br Telomere Dysfunction Can Occur

Telomere Dysfunction Can Occur Irrespectively of Length So far, the majority of studies have given evidence to senescence as a result of telomere shortening. However, several reports now suggest that telomere dysfunction can also occur in a length-independent manner (Fig. 1). For example, persistent telomeric DDR signaling in response to genotoxic stress has been shown to occur irrespectively of length in human fibroblasts in vitro and in mouse neurons in vivo (Fumagalli et al., 2012). Longer telomeres signaling a DDR have also been implicated during the ageing process in vivo. Initially, it was believed that murine cell senescence was mainly mediated by oxidative damage, and occurred independently of telomeres (Parrinello et al., 2003). However, recent studies suggest otherwise; telomeric DNA damage has been shown to increase with age in the gut and liver of mice, which occurred irrespectively of length (Hewitt et al., 2012; Jurk et al., 2014). Length-independent telomere damage has also been observed in hippocampal neurons and liver of baboons with age, possibly indicating that this plays a role in age-related tissue dysfunction by inducing cellular senescence (Fumagalli et al., 2012). Moreover, analysis of individual telomeres in small airway epithelial kinesin 5 in the lung of COPD patients, which show senescence markers such as increased p16 expression, has also revealed that damaged telomeres are not significantly shorter than those not associated with DDR proteins (Birch et al., 2015). This phenomenon also extends to oncogene-induced senescence (OIS), as a study has demonstrated that dysfunctional telomeres in melanocytic nevi were not shorter than functional ones (Suram et al., 2012). In fact, TRF2 was shown to still be present at a fraction of dysfunctional telomeres, suggesting that shortening and uncapping (i.e. loss of shelterin proteins) are not the only causes of DDR activation (Suram et al., 2012). In accordance to this, length-independent telomere dysfunction has also been shown to occur in replicative senescent cells, where TRF2 and RAP1 were still present in some telomeres positive for DDR proteins, suggesting that DDR activation at telomeres is not always a consequence of loss of shelterin components (Kaul et al., 2012). It has been suggested that DNA damage is more likely to occur at long telomeres as they represent a more abundant target for lesion formation, possibly explaining the occurrence of length-independent DDR activation (Fumagalli et al., 2012). Indeed, in a study where telomeres were elongated in human cancer cells, it was shown that cells with very long telomeres were more sensitive to ionizing irradiation, suggesting that telomeres above a critical length are more likely to accumulate DSBs (Fairlie and Harrington, 2015). Moreover, a recent Mendelian randomization study has shown strong associations between long telomeres due to germline genetic variation and increased risk for certain types of cancer (Haycock et al., 2017). However, thus far evidence does not clearly point out that longer telomeres are preferential targets of damage over shorter ones. Studies where individual telomere length was analyzed both in melanocytic nevi and in mice were unable to identify a significant difference in length between damaged and undamaged telomeres (Hewitt et al., 2012; Suram et al., 2012). One possibility is that the sensitivity of current techniques available to measure individual telomere length in tissues is not great enough to detect very short telomeres, masking any possible significant differences in length between dysfunctional and functional telomeres. Roger Reddel\'s group have suggested that telomeres may exist in three different states, providing an explanation to DNA damage signaling at telomeres that still contain shelterin proteins. The first state is the so-called fully capped or closed state, where telomeric repeats are sufficiently long such that the T-loop conformation is not compromised, and shelterin proteins inhibit NHEJ, preventing DDR activation. However, if T-loop conformation is lost, telomeres may adopt an intermediate state, activating a DDR without leading to end-to-end fusions due to the presence of sufficient shelterin proteins. This may occur in a length-dependent and –independent manner, possibly explaining the presence of DDR proteins at longer telomeres that still contain components of the shelterin complex. Lastly, extreme telomere shortening can lead to disruption of the T-loop, causing DDR activation, a state also known as fully uncapped. In the latter, telomeric repeats have reached critical lengths such that there is a loss of shelterin proteins and NHEJ is no longer inhibited at telomeres, causing end-to-end fusions (Cesare et al., 2009). Moreover, recent data from our lab and others indicate that a fourth state may also exist, whereby persistent DDR signaling within telomeric repeats can occur even in the presence of an undisrupted T-loop and shelterin components.