A Critical Examination of the "Bad Luck" Explanation of Cancer Risk

[Why did I get cancer?]

Two interesting papers explore the beliefs of individuals (general population, or cancer survivors) on the causes of cancer. They reveal a huge discrepancy between scientifically proven factors and spontaneous opinions, that consider "stress" as the major cause of cancer. This is understandable in terms of psychological needs, and must be taken into account in cancer information campaigns.

Stem cell mutations, associated cancer risk, and consequences for regenerative medicine

Mutation accumulation in stem cells has been associated with cancer risk. However, the presence of numerous mutant clones in healthy tissues has raised the question of what limits cancer initiation. Here, we review recent developments in characterizing mutation accumulation in healthy tissues and compare mutation rates in stem cells during development and adult life with corresponding cancer risk. A certain level of mutagenesis within the stem cell pool might be beneficial to limit the size of malignant clones through competition. This knowledge impacts our understanding of carcinogenesis with potential consequences for the use of stem cells in regenerative medicine.

Unblinding the watchmaker: cancer treatment and drug design in the face of evolutionary pressure

INTRODUCTION

Death due to cancer is mostly associated with therapy ineffectiveness, i.e. tumor cells no longer responding to treatment. The underlying dynamics that facilitate this mutational escape from selective pressure are well studied in several other fields and several interesting approaches exist to combat this phenomenon, for example in the context of antibiotic-resistance in bacteria.

AREAS COVERED

Ninety percent of all cancer-related deaths are associated with treatment failure. Here, we discuss the common treatment modalities and prior attempts to overcome acquired resistance to therapy. The underlying molecular mechanisms are discussed and the implications of emerging resistance in other systems, such as bacteria, are discussed in the context of cancer.

EXPERT OPINION

Reevaluating emerging therapy resistance in tumors as an evolutionary mechanism to survive in a rapidly and drastically altering fitness landscape leads to novel treatment strategies and distinct requirements for new drugs. Here, we propose a scheme of considerations that need to be applied prior to the discovery of novel therapeutic drugs.

Cancer types with high numbers of driver events are largely preventable

There is a long-standing debate on whether cancer is predominantly driven by extrinsic risk factors such as smoking, or by intrinsic processes such as errors in DNA replication. We have previously shown that the number of rate-limiting driver events per tumor can be estimated from the age distribution of cancer incidence using the gamma/Erlang probability distribution. Here, we show that this number strongly correlates with the proportion of cancer cases attributable to modifiable risk factors for all cancer types except the ones inducible by infection or ultraviolet radiation. The correlation was confirmed for three countries, three corresponding incidence databases and risk estimation studies, as well as for both sexes: USA, males (r = 0.80, P = 0.002), females (r = 0.81, P = 0.0003); England, males (r = 0.90, P < 0.0001), females (r = 0.67, P = 0.002); Australia, males (r = 0.90, P = 0.0004), females (r = 0.68, P = 0.01). Hence, this study suggests that the more driver events a cancer type requires, the more of its cases are due to preventable anthropogenic risk factors.

The role of chance in cancer causation

In the last years, the discussion about the role of chance in the causation of cancer has generated much scientific and public debate. The concept that chance, or "bad luck", as responsible for a majority of the variation of cancer incidence, may be misleading, possibly causing an underestimation of the role played by known risk factors. In this commentary we discuss how host and external factors interact with chance in cancer causation in different ways, and provide examples of situations where chance appears to play only a minor role on cancer onset.

Determining cancer risk: the evolutionary multistage model or total stem cell divisions?

A recent hypothesis proposed that the total number of stem cell divisions in a tissue (TSCD model) determine its intrinsic cancer risk; however, a different model-the multistage model-has long been used to understand how cancer originates. Identifying the correct model has important implications for interpreting the frequency of cancers. Using worldwide cancer incidence data, we applied three tests to the TSCD model and an evolutionary multistage model of carcinogenesis (EMMC), a model in which cancer suppression is recognized as an evolving trait, with natural selection acting to suppress cancers causing a significant mean loss of Darwinian fitness. Each test supported the EMMC but contradicted the TSCD model. This outcome undermines results based on the TSCD model quantifying the relative importance of 'bad luck' (the random accumulation of somatic mutations) versus environmental and genetic factors in determining cancer incidence. Our testing supported the EMMC prediction that cancers of large rapidly dividing tissues predominate late in life. Another important prediction is that an indicator of recent oncogenic environmental change is an unusually high mean fitness loss due to cancer, rather than a high lifetime incidence. The evolutionary model also predicts that large and/or long-lived animals have evolved mechanisms of cancer suppression that may be of value in preventing or controlling human cancers.

Unraveling Hematopoiesis through the Lens of Genomics

Hematopoiesis has long served as a paradigm of stem cell biology and tissue homeostasis. In the past decade, the genomics revolution has ushered in powerful new methods for investigating the hematopoietic system that have provided transformative insights into its biology. As part of the advances in genomics, increasingly accurate deep sequencing and novel methods of cell tracking have revealed hematopoiesis to be more of a continuous and less of a discrete and punctuated process than originally envisioned. In part, this continuous nature of hematopoiesis is made possible by the emergent outcomes of vast, interconnected regulatory networks that influence cell fates and lineage commitment. It is also becoming clear how these mechanisms are modulated by genetic variation present throughout the population. This review describes how these recently uncovered complexities are reshaping our concept of tissue development and homeostasis while opening up a more comprehensive future understanding of hematopoiesis.

Comparative Oncology: New Insights into an Ancient Disease

Cancer has deep evolutionary roots and is an important source of selective pressure in organismal evolution. Yet, we find a great deal of variation in cancer vulnerabilities across the tree of life. Comparative oncology offers insights into why some species vary in their susceptibility to cancer and the mechanisms responsible for the diversity of cancer defenses. Here we provide an overview for why cancer persists across the tree of life. We then summarize current data on cancer in mammals, reptiles, and birds in comparison with commonly reported human cancers. We report on both novel and shared mechanisms of cancer protection in animals. Cross-discipline collaborations, including zoological and aquarium institutions, wildlife and evolutionary biologists, veterinarians, medical doctors, cancer biologists, and oncologists, will be essential for progress in the field of comparative oncology. Improving medical treatment of humans and animals with cancer is the ultimate promise of comparative oncology.

Compare and contrast: pediatric cancer versus adult malignancies

Cancer is a leading cause of death in both adults and children, but in terms of absolute numbers, pediatric cancer is a relatively rare disease. The rarity of pediatric cancer is consistent with our current understanding of how adult malignancies form, emphasizing the view of cancer as a genetic disease caused by the accumulation and selection of unrepaired mutations over time. However, considering those children who develop cancer merely as stochastically "unlucky" does not fully explain the underlying aetiology, which is distinct from that observed in adults. Here, we discuss the differences in cancer genetics, distribution, and microenvironment between adult and pediatric cancers and argue that pediatric tumours need to be seen as a distinct subset with their own distinct therapeutic challenges. While in adults, the benefit of any treatment should outweigh mostly short-term complications, potential long-term effects have a much stronger impact in children. In addition, clinical trials must cope with low participant numbers when evaluating novel treatment strategies, which need to address the specific requirements of children.

Tumorigenesis as the Paradigm of Quasi-neutral Molecular Evolution

In the absence of both positive and negative selections, coding sequences evolve at a neutral rate (R = 1). Such a high genomic rate is generally not achievable due to the prevalence of negative selection against codon substitutions. Remarkably, somatic evolution exhibits the seemingly neutral rate R ∼ 1 across normal and cancerous tissues. Nevertheless, R ∼ 1 may also mean that positive and negative selections are both strong, but equal in intensity. We refer to this regime as quasi-neutral. Indeed, individual genes in cancer cells often evolve at a much higher, or lower, rate than R ∼ 1. Here, we show that 1) quasi-neutrality is much more likely when populations are small (N < 50); 2) stem-cell populations in single normal tissue niches, from which tumors likely emerge, have a small N (usually <50) but selection at this stage is measurable and strong; 3) when N dips below 50, selection efficacy decreases precipitously; and 4) notably, N is smaller in the stem-cell niche of the small intestine than in the colon. Hence, the ∼70-fold higher rate of phenotypic evolution (observed as cancer risk) in the latter can be explained by the greater efficacy of selection, which then leads to the fixation of more advantageous and fewer deleterious mutations in colon cancers. In conclusion, quasi-neutral evolution sheds a new light on a general evolutionary principle that helps to explain aspects of cancer evolution.

Flaxseed Lignans as Important Dietary Polyphenols for Cancer Prevention and Treatment: Chemistry, Pharmacokinetics, and Molecular Targets

Cancer causes considerable morbidity and mortality across the world. Socioeconomic, environmental, and lifestyle factors contribute to the increasing cancer prevalence, bespeaking a need for effective prevention and treatment strategies. Phytochemicals like plant polyphenols are generally considered to have anticancer, anti-inflammatory, antiviral, antimicrobial, and immunomodulatory effects, which explain their promotion for human health. The past several decades have contributed to a growing evidence base in the literature that demonstrate ability of polyphenols to modulate multiple targets of carcinogenesis linking models of cancer characteristics (i.e., hallmarks and nutraceutical-based targeting of cancer) via direct or indirect interaction or modulation of cellular and molecular targets. This evidence is particularly relevant for the lignans, an ubiquitous, important class of dietary polyphenols present in high levels in food sources such as flaxseed. Literature evidence on lignans suggests potential benefit in cancer prevention and treatment. This review summarizes the relevant chemical and pharmacokinetic properties of dietary polyphenols and specifically focuses on the biological targets of flaxseed lignans. The consolidation of the considerable body of data on the diverse targets of the lignans will aid continued research into their potential for use in combination with other cancer chemotherapies, utilizing flaxseed lignan-enriched natural products.

Big Bang Tumor Growth and Clonal Evolution

The advent and application of next-generation sequencing (NGS) technologies to tumor genomes has reinvigorated efforts to understand clonal evolution. Although tumor progression has traditionally been viewed as a gradual stepwise process, recent studies suggest that evolutionary rates in tumors can be variable with periods of punctuated mutational bursts and relative stasis. For example, Big Bang dynamics have been reported, wherein after transformation, growth occurs in the absence of stringent selection, consistent with effectively neutral evolution. Although first noted in colorectal tumors, effective neutrality may be relatively common. Additionally, punctuated evolution resulting from mutational bursts and cataclysmic genomic alterations have been described. In this review, we contrast these findings with the conventional gradualist view of clonal evolution and describe potential clinical and therapeutic implications of different evolutionary modes and tempos.

The Warburg Effect and Mass Spectrometry-based Proteomic Analysis

Compared to normal cells, cancer cells have a unique metabolism by performing lactic acid fermentation in the presence of oxygen, also known as the Warburg effect. Researchers have proposed several hypotheses to elucidate the phenomenon, but the mechanism is still an enigma. In this review, we discuss three typical models, such as "damaged mitochondria", "adaptation to hypoxia", and "cell proliferation requirement", as well as contributions from mass spectrometry analysis toward our understanding of the Warburg effect. Mass spectrometry analysis supports the "adaptation to hypoxia" model that cancer cells are using quasi-anaerobic fermentation to reduce oxygen consumption in vivo. We further propose that hypoxia is an early event and it plays a crucial role in carcinoma initiation and development.

Stem cell divisions, somatic mutations, cancer etiology, and cancer prevention

Cancers are caused by mutations that may be inherited, induced by environmental factors, or result from DNA replication errors (R). We studied the relationship between the number of normal stem cell divisions and the risk of 17 cancer types in 69 countries throughout the world. The data revealed a strong correlation (median = 0.80) between cancer incidence and normal stem cell divisions in all countries, regardless of their environment. The major role of R mutations in cancer etiology was supported by an independent approach, based solely on cancer genome sequencing and epidemiological data, which suggested that R mutations are responsible for two-thirds of the mutations in human cancers. All of these results are consistent with epidemiological estimates of the fraction of cancers that can be prevented by changes in the environment. Moreover, they accentuate the importance of early detection and intervention to reduce deaths from the many cancers arising from unavoidable R mutations.

Health Equity and the Fallacy of Treating Causes of Population Health as if They Sum to 100

Numerous examples exist in population health of work that erroneously forces the causes of health to sum to 100%. This is surprising. Clear refutations of this error extend back 80 years. Because public health analysis, action, and allocation of resources are ill served by faulty methods, I consider why this error persists. I first review several high-profile examples, including Doll and Peto's 1981 opus on the causes of cancer and its current interpretations; a 2015 high-publicity article in Science claiming that two thirds of cancer is attributable to chance; and the influential Web site "County Health Rankings & Roadmaps: Building a Culture of Health, County by County," whose model sums causes of health to equal 100%: physical environment (10%), social and economic factors (40%), clinical care (20%), and health behaviors (30%). Critical analysis of these works and earlier historical debates reveals that underlying the error of forcing causes of health to sum to 100% is the still dominant but deeply flawed view that causation can be parsed as nature versus nurture. Better approaches exist for tallying risk and monitoring efforts to reach health equity.

Do External or Internal Factors Lead to Tumor Development? It Is Still Unknown

Arguments supporting the "bad luck" hypothesis presented by C. Tomasetti and B. Vogelstein ((2015) Science, 347, 78-81) and A. V. Lichtenstein ((2017) Biochemistry (Moscow), 82, 75-80) are critically discussed. Those arguments are not sufficient for recognition of the "bad luck" hypothesis and the leading role of internal factors in spontaneous tumor development.

A population genetics perspective on the determinants of intra-tumor heterogeneity

Cancer results from the acquisition of somatic alterations in a microevolutionary process that typically occurs over many years, much of which is occult. Understanding the evolutionary dynamics that are operative at different stages of progression in individual tumors might inform the earlier detection, diagnosis, and treatment of cancer. Although these processes cannot be directly observed, the resultant spatiotemporal patterns of genetic variation amongst tumor cells encode their evolutionary histories. Such intra-tumor heterogeneity is pervasive not only at the genomic level, but also at the transcriptomic, phenotypic, and cellular levels. Given the implications for precision medicine, the accurate quantification of heterogeneity within and between tumors has become a major focus of current research. In this review, we provide a population genetics perspective on the determinants of intra-tumor heterogeneity and approaches to quantify genetic diversity. We summarize evidence for different modes of evolution based on recent cancer genome sequencing studies and discuss emerging evolutionary strategies to therapeutically exploit tumor heterogeneity. This article is part of a Special Issue entitled: Evolutionary principles - heterogeneity in cancer?, edited by Dr. Robert A. Gatenby.

Preventable Incidence and Mortality of Carcinoma Associated With Lifestyle Factors Among White Adults in the United States

IMPORTANCE

Lifestyle factors are important for cancer development. However, a recent study has been interpreted to suggest that random mutations during stem cell divisions are the major contributor to human cancer.

OBJECTIVE

To estimate the proportion of cases and deaths of carcinoma (all cancers except skin, brain, lymphatic, hematologic, and nonfatal prostate malignancies) among whites in the United States that can be potentially prevented by lifestyle modification.

DESIGN, SETTING, AND PARTICIPANTS

This prospective cohort study analyzes cancer and lifestyle data from the Nurses' Health Study, the Health Professionals Follow-up Study, and US national cancer statistics to evaluate associations between lifestyle and cancer incidence and mortality.

EXPOSURES

A healthy lifestyle pattern was defined as never or past smoking (pack-years <5), no or moderate alcohol drinking (≤1 drink/d for women, ≤2 drinks/d for men), BMI of at least 18.5 but lower than 27.5, and weekly aerobic physical activity of at least 75 vigorous-intensity or 150 moderate-intensity minutes. Participants meeting all 4 of these criteria made up the low-risk group; all others, the high-risk group.

MAIN OUTCOMES AND MEASURES

We calculated the population-attributable risk (PAR) by comparing incidence and mortality of total and major individual carcinomas between the low- and high-risk groups. We further assessed the PAR at the national scale by comparing the low-risk group with the US population.

RESULTS

A total of 89 571 women and 46 339 men from 2 cohorts were included in the study: 16 531 women and 11 731 men had a healthy lifestyle pattern (low-risk group), and the remaining 73 040 women and 34 608 men made up the high-risk group. Within the 2 cohorts, the PARs for incidence and mortality of total carcinoma were 25% and 48% in women, and 33% and 44% in men, respectively. For individual cancers, the respective PARs in women and men were 82% and 78% for lung, 29% and 20% for colon and rectum, 30% and 29% for pancreas, and 36% and 44% for bladder. Similar estimates were obtained for mortality. The PARs were 4% and 12% for breast cancer incidence and mortality, and 21% for fatal prostate cancer. Substantially higher PARs were obtained when the low-risk group was compared with the US population. For example, the PARs in women and men were 41% and 63% for incidence of total carcinoma, and 60% and 59% for colorectal cancer, respectively.

CONCLUSIONS AND RELEVANCE

A substantial cancer burden may be prevented through lifestyle modification. Primary prevention should remain a priority for cancer control.

Cell Fusion in the War on Cancer: A Perspective on the Inception of Malignancy

Cell fusion occurs in development and in physiology and rarely in those settings is it associated with malignancy. However, deliberate fusion of cells and possibly untoward fusion of cells not suitably poised can eventuate in aneuploidy, DNA damage and malignant transformation. How often cell fusion may initiate malignancy is unknown. However, cell fusion could explain the high frequency of cancers in tissues with low underlying rates of cell proliferation and mutation. On the other hand, cell fusion might also engage innate and adaptive immune surveillance, thus helping to eliminate or retard malignancies. Here we consider whether and how cell fusion might weigh on the overall burden of cancer in modern societies.

Intrinsic versus Extrinsic Cancer Risks: The Debate Continues

Deciphering the relative contribution of intrinsic (e.g., genetic) and extrinsic (e.g., life style, environmental) risk factors in cancer development is crucial for strategizing cancer prevention. The recent publication by Wu and colleagues in Nature appears as an important contribution to the debate previously initiated by Tomasetti and Vogelstein in Science, who proposed that two-thirds of cancers can be attributed to random mutations and hence 'bad luck'. By contrast, Wu and colleagues, using four lines of evidence, suggest that cancer risk is dominated by extrinsic factors, and intrinsic risk factors only contribute marginally. The debate remains open, and an approach focusing on the evolutionary ecology of organs could provide crucial insights.

Somatic mosaicism: on the road to cancer

Diversity is the basis of fitness selection. Although the genome of an individual is considered to be largely stable, there is theoretical and experimental evidence--both in model organisms and in humans--that genetic mosaicism is the rule rather than the exception. The continuous generation of cell variants, their interactions and selective pressures lead to life-long tissue dynamics. Individuals may thus enjoy 'clonal health', defined as a clonal composition that supports healthy morphology and physiology, or suffer from clonal configurations that promote disease, such as cancer. The contribution of mosaicism to these processes starts during embryonic development. In this Opinion article, we argue that the road to cancer might begin during these early stages.