361
Mutation Research, 38 ( 1 9 7 6 ) 3 6 1 - - 3 6 6 @) Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
SOME THOUGHTS ON THE EVALUATION OF ENVIRONMENTAL MUTAGENS *
F.H. S O B E L S
Department of Radiation Genetics and Chemical Mutagenesis, University of Leiden, Leiden (The Netherlands) (Received May 14th, 1976) (Accepted May 18th, 1976)
Chemical c o m p o u n d s in ever-increasing variety and kind are constantly being introduced into the human environment. Some of these may adversely affect the genetic material. Such effects deserve attention not only for reasons of protecting the genetic constitution of future generations, but are also of prime and direct concern to the present, in view of the striking concordance between the carcinogenic and mutagenic potential of most chemicals. That is, recent results with microbial assay systems and with Drosophila have convincingly demonstrated that the great majority of compounds capable of producing malignant transformation are also effective in inducing genetic changes in the form of heritable mutations. A task of immediate concern thus becomes one of how such genetic and carcinogenic hazards can be avoided and h o w adequate regulations to minimize exposure should be formulated. Obviously, the detection of mutagenic activity has first priority. It is gratifying to see how much progress in this field has been made over the past five years. Many new, imaginative assay systems to probe for the induction of genetic damage have now been developed, and are actually in operation. Moreover, a great deal of thought has been given to the evaluation of mutagenic activity. Elegant and systematic approaches to this problem have been developed in the form of the now well-known "three-tier p r o t o c o l " [4,6]. The need for studies on comparative mutagenesis
If clearly positive findings have been obtained with any particular fast assay system, what should we do next? It is m y opinion that regulatory measures ought to be p o s t p o n e d until a verification has been obtained from a battery of
* From the address p r e s e n t e d at the 7 t h Annual EMS Meeting in Atlanta, Georgia, U.S.A. (March 12--15, 1976), w h e n the a u t h o r r e c e i v e d the F i f t h Annual Award o f the A m e r i c a n E n v i r o n m e n t a l M u t a g e n Society.
362
different test systems. A proper evaluation does require, therefore, data on the sensitivity and detection capacity of the various assay systems used. At the present state of our knowledge, there is, however, a remarkable dearth of information on how, for example, results on the potency of mutagenic activity in the Salmonella test would compare with those in Drosophila, or how the frequency of induced micronuclei corresponds to that of heritable translocations, or of mutations in mammalian assay systems in vitro. To make more meaningful extrapolations concerning the genetic risk involved, it is exactly this kind of information that is most urgently required. This calls for systematic studies on comparative mutagenesis to explore the detection capacity of a variety of assay systems, measuring different end points of genetic damage. Such studies should be carried out for various different groups of mutagens over an extended range of concentrations. To gather meaningful data in this respect is beyond the facilities of any individual laboratory, and consequently calls for intensive planning and co-operation on an international level. It is with this general purpose in mind that, I hope, the next five-year program of the Contract Group "Mutagenicity Testing of the Environmental Research Programme of the European Economic C o m m u n i t y " will move. Extrapolation to man The most pertinent question facing all those involved in the evaluation of mutagenicity data, as obtained in experimental assay systems, concerns their significance to man. Since man is more closely related to other mammals than to Salmonella, fungi or Drosophila, it would seem obvious to assign high relevance to data obtained with mammalian assay systems. Bridges [4] for example, states: " I t is quite clear that compounds likely to be consumed by the general population must be tested for mutagenicity by tests involving mammals". The Committee 17 Report [5], after having praised the usefulness of rapid, inexpensive sub-mammalian tests for initial screening, then states that "compounds that are proposed for actual distribution, however, should also be subject to mammalian screening tests". Obviously, from a toxicological point of view, the intact mammal offers enormous advantages over any other assay system; information regarding pharmacokinetics, such as absorption, elimination, biotransformation both outside and within the target cell, can only be obtained by studies on mammals. With regard to the genetic data which can be obtained from the intact mammal I tend to be more pessimistic, however. From the point of view of genetic hazards, transmissible point mutations, and small deletions probably deserve the highest priority, particularly so since the induction of these correlates with the risk of carcinogenesis. For the detection of this class of genetic damage in mammals one has until now to rely entirely on specific locus tests. In view of the extreme costs and labor involved, there are only a few laboratories in the world where these can be carried out, and this places severe limitations on the number of chemicals and concentrations that can be investigated. Be necessity, experimentation is restricted to high concentrations, but, since dose-effect curves with chemical mutagens do not exhibit simple proportionality, the justification for linear extrapolation to low concentrations is not self-evident.
363 All other mammalian assay systems that can be used for routine screening purposes rely on the detection of chromosome breakage effects, whether these are studied in bone marrow, peripheral blood, testes or in dominant-lethal assays. Recent experiments with Drosophila have revealed that a large number of substances, the indirect carcinogens in particular, are highly effective in inducing mutations, b u t do n o t produce chromosome breakage effects at all, or if so, only at considerably higher concentrations [8--10]. Thus such compounds would spuriously register as safe in any existing routine mammalian assay system. Since it is particularly these substances that entail the risk of producing malignancy, mammalian tests relying on chromosome breakage cannot be considered as fully diagnostic. Salmonella and mammalian cells in culture with microsomal extracts, or Drosophila would seem preferable. The decision for compounds that merit testing in Bridges's tier I and tier 2 is very simple. If they raise the mutation frequency over the control level in a number of test systems (thus are mutagenic) and non-mutagenic substitutes are available, they should obviously be replaced by the latter. It is only with irreplaceable substances of high usefulness that we enter the sphere of risk evaluation (the tier 3 stage). It is becoming increasingly clear that a large number of uncertainties will be encountered in this area. When discussing extrapolation to man, the Committee 17 recommends extrapolation from mouse specific locus tests and cytogenetic tests. But for the reasons set out above, it appears that these systems may be of limited use for the purpose of extrapolation, since for specific locus mutations no dose-effect curves can be determined, and cytogenetic assays may yield negative results for some indirect mutagens and carcinogens in the relevant low concentration range. While I do not at all deny the usefulness or validity of mammalian assays for detecting the various kinds of genetic damage, I see several problems that exist here and suggest the need for prudence and caution. Since there are no precise data to provide a scientific basis on which risk estimates can be based, it seems to me that here we are entering an uncertain domain of compromise. On the one hand, there is a limited amount of scientific information; on the other, there are the pragmatic requirements from society in the form of regulating authorities, for simple categories and numbers. One sometimes feels that one's scientific integrity may be endangered when entering this sort of game; y e t it is a job we are called upon to do from time to time. Perhaps these functions are complementary, and the criteria used in the scientific and regulatory areas mutually exclusive. Yet, as scientists, we are obliged to carry our scientific integrity with us as far as humanly possible, and then make it quite clear when we are leaving the scientific realm and entering that of finding workable and useful formulations in the service of a society which is in need of protection from the adverse effects of environmental chemicals. The Committee 17 report states that for extrapolating mutation rates from test systems to man certain basic units should be used and suggests: (1) the rate-doubling concentration and (2) rem-equivalent chemical. I cannot muster great enthusiam for either. The rate-doubling concentration is the chemical equivalent of the doubling dose used for assessing the genetic effects of ionizing radiation. Application of the doubling dose concept requires that the dose-effect relationship is linear or follows simple power kinetics. The available evi-
364
dence for chemical mutagens suggests that linearity is not a priori to be expected, but rather that all kinds of intracellular factors tend to modify the shape of the dose-effect curve. This is particularly so at the higher concentrations used in most experimental situations; thus extrapolation to lower concentrations is fraught with considerable uncertainty. Briefly, the procedure of determining risks by means of doubling dose operates as follows. First, one determines the natural burden of genetic disease, malformation and early death. An estimate is then made of what fraction of this genetic load is maintained by recurring mutation. Here there is again considerable uncertainty. The more serious shortcoming is, in m y opinion, the unverified assumption that the induced average mutation rate is proportional to the average spontaneous rate. It is wellknown that (1) only a fraction of spontaneous mutations can be ascribed to radiation, (2) vast differences exist between spontaneous rates for individual loci, and {3) the same is true for induction rates; the data presented at this meeting by Sherman [7] provide an example in case for extreme specificity of action at the nucleotide level. If some loci are spontaneously more mutable than after treatment with a mutagenic agent or vice versa, the use of a single doubling dose for the entire genome to quantitate the effects expected after treatment with the agent under consideration will lose its validity. Thus lack of knowledge regarding (1} the shape of the dose-effect curve and (2) the extent to which the frequency of induced mutation is correlated with the spontaneous mutation frequency should caution against generalizations based on the use of rate-doubling concentrations. The application of the other suggested unit, the rem-equivalent chemical, appears attractively simple at first sight, but on closer inspection is also fraught with many uncertainties. Characteristic for chemical mutagens is their specificity of action, either with regard to the spectrum of genetic changes, or at the level of organisms, cell types, chromosome regions or genes. Thus, in Drosophila some chemicals, such as most indirect carcinogens, produce high levels of gene mutations, equivalent to radiation doses of 5000--7000 rad, b u t no translocations or dominant lethals. Another well-known example is provided by formaldehyde, the mutagenic effects of which are dependent on the constitution of the nutritional medium, cell stage, life cycle and sex. Auerbach [1] recently stated that under such conditions, standardization of genetic risks and the ranking of substances in order of risks is a dangerous procedure, because it will create the impression that our conclusions are meaningful, whereas in reality they are so full of uncertainty as to be practically meaningless. She suggests that the decision on the minimal degree of effect above which a chemical is considered risky should be established for the various assay systems. It goes without saying that, to obtain meaningful conclusions, sufficiently large samples and replica experiments are required. A similar opinion was expressed by Bochkov [2,3] at the recent International Symposium held at Zinkovy Castle in October 1975. For reasons set out above, I believe that, at the present state of the development of our field, attempts to quantify genetic risks ensuing from exposure of the human population to mutagenic chemicals are premature. However, adequate regulatory measures can be formulated on the basis of a verified positive response in a number of different assay systems, preferably those for which
365 comparative data defining their relative sensitivity for the detection of different end points of genetic damage are available.
International Commission on the Protection against Environmental Mutagens The problems and complexities involved in adequately protecting the human population against hazards arising from exposure to mutagens calls for intensive collaboration on a world-wide scale. What we need now is the establishment of an "International Commission on Protection against Environmental Mutagens", somewhat along the pattern of the "International Commission on Radiological Protection" which has exerted such a profound influence on the safegards established for the exposure to ionizing radiation. This commission should aim at collecting a b o d y of knowledge to enable an evaluation and a continuing survey of the extent of the hazards involved. What should, in fact, be developed on an authorative international level is a set of recommendations and guidelines on which regulatory measures in the national c o n t e x t can be based. The commission by itself should n o t be too large, so as to ensure maximum workability, b u t it should be advised by a number of. expert committees covering for example: (1) experimental studies dealing with the development, validation, application and comparison of assay systems; (2) an evaluation of the genetic data and whenever possible their translation into risk estimates; (3) epidemiological studies on those sections of the human population that are being exposed to known mutagenic agents; (4) a continuing review of our knowledge with regard to the levels of exposure, both chronic and acute; (5) the correlation between carcinogenesis and mutagenesis. This commission and its various committees should have fair representation in the field of genetics, carcinogenesis, toxicology, epidemiology, industry and international organization. I believe that here lies an important task ahead for the International Association of Environmental Mutagen Societies.
Acknowledgement I am much indebted to m y colleague Dr. K. Sankaranarayanan for critically reading the manuscript. Discussions with Prof. Per Oftedal contributed to the clarification of some of the ideas outlined at the end of this paper. The Drosophila work quoted in this paper was supported in part by PHS Research Grant No. ESO 1027-02 from the National Institute of Environmental Health Sciences and the contract between the EC Environmental Research Programme and the University of Leiden, contract No. C 30-74-JENVN.
References 1 A u e r b a c h , C., T h e e f f e c t s o f s i x y e a r s o f m u t a g e n t e s t i n g o n o u r a t t i t u d e t o t h e p r o b l e m s p o s e d b y it, M u t a t i o n R e s . , 3 3 ( 1 9 7 5 ) 3. 2 B o c h k o v , N.P., C o m m e n t s i n P a n e l o n E s t i m a t i o n o f R i s k , s y m p o s i u m o n N e w D e v e l o p m e n t s in M u t a -
366
3
4 5 6 7 8 9
10
g e n i c i t y T e s t i n g o f E n v i r o n m e n t a l C h e m i c a l s , Z i n k o v y Castle, C z e c h o s l o w a k i a , O c t . 1 4 - - 1 7 , 1 9 7 5 , see " C l o s i n g R e m a r k s " b y F . H . S o b e l s , M u t a t i o n Res., 41 ( 1 9 7 6 ) 1 7 9 - - 1 8 4 . B o c h k o v , N.P., R . J . g r a m , N.P. K u l e s h o v a n d V.S. Z h u r k o v , S y s t e m f o r t h e e v a l u a t i o n o f r i s k f r o m c h e m i c a l m u t a g e n s f o r m a n : basic p r i n c i p l e s a n d p r a c t i c a l r e c o m m e n d a t i o n s , M u t a t i o n Res., 3 8 (1976) 191--202. Bridges, B.A., T h e t h r e e - t i e r a p p r o a c h t o m u t a g e n i c i t y s c r e e n i n g a n d the c o n c e p t o f r a d i a t i o n equival e n t dose. M u t a t i o n Res., 26 ( 1 9 7 4 ) 3 3 5 . C o m m i t t e e 17 o f t h e C o u n c i l o f t h e E n v i r o n m e n t a l M u t a g e n S o c i e t y , E n v i r o n m e n t a l m u t a g e n i c h a z ards, C h a i r m a n J.W. D r a k e , S c i e n c e , 1 8 7 ( 1 9 7 5 ) 5 0 3 . Flamm, W.G., A tier approach to mutagen testing, Mutation Res., 26 (1974) 329. S h e r m a n , F. a n d S. C o n s a u l , C h a r a c t e r i z a t i o n s o f y e a s t m u t a n t s d e f e c t i v e in i s o - l - c y t o c h r o m e c: s t u d ies o f m u t a g e n i e specificities, M u t a t i o n Res., (in press). S o b e l s , F . H . a n d E. V o g e l , T h e c a p a c i t y o f D r o s o p h i l a f o r d e t e c t i n g r e l e v a n t g e n e t i c d a m a g e , M u t a t i o n Res., 4 1 ( 1 9 7 6 ) . V o g e l , E. a n d B. Leigh, C o n c e n t r a t i o n - e f f e c t s t u d i e s w i t h MMS, T E B , 2 , 4 , 6 - T r i C 1 - P D M T a n d D E N o n t h e i n d u c t i o n o f d o m i n a n t a n d recessive l e t h a l s , c h r o m o s o m e loss a n d t r a n s l o c a t i o n s in D r o s o p h i l i a sperm, Mutation Res., 29 (1975) 383. V o g e l , E. a n d F . H . S o b e l s , T h e f u n c t i o n o f D r o s o p h i l a in g e n e t i c t o x i c o l o g y t e s t i n g , in A. H o l l a e n d e r (ed.) C h e m i c a l M u t a g e n s . P r i n c i p l e s a n d M e t h o d s f o r t h e i r D e c t e c t i o n , Vol. 4, P l e n u m Press, N e w York, pp. 93--142.
Mutation Research, 38 ( 1 9 7 6 ) 3 6 1 - - 3 6 6 @) Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
SOME THOUGHTS ON THE EVALUATION OF ENVIRONMENTAL MUTAGENS *
F.H. S O B E L S
Department of Radiation Genetics and Chemical Mutagenesis, University of Leiden, Leiden (The Netherlands) (Received May 14th, 1976) (Accepted May 18th, 1976)
Chemical c o m p o u n d s in ever-increasing variety and kind are constantly being introduced into the human environment. Some of these may adversely affect the genetic material. Such effects deserve attention not only for reasons of protecting the genetic constitution of future generations, but are also of prime and direct concern to the present, in view of the striking concordance between the carcinogenic and mutagenic potential of most chemicals. That is, recent results with microbial assay systems and with Drosophila have convincingly demonstrated that the great majority of compounds capable of producing malignant transformation are also effective in inducing genetic changes in the form of heritable mutations. A task of immediate concern thus becomes one of how such genetic and carcinogenic hazards can be avoided and h o w adequate regulations to minimize exposure should be formulated. Obviously, the detection of mutagenic activity has first priority. It is gratifying to see how much progress in this field has been made over the past five years. Many new, imaginative assay systems to probe for the induction of genetic damage have now been developed, and are actually in operation. Moreover, a great deal of thought has been given to the evaluation of mutagenic activity. Elegant and systematic approaches to this problem have been developed in the form of the now well-known "three-tier p r o t o c o l " [4,6]. The need for studies on comparative mutagenesis
If clearly positive findings have been obtained with any particular fast assay system, what should we do next? It is m y opinion that regulatory measures ought to be p o s t p o n e d until a verification has been obtained from a battery of
* From the address p r e s e n t e d at the 7 t h Annual EMS Meeting in Atlanta, Georgia, U.S.A. (March 12--15, 1976), w h e n the a u t h o r r e c e i v e d the F i f t h Annual Award o f the A m e r i c a n E n v i r o n m e n t a l M u t a g e n Society.
362
different test systems. A proper evaluation does require, therefore, data on the sensitivity and detection capacity of the various assay systems used. At the present state of our knowledge, there is, however, a remarkable dearth of information on how, for example, results on the potency of mutagenic activity in the Salmonella test would compare with those in Drosophila, or how the frequency of induced micronuclei corresponds to that of heritable translocations, or of mutations in mammalian assay systems in vitro. To make more meaningful extrapolations concerning the genetic risk involved, it is exactly this kind of information that is most urgently required. This calls for systematic studies on comparative mutagenesis to explore the detection capacity of a variety of assay systems, measuring different end points of genetic damage. Such studies should be carried out for various different groups of mutagens over an extended range of concentrations. To gather meaningful data in this respect is beyond the facilities of any individual laboratory, and consequently calls for intensive planning and co-operation on an international level. It is with this general purpose in mind that, I hope, the next five-year program of the Contract Group "Mutagenicity Testing of the Environmental Research Programme of the European Economic C o m m u n i t y " will move. Extrapolation to man The most pertinent question facing all those involved in the evaluation of mutagenicity data, as obtained in experimental assay systems, concerns their significance to man. Since man is more closely related to other mammals than to Salmonella, fungi or Drosophila, it would seem obvious to assign high relevance to data obtained with mammalian assay systems. Bridges [4] for example, states: " I t is quite clear that compounds likely to be consumed by the general population must be tested for mutagenicity by tests involving mammals". The Committee 17 Report [5], after having praised the usefulness of rapid, inexpensive sub-mammalian tests for initial screening, then states that "compounds that are proposed for actual distribution, however, should also be subject to mammalian screening tests". Obviously, from a toxicological point of view, the intact mammal offers enormous advantages over any other assay system; information regarding pharmacokinetics, such as absorption, elimination, biotransformation both outside and within the target cell, can only be obtained by studies on mammals. With regard to the genetic data which can be obtained from the intact mammal I tend to be more pessimistic, however. From the point of view of genetic hazards, transmissible point mutations, and small deletions probably deserve the highest priority, particularly so since the induction of these correlates with the risk of carcinogenesis. For the detection of this class of genetic damage in mammals one has until now to rely entirely on specific locus tests. In view of the extreme costs and labor involved, there are only a few laboratories in the world where these can be carried out, and this places severe limitations on the number of chemicals and concentrations that can be investigated. Be necessity, experimentation is restricted to high concentrations, but, since dose-effect curves with chemical mutagens do not exhibit simple proportionality, the justification for linear extrapolation to low concentrations is not self-evident.
363 All other mammalian assay systems that can be used for routine screening purposes rely on the detection of chromosome breakage effects, whether these are studied in bone marrow, peripheral blood, testes or in dominant-lethal assays. Recent experiments with Drosophila have revealed that a large number of substances, the indirect carcinogens in particular, are highly effective in inducing mutations, b u t do n o t produce chromosome breakage effects at all, or if so, only at considerably higher concentrations [8--10]. Thus such compounds would spuriously register as safe in any existing routine mammalian assay system. Since it is particularly these substances that entail the risk of producing malignancy, mammalian tests relying on chromosome breakage cannot be considered as fully diagnostic. Salmonella and mammalian cells in culture with microsomal extracts, or Drosophila would seem preferable. The decision for compounds that merit testing in Bridges's tier I and tier 2 is very simple. If they raise the mutation frequency over the control level in a number of test systems (thus are mutagenic) and non-mutagenic substitutes are available, they should obviously be replaced by the latter. It is only with irreplaceable substances of high usefulness that we enter the sphere of risk evaluation (the tier 3 stage). It is becoming increasingly clear that a large number of uncertainties will be encountered in this area. When discussing extrapolation to man, the Committee 17 recommends extrapolation from mouse specific locus tests and cytogenetic tests. But for the reasons set out above, it appears that these systems may be of limited use for the purpose of extrapolation, since for specific locus mutations no dose-effect curves can be determined, and cytogenetic assays may yield negative results for some indirect mutagens and carcinogens in the relevant low concentration range. While I do not at all deny the usefulness or validity of mammalian assays for detecting the various kinds of genetic damage, I see several problems that exist here and suggest the need for prudence and caution. Since there are no precise data to provide a scientific basis on which risk estimates can be based, it seems to me that here we are entering an uncertain domain of compromise. On the one hand, there is a limited amount of scientific information; on the other, there are the pragmatic requirements from society in the form of regulating authorities, for simple categories and numbers. One sometimes feels that one's scientific integrity may be endangered when entering this sort of game; y e t it is a job we are called upon to do from time to time. Perhaps these functions are complementary, and the criteria used in the scientific and regulatory areas mutually exclusive. Yet, as scientists, we are obliged to carry our scientific integrity with us as far as humanly possible, and then make it quite clear when we are leaving the scientific realm and entering that of finding workable and useful formulations in the service of a society which is in need of protection from the adverse effects of environmental chemicals. The Committee 17 report states that for extrapolating mutation rates from test systems to man certain basic units should be used and suggests: (1) the rate-doubling concentration and (2) rem-equivalent chemical. I cannot muster great enthusiam for either. The rate-doubling concentration is the chemical equivalent of the doubling dose used for assessing the genetic effects of ionizing radiation. Application of the doubling dose concept requires that the dose-effect relationship is linear or follows simple power kinetics. The available evi-
364
dence for chemical mutagens suggests that linearity is not a priori to be expected, but rather that all kinds of intracellular factors tend to modify the shape of the dose-effect curve. This is particularly so at the higher concentrations used in most experimental situations; thus extrapolation to lower concentrations is fraught with considerable uncertainty. Briefly, the procedure of determining risks by means of doubling dose operates as follows. First, one determines the natural burden of genetic disease, malformation and early death. An estimate is then made of what fraction of this genetic load is maintained by recurring mutation. Here there is again considerable uncertainty. The more serious shortcoming is, in m y opinion, the unverified assumption that the induced average mutation rate is proportional to the average spontaneous rate. It is wellknown that (1) only a fraction of spontaneous mutations can be ascribed to radiation, (2) vast differences exist between spontaneous rates for individual loci, and {3) the same is true for induction rates; the data presented at this meeting by Sherman [7] provide an example in case for extreme specificity of action at the nucleotide level. If some loci are spontaneously more mutable than after treatment with a mutagenic agent or vice versa, the use of a single doubling dose for the entire genome to quantitate the effects expected after treatment with the agent under consideration will lose its validity. Thus lack of knowledge regarding (1} the shape of the dose-effect curve and (2) the extent to which the frequency of induced mutation is correlated with the spontaneous mutation frequency should caution against generalizations based on the use of rate-doubling concentrations. The application of the other suggested unit, the rem-equivalent chemical, appears attractively simple at first sight, but on closer inspection is also fraught with many uncertainties. Characteristic for chemical mutagens is their specificity of action, either with regard to the spectrum of genetic changes, or at the level of organisms, cell types, chromosome regions or genes. Thus, in Drosophila some chemicals, such as most indirect carcinogens, produce high levels of gene mutations, equivalent to radiation doses of 5000--7000 rad, b u t no translocations or dominant lethals. Another well-known example is provided by formaldehyde, the mutagenic effects of which are dependent on the constitution of the nutritional medium, cell stage, life cycle and sex. Auerbach [1] recently stated that under such conditions, standardization of genetic risks and the ranking of substances in order of risks is a dangerous procedure, because it will create the impression that our conclusions are meaningful, whereas in reality they are so full of uncertainty as to be practically meaningless. She suggests that the decision on the minimal degree of effect above which a chemical is considered risky should be established for the various assay systems. It goes without saying that, to obtain meaningful conclusions, sufficiently large samples and replica experiments are required. A similar opinion was expressed by Bochkov [2,3] at the recent International Symposium held at Zinkovy Castle in October 1975. For reasons set out above, I believe that, at the present state of the development of our field, attempts to quantify genetic risks ensuing from exposure of the human population to mutagenic chemicals are premature. However, adequate regulatory measures can be formulated on the basis of a verified positive response in a number of different assay systems, preferably those for which
365 comparative data defining their relative sensitivity for the detection of different end points of genetic damage are available.
International Commission on the Protection against Environmental Mutagens The problems and complexities involved in adequately protecting the human population against hazards arising from exposure to mutagens calls for intensive collaboration on a world-wide scale. What we need now is the establishment of an "International Commission on Protection against Environmental Mutagens", somewhat along the pattern of the "International Commission on Radiological Protection" which has exerted such a profound influence on the safegards established for the exposure to ionizing radiation. This commission should aim at collecting a b o d y of knowledge to enable an evaluation and a continuing survey of the extent of the hazards involved. What should, in fact, be developed on an authorative international level is a set of recommendations and guidelines on which regulatory measures in the national c o n t e x t can be based. The commission by itself should n o t be too large, so as to ensure maximum workability, b u t it should be advised by a number of. expert committees covering for example: (1) experimental studies dealing with the development, validation, application and comparison of assay systems; (2) an evaluation of the genetic data and whenever possible their translation into risk estimates; (3) epidemiological studies on those sections of the human population that are being exposed to known mutagenic agents; (4) a continuing review of our knowledge with regard to the levels of exposure, both chronic and acute; (5) the correlation between carcinogenesis and mutagenesis. This commission and its various committees should have fair representation in the field of genetics, carcinogenesis, toxicology, epidemiology, industry and international organization. I believe that here lies an important task ahead for the International Association of Environmental Mutagen Societies.
Acknowledgement I am much indebted to m y colleague Dr. K. Sankaranarayanan for critically reading the manuscript. Discussions with Prof. Per Oftedal contributed to the clarification of some of the ideas outlined at the end of this paper. The Drosophila work quoted in this paper was supported in part by PHS Research Grant No. ESO 1027-02 from the National Institute of Environmental Health Sciences and the contract between the EC Environmental Research Programme and the University of Leiden, contract No. C 30-74-JENVN.
References 1 A u e r b a c h , C., T h e e f f e c t s o f s i x y e a r s o f m u t a g e n t e s t i n g o n o u r a t t i t u d e t o t h e p r o b l e m s p o s e d b y it, M u t a t i o n R e s . , 3 3 ( 1 9 7 5 ) 3. 2 B o c h k o v , N.P., C o m m e n t s i n P a n e l o n E s t i m a t i o n o f R i s k , s y m p o s i u m o n N e w D e v e l o p m e n t s in M u t a -
366
3
4 5 6 7 8 9
10
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