The nuclear DNA in our cells is surrounded by a panoply of exceedingly efficient quality control and repair machinery, but nonetheless damage occurs: individual cells suffer all sorts of mutations over time as molecules react with DNA or pieces are lost or reshuffled during replication. This is more pronounced in long-lived cells, such as those in the central nervous system, or the stem cell populations that support specific tissues.
Cancer spawns from nuclear DNA damage, and the risk of cancer grows greatly with age - not just because of growing damage to nuclear DNA, but also due to the decline of the immune system's watchdogs and other related consequences of aging. But aside from cancer, does the accumulation of various forms of nuclear DNA damage scattered across our cells contribute meaningfully to dysfunction and decline? There is some debate on this topic, and while the consensus position is more or less "yes, of course," there is at this point no experiment by which one can conclusively demonstrate that this is the case.
Today I'll point you to an open access study in which researchers compare DNA sequencing data from the blood of a pair of 40-year-old twins and a pair of 100-year old twins. Blood cells cycle into and out of circulation on a timescale of a few months, but we might take nuclear DNA damage in blood cells as being representative of the damage present in the population of hematopoietic stem cells that generated those blood cells.
It has been postulated that aging is the consequence of an accelerated accumulation of somatic DNA mutations and that subsequent errors in the primary structure of proteins ultimately reach levels sufficient to affect organismal functions. The technical limitations of detecting somatic changes and the lack of insight about the minimum level of erroneous proteins to cause an error catastrophe hampered any firm conclusions on these theories.
In this study, we sequenced the whole genome of DNA in whole blood of two pairs of monozygotic (MZ) twins, 40 and 100 years old, by two independent next-generation sequencing (NGS) platforms (Illumina and Complete Genomics). Potentially discordant single-base substitutions supported by both platforms were validated extensively by Sanger, Roche 454, and Ion Torrent sequencing.
We demonstrate that the genomes of the two twin pairs are germ-line identical between co-twins, and that the genomes of the 100-year-old MZ twins are discerned by eight confirmed somatic single-base substitutions, five of which are within introns. Putative somatic variation between the 40-year-old twins was not confirmed in the validation phase.
We conclude from this systematic effort that by using two independent NGS platforms, somatic single nucleotide substitutions can be detected, and that a century of life did not result in a large number of detectable somatic mutations in blood.
I would have expected more differences and larger differences to turn up, but as the researchers note it is impossibly to detect mutations that have not spread to at least some degree (in this case that means spreading through the population of hematopoietic stem cells). A next step might be a survey of whole genome sequencing by tissue types in old twins, especially those with longer-lived cells, to see whether this low level of exhibited mutational damage is peculiar to blood or typical for most or all tissues.
The number of somatic variants may be substantially larger but those present in smaller fractions of cells go undetected. Consistent, detectable somatic variation likely includes somatic mosaicism in blood generated during development or clonal expansion of mutations generated at any point during the lifetime. The frequency of these variants is limited in blood even after 100 years of life.
In summary, this study shows that the number of detectable somatic variants in blood by using NGS is very low and that accumulation of somatic mutations is not necessarily a consequence of a century of life. Stochastic somatic variation occurring in less than 20% of cells will go undetected, however.