The significance of dynamic dysfunction of critically short telomeres in directing the evolution of chromosomal structural instability in human cells undergoing immortalization

Abstract

Nonrandom patterns of chromosomal aberrations are universally observed in all types of cancers as well as immortalized cell lines, for which the mechanism of initiation and progression is unknown. To examine whether dynamic telomere dysfunction contributes to the initiation and evolution of the preferential chromosomal aberrations during cellular immortalization, we developed a new technique which allowed, for the first time, telomere quantitative FISH and 24-colour spectral karyotyping in the same human cells. Two human ovarian surface epithelial cell lines (HOSE6-3, HOSE11-12), two esophageal cell lines (NE1-E6E7, NE6-E6E7), and one nasopharyngeal cell line (NP460-E6E7) immortalized by expression of HPV16 E6E7 oncogenes, were studied. We detected striking consistent differences in telomere signal intensities between homologous chromosomes and among non-homologous chromosomes in all five cell lines. The cell lines established from specimens from different donors had different distributions of the shortest telomeres. During cellular aging in vitro, chromosomes with the shortest telomeres were the first to show increased frequencies of chromosomal structural aberrations including dicentrics, translocations, insertions, deletions and duplications, independent of cell lines and cell types. With further cell proliferation towards crisis, progressive increases in frequencies of chromosomal structural aberrations occurred preferentially on specific chromosomes that had increasing frequencies of signal-free telomeres. None of analyzed pre-crisis or crisis metaphases had completely identical aberrations. In post-crisis (immortalized) cells, the majority of clonal stable aberrations again involved the chromosomes with signal-free telomeres before and during crisis. The existence of clonal aberrations in post-crisis cells demonstrates that chromosomes in post-crisis cells are far more stable than those during crisis. Parallel to this, the frequencies of chromosomal ends with undetectable telomeres dropped dramatically in post-crisis cells. Taken together, this study showed that the dysfunction of critically short telomeres played a fundamental role in directing the evolution of chromosomal structural instability leading to formation of nonrandom chromosomal aberrations in human cells undergoing immortalization, and this phenomenon appeared to be independent of cell lines and cell-types.