TERRA Expression Levels Do Not Correlate with Telomere Length and Radiation Sensitivity in Human Cancer Cell Lines

Abstract

Mammalian telomeres are transcribed into long non-coding telomeric repeat-containing RNA (TERRA) molecules that seem to play a role in the maintenance of telomere stability. In human cells, CpG-island promoters drive TERRA transcription and are regulated by methylation. It was suggested that the amount of TERRA may be related to telomere length. To test this hypothesis we measured telomere length and TERRA levels in single clones isolated from five human cell lines: HeLa (cervical carcinoma), BRC-230 (breast cancer), AKG and GK2 (gastric cancers), and GM847 (SV40 immortalized skin fibroblasts). However, these two parameters did not correlate with each other. Moreover, cell survival to γ-rays did not show a significant variation among the clones, suggesting that, in this cellular system, the intra-population variability in telomere length and TERRA levels does not influence sensitivity to ionizing radiation. This conclusion was supported by the observation that in a cell line in which telomeres were greatly elongated by the ectopic expression of telomerase, TERRA expression levels and radiation sensitivity were similar to the parental HeLa cell line.

Telomeric RNA Expression: Length Matters

“…Hence, quantification of TERRA by RT-PCR (Reverse Transcriptase-PCR) targeting subtelomeric parts of the RNA molecules, as initiated by Azzalin et al (3), appears to be more appropriate to assess TERRA expression independently of molecule length (Figures 1D,E). In this view, and fitting with our proposal that telomere transcription decreases upon elongation, close examination of Figure 3 data from Smirnova et al (1) revealed that there might be an inverse correlation between telomere length and the number of TERRA molecules measured by quantitative RT-PCR in various HeLa subclones. However, because the cellular systems used in Smirnova et al (1) are different from the ones described in Arnoult et al (2), we cannot exclude that differences may exist between these two studies regarding telomere length-dependent repression of TERRA.…”

“…A drawback of alkaline treatment however explaining why this is usually not applied for northern blots is that it releases RNA from the gel (Figures 1B,C), making quantifications rather difficult. Smirnova et al (1) do not appear to have included an alkaline treatment step as, despite high differences in telomere length, notably between HeLa parental cells (7 kb) and cells overexpressing telomerase (50 kb), TERRA profiles were similar (Figure 1 of their article). Most probably, a dot-blot analysis of total RNA would have revealed much higher levels of UUAGGG repeats in HeLa cells overexpressing telomerase.…”

“…We read with great interest the Research Article by Smirnova et al (1) in which sets of isogenic subclones isolated from human cancer cell lines were used to investigate the impact of telomere length on telomeric repeat-containing RNA (TERRA) expression. The first conclusion from this study, i.e., that telomere length does not impact on TERRA expression, was based on results obtained from a combination of Southern (telomeres) and northern (TERRA) blots.…”

Crosstalk between telomere maintenance and radiation effects: A key player in the process of radiation-induced carcinogenesis

“…Indeed, telomeres were found to be actively transcribed into TERRA, which localize to telomeres, and the presence of these molecules at the telomeres was suggested to cause delayed entry into senescence in yeast cells by promoting recombination-mediated telomere elongation at critically short telomeres [42]. Interestingly, however, a recent study showed no correlation between TL, TERRA levels, and radiosensitivity in several human cancer cell lines [43]. Shelterin proteins, in conjunction with DDR mechanisms, have also been shown to be involved in telomere protection and maintenance [11,17].…”

“…Late-passage normal human fibroblasts with short telomeres are more sensitive to IR than their younger counterparts with longer telomeres [196]. Whether this holds true in telomerase-positive cells is still debatable, as a recent study demonstrated no correlation between TL and radiosensitivity in this setting [43]. It has also been suggested that radiosensitivity is correlated with telomere loss, not mean TL, with no observable IR-induced changes in levels of telomerase expression [197].…”