Telomeres are specialized structures found at the ends of eukaryotic chromosomes serving as guardians of genome stability. In normal cells telomeres shorten with each cell division, but immortal cells undergoing multiple divisions constantly have to maintain telomere lengths above a critical level. This is accomplished either through expression of telomerase or the alternative recombination pathway (ALT). In the present study, we analyzed telomere dynamics of the telomerase positive human pancreatic tumor cell line MIAPaCa-2. The cells demonstrated genomic instability with a high frequency of chromosomal aberrations resulting in differences between individual karyotypes within the same cell population. The telomeres were short when compared with normal human fibroblasts, and about 39% of the chromosome ends did not have detectable telomere repeats as demonstrated by PNA-FISH. In many cases telomere signals were missing even when sister chromatids were strongly labeled. In addition, we used an internal PNA probe specific for the X chromosome, present in a single copy in these cells, in order to follow telomere dynamics on individual chromatids. High heterogeneity in telomere signals among individual X chromosomes as well as between their sister chromatids suggested sudden and stochastic loss or gain of telomere repeats. Such constant genomic instability often results in apoptosis and death of a fraction of cells present in the culture at all times. We discuss possible molecular mechanisms that may explain this observed telomere heterogeneity and possible adaptive repair mechanisms by which these cells maintain their chromosomes in order to survive such extreme and permanent genomic instability.

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