TELOMERASE ACTIVITY IS RESTORED IN HUMAN-CELLS BY ECTOPIC EXPRESSION OF HTERT (HEST2), THE CATALYTIC SUBUNIT OF TELOMERASE

The expression of telomerase, the enzyme that synthesizes telomeric DN A de novo, is suppressed in normal somatic human cells but is reactiva ted during tumorigenesis. This reactivation appears to arrest the norm al loss of telomeric DNA incurred as human cells divide, Since continu al loss of telomeric DNA is predicted to eventually limit cell prolife ration, activation of telomerase in cancer cells may represent an impo rtant step in the acquisition of the cell immortalization which occurs during tumor progression. The telomerase holoenzyme is composed of bo th RNA and protein subunits, In humans, mRNA expression of hTERT (hEST 2), the candidate telomerase catalytic subunit gene, appears to parall el the levels of telomerase enzyme activity, suggesting that induction of hTERT is necessary and perhaps sufficient for expression of telome rase activity in tumor cells. To test this model directly, we ectopica lly expressed an epitope-tagged version of hTERT in telomerase-negativ e cells and show that telomerase activity was induced to levels compar able to those seen in immortal telomerase-positive cells and that the expressed hTERT protein was physically associated with the cellular te lomerase activity. We conclude that synthesis of the hTERT telomerase subunit represents the rate-limiting determinant of telomerase activit y in these cells and that this protein, once expressed, becomes part o f the functional telomerase holoenzyme.