Telomeric fusions in cultured human fibroblasts as a source of genomic instability

In a human fibroblast clone we studied the evolution, during culture propagation, of a dicentric chromosome consisting of the end-to-end association of the short arm of chromosome 5 and the long arm of chromosome 16. Dual-color fluorescence in situ hybridization (FISH) with painting probes allowed us to define the structure of a variety of derivative chromosomes and to identify the mechanisms by which they originated. Asymmetric interchanges involving the intercentromeric region of the dicentric, bridge-breakage- fusion events, or breaks followed by sister chromatid fusion, originate unstable hetero- or homodicentric chromosomes with deletion or duplication; breakages not followed by reunion, or intradicentric recombination, presumably originate stable rearranged monocentric chromosomes. The variety of the derivatives is extremely large because the observed events may involve any site of the intercentromeric region, although the majority of them occurs after a break in 16qh. The results of this investigation document the evolution through successive steps of a telomeric fusion, a chromosome anomaly frequently observed in tumor and senescent cells. They also demonstrate that in cultured cells of normal origin, starting with this anomaly, various chromosomal mechanisms may produce translocations, duplications, and deletions. The karyotype instability produced by a telomeric fusion can be relevant for carcinogenesis because it may generate genetic changes critical in the multistep process of transformation.