Telomere shortening can lead to genomic instability and cancer, as telomeres protect the chromosome ends from being recognized as DNA breaks, which prevents chromosome end-to-end fusions. So what determines the cellular response to telomere shortening? Several groups have proposed that it is the average telomere length, but now, Carol Greider's group, reporting in Cell, show that it is the shortest telomere that initiates the response and affects cell survival.

To investigate this problem, the authors crossed mice that had long telomeres (heterozygous for telomerase; mTR +/−) with mice that had short telomeres — deleted for telomerase and interbred for six generations (mTR−/− G6). The offspring would have a similar average telomere length, which would be equivalent to mTR−/− G3 mice, and would be either null (F1 mTR−/−) or heterozygous (F1 mTR+/−) for telomerase activity. Late-generation mice (G4–G6) show fertility defects, but the mTR−/− G3 mice do not. Would the F1 mTR−/− and F1 mTR+/− mice? The F1 mTR+/− mice have a similar phenotype to the parental mTR+/− strain, but the F1 mTR−/− mice are more like the mTR−/− G6 mice than the mTR−/− G3 mice. Both strains have a correspondingly low testes weight, which is a reflection of the increased germ-cell apoptosis. The number of chromosome fusions is also higher than in the mTR−/− G3 mice. These results indicate that it is the presence of short telomeres, rather than the average telomere length, that elicits the cellular response.

Interestingly, when the telomere-length distribution was measured in these mice — using quantitative fluorescence in situ hybridization (Q-FISH) — the authors found that the average telomere length was only slightly higher in F1 mTR+/− mice than in F1 mTR−/− mice. There was, however, a significant difference in the number of short telomeres in the two mouse strains. It seems that the short telomeres in the F1 mTR+/− mice can be specifically recognized and elongated to restore telomere function.

To further characterize the cellular response to telomere shortening, the authors measured chromosome fusions and the number of short telomeres in mTR−/− mice. The fusion events were non-random — for example, fusions frequently involved chromosomes 5 and 19 in one mouse strain — and correlated with chromosomes that had short telomeres. Chromsome fusions are non-random in other mTR−/− strains, but do not involve the same chromosomes. The participation in fusion events is therefore not chromosome-specific. The authors concluded that a short telomere is most likely to lose function and fuse with another short telomere, and therefore that two short telomeres might be required for a fusion event to take place.

So what causes telomere dysfunction and chromosome fusion? Sequencing of the junctions between the two chromosomes showed that no telomere sequence is present; and FISH showed that microsatellite repeats are also decreased. However, regions of microhomology do exist within the remaining microsatellite repeats, which supports the previous hypothesis that non-homologous end joining is involved in chromosome fusions.

One problem that remains to be solved is how telomerase activity is targeted to the shortest telomeres. We await, with interest, further explanation of this mechanism.