Developmental noise, ageing and cancer

Mechanisms of Ageing and Development 124 (2003) 711
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Review

Developmental noise, ageing and cancer Armando Aranda-Anzaldo a,*, Myrna A.R. Dent b a

Laboratorio de Biologı´a Molecular, Facultad de Medicina, Universidad Auto´noma del Estado de Me´xico, Apartado Postal 428, C.P. 50000, Toluca, Edo. Me´x., Mexico b Laboratorio de Neurociencias, Facultad de Medicina, Universidad Auto´noma del Estado de Me´xico, Apartado Postal 428, C.P. 50000, Toluca, Edo. Me´x., Mexico Received 11 November 2002; received in revised form 17 March 2003; accepted 19 March 2003

Abstract Development is a very robust but far from perfect process, subjected to random variation due to the combined factors that constitute the so-called developmental noise. The effects of early developmental noise may have long-term consequences resulting from slight differences in the make-up and organisation of the former developing system. Here we present evidence suggesting that cancer is not an acquired but an intrinsic process resulting from random factors acting during early development, thus leading to a mixture of susceptibility types that may develop cancer sooner or later, depending on the combination of the environment acting upon such different susceptibility types. We discuss evidence suggesting that some supposedly tumour-suppressor functions, such as those associated with the p53 protein, actually evolved as buffering functions against the early effects of developmental noise that might compromise the stability of embryonic cells and hence of development. Ageing is a stochastic process characterised by progressive failure of somatic maintenance and repair. We put forth the notion that progressive loss of the morphological coherence of the organism (morphological disorder) is a form of ageing, and that morphological disorder is the common theme of most types of cancer. Thus, we suggest that the exhaustion of both developmental constraints and buffering developmental mechanisms link ageing and cancer. Moreover, we propose that cancer may represent one of the most radical forms of ageing, because it generally satisfies the criteria of senescence: intrinsicality, progressiveness and deleteriousness. # 2003 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Development; Chance; Morphogenesis; Oncogenes; p53; Reliability theory; Tumour suppressor genes

1. Introduction Embryonic development is a very complex process in which cell proliferation is coupled to cell migration and there is a high degree of precision in the specification of both the fate of individual cells and their spatial relationships with each other. The individual’s ability to produce a consistent phenotype under different environmental and genetic conditions is known as ‘canalisation’ (Waddington, 1942). However, development is not a perfect process nor it may be optimum. Yet, whichever the mechanisms or constraints associated with canalisation, they become non-relevant once the organism has achieved reproductive maturity that

* Corresponding author. Tel.: /52-722-217-3552x113; fax: /52722-217-4142/270-2899. E-mail address: [email protected] (A. Aranda-Anzaldo).

constitutes the end point of biologically significant development. It is well known that genetically identical individuals raised in the very same environmental conditions, nevertheless may show a slight but measurable degree of phenotypic variation. Such a variation in the absence of genetic or environmental factors is described as ‘developmental noise’ (Griffiths et al., 1996). Random processes occur in every developmental generation, but they are not selected for. At the individual’s level, these are one-time events that may result from random chemical fluctuations and thermodynamic noise within the embryo’s cells, as well as from random cellular processes such as the timing of individual cell division or the stochastic establishment of epigenetic modifications to the DNA nucleotide sequence, such as DNA methylation (Rakayan et al., 2001). The slight random molecular heterogeneity that occurs within the cell

0047-6374/03/$ - see front matter # 2003 Elsevier Science Ireland Ltd. All rights reserved. doi:10.1016/S0047-6374(03)00089-7

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coupled to the random thermodynamic effects, imply that the resulting phenotype is always the product of random events as well as of genetic and environmental influences.

genetic background is the source of cohort heterogeneity. Thus the intrinsic differences among similar individuals caused by developmental noise might remain latent until unmasked by the progressive erosion during lifetime of the homeostatic-buffering systems.

2. Developmental noise and ageing 3. Cancer incidence in the human population There seems to be a common agreement on the notion that ageing is a stochastic and non-deterministic process characterised by a progressive failure of maintenance and repair (Hayflick, 2000; Kirkwood and Austad, 2000; Rattan, 2000). Somatic maintenance is necessary to keep the organism in good conditions but only for as long as it reaches sexual maturity and reproductive success. Recently, Finch and Kirkwood have thoroughly argued that chance variations in form and function, arising through development, affect individual baseline functions and individual responses to the external environment, and so modify the outcomes of ageing (Finch and Kirkwood, 2000). They provide a wealth of evidence supporting their proposal. For example: identical twins who reach middle-age have life spans that differ almost as much as unrelated individuals in the same population (Finch and Kirkwood, 2000, p. 204). Moreover, in C. elegans it has been established that stochastic factors contribute significantly to senescence and that different tissues and cell types age at markedly different rates with extensive variability both among animals of the same age and between cells of the same type within individuals (Herndon et al., 2002). Some buffering systems may have evolved so as to reduce the effects of developmental noise and to ensure canalisation into normal development. At least one important buffering system has been identified and appears to be conserved in both plants and animals: the heat-shock protein 90 (Hsp90) that chaperones the maturation of many regulatory proteins that lie at the interface of several developmental pathways (Rutherford and Lindquist, 1998). Experiments with isogenic plants of Arabidopsis thaliana raised in identical environmental conditions, showed that lowering the activity of Hsp90 lead to the appearance of striking phenotypic differences among the isogenic plants, that can only be attributed to random events characteristic of developmental noise (Queitsch et al., 2002). The late-life mortality deceleration observed in large cohorts of experimental animals and in humans might be explained if the original cohort of individuals is heterogeneous, consisting of a mixture of frailty types (Partridge and Mangel, 1999). The fact that the deceleration of mortality at advanced age is also observed in genetically uniform strains of Drosophila raised in similar environmental conditions (Curtsinger et al., 1992), suggests that developmental noise and not the

As the average life expectancy world-wide has almost doubled since the mid 19th century the population at risk of cancer has grown. On average, among those surviving after the first year of life, one in three people will be diagnosed with cancer during their lifetime (see Cancer Official Websites). Cancer is very rare in children and the young, while the overwhelming incidence (77%) occurs in persons older than 50 years and in this group 90% of the tumours is epithelial in origin (Cancer Facts and Figures, 2002). There are over 200 different types of cancer, but five major types: lung, breast, stomach, prostate and colorectal, account for over half of all cases diagnosed world-wide (Pike and Forman, 1991; Cancer Official Websites 1). The most frequent tumour sites correspond to tissues that preserve a general proliferating potential after birth. Non-proliferating tissues such as the striated muscle, the heart and the brain are rarely the seats of malignant tumours. Lymphomas and leukaemia might be a kind of their own: these tumours are the best examples of the role of gene mutations and rearrangements involved in deregulation of cell proliferation and thus in the causality of cancer, but altogether they represent less than 8% of the total cancer incidence (Sheer, 1991; Alberts et al., 1994, p. 1257). Cancer incidence studies basically refer to the incidence of diagnosed cancers. Yet, when a typical solid tumour is first detected, it already contains between 108 and 109 cells (Alberts et al., 1994, p. 1258). Thus, it has been estimated that it should normally take several years for a cancer to grow before it becomes noticeable to either patients or clinicians (Retsky et al., 1987; Kothari and Mehta, 2001). This situation is confirmed by autopsy studies showing that a large number of cancers are never clinically significant nor they will ever be diagnosed during the lifetime of the affected individuals. Thus suggesting that the actual incidence of cancer is greater than usually estimated (Breslow et al., 1977; Tulinius, 1991; Sakr et al., 1993; Koyi et al., 2002; Etzioni et al., 2002; Hua Luo, 2002). Comprehensive studies of cancer incidence in cohorts of twins show that inherited genetic factors make a minor contribution to susceptibility to most types of sporadic cancers (Lichtenstein et al., 2000; Hemminiki and Li, 2002). Most cancers occur in people with no familial history of the disease and non-sporadic, familial type of cancers represent a very small percentage of the

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disease (Pike and Forman, 1991). For example, less than 5% of breast cancers are linked to the presence of inherited susceptibility genes for breast cancer such as BRCA1 and BRCA2 (Cancer Facts and Figures, 2002; Cancer Official Websites 1 and 3). Parasitic diseases are true examples of environmentally caused diseases, but parasites rarely have a cosmopolitan distribution. The same can be said about other environmental factors such as industrial pollution or specific diets. Thus, malaria affects a large number of individuals in specific regions of the world but it is almost unheard of in other parts of the world. However, there is no country free of a specific type of cancer. Indeed, cancer occurs everywhere in the world, but in excess nowhere, this, striking finding suggests that there is a basic incidence of cancer independently of environmental and genetic factors. (Segi and Kurihara, 1972; Breslow et al., 1977; Jackson et al., 1981; Kothari and Mehta, 2001; Gersten and Wilmoth, 2002). The aggregate incidence of cancer in the human population remains the same world over irrespective of ethnic and geographic variations (Parkin et al., 1992). Although the anatomic distribution of cancers in different parts of the world is rather varied, in aged persons the overall death rate from cancer at all sites is remarkably constant for humans world-wide (Smithers, 1960). Thus, the overall cancer incidence in humans is rather predictable (about one in three to four people during their lifetimes), yet it remains a matter of chance at the individual level (which one of those three or four gets the cancer).

4. Sporadic cancer is not the same as laboratory cancer Cancer is currently understood as a genetic or cellular disease that results either from the over-expression or lack of expression of certain genes (oncogenes and tumour suppressor genes), but also from the abnormal activity or lack of activity of the proteins coded by such altered genes (Watson et al., 1992; Alberts et al., 1994; Cooper, 1997; Matsudara et al., 2000). Mouse primary cells have been commonly used a test system for oncogene-associated transforming functions (Land et al., 1983). Mice are short-lived mammals, and at a difference of human cells, mouse cells are very easy to transform in vitro (Hahn et al., 1999; Zimonjic et al., 2001; Seger et al., 2002). Indeed, it was until 1999 that in vitro transformation of primary human cells was reported for the first time (Hahn et al., 1999). However, another group realised similar manipulations, targeting the same four genetic functions, without achieving any success in transforming primary human cells (Morales et al., 1999), and the same type of primary human cells has been transformed in vitro to a tumorigenic state by targeting different combinations of supposedly oncogenic functions (Hahn et al., 1999; Seger et al., 2002).

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Actually, the total number of mutant genes required to create a malignant human cell remains unclear. Moreover, it has been acknowledged that the in vitro transformation mechanisms described are in contrast with the processes operating in the genesis of most sporadic human cancers (Zimonjic et al., 2001). Cancer research favours the study of model systems where brief exposure to a strong carcinogen or outright genetic manipulation produces a high incidence of cancer within weeks or months. But most naturally occurring cancers arise at a slow rate after incubation periods of many years. A common misconception propagated in the literature about cancer is the notion that tumours are always constituted by rapidly dividing cells. Yet, classical cytokinetic studies have shown that many tumours grow much slower than some normal tissues such as the gut’s epithelium or the bone marrow (Steel, 1977; Retsky et al., 1990; Stockdale, 1995). Indeed, there is no evidence that tumours as a group grow faster than normal tissue (Skehan, 1986; Gyllenberg and Webb, 1989). Moreover, often the model tumours are lymphomas, leukaemias or sarcomas that altogether represent less than 10% of naturally occurring tumours, while almost 90% of human tumours are epithelial in origin (Cancer Facts and Figures, 2002). Indeed, epithelial tumours in mice are very rare representing less than 2% of the total incidence (DePinho, 2000). Comparison of the gene-expression profiles of thousands of genes suggest that many of the so-called oncogenes are actually non-expressed or lowly expressed in tumour tissues, and that each individual tumour is genetically different, and therefore, there is no common or typical pattern of gene expression characteristic of a specific kind of cancer (Rasnick and Duesberg, 1999; Alizadeh et al., 2000; Perou et al., 2000; Hua Luo, 2002; Chung et al., 2002; Hampton and Frierson, 2003). Moreover, it has been experimentally established that highly malignant mouse tumour cells injected into normal mouse blastocysts differentiate into normal tissues and contribute to full development of genetically mosaic but otherwise normal mice (Mintz and Illmensee, 1975). These results are unequivocal example of a nonmutational basis for malignant transformation and of the reversal to the normal state when the tumour cells are under the influence of the strong developmental constraints typical of an early developing system.

5. Development, ageing and cancer Ageing has been shown to occur in most animals exhibiting determinate growth, thus reaching a rather fixed size in adulthood (Hayflick, 2000). Yet, some animals such as several species of fish exhibit indeterminate growth, thus being able to keep growing even after reproductive maturity. Such animals show either no

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detectable ageing or significantly delayed senescence (Reznick et al., 2002). Also, several data from animal models suggest that ageing is directly linked with the actual cessation of development (Guarente and Kenyon, 2000). Caloric restriction is known to extend the life span of diverse organisms ranging from unicellular yeast to worms, flies and mammals, but it has progressively less effect the later in life it is begun (Weindruch and Walford, 1982; Finch, 1990). Indeed, in mice, caloric restriction significantly extends the life span provided that it begins after weaning but preferentially before the time of reproductive maturity (Weindruch et al., 1986; Weindruch and Walford, 1998), that most species of mice reach between 5 and 7 weeks of age (Johnson and Johnson, 1982; Mouse websites listed in the bibliography). So far, there is no clear explanation about the mechanisms linking caloric restriction with extended life span (Lin et al., 2002), but it is possible to speculate that part of the effect of early caloric restriction is the delay of final development in such a way that animals take longer to reach reproductive maturity. Hence the partial or complete infertility commonly observed in animals subjected to caloric restriction from early on in their lives, might be a manifestation of delayed reproductive maturity instead of the suggested evolved adaptation to possible temporary fluctuations in resource availability (Shanley and Kirkwood, 2000). The fact that in many tumours the cells are completely normal in their growth rate and control mechanisms, suggests that neoplastic transformation might be the consequence of tissue neogenesis resulting from an altered or inappropriate cellular recognition process (Skehan, 1986). Teratocarcinomas or embryonal carcinomas have their origin in a disorganised embryo, yet cells from such tumours transplanted into normal blastocysts give rise to normal individuals, suggesting that cancer is a consequence of tissue disorganisation rather than changes in gene structure (Mintz and Illmensee, 1975). Recently, it has been suggested that progressive loss of the morphological coherence of the organism (morphological disorder) is a form of ageing, and that all cancers, independently of their site of origin and clinical behaviour, can be understood as a conflict between an organised morphology represented by the organism and a part of such a morphology (the tumour) that drifts towards an amorphous state when the developmental constraints are deranged or exhausted (Aranda-Anzaldo, 2002a,b). The correlation between increasing physiological age and increasing probability of cancer, coupled to the fact that caloric restriction retards both, ageing and the incidence of cancer in animals with different average life spans (Weindruch, 1992), suggests that developmental exhaustion may be the link between ageing and cancer, because developmental constraints and buffering-homeostatic mechanisms that canalise development must act only for as long

as development is taking place, and once a developmental end point is achieved such constraints become superfluous.

6. Tumour suppressors or suppressors of developmental noise? It is widely claimed that there are specific traits and functions that have evolved so as to protect higher eukaryotes, such as mammals from developing cancer. Among such functions are those associated with p53, the paradigmatic tumour suppressor gene. Indeed, the p53 protein responds to a large variety of cellular stresses thus integrating a number of effector pathways that result in arrest of cell proliferation, cellular senescence and/or apoptosis, and so contribute to preserving the stability of the genome (Aranda-Anzaldo et al., 1999; Prives and Hall, 1999; Vousden, 2000; Donehower, 2002). On the other hand, in metazoan tissues cell replication, differentiation and movement must be tuned by the ability of the constituent cells to relate to each other. Yet, reversal or rectification in the event of failure is almost impossible, Thus, coupling cell stress, DNA damage and apoptosis, by some critical effector such as p53, might be an efficient way of coping with problems of tissue organisation (Rich et al., 2000). Mouse models have been widely used to establish the apparent tumour suppressive function of p53. Different laboratory strains of p53-null mice are viable but develop tumours between 3 and 9 months of age (Donehower et al., 1992; Jacks et al., 1994). However, in spite of the high tumour incidence, such mice do not really show a significant reduction of their life span when compared with mice living in the wild, because although several species of mice in captivity show average life-spans between 1 and 2 years, the same species in the wild show much reduced average life-spans ranging from 2 to 6 months at the most, which bears witness to the high extrinsic mortality suffered by these short lived mammals (Getz, 1960; Banfield, 1974; Reich, 1981; Johnson and Johnson, 1982; Mouse websites listed in the bibliography). For example, experiments involving the deletion or modification of p53 alleles have been carried out using mixed inbred mice C57Bl/6-129/Sv, very homogeneous in genetic background. The tumour spectrum of p53// /, p53/// and p53/// mice is remarkably the same showing a predominance of lymphomas and sarcomas, but rarely carcinomas (Tyner et al., 2002). Such mice with p53/// genotype have an average life span of 118 weeks and some mice live more than 3 years. That such laboratory mice achieve amazing life spans in captivity attests that laboratory conditions reduce environmental deleterious factors to a minimum. The p53/// mice have shorter life spans (no mice survived more than 110

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weeks) and display an 80% cancer incidence. The socalled p53//m mice (Tyner et al., 2002) have reduced life spans and show early signs of ageing (96 weeks average, 136 weeks maximum), but most die cancer free (less than 6% cancer incidence). Moreover, most if not all p53/// mice develop cancer and live no more than 40 weeks, which is less than half the average life span of p53/// mice but close to the life span of mice in the wild, estimated at 12
/40 weeks, with a maximum of 64 weeks (Getz, 1960; Banfield, 1974; Reich, 1981; Berry and Bronson, 1992; Mouse websites). If the incidence and rate of cancer in each group of mice were directly due to the cumulative effects of prenatal and postnatal somatic mutagenesis, then the possible load of mutagenic factors affecting each group of mice should be about the same, considering that all mice share the same genetic background and were reared in the same highly controlled laboratory conditions. Thus, in principle, any significant difference in the net mutational load should be the result of the specific p53 genotypes exhibited by each group of mice. However, if such were the case, then cancer incidence within each group should be clustered within a narrow time band, because the critical mutational load compatible with cancer should be on average achieved at about the same time by a significant number of cells within most mice belonging to a given p53-genotype (even though the chronological position of such a time band might vary among the different mouse genotypes). Yet, in any group of mice of a given p53-genotype tumour incidence is scattered in time: some mice develop cancer earlier than others (Tyner et al., 2002). This fact suggests that intrinsic factors proper to each individual mouse have a strong influence on the timing of tumour onset. Therefore, developmental noise remains as a very likely source of the observed heterogeneity in cancer onset and development. Moreover, the high tumour incidence in the p53/// mice is not really affecting the typical mouse life span, considering that individuals of most mouse species are unlikely to live for more than 6 months (24 weeks) in the wild1, thus casting a serious doubt on the need of selection for tumour suppressor functions in wild, normal mice unlikely to live till the age when such functions may become useful or relevant. Detailed studies of p53-null mice show that an important fraction of them display significant developmental abnormalities affecting several tissues, besides the fact that litter size is smaller on average with a relative deficiency of females (Armstrong et al., 1995; Sah et al., 1995). Thus, it has been thoughtfully suggested that p53 main function is to act as a teratological suppressor during early development (Ni1 For example, mortality in the meadow vole Microtus pennsylvanicus is 88% during the first 30 days after birth (Getz, 1960).

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col et al., 1995; Norimura et al., 1996; Hall and Lane, 1997). The fact that p53//m mice appear to have increased activity of p53 and thus be resistant to cancer but show early ageing and reduced life span by some 20% (Tyner et al., 2002), suggests that p53 is involved in the control of critical process during development. It has been proposed that the key factor affected by p53 hyperactivity might be the rate of stem cell production and distribution; that may affect the adult organism, by reducing its capacity for tissue repair thus accelerating the rate of ageing (Donehower, 2002). However, the same facts may be the result of an overall increase in the ‘quality control’ of cellular production due to an excessive p53 activity that may lead to outright apoptosis of slightly damaged or defective cells. Significant evidence suggests that over-expression of wild-type p53 is highly deleterious for the developing vertebrate embryo thus leading to early developmental arrest and lethality (Hoever et al., 1994; Tyner et al., 2002; Donehower, 2002). Indeed, excess of p53 activity seems to be harmful for normal developmental processes in vertebrates (Nakamura et al., 1995; Montes de Oca Luna et al., 1995; Jones et al., 1995; Godley et al., 1996; Migliorini et al., 2002; Pani et al., 2002). Moreover, the tight regulation of p53 action by p53-induced proteins such as MDM2 that exports p53 from the nucleus and targets it for degradation, guarantees that the p53 signal is transient (Rich et al., 2000). Recently ‘‘p53-supermice’’ carrying extra alleles of p53 but under control of their normal promoters were reported. In these mice the basal levels of p53 activity are not different from those in the wild-type. Yet, the p53-supermice display an enhanced response to DNA damage, without showing any evidence of premature ageing (Garcı´a-Cao et al., 2002), Moreover, the p53supermice show significant resistance to chemical tumour induction when compared with the wild-type mice, and the preliminary data suggest that the incidence of spontaneous tumours is much reduced in the p53supermice when compared with wild-type mice (17 vs. 47%). However, the survival fraction at 2 years was very similar among wild-type (78%) and p53-supermice (70%), in spite the fact that given the reduced incidence of spontaneous tumours in the p53-supermice, we should expect a significant increase in their survival at 2 years. This poses the question of what other factors are killing the p53-supermice that would otherwise die from cancer. It would be worth exploring the possibility that excessive apoptosis is responsible for the lack of increased survival in p53-supermice. The mutation spectra in normal young mice (3.5 months) is very similar in different tissues independently of their proliferative history or potential, thus suggesting that mutations in young somatic tissues are the consequence of a mutation mechanism common to all cells and tissues (Dolle´ et al., 2002; Vijg and Dolle, 2002).

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Yet, the mutation spectra in the different organs diverge during the ageing process, and it shows clear differences among different tissues by 32 months of age (Dolle´ et al., 2002). However, the small intestine has the highest spontaneous mutation rate of all tissues tested. This is rather paradoxical considering that tumours of the small intestine are very rare. Indeed, of five tissues tested, the spleen shows the lowest frequency of five different kinds of mutations studied, while the brain, heart, liver and small intestine show on average higher mutation frequencies. These striking results show an uncoupling between the mutation frequency and cancer incidence, since tumours of either brain, heart, liver or small intestine are on the whole rare in mice, while lymphomas and leukaemias represent a significant percentage of the overall cancer incidence in mice (DePinho, 2000). Moreover, these results also show that significant mutation frequencies in mouse somatic tissues are achieved at ages unlikely to be attained in the wild. Indeed, recent experimental results with p53-null mice show that the absence of p53 is not an important determinant of mutation induction either spontaneously during development or after treatment with genotoxic agents (Giese et al., 2002).

7. Cell quality control and developmental success: the case of p53 The general theory of systems failure is known as reliability theory, and it constitutes a body of concepts directed to predict, estimate and optimise the life span distribution of systems and their components (Barlow et al., 1965). Considering that organisms are formed during ontogenesis by the self-assembly of new elements (cells) that are not subjected to external quality control, the reliability of such organisms is usually achieved not by assuring the initial high quality of all the elements but by their huge numbers, permitting the establishment of a high level of redundancy in the number of the constituting elements. The theory predicts that early life events may affect the outcomes in later adult life through the initial level of damage. Sets of elements (cells) will show different rates of failure early in life, but such differences will eventually vanish as the failure rates approach the upper limit determined by the rate of loss of the constituting elements (Gavrilov and Gavrilova, 2001). Therefore, a plausible explanation for the observed reduction of both ageing and cancer at very advanced ages in humans (Partridge and Mangel, 1999; MacieiraCoelho, 2001), is that the failure rate levels-off because redundancy in the number of elements is being gradually reduced. We suggest that the p53 pathway is one of those buffering systems that have evolved to ensure canalisation into normal development. Thus, p53 activity might

be a feature for improving the quality control of the de novo forming and externally untested elements (cells) of the developing system. This may be achieved by keeping a tightly controlled equilibrium between cell proliferation and apoptosis of defective cells. Blocking the activity of p53 in early embryos of Xenopus laevis results in inhibition of further differentiation of the blastomeres, and the embryos become tumour-like disorganised cellular masses (Wallingford et al., 1997), thus implying a very early loss of morphological coherence. Obviously, such tumour-like masses are unlikely to be the result of mutations in oncogenes and other functions highlighted by the somatic mutation theory of cancer. The disorganised cellular masses reflect the need to keep an equilibrium between cell proliferation and morphogenesis during early development, something that can be achieved by p53-induced apoptosis of defective cells that results in a tight control of the actual cell numbers involved in critical morphogenetic movements. We must bear in mind that the critical signal for induction of p53 activity is the presence of single stranded breaks in the DNA (Nelson and Kastan, 1994). Such breaks need not be related to the repair of previous DNA damage, but naturally occur at a high frequency in DNA of actively-replicating cells. Indeed, there is a tight coupling between cell proliferation and apoptosis, as shown by the fact that expression of growth promoting ‘‘oncogenes’’ induce the expression of p53, thus establishing a state of high sensitivity to DNA damage or cellular stress in the affected cells that either may become growth arrested or commit suicide (Sherr and Weber, 2000). The different instances of embryonic induction that occur during early development depend critically on the timing of their occurrence, because inductor and induced tissues must be synchronically competent for this phenomenon to occur (Mu¨ller, 1997). The activities of p53 may contribute to the timing of such a process by keeping a balance between cell proliferation, progression into the cell cycle and apoptosis. This is exemplified by the fact that suppression of p53 function accelerates cell differentiation and exit from the cell cycle in neural cells (Feirrera and Kosic, 1996; Eizenberg et al., 1996). However, excess of p53 function during early development may reduce cell variability and embryonic plasticity resulting from the natural heterogeneity of large cell populations. Extreme quality control during tissue proliferation; namely: p53-induced cell arrest and apoptosis in slightly damaged cells, works against the process for which this proliferation is needed (namely: development). The gene p63 is the most ancient member of the p53 family. The p63-knockout mice die at birth and lack limbs and several tissues. Moreover, some dominant human syndromes affecting limb development have been mapped to the gene encoding p63 (Van Bokhoven

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and McKeon, 2002). Thus, it seems that members of the p53 family constitute a set of functions evolved to play a critical role during vertebrate development, and as such these functions are non-relevant after reproductive maturity (the usual end point of development). Therefore, it should not be surprising that either the p53 gene or genes that regulate the activation of p53 (such as ARF/INK4A) are frequently found in a mutated state in a large number of tumours in aged people (Prives and Hall, 1999; Woods and Vousden, 2001), because such genes are not critical for cell survival in the short term and as such they are prime targets for genetic drift in absence of selection. Indeed, the ultimate fate of a gene that ceases to be useful is to accumulate mutations in the absence of selective pressure by mere genetic drift (Lewin, 2000; Watson et al., 1992, p. 443). The oncogenes and tumour suppressor genes so far described are involved either in cell signalling, control of the cell cycle or control of gene expression, but they do not code for essential metabolic enzymes or essential structural proteins. Therefore, somatic mutations in such genes do not put at risk the survival of the individual cell (at least in the short term). Mutations in key cellular functions lead to cell death, thus in a given tumour only the cells displaying mutations in functions that are not critical for cell survival are available for sampling, this situation may produce the false impression that the observed somatic mutations are the cause of cancer (ArandaAnzaldo, 2001).

8. Cancer as an intrinsic process In spite of decades of concerted research efforts, the current lack of well-identified, necessary and sufficient causes for most sporadic cancers, coupled to the contradictory evidence about the role of specific environmental factors in the causality of sporadic cancers, and the constant distribution of cancer incidence and mortality in the human population, point towards a common factor shared by all human beings as the source of the intrinsic and apparently unavoidable incidence of cancer world-wide. If ageing is understood not only as an increase in mortality and decline in fertility with advancing age (Partridge and Mangel, 1999), but also as increasing molecular and morphological disorder (Hayflick, 2000; Aranda-Anzaldo, 2002a,b), then cancer may represent one of the most radical forms of ageing, because is the evidence of the loss of morphological coherence within the organism resulting from exhaustion or alteration of the normal dynamics of development. The chancy effects of developmental noise must leave a hallmark in each human so that postnatal development of each individual may be more or less susceptible to be disturbed by a number of environmental factors.

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Moreover, as the buffering-homeostatic mechanism that work to guarantee the achievement of full development and reproductive maturity, become progressively weakened or exhausted past the reproductive age, then the intrinsic defects of each individual’s organisation would become manifest through the continued interaction between the organism and the environment. Thus, the morphological coherence of the organism may become eroded sooner or later. The site where such morphological disorganisation becomes manifest is determined by two factors: local site intrinsic instability due to previous developmental noise, and exposure of such a site to the environment. Considering that ageing is not a programmed process, there must be no specific time for its instalment. Thus, although natural selection must favour whichever mechanisms guarantee most members of a given species reach reproductive maturity without showing significant evidence of ageing, in a large cohort there will always be individuals displaying premature signs of ageing, including early onset of cancer. This formulation is consistent with the fact that there is an incidence of cancer throughout the human life span, but also with the fact that cancer incidence increases after the reproductive age but then, after age 75
/80, the frequency levels off or declines (Macieira-Coelho, 2001), because the cohort of individuals is heterogeneous (due to developmental noise), consisting of a mixture of frailty types from the developmental point of view. Hence, the frailest subgroups will develop cancer first, leaving the more robust cancer-free for a longer time until finally there is a levelling-off of cancer incidence at very advanced ages. Moreover, this is also consistent with the fact that those tissues and structures that usually are more exposed to changes in the external and internal environment (skin, lungs, digestive tract, breast and prostate) are the commonest seats of cancer. This view is also consistent with the general discussion on the role of chance in development and ageing expounded by Finch and Kirkwood (2000), and with the basic postulates of the ‘disposable soma theory’ which is based on optimal allocation of metabolic resources between somatic maintenance and reproduction (Kirkwood and Austad, 2000), since an energy saving strategy of reduced accuracy in somatic repair and maintenance may speed up development and reproduction, but may also put in evidence the organism’s intrinsic frailty resulting from developmental noise.

9. Conclusion Considering that for some 99.99% of Homo sapiens history life expectancy at birth was no greater than 27
/ 29 years (Gavrilov and Gavrilova, 2002), cancer, like senescence, must have been a very rare phenomenon

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throughout human history, and certainly of no impact whatsoever upon humankind fitness. Thus, it might be the case that cancer, understood as the loss of morphological coherence within a living system, is just a natural process resulting in most cases from living beyond the normal life expectancy in the wild. Indeed, other socalled age-related ‘‘diseases’’ that currently get a lot of attention, such as Alzheimer disease, might just be a particular manifestation of senescence (Chen and Ferna´ndez, 2001). As a temporal process, cancer might already being embedded, as a result of early developmental noise, in most individuals. According to Strehler, the criteria of senescence are: intrinsicality, progressiveness and deleteriousness (Strehler, 1977), therefore, cancer is a form of senescence, because at whatever age it occurs it seems to be an intrinsic process that increases the probability of death. This means that an understanding of the stochastic components involved in the process of ageing will be required for a full understanding of cancer. Currently, life expectancy at birth is higher in Japan than in any other country, while the overall trend for cancer mortality in Japan has been decreasing since the 1960s, in spite of the fact that cancer remains the leading cause of death in such a country (Gersten and Wilmoth, 2002). This suggests that a general improvement in living standard and public health leads to optimising the processes of ontogenesis and this is enough to counteract or retard the deleterious effects of cancer in such a way that people may live a very long and productive life.

Acknowledgements A. Aranda-Anzaldo and M.A.R. Dent acknowledge the continued support from CONACYT-Me´xico, grants 33539-N and 33540-N respectively. We thank Professor Tom Kirkwood and three anonymous referees for their comments on an early draft of this paper.

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