Telomere maintenance by telomerase and by recombination can coexist in human cells

Immortal human cells maintain their telomeres by two independent mechanisms , a prevalent one dependent on de novo synthesis of telomeric DNA by telome rase, and a rarer one based on telomere recombination [alternative lengthen ing of telomeres (ALT)]. Studies with yeast have indicated that expression of telomerase inhibits telomere recombination. In the present study, we hav e investigated whether expression of telomerase in cells that use ALT would similarly reveal dominance of telomere elongation by telomerase over telom ere recombination. Telomerase-negative WI38 VA13/2RA ALT cells were reconst ituted for telomerase activity through ectopic expression of the enzyme sub units, hTERT and hTR, and the presence and function of telomerase and ALT w ere monitored during long term cell growth by enzymatic assays, detection o f the ALT-associated PML bodies (APBs) and analysis of telomere dynamics. O ur results indicate that telomerase activity and APBs persisted in the cell s over at least 90 population doublings. The activity of both pathways on t elomeres was determined by analysis of telomere length versus time by gel e lectrophoresis and in situ hybridization. ALT cells are characterized by ve ry heterogeneous telomeres with a much longer average size than the telomer es of telomerase-positive cells. Telomere dynamics in our cells were compat ible with both ALT and telomerase being biologically active since the long telomeres typical of ALT were maintained, while short telomeres, thought to be the preferential substrate of telomerase, were elongated. These finding s, indicating that human cells may be capable of concomitantly utilizing bo th mechanisms of telomere maintenance without effects on their growth and v iability, have implications for cancer therapy.