Aging is a complex process that results in loss of the ability to reattain homeostasis following stress, leading, thereby, to increased risk of morbidity and mortality. Many factors contribute to aging, such as the time-dependent accumulation of macromolecular damage, including DNA damage. The integrity of the nuclear genome is essential for cellular, tissue, and organismal health. DNA damage is a constant threat because nucleic acids are chemically unstable under physiological conditions and vulnerable to attack by endogenous and environmental factors. To combat this, all organisms possess highly conserved mechanisms to detect and repair DNA damage. Persistent DNA damage (genotoxic stress) triggers signaling cascades that drive cells into apoptosis or senescence to avoid replicating a damaged genome. The drawback is that these cancer avoidance mechanisms promote aging. Here, we review evidence that DNA damage plays a causal role in aging. We also provide evidence that genotoxic stress is linked to other cellular processes implicated as drivers of aging, including mitochondrial and metabolic dysfunction, altered proteostasis and inflammation. These links between damage to the genetic code and other pillars of aging support the notion that DNA damage could be the root of aging.

Over the course of a human lifespan, genome integrity erodes, leading to an increased abundance of several types of chromatin changes. The abundance of DNA lesions (chemical perturbations to nucleotides) increases with age, as does the number of genomic mutations and transcriptional disruptions caused by replication or transcription of those lesions, respectively. At the epigenetic level, precise DNA methylation patterns degrade, likely causing increasingly stochastic variations in gene expression. Similarly, the tight regulation of histone modifications begins to unravel. The genomic instability caused by these mechanisms allows transposon element reactivation and remobilization, further mutations, gene dysregulation, and cytoplasmic chromatin fragments. This cumulative genomic instability promotes cell signaling events that drive cell fate decisions and extracellular communications known to disrupt tissue homeostasis and regeneration. In this Review, we focus on age-related epigenetic changes and their interactions with age-related genomic changes that instigate these events.

An insulin-like growth factor-1 receptor (IGF1R) variant in exon 6 (Arg-407-His) in Ashkenazi Jewish centenarians was previously found to be associated with reduced IGF1R activity. To further study this longevity associated IGF1R variant, we generated a novel mouse line carrying the R407H variant in exon 6 of the Igf1r gene by employing CRISPR/Cas9 genome editing technology. Here, we show that the Igf1r gene can be edited in mouse embryos by zygotic electroporation of Cas9 protein and a single-guide RNAs together with a single stranded oligonucleotide donor containing the desired key nucleotide changes at the Igf1r locus. Sequence analysis of F0 and F1 mice following targeted editing demonstrated the robustness of this approach in mice using CRISPR/Cas9 directed homologous recombination (HDR). Western blot analysis indicates that mice heterozygous for the variant have a significant decrease in IGF1R phosphorylation in various tissues, including skeletal muscle, compared to wildtype. In addition, depletion of IGF1R signaling specifically in skeletal muscle of progeroid Ercc1 -/∆ mice resulted in extended health span and median lifespan providing the rationale for long term lifespan studies in Igf1r hR407H variant mice. This mouse line will be a valuable genetic tool to help determine the impact of IGF1R signaling on aging and longevity. The CRISPR editing approach represents a prototype for generating additional longevity associated gene variant mouse lines to study relevance to human exceptional longevity.

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