Repression of an alternative mechanism for lengthening of telomeres in somatic cell hybrids

Some immortalized cell lines maintain their telomeres in the absence of det ectable telomerase activity by an alternative (ALT) mechanism. To study how telomere maintenance is controlled in ALT cells, we have fused an ALT cell Line GM847 (SV40 immortalized human skin fibroblasts) with normal fibrobla sts or with telomerase positive immortal human cell lines and have examined their proliferative potential and telomere dynamics. The telomeres in ALT cells are characteristically very heterogeneous in length, ranging from ver y short to very long, The ALT x normal hybrids underwent a rapid reduction in telomeric DIVA and entered a senescence-like state. Immortal segregants rapidly reverted to the ALT telomere phenotype. Fusion of ALT cells to telo merase-positive immortal cells in the same immortalization complementation group resulted in hybrids that appeared immortal and also exhibited repress ion of the ALT telomere phenotype. In these hybrids, which were all telomer ase-positive, we observed an initial rapid loss of most long telomeres, fol lowed either by gradual loss of the remaining long telomeres at a rate simi lar to the rate of telomere shortening in normal telomerase-negative cells, or by maintenance of shortened telomeres. These data indicate the existenc e of a mechanism of rapid telomere deletion in human cells. They also demon strate that normal cells and at least some telomerase-positive immortal cel ls contain repressors of the ALT telomere phenotype.