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Monday, June 22, 2009

Grey hair may be protecting us from cancer

Another article from New Scientist documenting the slow decline of that journal into a typical supermarket rag [Grey hair may be protecting us from cancer ].
GREY hair may be unwelcome, but the processes that produce it are now better understood and could be protecting us from cancer.
First off, I want to make it clear that many of us with grey hair do not find it "unwelcome" in spite of societal pressures to make us feel embarrassed.

Second, here's the actual paper and abstract [doi:10.1016/j.cell.2009.03.037].

Inomata, K., Aoto, T., Binh, N.T., Okamoto, N., Tanimura, S., Wakayama, T., Iseki, S., Hara, E., Masunaga, T., Shimizu, H., and Nishimura, E.K. (2009) Genotoxic Stress Abrogates Renewal of Melanocyte Stem Cells by Triggering Their Differentiation. Cell 137: 1088-1099.
Somatic stem cell depletion due to the accumulation of DNA damage has been implicated in the appearance of aging-related phenotypes. Hair graying, a typical sign of aging in mammals, is caused by the incomplete maintenance of melanocyte stem cells (MSCs) with age. Here, we report that irreparable DNA damage, as caused by ionizing radiation, abrogates renewal of MSCs in mice. Surprisingly, the DNA-damage response triggers MSC differentiation into mature melanocytes in the niche, rather than inducing their apoptosis or senescence. The resulting MSC depletion leads to irreversible hair graying. Furthermore, deficiency of Ataxia-telangiectasia mutated (ATM), a central transducer kinase of the DNA-damage response, sensitizes MSCs to ectopic differentiation, demonstrating that the kinase protects MSCs from their premature differentiation by functioning as a “stemness checkpoint” to maintain the stem cell quality and quantity.
The idea is that DNA damage causes stem cells to differentiate into melanocytes that eventually die. Since there are fewer stem cells there will be fewer melanocytes produced over time and hair becomes grey. The fact that damaged stem cells undergo terminal differentiation instead of remaining as stem cells means that they are probably less likely to serve as the progenitors of a cancerous cell line.

Whether this has any real effect on protecting us from cancer is an open question. I doubt it very much but it's an easy hypothesis to test. Is it true that people with grey hair develop fewer cancers than people of the same age with darker hair?


  1. Wouldn't it have to be "people with darker hair" (or less grey hair really, since hair colour at least to some extent correlates with skin colour, which correlates with skin cancer risk, iirc) and similar levels of sun exposure?

  2. I'm bald, so I shouldn't have to worry about anything.

  3. Thanks for the info. Now i understand why hair turns gray.

  4. Might changes in MSC regulation also come w/ a cancer risk? I guess it really depends on exactly what type of cancer you're interested in (i.e. which cells become cancerous). Other test ideas assuming you could reasonably control for "DNA damage" levels:

    1. Do patterns of gray on individuals correspond to sun exposure or other sources of DNA damage? What about cancer development?

    2. Do blonds go gray as frequently as dark-haired individuals?

    3. Do darker skinned people (more protected DNA?) go gray at frequently as lighter skinned people (presumable more easily damaged DNA?)?

    Other than the mammal connection, is this mechanism likely to be seen in people, or just mice??

  5. Purposefully over-interpreting the data is the bread and butter of science fiction, or science-informed fiction.

    Maybe that's what New Scientists is morphing into.

  6. If you have to invent an evolutionary purpose for grey hair, its use as an indicator to predator animals that this post-reproductive individual is a good candidate for culling from the herd comes to mind. Likewise the grey hair is a good message to those younger members of the herd to turn elsewhere when looking for a reproductive partner.

  7. if grey hair increases the level of solar radiation that reaches the scalp, could it be a means of increasing vitamin D production in older people, given that vitamin D production in oldies is less efficient than in younger people? Of course this would also increase risk of melanoma, but maybe that is evolutionarily desirable in that it knocks off older people quickly. So my hypothesis is that greying hair (and baldness and hair thinning) increases the usefulness of older people in raising their grandchildren by improving their health via increased vitamin D production, and if this comes at the expense of skin cancer, then they do not remain a burden for their offspring for long.