The discovery that the estimated number of stem-cell divisions in a tissue correlates with cancer incidence suggests that the varying probability of developing cancer in different tissues is mostly down to random mutations.
Cancer arises through the accumulation of molecular changes that together allow cells to grow in an uncontrolled manner. Many factors contribute to this process, including hereditary genetic mutations and environmental hazards, such as exposure to smoking or radiation. But cancer can still emerge in the absence of these factors, as a result of random mutations that arise during cell division1. Furthermore, little is known about how each risk factor affects different tissues, in which cancers arise at varying frequencies. Writing in Science, Tomasetti and Vogelstein2 show that about 65% of the differences in cancer incidence between tissues can be simply explained by the estimated total number of stem-cell divisions in those tissues. This result suggests that, rather than environmental and heredity influences, stochastic accumulation of mutations during DNA replication is the major cause of variations in cancer incidence between tissues.
Estimating the number of stem-cell divisions that occur in a tissue is a complex task. Tomasetti and Vogelstein performed an extensive literature search to obtain information about stem-cell numbers in various tissues. They then used mathematical and statistical models to estimate the number of stem-cell divisions in each tissue during the average human lifespan, and correlated this with publicly available cancer-incidence data. Although some cancers are certainly driven by genetic predisposition or environmental risk factors, the authors' analysis indicated that these factors explain only one-third of the variation in cancer incidence between tissues (Fig. 1). The rest of the variability was explained by the different number of stem-cell divisions estimated to occur in different tissues. However, some commonly occurring cancers, such as breast and prostate, were not included in the analysis, and these must be examined when data become available. Importantly, although this paper attempts to explain the variation in cancer incidence among tissues, it does not try to quantify the percentage of cancers that arise owing to random accumulation of mutations alone, which is a different measure.
This study suggests that processes governing the evolution of cancer cells need to be better understood, so that they can be manipulated to delay the onset of cancer. One drug that might affect evolutionary dynamics is aspirin, which protects against a variety of cancers3. Aspirin modulates evolutionary parameters that determine the rate of cancer development, including cell-division and cell-death rates4, and the persistence of cells with elevated mutation rates5.
In addition to experimental studies, mathematical models are crucial for investigating the evolutionary dynamics of cancer6. Such models have led to the development of several principles that define how random mutations and subsequent selection influence the emergence and growth of tumour cells in certain settings. For example, stochastic evolutionary models have been used to study the emergence and spread of tumour cells in healthy tissue7,8. Furthermore, models that take into account stem-cell dynamics have been used to study the evolutionary pathways by which cancerous cells escape the constraints of tissue regulation9.
Nonetheless, much remains to be done, and Tomasetti and Vogelstein's paper highlights some intriguing directions for future research. For instance, although the authors assumed that random cancer-causing mutations occur during stem-cell division, it is actually unknown in which tissues stem cells (rather than more-differentiated cells) become cancerous10. The correlation they report could arise either way, because the number of stem-cell divisions probably correlates with the number of divisions undertaken by the differentiating daughters of stem cells.
Another gap in our knowledge concerns the relative importance for cancer evolution of embryonic cell divisions compared with divisions after birth11. Although many cell divisions occur after birth in most tissues included in this study, almost no cell division occurs after birth in the cells that give rise to glioblastoma12, an aggressive brain tumour. But Tomasetti and Vogelstein found that the incidence of glioblastoma fits the same trend as the other cancers they studied.
Last but not least, we need to understand how various environmental selection pressures influence the fate of the mutant cells that are generated by chance. Microenvironmental conditions, such as immune responses, inflammation or the presence of cancer-causing molecules, can affect the fitness of specific mutants and thus their ability to grow and give rise to disease. These conditions can change during ageing, resulting in environments that are conducive to the growth of cancerous cells13. Although the correlation observed by Tomasetti and Vogelstein is certainly intriguing, and although accumulation of mutations clearly has a central role in causing variability between tissues, much research is needed to disentangle the complex, multifactorial interactions that result in disease.
A landmark paper14 published in 1981 suggested that most cancers could be averted by removing various lifestyle, behavioural and environmental risk factors prevalent in the population, and thus that much of the risk of cancer could be controlled. Risk factors are certainly involved in promoting the occurrence of many cancers, especially the more common ones. It therefore makes sense to use the presence or absence of risk factors to identify individuals who should be screened for cancer. If, however, random genetic changes are more-relevant drivers of carcinogenesis, then biomarkers and early-detection methods will have to be developed to prevent cancer mortality in the general population.
This might be problematic. Screening for many of the cancers included in Tomasetti and Vogelstein's study is currently difficult, particularly for rare cancers. Screening is successful only when several criteria are fulfilled: that tests are available that detect early disease; that the test's sensitivity and specificity for the given cancer is high; that people are willing to be screened; and that effective treatments exist that can be applied to early-stage cancers to prevent death. Furthermore, the benefits of screening must outweigh the risks of offering screening to the general population. The 'number needed to screen' to prevent one cancer death must be feasible, given the resources required to perform screens on the population as a whole (see go.nature.com/61efy9).
In summary, Tomasetti and Vogelstein's findings emphasize the role of basic evolutionary mechanisms in cancer development, and might lead to new chemoprevention therapies that slow the evolutionary processes at work. Although screening according to risk factors remains a crucial intervention strategy, an improvement in our cancer-prevention efforts in the general population will require the generation of early-detection techniques.
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Chaos: An Interdisciplinary Journal of Nonlinear Science (2017)