April 8, 2019History of Medicine
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Sir Richard Peto attended Taunton's School in Southampton and subsequently studied the Natural Sciences Tripos at Queens' College, Cambridge followed by a Master of Science degree in Statistics at Imperial College London. His career has included collaborations with Richard Doll beginning at the Medical Research Council Statistical Research Unit in London. He set up the Clinical Trial Service Unit (CTSU) in Oxford in 1975 and is currently co-director. Peto's Paradox is named after him. Peto was elected a Fellow of the Royal Society in 1989 for his contributions to the development of meta-analysis. He is a leading expert on deaths related to tobacco use and was knighted for his services to epidemiology and to cancer prevention in 1999. He received an honorary Doctor of Medical Sciences degree from Yale University in 2011. His brother Julian Peto, with whom he has published work in mathematical statistics (e.g. on the logrank test), is also a distinguished epidemiologist. His family runs a Thai restaurant in the Covered Market, Oxford, of whose parent company he is a director.
Clinical Trial Service Unit and Epidemiological Studies Unit
Sir Richard Peto, FRS, is currently Professor of Medical Statistics and Epidemiology at the University of Oxford, and co-director (with Professor Sir Rory Collins) of the Clinical Trial Service Unit and Epidemiological Studies Unit (CTSU). He was made a Fellow of the Royal Society of London in 1989 for introducing meta-analyses of randomized trials, was knighted by Queen Elizabeth in 1999 for services to epidemiology, and received in 2010 and 2011 the Cancer Research UK and the BMJ Lifetime Achievement Award. Richard Peto, Rory Collins and others in the Oxford CTSU have, by their large randomized trials, large prospective studies and worldwide meta-analyses, increased substantially the estimated importance of blood lipids, blood pressure and smoking as causes of premature death. Peto has recently collaborated in major studies of alcohol in Russia and of malaria in Africa and India. His investigations into the worldwide health effects of smoking and benefits of stopping at particular ages have helped to communicate effectively the vast and growing burden of disease from tobacco use, have helped change national and international attitudes about smoking and public health, and have helped many smokers to stop. He was the first to describe clearly the future worldwide health effects of current smoking patterns, predicting one billion deaths from tobacco in the present century if current smoking patterns persist, as against ?only' 100 million in the 20th century.
Does the incidence of cancer correlate with the number of cells in an organism?
Because cancer develops through the accumulation of mutations, each proliferating cell is at risk of malignant transformation, assuming all proliferating cells have similar probabilities of mutation. Therefore, if an organism has more cells, i.e. more chances to initiate a tumor, the probability of getting cancer should increase. Similarly, if an organism has an extended lifespan, its cells have more time to accumulate mutations. Because the probability of carcinogenesis is an increasing function of age, an organism's lifetime risk of cancer should also scale with its lifespan. It is well known that larger organisms generally have longer lifespans which exacerbates this problem.
Peto's paradox is the observation, named after Richard Peto, that at the species level, the incidence of cancer does not appear to correlate with the number of cells in an organism. For example, the incidence of cancer in humans is much higher than the incidence of cancer in whales. This is despite the fact that a whale has many more cells than a human. If the probability of carcinogenesis were constant across cells, one would expect whales to have a higher incidence of cancer than humans.
Peto first formulated the paradox in 1977. Writing an overview of the multi-stage model of cancer, Peto noted that, on a cell-for-cell basis, humans were much less susceptible to cancer than mice. A man has 1000 times as many cells as a mouse and we usually live at least 30 times as long as mice. Exposure of two similar organisms to risk of carcinoma, one for 30 times as long as the other, would give perhaps 304 or 306 (i.e., a million or a billion) times the risk of carcinoma induction per epithelial cell. However, it seems that, in the wild, the probabilities of carcinoma induction in mice and in men are not vastly different. Are our stem cells really, then, a billion or a trillion times more cancer-proof than murine stem cells? This is biologically pretty implausible; if human DNA is no more resistant to mutagenesis in vitro than mouse DNA, why don't we all die of multiple carcinomas at an early age? Peto went on to suggest that evolutionary considerations were likely responsible for varying per-cell carcinogenesis rates across species (Epidemiology and Multistage Models - 1977).
Within members of the same species, cancer risk and body size appear to be positively correlated, even once other risk factors are controlled for. A 25-year longitudinal study of 17,738 male British civil servants, published in 1998, showed a positive correlation between height and cancer incidence with a high degree of statistical confidence, even after risk factors like smoking were controlled for. A similar 2011 study of more than one million British women found strong statistical evidence of a relationship between cancer and height, even after controlling for a number of socioeconomic and behavioral risk factors. A 2011 analysis of the causes of death of 74,556 domesticated North American dogs found that cancer incidence was lowest in the smaller breeds, confirming the results of earlier studies.
Across species, however, the relationship breaks down. A 2015 study, using data from necropsies performed by the San Diego Zoo, surveyed results from 36 different mammalian species, ranging in size from the 51-gram striped grass mouse to the 4,800-kilogram elephant, nearly 100,000 times larger. The study found no relationship between body size and cancer incidence, offering empirical support for Peto's initial observation. The evolution of multicellularity has required the suppression of cancer to some extent, and connections have been found between the origins of multicellularity and cancer. In order to build larger and longer-lived bodies, organisms required greater cancer suppression. Evidence suggests that large organisms such as elephants have more adaptations that allow them to evade cancer. The reason that intermediate-sized organisms have relatively few of these genes may be because the advantage of preventing cancer these genes conferred was, for moderately-sized organisms, outweighed by their disadvantages - particularly reduced fertility.
Various species have evolved different mechanisms for suppressing cancer. A paper in Cell Reports in January 2015 claimed to have found genes in the bowhead whale (Balaena mysticetus) that may be associated with longevity. Around the same time, a second team of researchers identified a polysaccharide in the naked mole-rat that appeared to block the development of tumors. In October 2015, two independent studies showed that elephants have 20 copies of tumor suppressor gene TP53 in their genome, where humans and other mammals have only one. Additional research showed 14 copies of the gene present in the DNA of preserved mammoths, but only one copy of the gene in the DNA of manatees and hyraxes, the elephant's closest living relatives. The results suggest an evolutionary relationship between animal size and tumor suppression, as Peto had theorized.
A 2014 paper in Evolutionary Applications by Maciak and Michalak emphasized what they termed a largely underappreciated relation of cell size to both metabolism and cell-division rates across species as key factors underlying the paradox, and concluded that larger organisms have bigger and slowly dividing cells with lower energy turnover, all significantly reducing the risk of cancer initiation. Maciak and Michalak argue that cell size is not uniform across mammalian species, making body size an imperfect proxy for the number of cells in an organism. (For example, the volume of an individual red blood cell of an elephant is roughly four times that of one from a common shrew). Furthermore, larger cells divide more slowly than smaller ones, a difference which compounds exponentially over the life-span of the organism. Fewer cell divisions means fewer opportunities for cancer mutations, and mathematical models of cancer incidence are highly sensitive to cell-division rates. Additionally, larger animals generally have lower basal metabolic rates, following a well-defined inverse logarithmic relationship. Consequently, their cells will incur less damage over time per unit of body mass. Combined, these factors may explain much of the apparent paradox. The apparent ability of bigger animals to suppress cancer across very large numbers of cells has spurred an active field of medical research. In one experiment, laboratory mice were genetically altered to express active TP53 tumor antigens, similar to the ones found in elephants. The mutated mice exhibited increased tumor suppression ability, but also showed signs of premature aging.
Sources: Oxford University; NIH.gov; Wikipedia