REPROGRAMMING OF TELOMERASE BY EXPRESSION OF MUTANT TELOMERASE RNA TEMPLATE IN HUMAN-CELLS LEADS TO ALTERED TELOMERES THAT CORRELATE WITH REDUCED CELL VIABILITY

Telomerase synthesizes telomeric DNA by copying the template sequence of its own RNA component. In Tetrahymena thermophila and yeast (G. Yu, J. D. Bradley, L. D. Attardi, and E. H. Blackburn, Nature 344:126-131 , 1990; M. McEachern and E. H. Blackburn, Nature 376:403-409, 1995), m utations in the template domain of this RNA result in synthesis of mut ant telomeres and in impaired cell growth and survival. We have invest igated whether mutant telomerase affects the proliferative potential a nd viability of immortal human cells. Plasmids encoding mutant or wild -type template RNAs (hTRs) of human telomerase and the neomycin resist ance gene were transfected into human cells to generate stable transfo rmants. Expression of mutant hTR resulted in the appearance of mutant telomerase activity and in the synthesis of mutant telomeres. Transfor med cells were not visibly affected in their growth and viability when grown as mass populations. However, a reduction in plating efficiency and growth rare and an increase in the number of senescent cells were detected in populations with mutant telomeres by colony-forming assay s. These results suggest that the presence of mutant telomerase, even if coexpressed with the wild-type enzyme, can be deleterious to cells, likely as a result of the impaired function of hybrid telomeres.