CORE CONCEPTS

Solving Peto’s Paradox to better understand cancerViviane Callier, Science Writer

Cancer is as ancient as multicellularity itself. But not allanimals get cancer at the same rate. Some, such aselephants and naked mole rats, rarely get it at all,whereas others, such as ferrets and dogs, have cancerat unusually high rates. The question is why.

In 1977, British epidemiologist Richard Peto rea-soned that the cells in large-bodied, long-lived animalsundergo more cell divisions, and every cell divisioncarries a small but nonnegligible risk of introducingmutations in the daughter cells. Some of those muta-tions could lead to cancer. So all else being equal, onewould expect that large-bodied, long-lived animalswould have a greater risk of cancer than small, short-lived ones. But when Peto looked into cancer incidencein some of these animals, that’s not what he found. This

seemingly counterintuitive phenomenon was dubbedPeto’s paradox (1).

To unravel themystery of Peto’s paradox, researchersare studying the genome sequences of animals acrossthe tree of life, especially those that are particularly largeor particularly long-lived. But there’s no one answer. Ev-ery species studied so far seems to have solved thisparadox in a different way, possibly because of differentlife histories and evolutionary selective pressures.

Such work could offer leads for treating or prevent-ing human cancers, says Joshua Schiffman, a pediatriconcologist at the Huntsman Cancer Institute at theUniversity of Utah. His research shows that the set ofgenes and signaling pathways that are deficient orbroken in patients with a high genetic risk of cancer

Despite their size and lifespan, elephants are able to stave off cancer by having 20 copies of the tumor suppressor geneTP53 and 11 extra copies of LIF. Image credit: Shutterstock.com/ronnybas frimages.

Published under the PNAS license.

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are actually the same ones that are protecting theanimals (2). “Now we can say, ‘Nature has really put aspotlight on these pathways in cancer resistance, sothese are the proteins and pathways that we want togo after when we start thinking about making newdrugs for our patients,’” Schiffman says.

An Elephantine SecretIn 2015, Schiffman’s team and his collaborators, alongwith another group working independently, led byevolutionary biologist Vincent Lynch at the Universityof Chicago, began to unravel the elephant’s secret toPeto’s paradox: these giants have 20 copies of the tumorsuppressor gene TP53 (or “tumor protein” p53) (2, 3).Once TP53, which is also present in humans and mostother animals, detects irreparable DNA damage thatcouldmake a cell cancerous, the p53 protein triggers celldeath. People born with a mutation in TP53 develop Li-Fraumeni syndrome and have a lifetime risk of de-veloping cancer approaching 100%.

To uncover the elephants’ secrets to cancer re-sistance, the researchers scoured the elephant ge-nome, discovering those extra TP53 copies. Using RT-PCR, the researchers showed that these extra copiesare transcribed into mRNAs. To understand their im-pact on cellular function, the researchers subjectedelephant lymphocytes and fibroblasts to DNA dam-age using two methods: ionizing radiation and doxo-rubicin. Compared with the control human cell lines,the elephant lymphocytes and fibroblasts underwentapoptosis at significantly higher rates in response tothe treatments, suggesting that those extra TP53copies in elephants may confer a higher sensitivity toDNA damage—and, hence, the ability to cull poten-tially cancerous cells earlier.

Some of the elephants’ extra copies of TP53—called TP53 retrogenes because they were reverse-transcribed and reinserted into the genome overthe course of millions of years of evolution—carry

mutations that result in a truncated p53 protein. So,based on the gene sequence, the researchers pre-dicted that the extra TP53 copies might not be func-tional. But the cell-based assays suggest otherwise.

To further decipher the role of each of the TP53copies, Schiffman’s team isolated one of them andintroduced it into a human cancer cell line. At the In-ternational Society for Evolutionary Medicine andPublic Health meeting in Utah in August, cancer bi-ologist Lisa Abegglen, who works with Schiffman,reported that doing so caused increased cell death inresponse to DNA damage compared with the samehuman cancer cell line without the elephant TP53retrogene. Lynch and his team also showed that ele-phant cells induce cell death at lower levels of DNAdamage than the cells of their closest living relatives,including the African rock hyrax, the East Africanaardvark, and the southern three-banded armadillo.

Schiffman is teaming up with scientists from theTechnion-Israel Institute of Technology and the Uni-versity of Utah to explore the possibility of attackingtumors by deftly delivering this elephant TP53 retro-gene via nanoparticles—although Schiffman empha-sizes that it’s very early days. The researchers havecreated a start-up company called PEEL Therapeutics(peel is the Hebrew word for elephant).

Alternate RoutesThis isn’t the elephants’ only secret. A 2018 study fromLynch’s laboratory shows that elephants also have11 extra copies of a gene called leukemia inhibitoryfactor (LIF). One of those copies, LIF6, is activated byTP53 in response to DNA damage (4). Overexpressionof LIF6 was sufficient to induce apoptosis in the ab-sence of DNA damage or activation by TP53. Whenactivated, LIF proteins enter the mitochondria, wherethey trigger leakage of the mitochondrial membraneand, ultimately, cell death. As in the case of extra copiesof TP53, this essentially makes the elephant cells moresensitive to DNA damage.

And cell-death triggers may not be the only meansof suppressing cancer in these animal outliers. Nakedmole rats (Heterocephalus glaber) have some peculiarcharacteristics outside their cells, in the extracellularmatrix, that help them stave off tumorigenesis.

The ground-dwelling, mouse-sized naked mole ratlives up to 30 years—more than seven times the life-span of a mouse. The animals have an extraordinaryhive-like behavior, unlike any other mammal. Theyalso hardly ever get cancer.

Studies show that the cells of the naked mole ratshave evolved really sensitive contact inhibition. “Thecells don’t like to be crowded,” says Lynch. Main-taining space among cells is a nice way to reduce yourcancer risk, he explains. When nakedmole rat cells gettoo crowded, cell signaling networks tell the cells tostop dividing. This hypersensitivity to contact inhibitionis due to unusually high-mass hyaluronan, a carbohy-drate polymer that is found throughout the extracellularmatrix (5, 6). When researchers degraded hyalur-onan in the extracellular matrix by overexpressing

To ward off cancer, the naked mole rat has evolved very sensitive contactinhibition—when its cells get too crowded, cell signaling networks tell the cellsto stop dividing. Image credit: Shutterstock.com/belizar.

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an enzyme that chews it up, the naked mole rat cellsreadily formed tumors.

So might, then, the hyaluronan offer a target forpreventing or treating cancer in humans? If so, theremedy won’t simply be injecting high-mass hyalur-onan into human tumors—the cellular signaling net-works are too different. But human tumors frequentlyshow an accumulation of hyaluronan, stymieing can-cer drugs. So one therapeutic approach involvesnanoparticle-based treatments targeting the hyalur-onan itself, degrading it so that the drugs can reachtheir intended targets (7).

Other animals boast different evolutionary advan-tages—and, hence, different potential cancer-treatingstrategies. Weighing more than 60 kg and typicallyliving for about 10 years, the capybara is a large ro-dent native to South America. The capybara genome,recently published on bioRxiv (8), reveals several in-teresting changes, compared with their smaller rodentancestors, that could elucidate capybara cancer re-sistance: The animals’ insulin signaling pathway allowedthem to grow larger than their ancestors. But as withhumans, increased stature comes with an increasedrisk of cancer (9). To compensate, capybaras appear tohave an expanded family of immune-related genesthat made their immune system hypervigilant againstcancer cells.

Those two changes probably coevolved in re-sponse to each other, says Santiago Herrera-Alvarez,an evolutionary biologist and coauthor of the bioRxivpreprint. Increased insulin signaling promotes growth,but that same signaling pathway is often hijacked bycancer cells to trigger their growth and proliferation. Acompensatory mechanism had to evolve to reduce therisk of cancer, he explains. “So what we were trying tounderstand is, how are those mechanisms that areinvolved in growth regulation and cancer suppressioncoevolving?” Herrera-Alvarez says.

Additional clues may come from some species ofbats, which can live 45 years. Their longevity stems notonly from extra copies of TP53 in some cases but alsofrom resilient telomeres that remain long despite ad-vanced age (10). Short telomeres cause the cells tosenesce and die rapidly whereas long telomeres allowthe cells (and thus the animals) to grow old—the extracopies of TP53 cull DNA-damaged cells, preventingtumors from forming. Early studies of the bowheadwhale, which boasts an incredible lifespan of more

than 200 years, suggest that they manage their in-credible longevity without extra TP53 genes (11).“There has to be some kind of way that they’re doingit,” says Lynch. “It just means that it’s not the mostobvious way.”

Diverse Strategies, Common ThemesTo better understand Peto’s Paradox and the evolu-tionary roots of cancer, some researchers are tacklinga related mystery: Why high cancer rates appear to bemore common in mammals (12). Evolutionary bi-ologist Amy Boddy at the University of California,Santa Barbara is exploring the hypothesis that thediscrepancy boils down to how mammals reproduce(13). In mammalian pregnancies, the placenta is fetaltissue that invades the maternal uterus, triggering aproliferation of blood vessels and suppressing thematernal immune system so that the mother can tol-erate the fetus’s genetically different cells. Like an in-vasive placenta, a metastatic tumor consists of geneticallydifferent tissue that invades the host’s body and sup-presses the immune system. After mammals evolved thisplacenta, perhaps tumors co-opted those genetic mech-anisms to do the same thing.

There are many benefits to having an invasiveplacenta, including more nutrients for the offspring,Boddy says. “But the tradeoff is that later on, this in-vasive cellular phenotype can get turned back on anddo some damage to the body,” she notes. This phe-nomenon, known as antagonistic pleiotropy, occurswhen a gene regulates more than one function andthose functions come in direct conflict.

But thus far, Boddy’s data show no relationshipbetween the degree of placental invasiveness andcancer incidence—only that mammals as a whole tendto have a higher cancer incidence than other groups.Because placental mammals evolved almost 100 millionyears ago, compensatory mechanisms may havecoevolved with invasive placentation, she suggests.

Peto’s paradox has yet to be completely solved,but investigating the phenomenon has certainly becomea fertile research area. Investigating the strategies thatdifferent animals have evolved, says Schiffman, mayeventually offer a variety of therapeutic avenues, eachsuited to a different subset of cancer patients. “I thinkthe fact that each animal took different routes throughnature, through evolution,” he says, “really is veryexciting.”

1 Peto R (1977) Epidemiology, multistage models, and short-term mutagenicity tests. Origins of Human Cancer, eds Hiatt HH,Watson JD, Winsten JA (Cold Spring Harbor Laboratory Press, New York), pp 1403–1428.

2 Abegglen LM, et al. (2015) Potential mechanisms for cancer resistance in elephants and comparative cellular response to DNAdamage in humans. JAMA 314:1850–1860.

3 Sulak M, et al. (2016) TP53 copy number expansion is associated with the evolution of increased body size and an enhanced DNAdamage response in elephants. eLife 5:e11994.

4 Vazquez JM, Sulak M, Chigurupati S, Lynch VJ (2018) A zombie LIF gene in elephants is upregulated by TP53 to induce apoptosis inresponse to DNA damage. Cell Reports 24:1765–1776.

5 Seluanov A, et al. (2009) Hypersensitivity to contact inhibition provides a clue to cancer resistance of naked mole-rat. Proc Natl AcadSci USA 106:19352–19357.

6 Tian X, et al. (2013) High-molecular-mass hyaluronan mediates the cancer resistance of the naked mole rat. Nature 499:346–349.7 Rankin KS, Frankel D (2016) Hyaluronan in cancer - from the naked mole rat to nanoparticle therapy. Soft Matter 12:3841–3848.8 Herrera-Alvarez S, Karlsson E, Ryder OA, Lindblad-Toh K, Crawford A (2018) How to make a rodent giant: Genomic basis andtradeoffs of gigantism in the capybara, the world’s largest rodent. bioRxiv:424606.

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9 Nunney L (2018) Size matters: Height, cell number and a person’s risk of cancer. Proc Biol Sci 285:2018743.10 Foley NM, et al. (2018) Growing old, yet staying young: The role of telomeres in bats’ exceptional longevity. Sci Adv 4:eaao0926.11 Keane M, et al. (2015) Insights into the evolution of longevity from the bowhead whale genome. Cell Reports 10:112–122.12 Effron M, Griner L, Benirschke K (1977) Nature and rate of neoplasia found in captive wild mammals, birds, and reptiles at necropsy.

J Natl Cancer Inst 59:185–198.13 D’Souza AW, Wagner GP (2014) Malignant cancer and invasive placentation: A case for positive pleiotropy between endometrial and

malignancy phenotypes. Evol Med Public Health 2014:136–145.

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