carcinoma initiation and development is being seen with different perspectives including genetic and epigenetic tools and events. In view of several tumor progenitors genes (also referred as epigenetic mediators), telomerase is placed as key enzyme to achieve the carcinoma phenotype and sustain during the progression. It is true that swaying of telomerase in carcinoma could be facilitated with dedicated set of epigenetic modulators and modifiers players. These epigenetic alterations are heritable, potentially reversible, and seen as the epigenetic signature of carcinoma. Several papers converge to suggest that DNA methylation, histone modification, and small non-coding RNAs are the widely appreciated epigenetic changes towards hTERT modulation. In this review, we summarize the contribution of epigenetic factors in the telomerase activation and discuss potential avenues to achieve therapeutic intervention in carcinoma.

Linking Telomere Regulation to Stem Cell Pluripotency

Embryonic stem cells (ESCs), somatic cell nuclear transfer ESCs, and induced pluripotent stem cells (iPSCs) represent the most studied group of PSCs. Unlimited self-renewal without incurring chromosomal instability and pluripotency are essential for the potential use of PSCs in regenerative therapy. Telomere length maintenance is critical for the unlimited self-renewal, pluripotency, and chromosomal stability of PSCs. While telomerase has a primary role in telomere maintenance, alternative lengthening of telomere pathways involving recombination and epigenetic modifications are also required for telomere length regulation, notably in mouse PSCs. Telomere rejuvenation is part of epigenetic reprogramming to pluripotency. Insights into telomere reprogramming and maintenance in PSCs may have implications for understanding of aging and tumorigenesis. Here, I discuss the link between telomere elongation and homeostasis to the acquisition and maintenance of stem cell pluripotency, and their regulatory mechanisms by epigenetic modifications.

Telomere status in chronic lymphocytic leukemia with TP53 disruption

In chronic lymphocytic leukemia (CLL), telomere dysfunction is associated with poor outcomes. TP53 is involved in cellular responses to dysfunctional telomeres, and its inactivation is the strongest adverse prognostic factor for CLL. Given the biological relationship between TP53 and telomeres, and their prognostic value, it is important to improve our understanding of the impact of TP53 alterations on telomeres. We performed a comprehensive study of the deletions and mutations of the TP53 gene and telomere parameters, including hTERT and the shelterin complex, in 115 CLL patients. We found that any type of TP53 alteration was associated with very short telomeres and high hTERT expression, independently of other biological CLL features. Patients with disrupted TP53 showed telomere deletions and chromosomal end-to-end fusions in cells with complex karyotypes. TP53 disruption was characterized by downregulation of shelterin genes. Interestingly, low expression of POT1, TPP1 and TIN2 was also found in some patients with wild-type TP53 and had an adverse impact on progression-free survival after standard genotoxic therapy. In conclusion, we have demonstrated that patients with disrupted TP53 have severe telomere dysfunction and high genomic instability. Thus, the telomeric profile could be tested as a biomarker in CLL patients treated with new therapeutic agents.

Shorter telomeres and high telomerase activity correlate with a highly aggressive phenotype in breast cancer cell lines

Maintenance of telomere length is one function of human telomerase that is crucial for the survival of cancer cells and cancer progression. Both telomeres and telomerase have been proposed as possible biomarkers of cancer risk and cancer invasiveness; however, their clinical relevance is still under discussion. In order to improve our understanding of the relationship between telomere length and telomerase activity with cancer invasiveness, we studied telomere length as well as telomerase levels, activity, and intracellular localization in breast cancer cell lines with diverse invasive phenotypes. We found an apparently paradoxical coincidence of short telomeres and enhanced telomerase activity in the most invasive breast cancer cell lines. We also observed that hTERT intracellular localization could be correlated with its level of activity. There was no association between human telomerase reverse transcriptase (hTERT) protein expression levels and invasiveness. We propose that simultaneous evaluation of these two biomarkers-telomere length and telomerase activity-could be useful for the assessment of the invasive capacity and aggressiveness of tumor cells from breast cancer patients.