On the apparent rarity of epithelial cancers in captive chimpanzees

Human-specific elimination of epithelial Siglec-XII suppresses the risk of inflammation-driven colorectal cancers

Carcinomas are common in humans but rare among closely related "great apes." Plausible explanations, including human-specific genomic alterations affecting the biology of sialic acids, are proposed, but causality remains unproven. Here, an integrated evolutionary genetics-phenome-transcriptome approach studied the role of SIGLEC12 gene (encoding Siglec-XII) in epithelial transformation and cancer. Exogenous expression of the protein in cell lines and genetically engineered mice recapitulated approximately 30% of the human population in whom the protein is expressed in a form that cannot bind ligand because of a fixed, homozygous, human-universal missense mutation. Siglec-XII-null cells/mice recapitulated the remaining approximately 70% of the human population in whom an additional polymorphic frameshift mutation eliminates the entire protein. Siglec-XII expression drove several pro-oncogenic phenotypes in cell lines and increased tumor burden in mice challenged with chemical carcinogen and inflammation. Transcriptomic studies yielded a 29-gene signature of Siglec-XII-positive disease and when used as a computational tool for navigating human data sets, pinpointed with surprising precision that SIGLEC12 expression (model) recapitulates a very specific type of colorectal carcinomas (disease) that is associated with mismatch-repair defects and inflammation, disproportionately affects European Americans, and carries a favorable prognosis. They revealed a hitherto-unknown evolutionary genetic mechanism for an ethnic/environmental predisposition of carcinogenesis.

Pathogenic variants in human DNA damage repair genes mostly arose in recent human history

BACKGROUND

Genome stability is maintained by the DNA damage repair (DDR) system composed of multiple DNA repair pathways of hundreds of genes. Germline pathogenic variation (PV) in DDR genes damages function of the affected DDR genes, leading to genome instability and high risk of diseases, in particular, cancer. Knowing evolutionary origin of the PVs in human DDR genes is essential to understand the etiology of human diseases. However, answer to the issue remains largely elusive. In this study, we analyzed evolutionary origin for the PVs in human DDR genes.

METHODS

We identified 169 DDR genes by referring to various databases and identified PVs in the DDR genes of modern humans from ClinVar database. We performed a phylogenetic analysis to analyze the conservation of human DDR PVs in 100 vertebrates through cross-species genomic data comparison using the phyloFit program of the PHAST package and visualized the results using the GraphPad Prism software and the ggplot module. We identified DDR PVs from over 5000 ancient humans developed a database to host the DDR PVs ( https://genemutation.fhs.um.edu.mo/dbDDR-AncientHumans ). Using the PV data, we performed a molecular archeological analysis to compare the DDR PVs between modern humans and ancient humans. We analyzed evolution selection of DDR genes across 20 vertebrates using the CodeML in PAML for phylogenetic analysis.

RESULTS

Our phylogenic analysis ruled out cross-species conservation as the origin of human DDR PVs. Our archeological approach identified rich DDR PVs shared between modern and ancient humans, which were mostly dated within the last 5000 years. We also observed similar pattern of quantitative PV distribution between modern and ancient humans. We further detected a set of ATM, BRCA2 and CHEK2 PVs shared between human and Neanderthals.

CONCLUSIONS

Our study reveals that human DDR PVs mostly arose in recent human history. We propose that human high cancer risk caused by DDR PVs can be a by-product of human evolution.

Astrocytes Drive Divergent Metabolic Gene Expression in Humans and Chimpanzees

The human brain utilizes ∼20% of all of the body's metabolic resources, while chimpanzee brains use <10%. Although previous work shows significant differences in metabolic gene expression between the brains of primates, we have yet to fully resolve the contribution of distinct brain cell types. To investigate cell type-specific interspecies differences in brain gene expression, we conducted RNA-seq on neural progenitor cells, neurons, and astrocytes generated from induced pluripotent stem cells from humans and chimpanzees. Interspecies differential expression analyses revealed that twice as many genes exhibit differential expression in astrocytes (12.2% of all genes expressed) than neurons (5.8%). Pathway enrichment analyses determined that astrocytes, rather than neurons, diverged in expression of glucose and lactate transmembrane transport, as well as pyruvate processing and oxidative phosphorylation. These findings suggest that astrocytes may have contributed significantly to the evolution of greater brain glucose metabolism with proximity to humans.

Oncogenic BRCA1,2 Mutations in the Human Lineage-A By-Product of Sexual Selection?

In this review, we discuss the long-known problem of tissue-specific carcinogenesis in BRCA1 and BRCA2 mutation carriers: while the genes are expressed ubiquitously, increased cancer risk is observed mostly in the breast and ovaries, and to a much lesser extent, in some other tissues such as the prostate or pancreas. We reevaluate hypotheses on the evolutionary origin of these mutations in humans. Also, we align together the reports that at least some great apes have much lower risks of epithelial cancers in general and breast cancer in particular with the fact that humans have more voluminous breast tissue as compared to their closest extant relatives, particularly chimpanzees and bonobos. We conjecture that this disparity may be a consequence of sexual selection, augmented via selection for enhanced lactation. Further, we argue that there is an organ-specific enigma similar to the Peto paradox: breast cancer risk in humans is only minimally correlated with breast size. These considerations lead to the hypothesis that, along with the evolutionary development of larger breasts in humans, additional changes have played a balancing role in suppressing breast cancer. These yet-to-be-discovered mechanisms, while purely speculative, may be valuable to understanding human breast cancer, though they may not be exclusive to the mammary gland epithelial cells. Combining these themes, we review some anti-carcinogenesis preventive strategies and prospects of new interventions against breast cancer.

Comparative physiological anthropogeny: exploring molecular underpinnings of distinctly human phenotypes

Anthropogeny is a classic term encompassing transdisciplinary investigations of the origins of the human species. Comparative anthropogeny is a systematic comparison of humans and other living nonhuman hominids (so-called "great apes"), aiming to identify distinctly human features in health and disease, with the overall goal of explaining human origins. We begin with a historical perspective, briefly describing how the field progressed from the earliest evolutionary insights to the current emphasis on in-depth molecular and genomic investigations of "human-specific" biology and an increased appreciation for cultural impacts on human biology. While many such genetic differences between humans and other hominids have been revealed over the last two decades, this information remains insufficient to explain the most distinctive phenotypic traits distinguishing humans from other living hominids. Here we undertake a complementary approach of "comparative physiological anthropogeny," along the lines of the preclinical medical curriculum, i.e., beginning with anatomy and considering each physiological system and in each case considering genetic and molecular components that are relevant. What is ultimately needed is a systematic comparative approach at all levels from molecular to physiological to sociocultural, building networks of related information, drawing inferences, and generating testable hypotheses. The concluding section will touch on distinctive considerations in the study of human evolution, including the importance of gene-culture interactions.

Cross-species identification of cancer resistance-associated genes that may mediate human cancer risk

Cancer is a predominant disease across animals. We applied a comparative genomics approach to systematically characterize genes whose conservation levels correlate positively (PC) or negatively (NC) with cancer resistance estimates across 193 vertebrates. Pathway analysis reveals that NC genes are enriched for metabolic functions and PC genes in cell cycle regulation, DNA repair, and immune response, pointing to their corresponding roles in mediating cancer risk. We find that PC genes are less tolerant to loss-of-function (LoF) mutations, are enriched in cancer driver genes, and are associated with germline mutations that increase human cancer risk. Their relevance to cancer risk is further supported via the analysis of mouse functional genomics and cancer mortality of zoo mammals' data. In sum, our study describes a cross-species genomic analysis pointing to candidate genes that may mediate human cancer risk.

Reproductive one health in primates

One Health is a collaborative trans-disciplinary approach to health; integrating human, animal, and environmental health. The focus is often on infection disease transmission and disease risk mitigation. However, One Health also includes the multidisciplinary and comparative approach to disease investigation and health of humans, animals, and the environment. One key aspect of environmental/ecosystem health is conservation, the maintenance of healthy, actively reproducing wildlife populations. Reproduction and reproductive health are an integral part of the One Health approach: the comparative aspects of reproduction can inform conservation policies or breeding strategies (in situ and ex situ) in addition to physiology and disease. Differences in reproductive strategies affect the impact poaching and habitat disruption might have on a given population, as well as ex situ breeding programs and the management of zoo and sanctuary populations. Much is known about chimpanzees, macaques, and marmosets as these are common animal models, but there is much that remains unknown regarding reproduction in many other primates. Examining the similarities and differences between and within taxonomic groups allows reasonable extrapolation for decision-making when there are knowledge gaps. For example: (1) knowing that a species has very low reproductive rates adds urgency to conservation policy for that region or species; (2) identifying species with short or absent lactation anestrus allows ex situ institutions to better plan contraception options for specific individuals or prepare for the immediate next pregnancy; (3) recognizing that progestin contraceptives are effective contraceptives, but may be associated with endometrial hyperplasia in some species (in Lemuridae but not great apes) better guides empirical contraceptive choice; (4) recognizing the variable endometriosis prevalence across taxa improves preventive medicine programs. A summary of anatomical variation, endocrinology, contraception, pathology, and diagnostics is provided to illustrate these features and aid in routine physical and postmortem examinations as well as primate management.

Can the Mitochondrial Metabolic Theory Explain Better the Origin and Management of Cancer than Can the Somatic Mutation Theory?

A theory that can best explain the facts of a phenomenon is more likely to advance knowledge than a theory that is less able to explain the facts. Cancer is generally considered a genetic disease based on the somatic mutation theory (SMT) where mutations in proto-oncogenes and tumor suppressor genes cause dysregulated cell growth. Evidence is reviewed showing that the mitochondrial metabolic theory (MMT) can better account for the hallmarks of cancer than can the SMT. Proliferating cancer cells cannot survive or grow without carbons and nitrogen for the synthesis of metabolites and ATP (Adenosine Triphosphate). Glucose carbons are essential for metabolite synthesis through the glycolysis and pentose phosphate pathways while glutamine nitrogen and carbons are essential for the synthesis of nitrogen-containing metabolites and ATP through the glutaminolysis pathway. Glutamine-dependent mitochondrial substrate level phosphorylation becomes essential for ATP synthesis in cancer cells that over-express the glycolytic pyruvate kinase M2 isoform (PKM2), that have deficient OxPhos, and that can grow in either hypoxia (0.1% oxygen) or in cyanide. The simultaneous targeting of glucose and glutamine, while elevating levels of non-fermentable ketone bodies, offers a simple and parsimonious therapeutic strategy for managing most cancers.

Is cancer a disease set up by cellular stress responses?

For several decades, the somatic mutation theory (SMT) has been the dominant paradigm on cancer research, leading to the textbook notion that cancer is fundamentally a genetic disease. However, recent discoveries indicate that mutations, including "oncogenic" ones, are widespread in normal somatic cells, suggesting that mutations may be necessary but not sufficient for cancer to develop. Indeed, a fundamental but as yet unanswered question is whether or not the first step in oncogenesis corresponds to a mutational event. On the other hand, for some time, it has been acknowledged the important role in cancer progression of molecular processes that participate in buffering cellular stress. However, their role is considered secondary or complementary to that of putative oncogenic mutations. Here we present and discuss evidence that cancer may have its origin in epigenetic processes associated with cellular adaptation to stressful conditions, and so it could be a direct consequence of stress-buffering mechanisms that allow cells with aberrant phenotypes (not necessarily associated with genetic mutations) to survive and propagate within the organism. We put forward the hypothesis that there would be an inverse correlation between the activation threshold of the cellular stress responses (CSRs) and the risk of cancer, so that species or individuals with low-threshold CSRs will display a higher incidence or risk of cancer.

Diagnosis and treatment of a poorly differentiated carcinoma in a male chimpanzee (Pan troglodytes)-A case study

This study reports the occurrence of a poorly differentiated carcinoma in a captive-born 28 year-old male chimpanzee (Pan troglodytes) who has a familial history of cancer. Pathological findings, surgical interventions, and experimental treatments are discussed.

The influence of evolutionary history on human health and disease

Nearly all genetic variants that influence disease risk have human-specific origins; however, the systems they influence have ancient roots that often trace back to evolutionary events long before the origin of humans. Here, we review how advances in our understanding of the genetic architectures of diseases, recent human evolution and deep evolutionary history can help explain how and why humans in modern environments become ill. Human populations exhibit differences in the prevalence of many common and rare genetic diseases. These differences are largely the result of the diverse environmental, cultural, demographic and genetic histories of modern human populations. Synthesizing our growing knowledge of evolutionary history with genetic medicine, while accounting for environmental and social factors, will help to achieve the promise of personalized genomics and realize the potential hidden in an individual's DNA sequence to guide clinical decisions. In short, precision medicine is fundamentally evolutionary medicine, and integration of evolutionary perspectives into the clinic will support the realization of its full potential.

Surprisingly long survival of premature conclusions about naked mole-rat biology

Naked mole-rats express many unusual traits for such a small rodent. Their morphology, social behaviour, physiology, and ageing have been well studied over the past half-century. Many early findings and speculations about this subterranean species persist in the literature, although some have been repeatedly questioned or refuted. While the popularity of this species as a natural-history curiosity, and oversimplified story-telling in science journalism, might have fuelled the perpetuation of such misconceptions, an accurate understanding of their biology is especially important for this new biomedical model organism. We review 28 of these persistent myths about naked mole-rat sensory abilities, ecophysiology, social behaviour, development and ageing, and where possible we explain how these misunderstandings came about.

Evolutionary transcriptomics implicates HAND2 in the origins of implantation and regulation of gestation length

The developmental origins and evolutionary histories of cell types, tissues, and organs contribute to the ways in which their dysfunction produces disease. In mammals, the nature, development and evolution of maternal-fetal interactions likely influence diseases of pregnancy. Here we show genes that evolved expression at the maternal-fetal interface in Eutherian mammals play essential roles in the evolution of pregnancy and are associated with immunological disorders and preterm birth. Among these genes is HAND2, a transcription factor that suppresses estrogen signaling, a Eutherian innovation allowing blastocyst implantation. We found dynamic HAND2 expression in the decidua throughout the menstrual cycle and pregnancy, gradually decreasing to a low at term. HAND2 regulates a distinct set of genes in endometrial stromal fibroblasts including IL15, a cytokine also exhibiting dynamic expression throughout the menstrual cycle and gestation, promoting migration of natural killer cells and extravillous cytotrophoblasts. We demonstrate that HAND2 promoter loops to an enhancer containing SNPs implicated in birth weight and gestation length regulation. Collectively, these data connect HAND2 expression at the maternal-fetal interface with evolution of implantation and gestational regulation, and preterm birth.

Possible Nutrition-Related Mechanisms of Metabolic Management in Cancer Treatment

Context: Somatic mutation theory has been considered as a potential cause for cancer. However, major inconsistencies with the gene theory have necessitated serious reconsideration of this assumption. According to these inconsistencies, cancer may be considered as a metabolic disorder. According to the mitochondrial metabolic theory, substrate-level phosphorylation has been suggested to be superior to oxidative phosphorylation in cancer cells. Cancer metabolic therapies such as ketogenic diets (KD) and limitation in glutamine and calorie can be beneficial and are in line with this theory. In this study, we have reviewed the potential effects of KD as well as glutamine and calorie restriction in various types/stages of cancer with a focus on possible mechanisms. Evidence Acquisition: A comprehensive electronic search of different databases was performed using “cancer”, “ketogenic diet”, and “metabolic” as the main keywords. A comprehensive electronic search of different databases was performed using “cancer”, “ketogenic diet”, and “metabolic” as the main keywords. Results: Emerging evidence has indicated that KD can affect tumor cells by reducing glucose availability and simultaneous elevation of ketone bodies as non-fermentable metabolic fuels. KD has been suggested to be more effective as a non-toxic therapeutic measure in combination with glutamine targeting agents, chloroquine for lysosomal targeting, hyperbaric oxygen therapy, and calorie restriction. Conclusions: This metabolic approach can be considered as a promising non-toxic strategy for cancer management.

Human-specific polymorphic pseudogenization of SIGLEC12 protects against advanced cancer progression

Compared with our closest living evolutionary cousins, humans appear unusually prone to develop carcinomas (cancers arising from epithelia). The SIGLEC12 gene, which encodes the Siglec-XII protein expressed on epithelial cells, has several uniquely human features: a fixed homozygous missense mutation inactivating its natural ligand recognition property; a polymorphic frameshift mutation eliminating full-length protein expression in ~60%-70% of worldwide human populations; and, genomic features suggesting a negative selective sweep favoring the pseudogene state. Despite the loss of canonical sialic acid binding, Siglec-XII still recruits Shp2 and accelerates tumor growth in a mouse model. We hypothesized that dysfunctional Siglec-XII facilitates human carcinoma progression, correlating with known tumorigenic signatures of Shp2-dependent cancers. Immunohistochemistry was used to detect Siglec-XII expression on tissue microarrays. PC-3 prostate cancer cells were transfected with Siglec-XII and transcription of genes enriched with Siglec-XII was determined. Genomic SIGLEC12 status was determined for four different cancer cohorts. Finally, a dot blot analysis of human urinary epithelial cells was established to determine the Siglec-XII expressors versus non-expressors. Forced expression in a SIGLEC12 null carcinoma cell line enriched transcription of genes associated with cancer progression. While Siglec-XII was detected as expected in ~30%-40% of normal epithelia, ~80% of advanced carcinomas showed strong expression. Notably, >80% of late-stage colorectal cancers had a functional SIGLEC12 allele, correlating with overall increased mortality. Thus, advanced carcinomas are much more likely to occur in individuals whose genomes have an intact SIGLEC12 gene, likely because the encoded Siglec-XII protein recruits Shp2-related oncogenic pathways. The finding has prognostic, diagnostic, and therapeutic implications.

Consideration of Ketogenic Metabolic Therapy as a Complementary or Alternative Approach for Managing Breast Cancer

Breast cancer remains as a significant cause of morbidity and mortality in women. Ultrastructural and biochemical evidence from breast biopsy tissue and cancer cells shows mitochondrial abnormalities that are incompatible with energy production through oxidative phosphorylation (OxPhos). Consequently, breast cancer, like most cancers, will become more reliant on substrate level phosphorylation (fermentation) than on oxidative phosphorylation (OxPhos) for growth consistent with the mitochondrial metabolic theory of cancer. Glucose and glutamine are the prime fermentable fuels that underlie therapy resistance and drive breast cancer growth through substrate level phosphorylation (SLP) in both the cytoplasm (Warburg effect) and the mitochondria (Q-effect), respectively. Emerging evidence indicates that ketogenic metabolic therapy (KMT) can reduce glucose availability to tumor cells while simultaneously elevating ketone bodies, a non-fermentable metabolic fuel. It is suggested that KMT would be most effective when used together with glutamine targeting. Information is reviewed for suggesting how KMT could reduce systemic inflammation and target tumor cells without causing damage to normal cells. Implementation of KMT in the clinic could improve progression free and overall survival for patients with breast cancer.

Human species-specific loss of CMP-N-acetylneuraminic acid hydroxylase enhances atherosclerosis via intrinsic and extrinsic mechanisms

Cardiovascular disease (CVD) events due to atherosclerosis cause one-third of worldwide deaths and risk factors include physical inactivity, age, dyslipidemia, hypertension, diabetes, obesity, smoking, and red meat consumption. However, ∼15% of first-time events occur without such factors. In contrast, coronary events are extremely rare even in closely related chimpanzees in captivity, despite human-like CVD-risk-prone blood lipid profiles, hypertension, and mild atherosclerosis. Similarly, red meat-associated enhancement of CVD event risk does not seem to occur in other carnivorous mammals. Thus, heightened CVD risk may be intrinsic to humans, and genetic changes during our evolution need consideration. Humans exhibit a species-specific deficiency of the sialic acid N-glycolylneuraminic acid (Neu5Gc), due to pseudogenization of cytidine monophosphate-N-acetylneuraminic acid (Neu5Ac) hydroxylase (CMAH), which occurred in hominin ancestors ∼2 to 3 Mya. Ldlr ^-/- mice with human-like Cmah deficiency fed a sialic acids (Sias)-free high-fat diet (HFD) showed ∼1.9-fold increased atherogenesis over Cmah wild-type Ldlr ^-/- mice, associated with elevated macrophage cytokine expression and enhanced hyperglycemia. Human consumption of Neu5Gc (from red meat) acts as a "xeno-autoantigen" via metabolic incorporation into endogenous glycoconjugates, as interactions with circulating anti-Neu5Gc "xeno-autoantibodies" potentiate chronic inflammation ("xenosialitis"). Cmah ^-/- Ldlr ^-/- mice immunized with Neu5Gc-bearing antigens to generate human-like anti-Neu5Gc antibodies suffered a ∼2.4-fold increased atherosclerosis on a Neu5Gc-rich HFD, compared with Neu5Ac-rich or Sias-free HFD. Lesions in Neu5Gc-immunized and Neu5Gc-rich HFD-fed Cmah ^-/- Ldlr ^-/- mice were more advanced but unexplained by lipoprotein or glucose changes. Human evolutionary loss of CMAH likely contributes to atherosclerosis predisposition via multiple intrinsic and extrinsic mechanisms, and future studies could consider this more human-like model.

Comparative Serum Challenges Show Divergent Patterns of Gene Expression and Open Chromatin in Human and Chimpanzee

Humans experience higher rates of age-associated diseases than our closest living evolutionary relatives, chimpanzees. Environmental factors can explain many of these increases in disease risk, but species-specific genetic changes can also play a role. Alleles that confer increased disease susceptibility later in life can persist in a population in the absence of selective pressure if those changes confer positive adaptation early in life. One age-associated disease that disproportionately affects humans compared with chimpanzees is epithelial cancer. Here, we explored genetic differences between humans and chimpanzees in a well-defined experimental assay that mimics gene expression changes that happen during cancer progression: A fibroblast serum challenge. We used this assay with fibroblasts isolated from humans and chimpanzees to explore species-specific differences in gene expression and chromatin state with RNA-Seq and DNase-Seq. Our data reveal that human fibroblasts increase expression of genes associated with wound healing and cancer pathways; in contrast, chimpanzee gene expression changes are not concentrated around particular functional categories. Chromatin accessibility dramatically increases in human fibroblasts, yet decreases in chimpanzee cells during the serum response. Many regions of opening and closing chromatin are in close proximity to genes encoding transcription factors or genes involved in wound healing processes, further supporting the link between changes in activity of regulatory elements and changes in gene expression. Together, these expression and open chromatin data show that humans and chimpanzees have dramatically different responses to the same physiological stressor, and how a core physiological process can evolve quickly over relatively short evolutionary time scales.

Biochemical, Cellular, Physiological, and Pathological Consequences of Human Loss of N-Glycolylneuraminic Acid

About 2-3 million years ago, Alu-mediated deletion of a critical exon in the CMAH gene became fixed in the hominin lineage ancestral to humans, possibly through a stepwise process of selection by pathogen targeting of the CMAH product (the sialic acid Neu5Gc), followed by reproductive isolation through female anti-Neu5Gc antibodies. Loss of CMAH has occurred independently in some other lineages, but is functionally intact in Old World primates, including our closest relatives, the chimpanzee. Although the biophysical and biochemical ramifications of losing tens of millions of Neu5Gc hydroxy groups at most cell surfaces remains poorly understood, we do know that there are multiscale effects functionally relevant to both sides of the host-pathogen interface. Hominin CMAH loss might also contribute to understanding human evolution, at the time when our ancestors were starting to use stone tools, increasing their consumption of meat, and possibly hunting. Comparisons with chimpanzees within ethical and practical limitations have revealed some consequences of human CMAH loss, but more has been learned by using a mouse model with a human-like Cmah inactivation. For example, such mice can develop antibodies against Neu5Gc that could affect inflammatory processes like cancer progression in the face of Neu5Gc metabolic incorporation from red meats, display a hyper-reactive immune system, a human-like tendency for delayed wound healing, late-onset hearing loss, insulin resistance, susceptibility to muscular dystrophy pathologies, and increased sensitivity to multiple human-adapted pathogens involving sialic acids. Further studies in such mice could provide a model for other human-specific processes and pathologies involving sialic acid biology that have yet to be explored.

Evolutionary constraints over microsatellite abundance in larger mammals as a potential mechanism against carcinogenic burden

Larger organisms tend to live longer, have more potentially carcinogenic cells, and undergo more cell divisions. While one might intuitively expect cancer incidence to scale with body size, this assertion does not hold over the range of different mammals. Explaining this lack of correlation, so-called 'Peto's paradox' can likely increase our understanding of how cancer defense mechanisms are shaped by natural selection. Here, we study the occurrence of microsatellite in mammal genomes and observe that animals with expanded body size restrain the number of microsatellite. To take into account of higher mutation rate in the microsatellite region compared to that of genome, limiting the abundance of somatic mutations might explain how larger organisms could overcome the burden of cancer. These observations may serve as the basis to better understand how evolution has modeled protective mechanisms against cancer development.

Peto's paradox and the hallmarks of cancer: constructing an evolutionary framework for understanding the incidence of cancer

An evolutionary perspective can help unify disparate observations and make testable predictions. We consider an evolutionary model in relation to two mechanistic frameworks of cancer biology: multistage carcinogenesis and the hallmarks of cancer. The multistage model predicts that cancer risk increases with body size and longevity; however, this is not observed across species (Peto's paradox), but the paradox is resolved by invoking the evolution of additional genetic mechanisms to suppress cancer in large, long-lived species. It is when cancer cells overcome these defence mechanisms that they exhibit the hallmarks of cancer, driving the ongoing evolution of these defences, which in turn is expected to create the differences observed in the genetics of cancer across species and tissues. To illustrate the utility of an evolutionary model we examined some recently published data linking stem-cell divisions and cancer incidence across a range of tissues and show why the original analysis was faulty, and demonstrate that the data are consistent with a multistage model varying from three to seven mutational hits across different tissues. Finally, we demonstrate how an evolutionary model can both define patterns of inherited (familial) cancer and explain the prevalence of cancer in post-reproductive years, including the dominance of epithelial cancers.

Peto's paradox and the promise of comparative oncology

The past several decades have seen a paradigm shift with the integration of evolutionary thinking into studying cancer. The evolutionary lens is most commonly employed in understanding cancer emergence, tumour growth and metastasis, but there is an increasing realization that cancer defences both between tissues within the individual and between species have been influenced by natural selection. This special issue focuses on discoveries of these deeper evolutionary phenomena in the emerging area of 'comparative oncology'. Comparing cancer dynamics in different tissues or species can lead to insights into how biology and ecology have led to differences in carcinogenesis, and the diversity, incidence and lethality of cancers. In this introduction to the special issue, we review the history of the field and outline how the contributions use empirical, comparative and theoretical approaches to address the processes and patterns associated with 'Peto's paradox', the lack of a statistical relationship of cancer incidence with body size and longevity. This burgeoning area of research can help us understand that cancer is not only a disease but is also a driving force in biological systems and species life histories. Comparative oncology will be key to understanding globally important health issues, including cancer epidemiology, prevention and improved therapies.