Telomerase-mediated telomere addition counteracts telomere shortening due to incomplete DNA replication. Short telomeres are the preferred substrate for telomere addition by telomerase; however, the mechanism by which telomerase recognizes short telomeres is unclear. In yeast, the Ataxia telangiectasia mutated (Atm) homolog, Tel1, is necessary for normal telomere length regulation likely by altering telomere structure, allowing telomerase recruitment to short telomeres. To examine the role of Atm in establishing preference for elongation of short telomeres in mice, we examined telomerase-mediated elongation of short dysfunctional telomeres in the presence or absence of Atm. Here we show that Atm is dispensable for elongation of short telomeres by telomerase, suggesting that telomerase recruitment in mammalian cells and in yeast may be regulated differently.

Eukaryotic cells undergo arrest and enter apoptosis in response to short telomeres. T cells from late generation mTR(-/-) mice that lack telomerase show increased apoptosis when stimulated to enter the cell cycle. The increased apoptosis was not inhibited by colcemid, indicating that the response did not result from breakage of dicentric chromosomes at mitosis. The damage response protein gamma-H2AX localized to telomeres in metaphases from T cells and fibroblasts from mTR(-/-) cells with short telomeres. These data suggest that the major mechanism for induction of apoptosis in late generation mTR(-/-) cells is independent of chromosome segregation and that loss of telomere function through progressive telomere shortening in the absence of telomerase leads to recognition of telomeres as DNA breaks.

To examine the role of ataxia-telangiectasia mutated (Atm) in telomere function, we generated Atm and telomerase null mice (Atm(-/-) mTR(-/-) iG6 mice). These mice exhibited increased germ cell death and chromosome fusions compared with either Atm(-/-) or mTR(-/-) iG6 mice. Furthermore, the Atm(-/-) mTR(--) iG6 mice had a delayed onset and reduced incidence of thymic lymphoma compared with Atm(-/-) mice. The tumors in the Atm(-/-) mTR(-/-) iG6 mice showed increased apoptosis and anaphase bridges. Finally, lymphomas from Atm(-/-) mTR(-/-) iG6 mice were derived from CD8 immature, single-positive T cells, whereas Atm(-/-) lymphomas were from CD4(+)CD8(+) double-positive T cells. We propose that Atm protects short telomeres and that Atm deficiency cooperates with short telomeres, leading to increased cell death, decreased tumorigenesis, and increased overall survival.

Telomeres are usually maintained about an equilibrium length, and the set point for this equilibrium differs between species and between strains of a given species. To examine the requirement for telomerase in mediating establishment of a new telomere length equilibrium, we generated interspecies crosses with telomerase mTR knockout mice. In crosses between C57BL/6J (B6) and either of two unrelated mouse species, CAST/Ei and SPRET/Ei, telomerase mediated establishment of a new telomere length equilibrium in wild-type mTR+/+ mice. This new equilibrium was characterized by elongation of the short telomeres of CAST/Ei or SPRET/Ei origin. In contrast, mTR−/− offspring of interspecies crosses failed to elongate telomeres. Unexpectedly, haploinsufficiency was observed in mTR+/− heterozygous interspecies mice, which had an impaired ability to elongate short SPRET/Ei or CAST/Ei telomeres to the new equilibrium set point that was achieved in wild-type mTR+/+ mice. These results demonstrate that elongation of telomeres to a new telomere set point requires telomerase and indicate that telomerase RNA may be limiting in vivo.

Telomere dysfunction results in fertility defects in a number of organisms. Although data from fission yeast and Caenorhabditis elegans suggests that telomere dysfunction manifests itself primarily as defects in proper meiotic chromosome segregation, it is unclear how mammalian telomere dysfunction results in germ cell death. To investigate the specific effects of telomere dysfunction on mammalian germ cell development, we examined the meiotic progression and germ cell apoptosis in late generation telomerase null mice. Our results indicate that chromosome asynapsis and missegregation are not the cause of infertility in mice with shortened telomeres. Rather, telomere dysfunction is recognized at the onset of meiosis, and cells with telomeric defects are removed from the germ cell precursor pool. This germ cell telomere surveillance may be an important mechanism to protect against the transmission of dysfunctional telomeres and chromosomal abnormalities.

Loss of telomere function can induce cell cycle arrest and apoptosis. To investigate the processes that trigger cellular responses to telomere dysfunction, we crossed mTR−/− G6 mice that have short telomeres with mice heterozygous for telomerase (mTR+/−) that have long telomeres. The phenotype of the telomerase null offspring was similar to that of the late generation parent, although only half of the chromosomes were short. Strikingly, spectral karyotyping (SKY) analysis revealed that loss of telomere function occurred preferentially on chromosomes with critically short telomeres. Our data indicate that, while average telomere length is measured in most studies, it is not the average but rather the shortest telomeres that constitute telomere dysfunction and limit cellular survival in the absence of telomerase.