Telomere dynamics, end-to-end fusions and telomerase activation during thehuman fibroblast immortalization process

Loss of telomeric repeats during cell proliferation could play a role in se nescence, It has been generally assumed that activation of telomerase preve nts further telomere shortening and is essential for cell immortalization, In this study, we performed a detailed cytogenetic and molecular characteri zation of four SV40 transformed human fibroblastic cell lines by regularly monitoring the size distribution of terminal restriction fragments, telomer ase activity and the associated chromosomal instability throughout immortal ization, The mean TRF lengths progressively decreased in pre-crisis cells d uring the lifespan of the cultures, At crisis, telomeres reached a critical size, different among the cell lines, contributing to the peak of dicentri c chromosomes, which resulted mostly from telomeric associations. We observ ed a direct correlation between short telomere length at crisis and chromos omal instability. In two immortal cell lines, although telomerase was detec ted, mean telomere length still continued to decrease whereas the number of dicentric chromosomes associated was stabilized. Thus telomerase could pro tect specifically telomeres which have reached a critical size against end- to-end dicentrics, while long telomeres continue to decrease, although at a slower rate as before crisis. This suggests a balance between elongation b y telomerase and telomere shortening, towards a stabilized 'optimal' length .