The quest for immortality has long been pursued by humans — as yet to no avail — but it is also a critical step on the path to tumour formation, and cancer cells will use any means at their disposal to achieve this end. Rustgi and colleagues, reporting in the September issue of The Journal of Clinical Investigation, provide a model for how this can be achieved by the alternative (ALT) pathway — a telomerase-independent mechanism.

The authors analysed the effects on cellular immortality of genetic changes that occur in oral-oesophageal squamous cell carcinoma, using oral keratinocytes as a model. They first overexpressed the cyclin D1 oncogene, the most common alteration in this cancer type. The effect was twofold: the cell-cycle distribution changed to include 20% more cells in S phase, and cells had an extended lifespan, undergoing 80 population doublings before entering senescence, rather than 40.

Combining this with a dominant-negative TP53 mutation — another frequent genetic change in cancer — exacerbated the cell-growth phenotype. These cells now had a lifespan of greater than 160 population doublings, and showed no signs of arrest. So, what was the mechanism of immortality?

Two mechanisms can facilitate immortality: activation of telomerase and activation of the ALT pathway. Rustgi and colleagues did not find evidence of the first possibility, as telomerase activity was absent from both the parent oral keratinoctyes and the ones expressing cyclin D1 and dominant-negative p53. Consistent with a lack of telomerase activity, telomere length — measured by Southern blotting with a telomere-specific probe — was initially shown to decrease with cell age. However, keratinocytes expressing cyclin D1 and dominant-negative p53 were found to have longer, albeit heterogeneous, telomeres by 160 population doublings. The heterogeneity, coupled with a karyotypic analysis that revealed aneuploidy and chromosomal changes, supported the hypothesis that the ALT pathway, which maintains telomeres through non-homologous recombination, was involved.

Interestingly, injection of these cells into nude mice did not result in tumour formation, indicating that they were not malignantly transformed. These keratinocytes therefore provide an interesting model that will allow the investigation of two problems: what additional changes are needed for transformation, and what is the mechanism by which the ALT pathway stabilizes telomeres?