Old age – protection from cancer
Rare cell division explained the decline in cancer incidence in the elderly
Polina Loseva, N+1
American biologists have found that with age, stem cells in the human body begin to divide less often and confirmed this on tissue samples from different parts of the intestine. It turned out that the less often a cell multiplies, the fewer mutations its descendants accumulate. This is how the researchers propose to explain the fact that after a certain age, older people have fewer and fewer tumors. The work was published in the journal PNAS (Tomasetti et al., Cell division rates decrease with age, providing a potential explanation for the age-dependent deceleration in cancer incident).
Tumor development begins with point mutations in cells. It has long been known that the older the body, the more of these mutations: therefore, cancer is considered an age-related disease and occurs more often in the elderly than in the young. However, after a certain age (according to some estimates, after 70 years), the incidence of cancer decreases, which is essentially paradoxical, since mutations cannot disappear anywhere.
Cristian Tomasetti and his colleagues from the Johns Hopkins University School of Medicine have proposed their own model of such dependence. They suggested that the number of mutations increases non-linearly with age, and at some point the growth slows down: this could explain the cancer incidence data.
However, it is unclear exactly which mechanism of mutation formation could lose its force with age. For example, one of the sources of DNA breakdowns is the "misses" of the repair system, but with age this system does not begin to work better. Another source is the errors of polymerases, enzymes that copy DNA, but these proteins do not become "neater" over time.
Scientists have suggested that the matter is in the rate of division of stem cells: the more often they multiply, the more often polymerases copy DNA – and, therefore, more often they make mistakes. To test their hypothesis, the researchers took tissue samples from the rectum of young (20-29 years old) and elderly (80-89 years old) people. They stained the resulting preparations with markers characteristic of dividing cells: thus, they managed to fix an average of 26% of dividing stem cells in young tissues and only 15% in old ones.
On micrographs of intestinal villi (at the top – a young man, at the bottom – an old one), the nuclei of dividing cells are highlighted in red – VM.
Scientists rechecked their data in several other ways. They tried to color the cells differently, transfer the drugs to independent experts and take samples from other parts of the gastrointestinal tract - the esophagus and duodenum. In all experiments, scientists have shown that dividing cells become much smaller with age.
Then the authors of the work conducted similar studies on mice: it is known that their frequency of tumor development does not change with age. In accordance with this, scientists could not find any difference in the number of dividing cells: the elderly mice had the same number of them as the young ones.
The fact that the number of stem cells decreases with age has been known for a long time, and now the authors of the work have found out that they also multiply less often. Scientists note that following this, it would be interesting to study what happens to the length of telomeres: if stem cells divide less often, then perhaps their telomeres shorten more slowly after a certain age. This could be an important discovery, because now telomeres are increasingly being used as a marker of biological age to assess the risk of disease or death in individuals.
There is a complex, not fully understood relationship between aging and cancer. Despite the fact that we perceive tumors as natural companions of old age, scientists have calculated that cancer is the only alternative to aging at the cellular level.
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