Telomere dynamics, end-to-end fusions and telomerase activation during the human fibroblast immortalization process.

Loss of telomeric repeats during cell proliferation could play a role in senescence. It has been generally assumed that activation of telomerase prevents further telomere shortening and is essential for cell immortalization. In this study, we performed a detailed cytogenetic and molecular characterization of four SV40 transformed human fibroblastic cell lines by regularly monitoring the size distribution of terminal restriction fragments, telomerase activity and the associated chromosomal instability throughout immortalization. The mean TRF lengths progressively decreased in pre-crisis cells during the lifespan of the cultures. At crisis, telomeres reached a critical size, different among the cell lines, contributing to the peak of dicentric chromosomes, which resulted mostly from telomeric associations. We observed a direct correlation between short telomere length at crisis and chromosomal instability. In two immortal cell lines, although telomerase was detected, mean telomere length still continued to decrease whereas the number of dicentric chromosomes associated was stabilized. Thus telomerase could protect 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 by telomerase and telomere shortening, towards a stabilized 'optimal' length.

Immunosenescence in HIV pathogenesis.

Telomeres are complex protein-DNA structures located at the ends of eukaryotic chromosomes. In a normal cell, telomere DNA shortens with cell divisions. Such a telomere loss may act as a mitotic clock to eventually signal cell cycling exit and cellular senescence. In a transversal study, we found a marked decrease in telomere length of peripheral blood mononuclear cells in HIV-infected patients with advanced immunodeficiency. This telomere reduction concerns T4, T8, and B lymphocytes, providing evidence of high turnover of these cells in the course of HIV infection. These data suggest that replicative senescence could be involved in the final immunosuppression and may have important therapeutical implications.

Chromosomal instability and alteration of telomere repeat sequences.

The very end of the chromosome is called the telomere and is composed of DNA repeat sequences and associated proteins. Genetic and biochemical analyses of this complex, the telosome, lead to the hypothesis that transcription and DNA replication are submitted to position effects mediated by the telomere proximity. Telomere length reduction and alterations of the telomeric chromatin assembly might explain the chromosome instability which occurs during the senescence and the immortalization process in vitro. A particular polymerase, the telomerase, is able to lengthen the telomeres. A telomerase activity was characterized in yeast, Tetrahymena, but also in transformed and in germline cells. We reviewed the involvement of telomeres in the aging process. We proposed that the short size of the telomere repeat at each chromosome could direct the loss of heterozygosity, thus telomere length could play a role in individual and tissular susceptibility to develop cancer. Antitelomerase strategy for cancer therapy is attractive but limited by the short decrease of the telomere length at each cell division.