The replicative lifespan of cells is thought to be determined by one of two mechanisms that ultimately trigger senescence — cumulative DNA damage or the progressive shortening of telomeres. Robert Weinberg and colleagues, reporting in Nature Genetics, now challenge the 'telomere length' hypothesis.

Weinberg and colleagues examined the possible role of the 3′ telomeric overhang — a key component of the telomere structure that is composed of several hundred nucleotides of the G-rich strand extended beyond the end of the C-rich strand — during senescence. For this, they used a newly developed method, the telomere-oligonucleotide ligation assay (T-OLA), in which radiolabelled oligonucleotides complementary to the overhang are annealed, and then ligated, to genomic DNA. The length and intensity of the ligation products, as measured by electrophoretic separation and densitometric analysis, respectively, indicate the length of the overhangs present in the sample.

Using T-OLA, the authors measured the telomeric overhang in BJ fibroblast cell lines. By population doubling 106, when virtually all BJ cells were senescent, the maximal detected length was 384 nucleotides, compared with 624 nucleotides in early-passage cells, and the overall signal intensity had decreased to 19% of that in early-passage cells. Similar results were found in a second fibroblast cell line.

Expression of telomerase is known to prevent senescence, which Weinberg and colleagues explain by the enzyme's ability to extend the 3′ overhang. So is the overhang loss the cause or the consequence of senescence? To test this, the authors examined whether the telomeric overhang was lost in cells that avoid senescence by inactivation of p53 and retinoblastoma-associated protein (RB). Overhang loss in these cells was similar to that in senescent cells, indicating that it was a result of continuous cell division rather than senescence itself. In addition, the authors found no evidence that the quiescent state of senescent cells or physiological stress are responsible for the extent of telomeric overhang loss seen in senescent cells.

So, the authors conclude that “...overhang loss, rather than shortening of overall telomere length, is the molecular signal that triggers senescence”. They propose that the overhang loss might affect a crucial telomeric structure, such as the T-loop.