RECONSTITUTION OF TELOMERASE ACTIVITY IN NORMAL HUMAN-CELLS LEADS TO ELONGATION OF TELOMERES AND EXTENDED REPLICATIVE LIFE-SPAN

Normal somatic cells have a finite life span [1] and lose telomeric DN A, present at the ends of chromosomes, each time they divide as a func tion of age in vivo or in culture [2-4], In contrast, many cancer cell s and cell lines established from tumours maintain their telomere leng th by activation of an RNA-protein complex called telomerase, an enzym e originally discovered in Tetrahymena [5], that synthesizes telomeric repeats [6-8], These findings have led to the formation of the 'telom ere hypothesis', which proposes that critical shortening of telomeric DNA due to the end-replication problem [9] is the signal for the initi ation of cellular senescence [10,11]. In yeast, the EST2 gene product, the catalytic subunit of telomerase, is essential for telomere mainte nance in vivo [12-14], The recent cloning of the cDNA encoding the cat alytic subunit of human telomerase (hTERT) [15,16] makes it possible t o test the telomere hypothesis, In this study, we expressed hTERT in n ormal human diploid fibroblasts, which lack telomerase activity, to de termine whether telomerase activity could be reconstituted leading to extension of replicative life span, Our results show that retroviral-m ediated expression of hTERT resulted in functional telomerase activity in normal aging human cells, Moreover, reconstitution of telomerase a ctivity in vivo led to an increase in the length of telomeric DNA and to extension of cellular life span, these findings provide direct evid ence in support of the telomere hypothesis, indicating that telomere l ength is one factor that can determine the replicative life span of hu man cells.