Telomere maintenance in sarcomas

PML body meets telomere: the beginning of an ALTernate ending?

The unlimited proliferation potential of cancer cells requires the maintenance of their telomeres. This is frequently accomplished by reactivation of telomerase. However, in a significant fraction of tumors an alternative lengthening of telomeres (ALT) mechanism is active. The molecular mechanism of the ALT pathway remains elusive. In particular, the role of characteristic complexes of promyelocytic leukemia nuclear bodies (PML-NBs) with telomeres, the ALT-associated PML-NBs (APBs), is currently under investigation. Here, we review recent findings on the assembly, structure and functions of APBs. It is discussed how genomic aberrations in ALT-positive cancer cells could result in the formation of APBs and in ALT activity. We conclude that they are important functional intermediates in what is considered the canonical ALT pathway and discuss deregulations of cellular pathways that contribute to the emergence of the ALT phenotype.

Prediction and diagnosis of bladder cancer recurrence based on urinary content of hTERT, SENP1, PPP1CA, and MCM5 transcripts

Background

Identification of urinary biomarkers for detection of bladder cancer recurrence would be beneficial to minimize the frequency of cystoscopy. Our objective was to determine the usability of urine content of mRNA in the detection and prediction of bladder cancer recurrence.

Methods

We analyzed 123 prospectively cross-sectional collected urine samples from 117 patients with bladder cancer (12 incident cancers and 111 control visits). We used biopsies from cystoscopies as diagnostic criteria for recurrence, and followed the patients for a median time of 28.5 months (range 0-44 months). We measured the levels of hTERT, SENP1, PPP1CA, and MCM5 mRNA in urine by q-RT- PCR.

Results

We found significant differences in urinary content of hTERT (p < 0.001), SENP1 (p < 0.001), MCM5 (p < 0.001), and PPP1CA (p < 0.001) transcripts, when comparing urine samples from patients with and without tumor present in the bladder. We obtained sensitivity and specificity values for hTERT: 63/73, SENP1: 56/78, MCM5: 63/66, and PPP1CA: 69/63, respectively. Including follow-up data resulted in sensitivity and specificity values for hTERT: 62/84, SENP1:53/84, MCM5: 61/73, and PPP1CA: 65/66. Interestingly, at non-tumor visits the urinary content of especially hTERT (p = 0.0001) and MCM5 (p = 0.02) were significantly associated with subsequent tumour recurrence. Combining the markers with cytology improved the detection. The best combination was hTERT and cytology with a sensitivity of 71% and a specificity of 86% after follow-up. Further prospective validation or registration studies needs to be carried out before clinical use.

Conclusions

We could use the urinary content of hTERT, SENP1, PPP1CA, and MCM5 to detect bladder cancer recurrence. All markers showed a higher sensitivity than cytology. The detection rate improved when including cytology results, but also the combination of hTERT and MCM5 increased the detection rate. Furthermore, hTERT and MCM5 levels predicted subsequent tumor recurrences.

Dynamics of telomeres and promyelocytic leukemia nuclear bodies in a telomerase-negative human cell line

Telomerase-negative tumor cells maintain their telomeres via an alternative lengthening of telomeres (ALT) mechanism. This process involves the association of telomeres with promyelocytic leukemia nuclear bodies (PML-NBs). Here, the mobility of both telomeres and PML-NBs as well as their interactions were studied in human U2OS osteosarcoma cells, in which the ALT pathway is active. A U2OS cell line was constructed that had lac operator repeats stably integrated adjacent to the telomeres of chromosomes 6q, 11p, and 12q. By fluorescence microscopy of autofluorescent LacI repressor bound to the lacO arrays the telomere mobility during interphase was traced and correlated with the telomere repeat length. A confined diffusion model was derived that describes telomere dynamics in the nucleus on the time scale from seconds to hours. Two telomere groups were identified that differed with respect to the nuclear space accessible to them. Furthermore, translocations of PML-NBs relative to telomeres and their complexes with telomeres were evaluated. Based on these studies, a model is proposed in which the shortening of telomeres results in an increased mobility that could facilitate the formation of complexes between telomeres and PML-NBs.

Telomeres do the (un)twist: helicase actions at chromosome termini

Telomeres play critical roles in protecting genome stability, and their dysfunction contributes to cancer and age-related degenerative diseases. The precise architecture of telomeres, including their single-stranded 3' overhangs, bound proteins, and ability to form unusual secondary structures such as t-loops, is central to their function and thus requires careful processing by diverse factors. Furthermore, telomeres provide unique challenges to the DNA replication and recombination machinery, and are particularly suited for extension by the telomerase reverse transcriptase. Helicases use the energy from NTP hydrolysis to track along DNA and disrupt base pairing. Here we review current findings concerning how helicases modulate several aspects of telomere form and function.

Detection of circular telomeric DNA without 2D gel electrophoresis

The end of linear chromosomes forms a lasso-like structure called the t-loop. Such t-loops resemble a DNA recombination intermediate, where the single-stranded 3' overhang is arrested in a stretch of duplex DNA. Presumably, such a t-loop can also be deleted via a recombination process. This would result in the occurrence of circular extrachromosomal telomeric DNA (t-circles), which are known to be abundantly present in immortal cells engaging the recombination-based alternative lengthening of telomeres pathway (ALT pathway). Little is known about the basic mechanism of telomeric recombination in these cells and what ultimately causes the generation of such t-circles. Current standard procedures for detecting these molecules involve 2D gel electrophoresis or electron microscopy. However, both methods are labor intense and sophisticated to perform. Here, we present a simpler, faster, and equally sensitive method for detecting t-circles. Our approach is a telomere restriction fragment assay that involves the enzymatic preservation of circular DNA with Klenow enzyme followed by Bal31 degradation of the remaining linear DNA molecules. We show that with this approach t-circles can be detected in ALT cell lines, whereas no t-circles are present in telomerase-positive cell lines. We consider our approach a valid method in which t-circle generation is the experimental readout.

The many faces of telomerase: emerging extratelomeric effects

Telomeres, the ends of chromosomes, shorten with each cell division. To expand their replicative potential, various cell types use the ribonucleoprotein telomerase, which lengthens telomeres by its reverse transcriptase activity. Because of its ability to immortalize cancer cells, telomerase also plays a significant role in tumor growth. However, in recent years, a wide variety of non-canonical effects of telomerase that are independent of telomere lengthening have been discovered, and even the notion that telomerase is restricted to very few cell types has been questioned. These effects also seem to be important in carcinogenesis and might explain the tumor-promoting effects of telomerase independently of telomere elongation. Here, the current understanding of the extratelomeric roles of telomerase and their physiological and pathological significance is reviewed.

Mating type influences chromosome loss and replicative senescence in telomerase-deficient budding yeast by Dnl4-dependent telomere fusion

As we age, the majority of our cells gradually lose the capacity to divide because of replicative senescence that results from the inability to replicate the ends of chromosomes. The timing of senescence is dependent on the length of telomeric DNA, which elicits a checkpoint signal when critically short. Critically short telomeres also become vulnerable to deleterious rearrangements, end-degradation and telomere-telomere fusions. Here we report a novel role of non-homologous end-joining (NHEJ), a pathway of double-strand break repair in influencing both the kinetics of replicative senescence and the rate of chromosome loss in telomerase-deficient Saccharomyces cerevisiae. In telomerase-deficient cells, the absence of NHEJ delays replicative senescence, decreases loss of viability during senescence, and suppresses senescence-associated chromosome loss and telomere-telomere fusion. Differences in mating-type gene expression in haploid and diploid cells affect NHEJ function, resulting in distinct kinetics of replicative senescence. These results suggest that the differences in the kinetics of replicative senescence in haploid and diploid telomerase-deficient yeast are determined by changes in NHEJ-dependent telomere fusion, perhaps through the initiation of the breakage-fusion-bridge cycle.