Human fibroblasts expressing the catalytic component of human telomerase (hTERT) have been followed for 250 -400 population doublings. As expected, telomerase activity declined in long term culture of stable transfectants. Surprisingly, however, clones with average telomere lengths several kilobases shorter than those of senescent parental cells continued to proliferate. Although the longest telomeres shortened, the size of the shortest telomeres was maintained. Cells with subsenescent telomere lengths proliferated for an additional 20 doublings after inhibiting telomerase activity with a dominant-negative hTERT mutant. These results indicate that, under conditions of limiting telomerase activity, cis-acting signals may recruit telomerase to act on the shortest telomeres, argue against the hypothesis that the mortality stage 1 mechanism of cellular senescence is regulated by telomere positional effects (in which subtelomeric loci silenced by long telomeres are expressed when telomeres become short), and suggest that catalytically active telomerase is not required to provide a protein-capping role at the end of very short telomeres.Normal human fibroblasts have a limited ability to proliferate in culture (1, 2). The use of conditionally expressed viral oncogenes led to the definition of two separate mechanisms regulating this phenomenon (3). Mortality stage 1 (M1) 1 occurs when the functional activation of pathways requiring both p53 and pRB causes the growth arrest associated with cellular senescence (4, 5). Viral oncogenes that bind and inactivate p53 and pRB block M1 and permit continued cell division for an additional 20 -40 doublings until an independent blockade to cell proliferation, the M2 mechanism, occurs. The balance of cell division and cell death at M2 (crisis) eventually tips in favor of cell death, so that the culture deteriorates and is generally lost. In human fibroblast cultures, some clones can spontaneously escape M2 and become immortal at a frequency of approximately 10 Ϫ7 (6). DNA polymerase ␣ cannot replicate the very end of a linear chromosome (7,8), and consequently the compensatory action of telomerase is required to maintain telomere length. Because telomerase is turned off in most human tissues during development (9) and cultured human fibroblasts lack telomerase activity (10, 11), telomeres shorten progressively with ongoing cell divisions. A causal relationship between telomere shortening and proliferative limits was firmly established by the demonstration that telomere shortening controlled M2 (12). Telomerase was repressed in hybrids between normal young fibroblasts with long telomeres and SV40 T-antigen immortalized fibroblasts whose telomeres had been experimentally manipulated to an average size of either 2.5 or 5 kb. The 20 extra population doublings obtained in the hybrids with the 5-kb starting telomere length established that telomere length was the limiting factor (12). Since T-antigen would have blocked the M1 mechanism in these hybrids, these results showed that telomere ...

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Recently a novel hematopoietic growth factor, stem cell factor (SCF), was cloned and demonstrated to be the ligand for the c-kit tyrosine kinase receptor. In the mouse, SCF is encoded by Sl (steel), a gene critical to the development of several distinct cell lineages during embryonic life and which has important effects on hematopoiesis in the adult animal. The Sl/SCF locus maps to the distal region of mouse chromosome 10, in the vicinity of genes that have been mapped to human chromosome 12. Here we report the use of somatic cell hybrid lines to localize SCF to the long arm of human chromosome 12, between 12q14.3 and 12qter. In addition to localizing the Sl homolog in man, these data provide further evidence for the conservation of synteny between the long arm of human chromosome 12 and the distal end of mouse chromosome 10.

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