Rapid induction of alternative lengthening of telomeres by depletion of the histone chaperone ASF1.

The mechanism of activation of the alternative lengthening of telomeres (ALT) pathway of mammalian chromosome-end maintenance has been unclear. We have now discovered that co-depletion of the histone chaperones ASF1a and ASF1b in human cells induced all hallmarks of ALT in both primary and cancer cells. These included the formation of ALT-associated PML (promyelocytic leukemia) bodies (APBs), the presence of extrachromosomal telomeric DNA species, an elevated frequency of telomeric sister chromatid exchanges (t-SCE) events and intertelomeric exchange of an integrated tag.

5' C-rich telomeric overhangs are an outcome of rapid telomere truncation events.

A subset of human tumors ensures indefinite telomere length maintenance by activating a telomerase-independent mechanism known as Alternative Lengthening of Telomeres (ALT). Most tumor cells of ALT origin share a constellation of unique characteristics, which include large stores of extra-chromosomal telomeric material, chronic telomere dysfunction and a peculiar enrichment in chromosome ends with 5' C-rich overhangs. Here we demonstrate that acute telomere de-protection and the subsequent DNA damage signal are not sufficient to facilitate formation of 5' C-overhangs at the chromosome end.

Mammalian 5' C-rich telomeric overhangs are a mark of recombination-dependent telomere maintenance.

Recent evidence for 5'-cytosine (C)-rich overhangs at the telomeres of the nematode Caenorhabditis elegans provided the impetus to re-examine the end structure of mammalian telomeres. Two-dimensional (2D) gel electrophoresis, single telomere-length analysis (STELA), and strand-specific exonuclease assays revealed the presence of a 5'-C-rich overhang at the telomeres of human and mouse chromosomes. C-overhangs were prominent in G1/S arrested as well as terminally differentiated cells, indicating that they did not represent replication intermediates.

Telomeric armor: the layers of end protection.

The linear nature of eukaryotic chromosomes necessitates protection of their physical ends, the telomeres, because the DNA-repair machinery can misconstrue the ends as double-stranded DNA breaks. Thus, protection is crucial for avoiding an unwarranted DNA-damage response that could have catastrophic ramifications for the integrity and stability of the linear genome. In this Commentary, we attempt to define what is currently understood by the term ;telomere protection'.

Telomerase recognizes G-quadruplex and linear DNA as distinct substrates.

Telomeric DNA can assemble into a nonlinear, higher-order conformation known as a G-quadruplex. Here, we demonstrate by electrospray ionization mass spectrometry that the two repeat telomeric sequence d(TGGGGTTGGGGT) from Tetrahymena thermophila gives rise to a novel parallel four-stranded G-quadruplex in the presence of sodium. The G-quadruplex directly interacts with the catalytic subunit of Tetrahymena telomerase (TERT) with micromolar affinity, and the presence of telomerase RNA is not obligatory for this interaction.