Molecular characterization of inter-telomere and intra-telomere mutations in human ALT cells

Abstract

Telomeres in most immortal cells are maintained by the enzyme telomerase, allowing cells to divide indefinitely. Some telomerase-negative tumors and immortal cell lines maintain long heterogeneous telomeres by the ALT (alternative lengthening of telomeres) mechanism; such tumors are expected to be resistant to anti-telomerase drug therapies. Occasionally telomerase-negative Saccharomyces cerevisiae mutants survive, and 10% of them (type II survivors) have unstable telomeres. As in human ALT+ cells, short telomeres in yeast type II survivors lengthen abruptly; in yeast, this is dependent on the recombination proteins Rad52p and Rad50p. In human cells, ALT involves copying of sequence from a donor to a recipient telomere. We have characterized for the first time a class of complex telomere mutations seen only in ALT+ cells. The mutant telomeres are defined by the replacement of the progenitor telomere at a discrete point (fusion point) with a different telomere repeat array. Among 19 characterized fusion points, one occurred within the first six repeats of the telomere, indicating that these recombination-like events can occur anywhere within the telomere. One mutant telomere may have been involved in a secondary recombination-like mutation event, suggesting that these mutations are sporadic but ongoing in ALT+ cells. We also identified simple intra-allelic mutations at high frequency, which evidently contribute to telomere instability in ALT+ cells.

The DNA damage response at dysfunctional telomeres, and at interstitial and subtelomeric DNA double-strand breaks

“…This mechanism is called alternative lengthening of telomeres (ALT) (reviewed by Cesare and Reddel, 2010;Pickett and Reddel, 2015). Telomeres maintained by ALT are highly heterogeneous in length due to both gradual and rapid changes in telomere length (Murnane et al, 1994), show a greatly elevated level of recombination at telomeres (Dunham et al, 2000;Varley et al, 2002; reviewed by Cesare and Reddel, 2010), and contain ECTRs (Cesare and Griffith, 2004), which can serve as replication templates to elongate telomeres. The ALT mechanism requires the MRN complex to process the chromosome ends to form a single-stranded 3' overhang for recombination, as well as HRR factors, such as RAD51, RAD52, BLM and BRCA1.…”

Alternative Lengthening of Telomeres Is Characterized by High Rates of Telomeric Exchange

“…2), and recent evidence indicates that this mechanism is common in various types of cancer including osteosarcoma (3) and glioblastoma multiforme, the most common type of primary brain tumor in adults (4). ALT is a recombination-based DNA replication mechanism that results in lengthening of telomeres (5)(6)(7). Given the evidence that the telomerase and ALT mechanisms coexist in some tumors (3,4,8) and the possibility that treatment of telomerase-positive tumors with effective telomerase inhibitors will result in activation of ALT (9), therapeutic approaches aimed at inhibiting telomerase activity may need to be complemented with others that target ALT.…”

Telomerase- and recombination-independent immortalization of budding yeast

“…These cells use a mechanism that has been termed ALT (Alternative Lengthening of Telomeres) (Henson et al 2002). The ALT mechanism is also thought to be recombination-dependent (Dunham et al 2000;Varley et al 2002).…”