Are telomerase and ALT two equivalent pathways to immortality? This is the question that Robert Weinberg and colleagues have addressed in a report in the 1 October issue of Proceedings of the National Academy of Sciences.

Both pathways result in the addition of telomeric sequence onto the ends of chromosomes, preventing crisis, and allowing cells to proliferate indefinitely. But ALT seems to occur less frequently than might be expected.

Cells that have been immortalized with TERT — the catalytic component of telomerase — can be converted from a non-tumorigenic to a tumorigenic state by the addition of the HRAS-V12 oncoprotein; is the same true for cells that have been immortalized by ALT? The GM847 cell line lacks telomerase activity, and maintains its long telomeres using ALT; however, the addition of HRAS-V12 did not produce cells that were able to induce tumour formation when injected into immunodeficient mice. This ability was restored when TERT was also introduced.

So does telomerase replace ALT as the mechanism by which cells maintain their telomeres? GM847 cells that have been transfected with HRAS-V12 and TERT still have AA-PML bodies — a marker for ALT cells — that contain the characteristic proteins TRF2 and PML, indicating that ALT remains active even when TERT is expressed.

Interestingly, the ability of telomerase to elongate telomeres is not even required for the tumorigenic phenotype, as TERT that is tagged at its carboxyl terminus with HA — which has in vitro telomerase activity, but can not elongate telomeres in vivo — is still able to confer tumorigenic potential when transfected into GM847 cells with HRAS-V12.

So what additional role does TERT perform in these cells? In normal tissue culture conditions, the growth rates of GM847 cells that have been transfected with HRAS-V12 were similar, regardless of whether TERT was also expressed; however, when the growth conditions were changed to those of limiting nutrient and oxygen levels, the expression of TERT resulted in an increased growth rate.

ALT and telomerase are therefore not functionally equivalent — despite the fact that ALT can elongate telomeres to lengths beyond those achieved with telomerase. Perhaps the reason that ALT occurs at lower levels in cancer than expected is because an additional mutation is required — compared with reactivation of telomerase — before cells can obtain a comparable growth advantage and become tumorigenic.