Today's open access research is an assessment of DNA damage accumulation in a variety of species, showing the pace of mutational damage correlates with species life span, at least as assessed here in immune cells from blood samples, and using a marker that identifies the response to short telomeres as well as forms of DNA damage. The DNA of the cell nucleus, the genetic blueprint for near all of the proteins produced in a cell, accumulates damage over time due to the normal haphazard chemical reactions that take place constantly inside cells. These mutational changes are largely irrelevant to cellular operation, but some can cause disruption in metabolism, or, worse, make a cell cancerous, by causing certain proteins to be produced in a broken or altered state. Near all mutational damage to DNA is quickly repaired by the highly efficient array of DNA repair mechanisms that a cell is equipped with. But some inevitably slips past.
The way in which mutation leads to cancer is fairly straightforward, but it is less obvious as to how random mutation in single cells can contribute meaningfully to other aspects of aging, such as widespread tissue dysfunction. The present consensus is that the important mutations are those that occur in stem cells and progenitor cells, able to spread widely throughout a tissue via the replication of daughter somatic cells created by those stem cells and progenitor cells. It was also recently suggested that DNA damage, even when repaired, and more or less regardless of what is damaged in DNA, leads to epigenetic changes characteristic of aging, and these epigenetic changes are what causes cell function to decline. In this view, a larger amount of unrepaired DNA damage, the marker usually measured, is indicative of the true cause of harm, which is more frequent DNA repair and thus epigenetic change.
The authors here are primarily focused on DNA damage markers that occur due to critically short telomeres in additional to mutational damage. Average telomere length in tissues, and the fraction of cells with critically short telomeres, is most likely downstream of stem cell function. Telomeres shorten inexorably with each cell division in somatic cells, and cells eventually self-destruct, or become senescent and are destroyed by the immune system. Stem cells use telomerase to maintain long telomeres, and deliver daughter somatic cells with long telomeres into tissues to make up the losses. So telomere length in tissues is a function of how rapidly cells divide and how frequently replacement cells are delivered by the supporting stem cell population - the pace of the latter is well known to decline with age.
Different species have very different lifespans ranging from less than 1 day for mayflies to more than 400 years for the Greenland shark. However, the exact cause of these differences in longevity are still largely unknown. Our group recently showed that the rate of telomere shortening with age correlates with lifespan in a variety of species from birds to mammals. Species with very fast telomere shortening rates such as mice have very short lifespans, and species with very slow telomere shortening rates such as humans have very long lifespans. It is interesting to note that species that share a similar longevity in spite of being evolutionarily distant like flamingos and elephants, also show a similar rate of telomere shortening, while evolutionarily closer species like mice and elephants, show very different longevities and also have very different rates of telomere shortening.
These findings suggest that longevity can be determined, at least in part, by epigenetic traits, such as the rate of telomere shortening. Furthermore, these findings pose the interesting question of which is the molecular determinant by which higher telomere shortening rates lead to shorter longevities. An obvious answer is that higher rates of telomere shortening will be associated to faster accumulation of critically short/dysfunctional telomeres, which are known to contribute to activation of a persistent DNA damage response stemming from telomeres, which leads to loss of cell viability and aging phenotypes. Thus, species that shorten telomeres at faster rates will reach telomere exhaustion and trigger a persistent DNA damage response earlier than those species that are able to maintain telomeres protected for a longer period of time. A short/dysfunctional telomere is recognized by the cell as an irreparable DNA double strand break (DSB), triggering a persistent DNA damage response which results in phosphorylation of γH2AX, and which eventually leads to cell death and/or senescence. In turn, induction of cellular senescence either owing to critically short telomeres or to other insults is also associated with increased γH2AX levels, involving in some instances the mTOR pathway. Thus, accumulation of cells with DNA damage throughout lifespan should also correlate with species longevity.
Here, we find that increased global rates of DNA damage, as determined by the DNA damage marker γH2AX which detects occurrence of double stranded DNA breaks in the genome, inversely correlates with species longevity. In particular, we determined here the rates of increase of the DNA damage marker γH2AX in leukocytes of phylogenetically distant species of birds and mammals in parallel and using the same experimental method. Previous studies have also shown a correlation between certain types of DNA damage and aging. Indeed, DNA damage accumulation with aging and telomere shortening may be related processes. Critically short telomeres as the result of cell proliferation throughout life to repair damaged tissues trigger a DNA damage signal specifically at telomeres.
We also measured the percentage of short telomeres of the species in this study, and we found that all of the species showed an increase in the percentage of short telomeres with age. This result is concomitant with the fact that average telomere length shortens with age in many species. Several studies have suggested that the percentage of short telomeres is more indicative of health and senescence than average telomere length. The percentage of short telomeres is an important metric since it is the length of the shortest telomere in a cell that induces a DNA damage response and cell senescence rather than the average telomere length of the telomeres on all of the chromosomes. Here we also noticed a mild trend for species with longer maximum lifespans to have a lower rate of increase of percent short telomeres, thus accumulating short telomeres more slowly with age. We also observed that species with the highest rates of γH2AX increase have the highest rates of increase of percent short telomeres with age. These results make a connection between γH2AX DNA damage, short telomeres, and lifespan. As cells accumulate DNA damage and short telomeres, they will enter into a state of senescence, thus accelerating the aging process and shortening lifespan.