To develop the INK4A-luciferase mouse, the authors 'knocked in' the firefly luciferase cDNA to the translational start site of INK4A (which is encoded, along with ARF, by Cdkn2a). The resulting mouse, Ink4aLUC/LUC, was null for INK4A but retained expression of ARF. Heterozygous Ink4a+/LUC mouse embryonic fibroblasts produced nearly identical levels of Ink4a and luciferase mRNA, and in vivo luciferase activation mirrored normal Ink4a expression under physiological conditions, indicating that the luciferase construct faithfully reports endogenous Ink4a expression.
The accumulation of senescent cells that express INK4A has been associated with ageing. Indeed, Ink4a+/LUC mice showed an exponential increase in total body luciferase (TBL) activity with age. However, there was considerable variability in luciferase expression between individual mice; so, given the association between cellular senescence and age-related phenotypes, they hypothesized that higher TBL levels would predict the development of age-related phenotypes and mortality. Surprisingly, TBL was not predictive of mortality, which was uniformly caused by the development of spontaneous malignancy. Therefore, expression of the INK4A marker of senescence is not associated with risk of spontaneous cancers in experimentally housed mice. However, they did not analyse other age-related phenotypes.
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