Telomere elongation involves intra-molecular DNA replication in cells utilizing alternative lengthening of telomeres.

Alternative lengthening of telomeres (ALT) is a telomere length maintenance mechanism based on recombination, where telomeres use other telomeric DNA as a template for DNA synthesis. About 10% of all human tumors depend on ALT for their continued growth, and understanding its molecular details is critically important for the development of cancer treatments that target this mechanism. We have previously shown that telomeres of ALT-positive human cells can become lengthened via inter-telomeric copying, i.e. by copying the telomere of another chromosome. The possibility that such telomeres could elongate by using other sources of telomeric DNA as copy templates has not been investigated previously. In this study, we have determined whether a telomere can become lengthened by copying its own sequences, without the need for using another telomere as a copy template. To test this, we transduced an ALT cell line with a telomere-targeting construct and obtained clones with a single tagged telomere. We showed that the telomere tag can be amplified without the involvement of other telomeres, indicating that telomere elongation can also occur by intra-telomeric DNA copying. This is the first direct evidence that the ALT mechanism involves more than one method of telomere elongation.

DNA damage induces alternative lengthening of telomeres (ALT) associated promyelocytic leukemia bodies that preferentially associate with linear telomeric DNA.

The linear chromosomes of vertebrates terminate in telomeres that consist of a tandemly repeated hexameric sequence, 5'TTAGGG3'. Telomeres form a protective loop structure (t-loop), which is thought to prevent them from being recognized as a double-strand break. Approximately 10% of human tumors prevent shortening of their telomeres by using a recombination-mediated alternative lengthening of telomeres (ALT) mechanism. ALT-positive human cells contain extrachromosomal telomere repeat (ECTR) DNA that may either be circular or linear. It has been proposed that ECTR may be generated by recombination events involving the t-loop. A proportion of the cells within ALT-positive cell populations contain promyelocytic leukemia (PML) nuclear bodies that contain telomeric DNA and telomere-binding proteins that are called ALT-associated PML bodies (APB). Although the presence of APBs is very useful for determining whether tumors and cell lines use the ALT mechanism, the function of APBs is unknown. It has previously been shown that telomeric DNA is particularly susceptible to damage by hydrogen peroxide and N-methyl-N'-nitro-N-nitrosoguanidine. We report here that these DNA-damaging agents induce both linear and circular ECTR DNA in ALT cells and increase the proportion of cells that contain APBs. We partially purified APBs and showed that the telomeric repeat DNA they contain is predominantly linear. We propose that a function of APBs is to sequester linear telomeric DNA.

Molecular changes associated with oral dysplasia progression and acquisition of immortality: potential for its reversal by 5-azacytidine.

This study has identified molecular changes characteristic of early oral cancer progression. We reported previously that acquisition of the immortal phenotype is an early event in oral cancer development (F. McGregor et al., Cancer Res., 57: 3886-3889, 1997); our current data indicate that about half of oral dysplasia cultures are immortal, and this is associated with loss of expression of retinoic acid receptor (RAR)-beta and the cell cycle inhibitor p16(ink4a) (p16), p53 mutations, and increased levels of telomerase/human telomerase reverse transcriptase mRNA. In contrast, increased expression of the epidermal growth factor receptor, known to be a characteristic of oral cancer, does not occur until after the dysplasia stage in squamous cell carcinomas. Acquisition of invasive properties as judged by an in vitro Matrigel invasion assay also does not occur until the carcinoma stage and is further increased in metastases. Interestingly, one atypical mortal dysplasia with a considerably extended life span has lost expression of RAR-beta and p16, but it still expresses only wild-type p53 (albeit at a higher level than normal) and has not activated telomerase. RAR-beta and/or p16 re-expression can be induced by treatment with 5-aza-2-deoxycytidine (Aza-C) in some immortal dysplasias, and this has been shown to be due to silencing of gene expression by promoter methylation. Aza-C treatment also down-regulated telomerase activity and human telomerase reverse transcriptase mRNA. Interestingly, with one dysplasia, Aza-C was able to reverse its immortal phenotype, as judged by morphological criteria and expression of the senescence-associated acid beta-galactosidase activity during terminal growth arrest; this immortal dysplasia was the only one in which Aza-C treatment not only down-regulated telomerase activity but also induced re-expression of both RAR-beta and p16. The possibility of reversing the immortal phenotype of some dysplasias by Aza-C may be of clinical usefulness.

Senescing oral dysplasias are not immortalized by ectopic expression of hTERT alone without other molecular changes, such as loss of INK4A and/or retinoic acid receptor-beta: but p53 mutations are not necessarily required.

Our previous work showed that acquisition of immortality at the dysplasia stage of oral cancer progression was consistently associated with four changes: loss of retinoic acid receptor (RAR)-beta and p16INK4A expression, p53 mutations and activation of telomerase. One atypical dysplasia (D17) that underwent delayed senescence after an extended lifespan showed loss of RAR-beta and p16INK4A/p14ARF expression, but retained functional wild-type p53 and telomerase was not activated. We now demonstrate that retroviral delivery of hTERT results in telomere lengthening and immortalization of D17 without loss of functional wild-type p53 activity. In contrast, the expression of hTERT in two other typical mortal dyplasia cultures (that retain RAR-beta and p16INK4A expression) does not extend their lifespan, even though telomeres are lengthened.

Presence of alternative lengthening of telomeres mechanism in patients with glioblastoma identifies a less aggressive tumor type with longer survival.

Patients with glioblastoma (GBM) have variable clinical courses, but the factors that underlie this heterogeneity are not understood. To determine whether the presence of the telomerase-independent alternative lengthening of telomeres (ALTs) mechanism is a significant prognostic factor for survival, we performed a retrospective analysis of 573 GBM patients. The presence of ALT was identified in paraffin sections using a combination of immunofluorescence for promyelocytic leukemia body and telomere fluorescence in situ hybridization. Alternative lengthening of telomere was present in 15% of the GBM patients. Patients with ALT had longer survival that was independent of age, surgery, and other treatments. Mutations in isocitrate dehydrogenase (IDH1mut) 1 frequently accompanied ALT, and in the presence of both molecular events, there was significantly longer overall survival. These data suggest that most ALT+ tumors may be less aggressive proneural GBMs, and the better prognosis may relate to the set of genetic changes associated with this tumor subtype. Despite improved overall survival of patients treated with the addition of chemotherapy to radiotherapy and surgery, ALT and chemotherapy independently provided a survival advantage, but these factors were not found to be additive. These results suggest a critical need for developing new therapies to target these specific GBM subtypes.

The first molecular details of ALT in human tumor cells.

The activation of a telomere maintenance mechanism (TMM) is indispensable for cellular immortalization, a hallmark of human cancer. Although most human cancers use telomerase as their TMM, some use an alternative lengthening of telomeres (ALT) mechanism. The latter especially include specific subtypes of soft tissue sarcomas where ALT occurs most often in tumors with complex karyotypes, astrocytic brain tumors and osteosarcomas. The prognostic significance of ALT varies according to the type of tumor. Some ALT cells have atypical features, suggesting the possibility that there is more than one ALT mechanism. ALT cells are characterized by instability at a specific minisatellite locus (although they are stable at microsatellite loci) and by high rates of telomeric recombinational exchange. We propose a revised model whereby unequal telomeric exchange and asymmetrical chromosome segregation could result in telomere length maintenance in a cell population. In at least some ALT cells, telomere maintenance requires the integrity of the MRN (MRE11-RAD50-NBS1) recombination complex and is efficiently repressed by its sequestration. Microsatellite instability (MSI) often results in disruption of MRN, so ALT may usually be incompatible with MSI. We suggest that ALT in human tumors is a dysregulated version of an aspect of normal mammalian telomere homeostasis, which may be a vestige of the TMM used by ancient eukaryotes. Understanding the molecular basis of ALT has important implications for the diagnosis and treatment of tumors that use this TMM.