ALT is a mechanism of telomere maintenance that is utilized by a portion of cancers, particularly in tissues of mesenchymal origin.
Telomeres in ALT cells are heterogeneous in length due to rapid deletions and elongations, which are thought to occur through high rates of interchromosomal recombination including a process termed telomere sister chromatid exchange (T-SCE).
Significantly, there is no increase in rates of general homologous recombination in these cells, suggesting that the mechanism of hyper-recombination is telomere-specific.
ALT cells are also characterized by the presence of extrachromosomal linear and circular telomere DNA molecules.
The extrachromosomal t-circles can in principle be utilized as templates for telomere elongation by rolling-circle-replication and therefore play an important role in the process of telomere maintenance in the absence of telomerase.
t-circles formation in ALT cells occurs through a process that requires the recombination proteins X-ray repair cross-complementing 3 (XRCC3) and Nijmegen breakage syndrome 1 (NBS1), as down-regulation of either one of these factors causes a dramatic decrease in the levels of t-circles in these cells.
However, MUS81, an endonuclease implicated in the regulation of telomere recombination, does not influence t-circle formation in ALT cells, suggesting that the precise contribution of the recombination machinery and the possible involvement of other pathways to the formation and maintenance of t-circles in human ALT cells remain to be defined.
Ku70/80 is a heterodimer that was first identified as a component of non-homologous end joining (NHEJ), an error-prone pathway involved in the repair of DNA double-strand breaks.
More recently, Ku70/80 has been detected at telomeres and implicated in the regulation of telomere maintenance in a range of organisms including yeast, plants, mice and humans.
At first, since fusion of telomeres leads to the formation of dicentric chromosomes and chromosome breaks, it seemed paradoxical that Ku70/80, a protein that facilitates the joining of DNA ends, is localized at telomeres.
Further analysis however suggested that a likely explanation for these observations is that Ku70/80 plays distinct roles at pathological and natural DNA ends.
Indeed, Ku70/80 has been proposed to perform at least two functions at telomeres.
First, it protects telomeres ends from degradation, and second it regulates telomere length, possibly through a functional interaction with the telomerase holoenzyme.
However, how Ku70/80 operates at telomere seems to differ among species.
For example, cells from Ku70 or Ku80 knockout plants show extreme telomere elongation while yeast lacking Ku70/80 display loss of telomeric repeats.
Moreover, Ku70/80 has been shown to contribute both positively and negatively to telomere length homeostasis in mice.
While further studies are needed to reconcile these observations, it is likely that the differences in telomere length homeostasis induced by depletion of Ku70/80 in distinct systems reflect natural species-specific variations in Ku function and telomere biology.
Interestingly, recent studies have demonstrated that inactivation of Ku70/80 induces the formation of t-circles in plant cells and in human telomerase-positive cancer cells.
However, t-circle formation in Ku-deficient plants is not suppressed by inactivation of genes involved in the homologous recombination pathway, including XRCC3.
A key molecular partner of Ku70/80, the Werner syndrome protein (WRN), is also involved in t-circle formation.
Interestingly, as observed in Ku70 or Ku80-deficient plants, t-circles in telomerase-positive human fibroblasts lacking WRN are refractory to XRCC3 depletion.
Yet, WRN is required for the formation of t-circles in cells expressing TRF2DB, a mutant form of the telomere repeat binding factor TRF2 which induces rapid telomere deletions and t-circles formation in normal fibroblast.
Collectively, these results suggest that in a context-dependent manner, distinct proteins can differentially influence the formation of t-circles.
By investigating the role of Ku70/80 in ALT cells it was demonstrated that down-regulation of this factor inhibits cell proliferation and significantly reduces the levels of t-circles without affecting telomere length or overhang signal.
Thus, Ku70/80 is an essential factor for ALT cell survival and its function is required for the maintenance of extrachromosomal t-circles in these cells.