DNA replication stress, genome instability and aging

Abstract

Genome instability is a fundamentally important component of aging in all eukaryotes. How age-related genome instability occurs remains unclear. The free radical theory of aging posits oxidative damage to DNA and other cellular constituents as a primary determinant of aging. More recent versions of this theory predict that mitochondria are a major source of reactive oxygen species (ROS) that cause oxidative damage. Although substantial support for the free radical theory exists, the results of some tests of this theory have been contradictory or inconclusive. Enhanced growth signaling also has been implicated in aging. Many efforts to understand the effects of growth signaling on aging have focused on inhibition of oxidative stress responses that impact oxidative damage. However, recent experiments in the model organism Saccharomyces cerevisiae (budding yeast) and in higher eukaryotes suggest that growth signaling also impacts aging and/or age-related diseases--including cancer and neurodegeneration--by inducing DNA replication stress, which causes DNA damage. Replication stress, which has not been broadly considered as a factor in aging, may be enhanced by ROS that signal growth. In this article, we review evidence that points to DNA replication stress and replication stress-induced genome instability as important factors in aging.

Figure 1.

Factors that contribute to replication fork stalling (replication stress). Some of these factors overlap. For example, reduced levels of dNTPs could be a consequence of reduced availability of nutrient precursors or downregulation of proteins required for their synthesis.

Figure 2.

The unique structure of replicating DNA contributes to genome instability at replication forks, which is enhanced by replication fork stalling. (A) When non-replicating DNA molecules suffer single-strand lesions, the integrity of these molecules is maintained by hydrogen bond base pairing on either side of the lesions until they are repaired. (B) Replicating DNA molecules contain single-strand DNA at replication forks after unwinding of double-strand template DNA has occurred in preparation for template-directed DNA synthesis. This single-strand DNA is highly recombinogenic. (C) Single-strand DNA at replication forks is susceptible to single-strand lesions that effectively produce double-strand breaks. Double-strand breaks also occur at stalled replication forks in association with attempts to repair the stalled forks and/or due to replication fork collapse in the absence of checkpoint functions.

Figure 3.

Replication stress model of aging. Growth signaling inhibits mitochondrial biogenesis and respiration and increases ROS, leading to replication stress, genome instability, cellular senescence and aging. Replication stress is likely enhanced by ROS-dependent growth signaling and by growth signaling that occurs independently of ROS. Caloric restriction and mutational inactivation of growth-signaling pathways stimulate mitochondrial biogenesis, increase respiration and reduce ROS. Reduced ROS-dependent and -independent growth signaling reduces replication stress and genome instability and promotes life span. In mammals, exercise also extends life span extension and promotes mitochondrial biogenesis and increased respiration (77,89). The effects of replication stress on aging likely occur in parallel with oxidative damage to DNA and other cellular constituents.