Ph.D Thesis

Ph.D StudentYalon-Oren Michal
SubjectSister Chromatid Separation at Human Telomeric Regions
DepartmentDepartment of Medicine
Supervisors PROFESSOR EMERITUS Karl Skorecki
Full Thesis textFull thesis text - English Version


Telomeres are nucleoprotein complexes located at chromosome ends, vital for preserving chromosomal integrity. Telomeric DNA shortens with progressive rounds of cell division, culminating in replicative senescence. Previously, the supervisors’ laboratory has reported, on the basis of fluorescence in situ hybridization, that several human telomeric regions display solitary signals (singlets) in metaphase cells of pre-senescent fibroblasts, in comparison to other genomic regions that hybridize as twin signals (doublets). In the current study, we show that an additional 12 out of 12 telomeric regions examined also display single metaphase signals in pre-senescent cells, and that excess telomere-metaphase singlets also occurs in earlier passage cells harvested from elderly individuals. In cancer cell lines expressing telomerase and in pre-senescent fibroblasts ectopically expressing hTERT, this phenomenon is abrogated. Confocal microscope image analysis showed that the telomere-metaphase singlets represent regions that have replicated but not separated; this is presumably because of persistent cohesion. The introduction of mutations that interfere with the normal dissolution of cohesion at the metaphase to anaphase transition induced the cut (chromosomes untimely torn) phenotype in early passage fibroblasts, with predominantly telomeric rather than centromeric DNA, present on the chromatin bridges joinig daughter nuclei. These results suggest that telomeric regions in animal cells may be potential sites of persistent cohesion, and that this cohesion may be the basis for an observed excess of fluorescence in situ hybridization metaphase singlets at telomeres. Such persistent cohesion at telomeres may be associated with attempted DNA repair or alternatively with chromosomal abnormalities, which have been described in pre-senescent cells.