Telomere uncapping in progenitor cells with critical telomere shortening is coupled to S-phase progression in vivo.

Telomeres protect chromosome ends and serve as a substrate for telomerase, a reverse transcriptase that adds DNA repeats to the telomere terminus. In the absence of telomerase, telomeres progressively shorten, ultimately leading to telomere uncapping, a structural change at the telomere that activates DNA damage responses and leads to ligation of chromosome ends. Telomere uncapping has been implicated in aging and cancer, yet the precise mechanism of uncapping and its relationship to cell cycle remain to be defined. Here, we show that telomeres uncap in an S-phase-dependent manner in gastrointestinal progenitors of TERT(-/-) mice. We develop an in vivo assay that allows a quantitative kinetic assessment of telomere dysfunction-induced apoptosis and its relationship to cell cycle. By exploiting the mathematical relationship between rates of generation and clearance of apoptotic cells, we show that 86.2 +/- 8.8% of apoptotic gastrointestinal cells undergo programmed cell death either late in S-phase or in G2. Apoptosis is primarily triggered via a signaling cascade from newly uncapped telomeres to the tumor suppressor p53, rather than by chromosome fusion-bridge breakage, because mitotic blockade did not alter the rate of newly generated apoptotic bodies. These data support a model in which rapidly dividing progenitor cells within a tissue with short telomeres are vulnerable to telomere uncapping during or shortly after telomere replication.

Effects of spontaneous KitlSteel mutations on survival and red blood cells of mice.

Kit ligand (Kitl), which is a member of the helical cytokine superfamily, is encoded by the Steel (Sl) locus of mice and is essential for the development of hematopoietic cells, germ cells, and melanocytes. A large series of Kitl(Sl) alleles has been described, including some that arose spontaneously and others that were induced by either chemical or radiation mutagenesis. Here we describe the nucleotide sequence alterations in two spontaneous Kitl(Sl) alleles. The Kitl(Sl-18R) allele has a point mutation that introduces a premature termination codon, and the encoded protein is expected to be null functionally. The Kitl(Sl-5R) allele has an in-frame deletion that results in deletion of amino acids at position 31 and 32 of Kitl. While both mutations exert severe effects on blood cells and survival of homozygous mice, these effects are slightly milder than those of a previously characterized spontaneous deletion allele, Kitl(Sl-gb). Examination of the survival of compound heterozygotes provided strong genetic evidence that the Kitl(Sl-18R) and Kitl(Sl-5R) mutants are null functionally for mouse survival.