Coexistence of alternative lengthening of telomeres and telomerase in hTERT-transfected GM847 cells

It has been shown previously that some immortalized human cells maintain th eir telomeres in the absence of significant levels of telomerase activity b y a mechanism referred to as alternative lengthening of telomeres (ALT). Ce lls utilizing ALT have telomeres of very heterogeneous length, ranging from very short to very long. Here we report the effect of telomerase expressio n in the ALT cell line GM847. Expression of exogenous hTERT in GM847 (GM847 /hTERT) cells resulted in lengthening of the shortest telomeres; this is th e first evidence that expression of hTERT in ALT cells can induce telomeras e that is active at the telomere. However, rapid fluctuation in telomere le ngth still occurred in the GM847/hTERT cells after more than 100 population doublings. Very long telomeres and ALT-associated promyelocytic leukemia ( PML) bodies continued to be generated, indicating that telomerase activity induced by exogenous hTERT did not abolish the ALT mechanism. In contrast, when the GM847 cell line was fused with two different telomerase-positive t umor cell lines, the ALT phenotype was repressed in each case. These hybrid cells were telomerase positive, and the telomeres decreased in length, ver y rapidly at first and then at the rate seen in telomerase-negative normal cells. Additionally, ALT-associated PML bodies disappeared. After the telom eres had shortened sufficiently, they were maintained at a stable length by telomerase. Together these data indicate that the telomerase-positive cell s contain a factor that represses the ALT mechanism but that this factor is unlikely to be telomerase. Further, the transfection data indicate that AL T and telomerase can coexist in the same cells.