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Toxicology and Applied Pharmaco

Review

Scientific basis for the Precautionary Principle

Paolo Vineis

Imperial College London, St. Mary’s Campus, Norfolk Place, W2 1PG London, UK

Received 15 July 2004; accepted 24 November 2004

Available online 29 June 2005

Abstract

The Precautionary Principle is based on two general criteria: (a) appropriate public action should be taken in response to limited, but

plausible and credible, evidence of likely and substantial harm; (b) the burden of proof is shifted from demonstrating the presence of risk

to demonstrating the absence of risk. Not much has been written about the scientific basis of the precautionary principle, apart from the

uncertainty that characterizes epidemiologic research on chronic disease, and the use of surrogate evidence when human evidence cannot

be provided. It is proposed in this paper that a new scientific paradigm, based on the theory of evolution, is emerging; this might offer

stronger support to the need for precaution in the regulation of environmental risks. Environmental hazards do not consist only in direct

attacks to the integrity of DNA or other macromolecules. They can consist in changes that take place already in utero, and that condition

disease risks many years later. Also, environmental exposures can act as ‘‘stressors’’, inducing hypermutability (the mutator phenotype) as

an adaptive response. Finally, environmental changes should be evaluated against a background of a not-so-easily modifiable genetic

make-up, inherited from a period in which humans were mainly hunters–gatherers and had dietary habits very different from the current

ones.

D 2005 Elsevier Inc. All rights reserved.

Keywords: Precautionary Principle; Toxicology; Chronic disease epidemiology

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S658

Thrifty genotype/phenotype. Phenotypic plasticity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S659

The mutator phenotype in bacteria and in humans. The PNH model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S660

The case of diet: the delicate balance between prooxidant and antioxidant exposures. . . . . . . . . . . . . . . . . . . . . . S660

Discussion and relevance for the Precautionary Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S661

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S661

Introduction

Looking at the past may be useful to understand the

consequences of rejecting early warnings. The United States

had very few cases of limb defects from thalidomide

because Frances Kelsey, an officer at the US Food and

Drug Administration, delayed approval of the drug. Both

Lenz in Germany (who came too late since he described as

0041-008X/$ - see front matter D 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.taap.2004.11.033

E-mail address: p.vineis@imperial.ac.uk.

many as 4000 babies with malformations) and Kelsey

underwent strong criticism in the name of ‘‘objectivity’’ of

medicine and had to face threats to their professional

credibility (Daemmrich, 2002).

As discussed elsewhere in this volume, the Precautionary

Principle is based on two general criteria: (a) appropriate

public action should be taken in response to limited, but

plausible and credible, evidence of likely and substantial

harm; (b) the burden of proof is shifted from demonstrating

the presence of risk to demonstrating the absence of risk

(Grandjean et al., 2004). Most discussions about the

logy 207 (2005) S658 – S662

YTAAP-10364; No. of pages: 5; 4C:

P. Vineis / Toxicology and Applied Pharmacology 207 (2005) S658–S662 S659

principle have been political or ethical. Both good reasons

for adopting it (e.g. impossibility to set a threshold for

carcinogenic exposures; long induction periods for many

modern diseases; uncertainties on mechanisms of action;

effects on a large scale of new pollutants) and limitations

(e.g. what is the minimum level of suspicion that leads to

precautionary action; the principle can be paralizing, as

some examples suggest; risks tend to be evaluated

independently of benefits) have been stressed. However,

few comments have been heard about the scientific basis of

the Precautionary Principle, apart from the uncertainty that

characterizes epidemiologic research on chronic disease (see

for example Tickner, 2002 and Kriebel et al., 2001). My

proposal is that a new scientific paradigm might be

emerging that offers stronger support to the need for

precaution in the regulation of environmental risks. One

of the reasons for urgency is the lack of understanding of

many serious diseases, such as Alzheimer, Parkinson, and

ALS. The new paradigm should help in protecting people

from environmental hazards more effectively than in the

past.

The first paradigm that has been used for regulation was

toxicology, which was based on the investigation mainly of

acute intoxication/poisoning. Toxicology has been defined

very broadly as the study of adverse effects of xenobiotics and

as a multidisciplinary field. However, its practical interpre-

tation has usually been much narrower. A feature of the

toxicological approach often consisted in postulating the

existence of thresholds, which were in fact observed for acute

poisoning, but not for many chronic effects. This is

particularly true if we consider for example the levels of

recommended daily allowances (RDA) for micronutrients,

which are based on acute deficiencies, not on chronic effects.

The second paradigm was chronic disease epidemiology,

which investigated the role of prolonged exposure in

increasing the risk for chronic diseases such as cancer,

criticized the concept of thresholds, and stressed the un-

certainties associated with the study of free-living popula-

tions, with many competing and interacting environmental

exposures.

The new paradigm is emerging from a number of

investigations in different fields, including toxicology,

epidemiology, nutrition, and gene–environment interac-

tions. This paradigm has developed historically from the

study of ecologic problems and can be called the theory of

‘‘Evolutionary Medicine’’ (Trevathan et al., 1999). This

theory has generated the idea that different organisms, and

the biological and physical environments, are mutually

dependent. It overcomes both toxicology, with its model of

chemical damage with a threshold, and epidemiology, with

its model of multifactorial probabilistic causal networks,

suggesting that the scientific basis of the Precautionary

Principle is larger than simple ‘‘prudence derived from

uncertainty’’. Two main components of this paradigm are

the theory of the thrifty genotype/phenotype and the theory

of the acquired ‘‘mutator phenotype’’ in carcinogenesis.

Thrifty genotype/phenotype. Phenotypic plasticity

According to the model originally proposed by Baker

and Hales, the organism adapts to poor nutrition in early life

by programming its insulin metabolism to expect a similarly

depleted environment subsequently. This has been called

‘‘thrifty phenotype’’. If the environment improves following

the initial adaptation (e.g. exposure to a richer diet), the

programmed trait becomes inappropriate, hence maladapta-

tion. This model has been used to explain the onset of

diabetes, obesity, and hypertension. Others have described

the same phenomenon on a species scale (the ‘‘thrifty

genotype’’) due to the selection of the thrifty trait over

generations. Therefore, the thrifty genotype would be

relevant to population differences in, e.g. diabetes, whereas

the thrifty phenotype hypothesis is relevant to adaptation

within an individual’s lifespan (Wells, 2003). In general,

individuals who fix their phenotype early in development

(i.e. have a ‘‘setting’’ of their metabolism, gene expression,

etc., conditioned by early stress) suffer higher mortality due

to environmental change than do organisms which preserve

plasticity during development (Wells, 2003). Early pro-

gramming due to developmental stress implies lower

phenotypic plasticity in the rest of life.

What is phenotypic plasticity? Organisms are ‘‘buffered’’

or ‘‘canalized’’ (Waddington, 1942) against environmental

variation. Buffering molecules are called ‘‘chaperons’’, for

example, Hsp (Heat-shock protein) 90. In plants – and

probably in animals – the balance between stability of the

genome and potential for change in conditions of environ-

mental stress is made possible by chaperones (Pigliucci,

2002). In stressful conditions, Drosophila melanogaster

accumulates hidden genetic variation which is kept by

Hsp90 from affecting the phenotype. Chaperones, which are

among the most highly conserved proteins in all organisms,

help cells to survive stressful fluctuations in the environ-

ment, through protection of the macromolecules (Sangster et

al., 2004; Kultz, 2003).

An interesting property of chaperones is that they

normally inhibit the expression of cryptic polymorphisms,

which become apparent in conditions of stress. For example,

in one experiment, increased temperature and Hsp90

reduction resulted in functional dominance of underlying

heterozygous polymorphisms (Sangster et al., 2004). A

second response to stress is represented by apoptosis, when

the dose of stress overcomes the cell’s ability to maintain

genomic and macromolecular integrity. A third mechanism

is hypermutability that intervenes in bacteria and aims at

generating new phenotypes (mutants) that are able to

overcome the environmental challenge.

Mice deficient for Hsp70, generated by gene targeting,

displayed genetic instability in their cells after exposure to

radiation (Hunt et al., 2004), thus demonstrating that Hsp70

plays an essential role in maintaining genomic stability. In a

human experiment, mononuclear cells from a donor were

treated with the antineoplastic agent doxorubicin. DNA

P. Vineis / Toxicology and Applied Pharmacology 207 (2005) S658–S662S660

damage from the drug was more prominent after previous

heat shock, which was followed by increased expression of

Hsp70 and of mismatch repair proteins (MMR, a type of

DNA repair) (Nadin et al., 2003). Hsps have been shown to

stimulate base excision repair enzymes (Mendez et al.,

2003). Hsps are possibly involved also in the onset of

diseases other than cancer. Increased expression of Hsp60

was found in patients with Alzheimer’s disease; diabetes

may impair Hsp protection from oxidative damage, but

physical exercise seems to up-regulate Hsp protection (see

below).

The mutator phenotype in bacteria and in humans. The

PNH model

Bacteria under environmental stress undergo a condition

which is called SLAM or stressful lifestyle associated

mutation (or stationary-phase mutability). This condition

requires functional proteins of the double-strand break

repair recombination system, that is, recombination is part

of a unique mechanism by which stationary-phase mutations

form. In conditions of stress, mutations occur at a rate that is

10–1000 times higher than in control bacteria populations

(Rosenberg et al., 1998; Bjedov et al., 2003). However,

mutations do not occur in all cells, but only in subpopula-

tions, which leave the hypermutable state when the adaptive

mutation is generated (Hall, 1990). Survivors will all carry

the mutation. In this transition, MMR plays a central role,

being inhibited during the stationary-phase (Harris et al.,

1999). Similar modulation in stationary-phase has been

shown in human cells (Richards et al., 1997). Among the

conditions that trigger the stationary-phase are starvation,

physical, and chemical stress. The small minority of

heritable mutator mutants resembles the population of cells

observed in several examples of adaptive evolution (Rosen-

berg et al., 1998).

At least for one environmental contaminant, cadmium, at

the dose levels typical of human exposure, mutagenesis has

been shown to be related to inhibition of MMR. Jin et al.

(2003) found that chronic exposure of yeast to environ-

mentally relevant concentrations of cadmium resulted in

extreme hypermutability. This occurred by an indirect

mechanism, i.e. inhibition of DNA repair; in fact, in the

same experiment, cadmium inhibited mismatch removal

(MMR) in extracts of human cells.

Could the model of hypermutability in bacteria be a

model for human disease, i.e. could cancer and other

chronic diseases be due to indirect mutagenesis and

selection of mutants/mutators? In fact, one theory of cancer

(Finette et al., 2001) implies that cancer is not only based on

selection of mutated phenotypes but in fact of mutator

phenotypes, that is, cells which easily undergo mutations

acquire a selective advantage over others in stressful

conditions. It is commonly recognized that somatic muta-

tion (irreversible change in DNA information content)

initiates the process of carcinogenesis: the mutated cell(s)

are selected in vivo because of their growth advantage, loss

of contact inhibition, loss of apoptotic pathway(s), etc. In

addition to that, further selective advantage is conferred by

the mutator phenotype, i.e. the ability to mutate rapidly and

adapt to a difficult environment such as hypoxia.

An example of how the presence of adaptive mutations

can confer advantage in stressful circumstances is paro-

xysmal nocturnal hemoglobinuria (PNH), which is an

acquired stem cell disorder characterized by intravascular

hemolysis, hypercoagulability, and bone marrow failure.

The characteristic defect in paroxysmal nocturnal hemo-

globinuria is the somatic mutation of the PIG-A gene in

hematopoietic cells. One current hypothesis explaining the

disorder suggests that there are two components: (1)

hematopoietic stem cells with the characteristic defect are

present in the marrow of many if not all normal individuals

in very small numbers; (2) some aplastogenic influence

(e.g. an immune overreaction to drugs) suppresses the

normal stem cells but does not suppress the defective stem

cells, thus allowing the proportion of these cells to increase

(a ‘‘darwinian’’ interpretation) (Bessler et al., 1994). Those

who are not carriers of PIG-A mutations and are exposed to

the aplastogenic insult instead of developing PNH undergo

an even worse disease, aplastica anemia.

The hypothesis of an acquired (rather than inherited)

mutator phenotype is interesting and is consistent with the

interpretation of carcinogenesis as both a multistage process

and a highly non-linear process.

The case of diet: the delicate balance between prooxidant

and antioxidant exposures

Humans have evolved in the course of their history by

developing defenses against various types of environmental

stress, one being dietary overload of prooxidant molecules.

In addition to the evolution of enzymatic systems, an

obvious defense is high intake of antioxidants contained in

different types of fruits and vegetables.

In an experiment we have conducted (F. Saletta, G.

Matullo, unpublished data), a dietary supplementation was

proposed, within a randomized controlled trial, to 120

healthy males, heavy smokers (10–20 cigarettes/day, 35–70

years). After the run-in phase, 90 subjects were eligible. The

study aimed at evaluating the capacity of flavonoids to

decrease urinary mutagenicity and to inhibit the formation

of DNA adducts in smokers. Volunteers underwent a

1-month lasting group-specific diet period: (a) normal

isocaloric diet (with an adequate administration of fruit

and vegetables); (b) rich in flavonoids but not based on

supplementation; (c) based on supplementation of flavo-

noids in the form of green tea and soy products. We

investigated, in a pilot study on a small subset of 9 subjects

belonging to the ‘‘supplement’’ group, whether administra-

tion of foods rich in flavonoids could affect DNA repair

P. Vineis / Toxicology and Applied Pharmacology 207 (2005) S658–S662 S661

gene expression. An increase in flavonoids intake was

obtained by dietary modifications through special recipes

and instructions by a cook during an intensive course. No

other significant difference has been observed in micro-

nutrients intake estimates. All the repair genes showed

positive correlation between mRNA levels and smoking

which seems to up-regulate DNA repair genes. A negative

correlation with flavonoids intake and urinary phenolics has

been found for all the gene expression levels. Our pre-

liminary data seem to support a protective role of flavonoids

in preventing DNA damage and, consequently, decreasing

the need for DNA repair.

Discussion and relevance for the Precautionary Principle

To summarize, environmental hazards do not consist

only in direct attacks to the integrity of DNA or other

macromolecules. First, they can consist in changes that

take place already in utero, and that condition disease risks

many years later. The extremely high prevalence of

diabetes and obesity among Pima Indians is attributed to

a combination of thrifty genotype and phenotype followed

by exposure to the highly caloric North-American diet.

The Pleistocene humans and in general hunters–gatherers

had a diet extremely rich in fruit and vegetables, with an

intake of folate which was at least 5 times higher than in

current Western diet. This was the dietary environment

with which the metabolic genetic profile of humans

evolved. The combination of the same genes with rapid

changes in the environment justifies rapid changes in

disease rates as well.

Second, environmental exposures can act as ‘‘stressors’’,

inducing hypermutability (the mutator phenotype) as an

adaptive response. In general, indirect mutagenesis, such as

the type induced by cadmium, might be more important than

previously suspected.

A striking recent observation was the finding of a very

high proportion in healthy newborns of mutations in a gene

associated with lymphocytic leukemia (the mutation rate

was about 100 times higher than the cumulative incidence of

leukemia) (Mori et al., 2002). While the origin of such

mutations is not known – but could express exposure to in

utero stressors – it is clear that mutations per se are

insufficient to explain the onset of leukemia, which is

probably due to further ‘‘hits’’ that select cells with a

selective advantage (the PNH model).

The importance of a ‘‘darwinian’’ approach for the

Precautionary Principle is evident. It suggests that long-

term changes and not only short-term exposures should be

considered and monitored; that consequences can be

unpredictable on the basis of traditional toxicology (e.g. in

the case of cadmium); and that environmental changes

should be evaluated against a background of a not-so-easily

modifiable genetic make-up, inherited from a period in

which humans were mainly hunters–gatherers. Therefore, it

is true that an unmodifiable ‘‘natural’’ state does not exist,

but it is also true that human physiology has constraints that

are not only those discovered by classical toxicology. Also

for modern diseases, not only for marriages, Anna Kare-

nina’s principle applies: ‘‘All happy marriages are similar to

each other; each unhappy marriage is unhappy in its own

way’’.

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