Telomerase activity does not always imply telomere maintenance

The forced expression of the catalytic subunit of human telomerase, hTERT, produces telomerase activity, allows telomere maintenance, and extends the cellular life span of IMR90 human lung fibroblasts. The mutation D869A abol ishes both the catalytic activity of hTERT and its ability to extend cellul ar life span, demonstrating that the immortalizing capabilities of the enzy me are dependent on active catalysis. A second mutant of hTERT was examined that contains three copies of an HA epitope inserted at the C-terminus, Th is mutant produced telomerase activity in fibroblasts that was virtually in distinguishable from that of wild type telomerase when assayed in vitro. Ho wever, the forced expression of this mutant failed to maintain telomeres or extend cellular life span. Our results show that the catalytic activity of hTERT is required for cellular immortalization but that the presence of ac tive telomerase does not necessarily imply telomere maintenance and immorta lity. (C) 1999 Academic Press.