Clocking Mutations

Back in 2015 I wrote a blog on Peto’s paradox, the observation that at the species level the incidence of cancer does not appear to correlate with the number of cells in an organism (Bigger is Better). On the face of it there should be a correlation because cancers arise from the accumulation in cells of DNA damage (mutations). So, obviously, the bigger an animal (i.e. the more cells it has) and the longer it lives the more likely it will be to get cancer. As I said in Bigger is Better, it’s obvious but, this being cancer, it’s wrong. Thus for example, cancer is much more common in humans than in whales, despite whales having more cells than humans. Likewise with mice and men: we have about 1000 times the number of cells in a mouse and live 30 times longer. So we should be much more likely to get cancer but in fact our rates are  pretty similar. Peto’s paradox applies across species but seemingly not within species, a number of studies having shown that cancer incidence increases with height both for men and women.

In Bigger is Better we noted that we don’t have much in the way of explanation for Peto’s paradox but we had earlier met the naked mole rat (Not getting cancer: a sequel to sequencing and evolution), a little subterranean fellow with a very long lifespan (up to 30 years) but who never seems to get cancer. We also described a study of 36 different mammalian species in the San Diego Zoo, ranging in size from mice (50 grams) to elephants nearly 100,000 times larger (4,800 kg). The upshot confirmed Peto’s paradox: there was no relationship between body size and cancer incidence and, as for cancer mortality rates, for elephants it’s less than 5% whilst the human range is 11% to 25%.

Opening the ark

However, it has been difficult to get unequivocal evidence for Peto’s paradox mainly because it’s hard to work out the risk of cancer in animals. The problem has recently been attacked head-on by Orsolya Vincze and colleagues from the University of Montpellier and a large number of other centres across the world who have collected data for a veritable Noah’s ark of creatures — no fewer than 110,148 animals of 191 species. Rather than doing the rounds of the world’s zoos, they used the Zoological Information Management System, a comprehensive global database. 

As expected, they confirmed that cancer-driving mutations occur in all mammals but that the cancer mortality risk was highly variable across species — ranging from zero to nearly 60%. The species with the worst deal is the kowari, a small carnivorous marsupial native to the gibber deserts of central Australia. These lovely little chaps get by on a diet of insects and spiders with the odd lizard, bird and rodent (I think that’s what they call a spoiler alert these days).

A kowari in its Australian desert habitat. Photo: Nathan Beerkens.

At the other end of the spectrum are the cloven-hooved mammals. These include sheep, goats, cows, camels, etc. — in fact most of the large, land mammalian species. They’re Artiodactyla and they are the least cancer-prone mammalian order. {Aside: There’s about 5,000 species of mammals divided into 26 ‘orders’ — the largest orders being rodents and bats. After these come the group comprising hedgehogs, moles etc. and then the Primates (humans, apes, etc.)}.

The really interesting finding, however, came from the comparison of diet across species from which it emerged that animals who, by and large, dine exclusively on the raw meat of other animals have the highest risk of dying of cancer — the prime example being the kowari. This result clearly eliminates a major role for chemicals generated by cooking, although it leaves open the possibility the ingestion of cancer-promoting viruses as a factor.

Cancer mortality risk across species. The red bars are median values (i.e. the number in the middle of a lot of data points). From Vincze et al. 2022.

The upshot is that long-lived animals and those with larger bodies are not more likely to die of cancer than smaller creatures or those with shorter lifespans. So far so good in that this further confirmed Peto’s paradox.

The accumulation of mutations across species during the lifetime of the animal. The key point is that animals with shorter lifespans acquire mutations more quickly than those that live longer. Thus the end of life mutational burden per cell is similar across species (the red arrows all end with the same height). This may begin to explain Peto’s paradox — although mutations accumulate randomly, in the biggest animals (with the most cells) they do so more slowly. From Gorelick and Naxerova 2022.

In a second recent contribution to the Peto paradox question Alex Cagan, Michael Stratton, Iñigo Martincorena and others from the Wellcome Sanger Institute, Hinxton, UK tackled the tricky question of how to measure mutation rates in different animal species by doing single cell sequencing of regions of the gut called crypts — they’re folds in the colon made mostly of epithelial cells. The key finding was that cells in long-lived animals mutate much more slowly than those in short-lived species. Broadly speaking, somatic mutations (alterations in DNA occurring after conception not passed on to children) arose from endogenous events in all species (i.e. they’re caused by internal factors arising from normal cell metabolism). Thus the animals studied differed in life-span by 30-fold and in body mass by 40,000-fold. Nevertheless, the mutational burden accumulated over their life-span varied only by about 3-fold. They also found that mutational signatures in other species resemble those in humans.

These remarkable findings, revealing the different rates at which mutational clocks tick across species, are consistent with the evidence that big animals may have evolved mechanisms of cancer protection not found in humans — e.g., the extra copies of the genome guardian TP53 in elephants (Bigger is Better). They also raise the interesting matter of whether the low mutation rates in long-lived animals protect not only against cancer but against the ageing process.

An additional merit is that these studies firmly place humans where they belong —  namely as just one of Nature’s many wonderful species with molecular features shared by all.

References

Gorelick, A.N. and Naxerova, K. (2022). Mutational clocks tick differently across species. Nature 604, 435-436. doi: 10.1038/d41586-022-00976-w.

Vincze, O., Colchero, F., Lemaître, JF. et al. (2022). Cancer risk across mammals. Nature 601, 263–267. https://doi.org/10.1038/s41586-021-04224-5.

Cagan A, Baez-Ortega A, Brzozowska N, et al. (2022). Somatic mutation rates scale with lifespan across mammals. Nature. 2022 Apr;604(7906):517-524. https://pubmed.ncbi.nlm.nih.gov/35418684/

Bigger is Better

“Nonsense!” most males would cry, quite logically, given that we spend much of our time trying to persuade the opposite sex that size doesn’t matter. But we want to have it both ways: in the macho world of rugby one of the oldest adages is that ‘a good big ’un will always beat a good little ’un’.  Beethoven doubtless had a view about size – albeit unrecorded by history – but after he’d written his Eroica symphony, perhaps the greatest revolutionary musical composition of all, his next offering in the genre was the magical Fourth – scored for the smallest orchestra used in any of his symphonies. And on the theme of small can be good, the British Medical Journal, no less, has just told us that if we cut the size of food portions and put ’em on smaller plates we’ll eat less and not get fat!

Is bigger better?

All of which suggests that whether bigger is better depends on what you have in mind. Needless to say, in these pages what we have in mind is ‘Does it apply to cancer?’ – that is, because cancers arise from the accumulation in cells of DNA damage (mutations), it would seem obvious that the bigger an animal (i.e. the more cells it has) and the longer it lives the more likely it will be to get cancer.

Obvious but, this being cancer, also wrong.

Peto’s Paradox

The first person to put his finger on this point was Sir Richard Peto, most famous for his work with Sir Richard Doll on cancer epidemiology. It was Doll, together with Austin Bradford Hill, who produced statistical proof (in the British Doctors’ Study published in 1956) that tobacco smoking increased the risk of lung cancer. Peto joined forces with Doll in 1971 and they went on to show that tobacco, infections and diet between them cause three quarters of all cancers.

Whenever this topic comes up I’m tempted to give a plug to the unfortunate Fritz Lickint – long forgotten German physician – who was actually the first to publish evidence that linked smoking and lung cancer and who coined the term ‘passive smoking’ – all some 30 years before the Doll study. Lickint’s findings were avidly taken up by the Nazi party as they promoted Draconian anti-smoking measures – presumably driven by the fact that their leader, Gröfaz (to use the derogatory acronym by which he became known in Germany as the war progressed – from Größter Feldherr aller Zeiten – Greatest Field Commander of all Time) was a confirmed non-smoker. Despite his usefulness, Lickint’s political views didn’t fit the ideology of the times. He lost his job, was conscripted, survived the war as a medical orderly and only then was able to resume his life as a doctor – albeit never receiving the credit he deserved.

Returning to Richard Peto, it was he who in 1975 pointed out that across different species the incidence of cancer doesn’t appear to be linked to the number of cells in animal – i.e. its size.   He based his notion on the comparison of mice with men – we have about 1000 times the number of cells in a mouse and typically live 30 times as long. So we should be about a million times more likely to get cancer – but in fact cancer incidence is another of those things where we’re pretty similar to our little furry friends. That’s Peto’s Paradox.

It doesn’t seem to apply within members of the same species, a number of surveys having shown that cancer incidence increases with height both for men and women. The Women’s Health Initiative found that a four inch increase in height raised overall cancer risk by 13% although for some forms (kidney, rectum, thyroid and blood) the risk went up by about 25%. A later study found a similar association for ovarian cancer: women who are 5ft 6in tall have a 23% greater risk than those who only make it to 5 feet. A similar risk links ovarian cancer to obesity (i.e. a rise in body mass index from 20 (slim) to 30 (slightly overweight) puts the risk up by 23%). Statistically sound though these results appear to be, it’s worth nothing that, as my colleague Paul Pharoah has pointed out, these risk changes are small. For example, the ovarian cancer finding translates to a lifetime risk of about 16-in-a-1000 for shorter women going up to 20-in-a-1000 as they rise by 6 inches.

It’s true that there may be a contribution from larger animals having bigger cells (whale red blood cells are about twice as big as those of the mouse) that divide more slowly but at most that effect seems small and doesn’t fully account for the fact that across species the association of size and age with cancer breaks down: Peto’s Paradox rules – humans are much more likely to get cancer than whales.

What did we know?

Well, since Peto picked up the problem, almost nothing about underlying causes. The ‘almost’ has been confined to the very small end of the scale and we’ve already met the star of the show – the naked mole rat – a rather shy chap with a very long lifespan (up to 30 years) but who never seems to get cancer. In that piece we described the glimmerings of an explanation but, thanks to Xiao Tian and colleagues of the University of Rochester, New York we now know that these bald burrowers make an extraordinarily large version of a polysaccharide (a polymer of sugars). These long strings of glucose-like molecules (called hyaluronan) form part of the extracellular matrix and regulate cell proliferation and migration. They’re enormous molecules with tens of thousands of sugars linked together but the naked mole rat makes versions about four times larger than those of mice or humans – and it seems that these extra-large sugar strings restrict cell behaviour and block the development of tumours.

Going up!

Our ignorance has just been further lifted with two heavyweight studies, one from Lisa Abegglen, Joshua Schiffman and chums from the University of Utah School of Medicine who went to the zoo (San Diego Zoo, in fact) and looked at 36 different mammalian species, ranging in size from the striped grass mouse (weighing in at 50 grams) to the elephant – at 4,800 kilogram nearly 100,000 times larger. They found no relationship between body size and cancer incidence, a result that conforms to Peto’s paradox. Comparing cancer mortality rates it transpires that the figure for elephants is less than 5% compared with the human range of 11% to 25%.

Cancer incidence across species by body size and lifespan. A selection of 20 of the 36 species studied is shown. Sizes range from the striped grass mouse to the elephant. As the risk of cancer depends on both the number of cells in the body and the number of years over which those cells can accumulate mutations, cancer incidence is plotted as a function of size (i.e. mass in grams × life span, years: y axis: log scale). Each species is represented by at least 10 animals (from Abegglen et al., 2015).

It can be seen at a glance that cancer incidence is not associated with mass and life span.

The Tasmanian devil stands out as a remarkable example of susceptibility to cancer through its transmission by biting and licking.

How does Jumbo do it?

In a different approach to Peto’s Paradox, Michael Sulak, Vincent Lynch and colleagues at the University of Chicago looked mainly at elephants – more specifically they used DNA sequencing to get at how the largest extant land mammal manages to be super-resistant to cancer. In particular they focused on the tumor suppressor gene P53 (aka TP53) because its expression is exquisitely sensitive to DNA damage and when it’s switched on the actions of the P53 protein buy time for the cell to repair the damage or, failing that, bring about the death of the cell. That’s as good an anti-cancer defence as you can imagine – hence P53’s appellation as the ‘guardian of the genome’. It turned out that elephants have no fewer than 20 copies of P53 in their genome, whereas humans and other mammals have only one (i.e. one copy per set of (23) chromosomes). DNA from frozen mammoths had 14 copies of P53 but manatees and the small furry hyraxes, the elephant’s closest living relatives, like humans have only one.

The Utah group confirmed that elephants have, in addition to one normal P53 gene, 19 extra P53 genes (they’re actually retrogenes – one type of the pseudogenes that we met in the preceding post) that have been acquired as the animals have expanded in size during evolution. Several of these extra versions of P53 were shown to be switched on (transcribed) and translated into proteins.

Consistent with their extra P53 fire-power, elephant cells committed P53-dependent suicide (programmed cell death, aka apoptosis) more frequently than human cells when exposed to DNA-damaging radiation. This suggests that elephant cells are rather better than human cells when it comes to killing themselves to avoid the risk of uncontrolled growth arising from defective DNA.

More genes anyone?

Those keen on jumping on technological bandwagons may wish to sign up for an extra P53 gene or two, courtesy of genetic engineering, so that bingo! – they’ll be free of cancers. Aside from the elephant, they may be encouraged by ‘super P53’ mice that were genetically altered to express one extra version of P53 that indeed significantly protected from cancer when compared with normal mice – and did so without any evident ill-effects.

We do not wish to dampen your enthusiasm but would be in dereliction of our duty is we did not add a serious health warning. We now know a lot about P53 – for example, that the P53 gene encodes at least 15 different proteins (isoforms), some of which do indeed protect against cancer – but there are some that appear to act as tumour promoters. In other words we know enough about P53 to realize that we simply haven’t a clue. So we really would be playing with fire if we started tinkering with our P53 gene complement – and to emphasise practicalities, as Mel Greaves has put it, we just don’t know how well the elephants’ defences would stack up if they smoked.

Nevertheless, on the bright side, light is at long last beginning to be shed on Peto’s Paradox and who knows where that will eventually lead us. Meanwhile Richard Peto’s activities have evolved in a different direction and he now helps to run a Thai restaurant in Oxford, a cuisine known for small things that pack a prodigious punch. Bit like Beethoven’s Fourth you could say.

References

Peto, R. et al. (1975). Cancer and ageing in mice and men. British Journal of Cancer 32, 411-426.

Doll, R. and Peto, R. (1976). Mortality in relation to smoking: 20 years’ observations on male British doctors. Br Med J. 2(6051):1525–36.

Maciak, S. and Michalak, P. (2015). “Cell size and cancer: A new solution to Peto’s paradox?”. Evolutionary Applications 8: 2.

Doll, R. and Hill, A.B. (1954). “The mortality of doctors in relation to their smoking habits”. BMJ 328 (7455): 1529.

Doll, R. and Hill, A.B. (November 1956). “Lung cancer and other causes of death in relation to smoking; a second report on the mortality of British doctors”. British Medical Journal 2 (5001): 1071–1081.

Tian, X. et al. (2013). High-molecular-mass hyaluronan mediates the cancer resistance of the naked mole rat. Nature 499, 346-349.

Abegglen, L.M., Schiffman, J.D. et al. (2015). Potential Mechanisms for Cancer Resistance in Elephants and Comparative Cellular Response to DNA Damage in Humans. JAMA. doi:10.1001/jama.2015.13134.

Sulak, M., Lindsey Fong, Katelyn Mika, Sravanthi Chigurupati, Lisa Yon, Nigel P. Mongan, Richard D. Emes, Vincent J. Lynch, V.J. (2015). TP53 copy number expansion correlates with the evolution of increased body size and an enhanced DNA damage response in elephants. doi: http://dx.doi.org/10.1101/028522.

García-Cao, I. et al. (2002). ‘Super p53’ mice exhibit enhanced DNA damage response, are tumor resistant and age normally. EMBO Journal 21, 6225–6235.