Molecular genetic approaches to the study of cellular senescence

Normal cells in culture exhibit limited division potential, which is used as a model for cellular aging. In contrast, tumor-derived, carcinogen- or virus-transformed cells are capable of dividing indefinitely (immortal). Fusion of normal with immortal human cells yielded hybrids having limited life span, indicating that cellular senescence is a dominant phenotype and that immortality is recessive. Fusions of various immortal human cell lines with each other led to the identification of four complementation groups for indefinite division. In order to identify the chromosomes and genes involved in growth regulation, that had been modified in immortal cells, we used the technique of microcell fusion to introduce either a normal human chromosome 11 or 4 into cell lines representative of the different complementation groups. Chromosome 11 had no effect on the in vitro life span of the different immortal human tumor lines. However, when a normal human chromosome 4 was introduced into cell lines assigned to complementation group B, the cells lost the immortal phenotype. No effect on the proliferation potential of cell lines representative of the other complementation groups was observed. These results suggest that a gene(s) on human chromosome 4 has been modified in immortal cell lines assigned to complementation group B, to allow escape from senescence. They also provide evidence for a genetic basis for cellular aging.