Cancer is the result of random mutational damage to nuclear DNA, but most such damage has no real effect, not even to the behavior of the affected cell. Cells in old tissues are riddled with mutations, but it is an open question as to how much this accumulated damage contributes to aging beyond cancer risk. Does it produce sufficient disarray in tissue function to be measured? A mutation capable of meaningfully altering cell behavior (a small subset of all possible mutations) can only have a noticeable affect when it occurs in many cells, a significant fraction of those present in a tissue. One slightly defective cell is a drop in the ocean, provided it isn’t actively cancerous.
Many researchers consider that the outcome of clonal expansion of mutations in adult tissue can be achieved when the original mutation occurs in a stem cell of some kind. The mutation can spread with the long-term delivery of a supply of daughter somatic cells and their descendants. Along these lines, the studies noted in the article below raise the possibility that cancer-associated mutations can also grant this ability to spread through excessive replication, yet without immediately resulting in the production of a tumor.
The field lacks definitive studies and models that would enable researchers to put numbers to the contribution of mutational damage to degenerative aging and age-related diseases other than cancer. Clearly the boundary between production of cancer and production of functional damage isn’t sharply drawn if expansion of mutations is a feature of the pre-cancerous state. Fixing the damage is usually the best way to proceed when answering this sort of question, but that is very hard to achieve for random DNA damage in isolation of all the other aspects of aging. Every cell needs custom work. More practically, delivering newly created, undamaged stem cell populations to replace old stem cell populations is a feasible form of future therapy, but it certainly doesn’t isolate DNA damage as the only altered variable.
Errors in DNA replication can alter a cell’s DNA sequence. If such alterations occur early enough in embryonic development, the changes are inherited by all of an organism’s cells. But if the alterations arise later in adult life, it is more difficult to track such changes in a small number of cells in a specific tissue, so the extent of these alterations in normal tissues is poorly understood. It is thought that cancer is initiated when cells acquire a minimum compendium of genetic alterations needed to trigger tumour formation. Understanding when such initiating mutations occur in normal cells is crucial for enabling reconstruction of the early events that lead to cancer.
Researchers have analysed the extent of mutations in human epithelial tissue from the healthy oesophagus, and how this relates to the processes that drive cancer development. They sequenced 74 cancer-associated genes in 844 tissue samples taken from the upper oesophagus of 9 healthy donors who differed in gender, age and lifestyle. For 21 of these samples, the authors also determined whole-genome sequences. A previous study assessing mutations in healthy skin cells reported between two and six mutations per million nucleotides of DNA. By contrast, here the mutations in oesophageal cells arose at a roughly tenfold lower rate. This difference is unsurprising, because skin cells are exposed to more DNA-damaging agents, such as ultraviolet light, than are oesophageal cells.
Instead, the surprise is that, compared with healthy skin, the healthy oesophagus has more mutations in cancer-associated genes. Moreover, at least a subset of these altered genes was under strong positive selection, meaning that the genetic alterations promoted cell proliferation, leading to the formation of cell clones. Compared with the samples from younger people, the overall number of mutations, the number of mutations in cancer-associated genes and the size of the clones were all greater in the samples from older people. The authors found that the donors’ samples had an average of about 120 different mutations in NOTCH1, a known cancer-associated gene, per square centimetre of normal oesophageal tissue.
The clonal expansion of normal oesophageal cells after cancer-promoting genes have mutated seems to be necessary, but not sufficient, to drive cancer, so something else must happen to the cells for tumours to form. For example, gaining a large-enough number of alterations in cancer-promoting genes might be needed. Few of the mutations were present in all the cells of the normal clones, and many of the cancer-promoting mutations were often found in spatially distinct subclones. This suggests that none of the normal cells had acquired enough cancer-promoting alterations to start cancer formation.