Many researchers see stochastic mutational damage to nuclear DNA as an important mechanism in aging, above and beyond its contribution to cancer risk. The challenge has always been that there don’t seem to be enough mutations to explain significant harm, if the harm remains restricted to only the cell in which the mutation occurs. One way to explain how DNA damage causes more general issues is through clonal expansion of detrimental mutations that occur in stem and progenitor cells. Another possible explanation, presently being energetically explored by the research community, is that DNA damage can cause cellular senescence. In this case, just a few senescent cells can cause outsized amounts of harm in surrounding tissue through the potent mix of signals they secrete: generating inflammation, remodeling the extracellular matrix, changing the behavior of other cells for the worse, and so on. We’ll be seeing a great many papers like this one in the years ahead, I think.
During an organism’s lifetime, cells are constantly exposed to exogenous and endogenous stressful agents. Cells can cope with these stressors by various response mechanisms, or in case of irreversible damage, programmed cell death (apoptosis), or permanent cell-cycle arrest (cellular senescence). Cellular senescence is characterized by a halt in cellular replication, accompanied by a specific molecular phenotype. This phenotype can be the result of a few factors, such as accumulation of DNA damage, telomere attrition, and various epigenetic alterations.
Cellular senescence is one of the cellular pathways contributing to organismal aging. Senescent cells can accumulate in tissues and organs and can ultimately result in tissue lesions that will cause organ dysfunction, such as through the senescence-associated secretory phenotype (SASP). Age-related accumulation of DNA damage has been studied thoroughly, showing correlation between age and damage levels or mutation frequency. In the presence of DNA lesions or abnormalities, the DNA damage response (DDR) is activated and can eventually lead to cell cycle arrest. In older organisms, accumulation of DNA damage and loss of regenerative potential consequently increase the number of senescent cells, leading to aging cells, tissues, organs, and inevitable death.
The accumulation of genomic abnormalities is influenced by the quality of the repair pathways, which may also decline with age. Researchers studied age-related DNA damage in peripheral blood cells using single nucleotide polymorphism (SNP) microarray data from over 50,000 individuals. The frequency of detectable genomic abnormalities was low (less than 0.5%) at birth and rose to 2-3% in 50-year-old donors. Peripheral blood cells were also studied using whole-exome sequencing data from DNA of 17,182 individuals lacking hematologic phenotypes. Somatic mutations were rare in young donors (~40 years old) but became more frequent with age. Furthermore, while studying subjects at 70-79 years, compared with 90-108 years, mutation frequency rose from 9.5 to 18.4%, respectively.
In conclusion, the connection between DNA damage and aging is emphasized by the secretion of senescence-associated proteins during cellular senescence, a phenotype which is activated by DNA damage and is common for both human and mice. Though much progress has been achieved, full understanding of these mechanisms has still a long way to go.