The reserve-capacity hypothesis: evolutionary origins and modern implications of the trade-off between tumor-suppression and tissue-repair.

Antagonistic pleiotropy, the evolutionary theory of senescence, posits that age related somatic decline is the inevitable late-life by-product of adaptations that increase fitness in early life. That concept, coupled with recent findings in oncology and gerontology, provides the foundation for an integrative theory of vertebrate senescence that reconciles aspects of the 'accumulated damage' 'metabolic rate', and 'oxidative stress' models. We hypothesize that (1) in vertebrates, a telomeric fail-safe inhibits tumor formation by limiting cellular proliferation. (2) The same system results in the progressive degradation of tissue function with age. (3) These patterns are manifestations of an evolved antagonistic pleiotropy in which extrinsic causes of mortality favor a species-optimal balance between tumor suppression and tissue repair. (4) With that trade-off as a fundamental constraint, selection adjusts telomere lengths--longer telomeres increasing the capacity for repair, shorter telomeres increasing tumor resistance. (5) In environments where extrinsically induced mortality is frequent, selection against senescence is comparatively weak as few individuals live long enough to suffer a substantial phenotypic decline. The weaker the selection against senescence, the further the optimal balance point moves toward shorter telomeres and increased tumor suppression. The stronger the selection against senescence, the farther the optimal balance point moves toward longer telomeres, increasing the capacity for tissue repair, slowing senescence and elevating tumor risks. (6) In iteroparous organisms selection tends to co-ordinate rates of senescence between tissues, such that no one organ generally limits life-span. A subsidiary hypothesis argues that senescent decline is the combined effect of (1) uncompensated cellular attrition and (2) increasing histological entropy. Entropy increases due to a loss of the intra-tissue positional information that normally regulates cell fate and function. Informational loss is subject to positive feedback, producing the ever-accelerating pattern of senescence characteristic of iteroparous vertebrates. Though telomere erosion begins early in development, the onset of senescence should, on average, be deferred to the species-typical age of first reproduction, the balance point at which selection on this trade-off should allow exhaustion of replicative capacity to overtake some cell lines. We observe that captive-rodent breeding protocols, designed to increase reproductive output, simultaneously exert strong selection against reproductive senescence and virtually eliminate selection that would otherwise favor tumor suppression. This appears to have greatly elongated the telomeres of laboratory mice. With their telomeric failsafe effectively disabled, these animals are unreliable models of normal senescence and tumor formation. Safety tests employing these animals likely overestimate cancer risks and underestimate tissue damage and consequent accelerated senescence.