Gray hair may have evolved as a protection against cancer

Aging comes with graying hair, which may be a sign of the body lowering its risk of cancer, a study suggests.

Cancer-causing triggers, such as ultraviolet (UV) light or certain chemicals, activate a natural defensive pathway that leads to premature graying but also reduces the incidence of cancer, the research found.

The researchers behind the study tracked the fate of the stem cells responsible for producing the pigment that gives hair its color. In mouse experiments, they found that these cells responded to DNA damage either by ceasing to grow and divide — leading to gray hair — or by replicating uncontrollably to ultimately form a tumor. The findings, reported in October in the journal Nature Cell Biology, underline the importance of these sorts of protective mechanisms that emerge with age as a defense against DNA damage and disease, the study authors say.

Graying hair as cancer defense
Healthy hair growth is dependent on a population of stem cells that constantly renews itself within the hair follicle. A tiny pocket within the follicle contains reserves of melanocyte stem cells — precursors to the cells that produce the melanin pigment that gives hair its color. "Every hair cycle, these melanocyte stem cells will divide and produce some mature, differentiated cells," said Dot Bennett, a cell biologist at City St George's, University of London who was not involved in the study. "These migrate down to the bottom of the hair follicle and start making pigment to feed into the hair." Graying occurs when these cells can no longer produce sufficient pigment to thoroughly color each strand.

"It's a sort of exhaustion called cell senescence," Bennett explained. "It's a limit to the total number of divisions that a cell can go through, and it seems to be an anti-cancer mechanism to prevent random genetic errors acquired over time propagating uncontrollably."

When the melanocyte stem cells reach this "stemness checkpoint," they cease to divide, meaning the follicle no longer has a source of pigment to color the hair. Ordinarily, this occurs with old age as the stem cells naturally reach this limit. However, Emi Nishimura, a professor of stem cell age-related medicine, and colleagues at the University of Tokyo were interested in how this same mechanism operates in response to DNA damage — a key trigger for cancer development.

https://www.livescience.com/health/ageing/gray-hair-may-have-evolved-as-a-protection-against-cancer-study-hints