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Somatic Darwinism in vivo
BioSystems, 12 (1980) 23-25
23
~ Elsevier/North-HollandScientificPublishers Ltd.
SOMATIC D A R W I N I S M IN VIVO JEAN-CLAUDE WEILL and CLAUDE-AGN]~SREYNAUD Service de Biochimie de la Diff~renciation, Institut de recherche en biologie moldculaire, Universit~ de Paris 7, Tour 43, 2 place Jussieu, 75221 Paris, Cedex 05, France
(:ReceivedMay 1st, 1979) (Revision received July 19th, 1979)
Several experimental results in favour of a variability within the somatic genome of eukaryotic organisms have recently emerged from several molecular biology laboratories. While these results are still sparse they nevertheless do allow us to consider the possible physiological significance of these phenomena. The existence of a variability within the somatic genome implies t h a t certain cells within a given population may express different phenotypical representations and t r a n s m i t them to their daughter cells. Several now classical experiments have shown t h a t one could observe within eukaryotic cells in culture conditions the same type of selection phenomena t h a t have been observed among prokaryotes. In general the agent of selective pressure is a drug which has a lethal action on these cells. The only cells which survive and ultimately are detected are those in which random mutations are favourable for growth under these altered conditions (Siminovitch, 1976). These mutations in eukaryotes, as with prokaryotes, generally consist of molecular changes in the composition of nucleic acids due to mechanisms of base substitution, deletion or addition. In the light of these observations, it seems t h a t the notion of the somatic genome as an integral and invariable unit must be reconsidered. Rather surprising phenomena such as the amplification of a cooing sequence or a somatic r e a r r a n g e m e n t between distant genes have been described in higher organisms.
These mechanisms may either be of a random nature and are then revealed under selective pressure: for instance, when mouse sarcoma cells are incubated with methotrexate, one can detect a cellular population resistant to this drug; a biochemical analysis showed t h a t the gene coOing for the enzyme inhibited by the methotrexate is amplified up to two hundred times in these cells (Schimke et al., 1977), and t h a t this amplification occurs on a distinct chromosome (Nunberg et al., 1978). Or these mechanisms may be part of a sequence of development and therefore explain certain steps of the differentiation process, as the somatic rearrangement of constant and variable regions of immunoglobulin light chain (Brack et al., 1978), the amplification of DNA sequences during chicken cartilage and neural retina differentiation (Strom et al., 1978), or the translocation of inverted repeat sequences during sea urchin embryogenesis (Dickinson and Baker, 1978). Even more complex are the notions of insertion of exogen sequences into the genome. These insertions may occur in an apparently silent m a n n e r but have unforeseen consequences on the behaviour of the target population (insertion of viral sequences at some sites, preferential or not, insertion of coOing sequences from an eukaryotic species into the genome of an other, as the gene for thymidine kinase, transferred to deficient mouse cells using total h u m a n DNA as donor (Wigler et' al., 1978)). It is therefore possible now to consider the
24 genome as a variable and non-static entity, the mechanisms experimentally described which maintain this variability are diverse, from simple mutations and mitotic recombinations, to gene amplification and insertions. This variability should be able to be largely amplified by external agents. Mutagens may increase up to a hundred times the number of favourable mutations on cultured cells. The great instability of our ecosystem allows us to think that these mechanisms of amplification operate effectively on living organisms. It is highly probable that, as a result of these variations which may occur with a relatively high frequency, there will be occasionally some "deviant" cells which will offer an advantage over other members of the population. If a favourable variation becomes stabilized, it will have a mitotic advantage and may then repopulate a part or perhaps all of an organ undergoing an aggression. The case of the pulmonary epithelium lesions caused by NO2, where it has been shown that a total in vivo restoration of tissue is obtained while maintaining the chemical aggressor (Evans et al., 1971), could very well illustrate this repairing mechanism. This hypothesis which we formulate as "somatic darwinism" is not without cytological foundations. It has, in fact, largely been shown that the rhythm of cellular divisions is subjected to a high degree of variability. Mitosis m a y be modulated according to physiological conditions, a cell remaining quiescent until its biochemical activity reaches a certain stage from which it is able to enter the cycle (Radley et al., 1976). Such a model could explain how a small number of living cells being obliged to assume a large amount of biochemical functions, e.g., in the case of regenerating liver, may proliferate very rapidly. Thus, equally in physiology of cell growth, biological mechanisms are not rigid and may be replaced by a more flexible frame when the organism is faced with great environmental diversity. It seems quite clear that this variability of
the somatic genome may be of consequence from the point of view of evolution. In effect, regions of high mutability may have been selected in the genome (e.g., inheritance of methylation sites (Bird, 1978)), this variability which would be in some way "coded" would stabilize the species, each organism having through its own existence the possibility to survive changes of the ecosystem. If such a scheme would be experimentally confirmed, it would go in some w a y against the "disposable soma theory" of senescence (Kirkwood, 1977). In effect there would be less advantage for a species to maintain a certain level of error in the germ line cells in order to allow for evolution. Moreover, the lower level of precision for macromolecular synthesis and repair predicted by the author for somatic cells would not be an obligatory source of "catastrophe". In conclusion, we would like to point out that the problem of variability of the soma is now very topical in molecular biology. The resulting speculations are very important and already permit to reconsider the problem of mutagens. In effect it is probable that a certain level of mutations is beneficial for the organism, as well as the amplifications through mutagenic factor that it may undergo. This benefit is obviously a rare event: somatic mutations are in general silent or repaired, or the cell undergoing the DNA modification is eliminated. However in certain cases, these mutations might allow a population of cells to escape from an external lethal aggression. There is also a negative aspect--which has been largely developed and maybe slightly oversimplified (Barrett and Ts'o, 1978) these mutation events being able to participate sometimes in a complex process ending with the cancerisation of a cell. This negative side could be the cost of somatic variability. In this new optic a systematic assimilation mutagen = cancer should be considered with more and more care, for it might be the result of an incomplete picture of the role of somatic mutations in the individual.
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References Barrett, J.C. and P.O.P. Ts'o, 1978, Relationship between somatic mutation and neoplastic transformation. Prec. :Natl. Acad. Sci. USA 75, 3217-3221. Bird, A.P., 1978, Use of restriction enzymes to study eukaryotic DNA methylation: II. The symmetry of methylated sites supports semi-conservative copying of the methylation pattern. J. Mol. Biol. 118, 49-60. Brack, C., M. Hirama, R. Lehnard-Schueller m.t S. Tonegawa, 1978, A complete immunoglobulin gene is created by somatic recombination. Cell 15, 1-14. Dickinson, D.G. and R.F. Baker, 1978, Evidence for translocation of DNA sequences during sea urchin embryogenesis. Prec. Natl. Acad. Sci. USA 75, 56275630. Evans, M.J., R.J. Stephens and G. Freeman, 1978, Effects of nitrogen dioxide on cell renewal in the rat lung. Arch. Int. Med. 128, 57-60. Kirkwood, T.B.L., 1977, Evolution of ageing. Nature 270, 301-304. Nunberg, J.H., R.J. Kaufman, R.T. Schimke, G. Urlaub and L.A. Chasin, 1978, Amplified dihydrofolate reduc-
tase genes are localized to a homogeneously staining region of a single chromosome in a methotrexate-resistant Chinese hamster ovary cell line. Proc. Natl. Acad. Sci. USA 75, 5553-5557. Radley, J.M., J.S. Hedgson and K.W. Koschel, 1976, Properties of Go cells: variations in the proliferative response following isoprenaline. Cell Tissue Kinet. 9, 371378. Schimke, R.T., F.W. Alt, R.E. Kellems, N. Kaufman and J.R. Bertino, 1977, Selective multiplication of dihydrofolate reductase gene in methotrexate-resistant variants of cultured murine cells. Cold Spring Harbor Syrup. Quant. Biol. 42, 649-657. Siminovitch, L., 1976, On the nature of hereditable variations in cultured somatic cells. Cell 7, 1-11. Strom, C.M., M. Moscona and A. Dorfman, 1978, Amplification of DNA sequences during chicken cartilage and neural retina differentiation. Proc. Natl. Acad. Sci. USA 75, 4459 A.!S2. Wigler, M., A. Pellicer, S. Silverstein and R. Axel, 1978, Biochemical transfer of single copy eukaryotic gene using total cellular DNA as donor. Cell 14, 725-731.
23
~ Elsevier/North-HollandScientificPublishers Ltd.
SOMATIC D A R W I N I S M IN VIVO JEAN-CLAUDE WEILL and CLAUDE-AGN]~SREYNAUD Service de Biochimie de la Diff~renciation, Institut de recherche en biologie moldculaire, Universit~ de Paris 7, Tour 43, 2 place Jussieu, 75221 Paris, Cedex 05, France
(:ReceivedMay 1st, 1979) (Revision received July 19th, 1979)
Several experimental results in favour of a variability within the somatic genome of eukaryotic organisms have recently emerged from several molecular biology laboratories. While these results are still sparse they nevertheless do allow us to consider the possible physiological significance of these phenomena. The existence of a variability within the somatic genome implies t h a t certain cells within a given population may express different phenotypical representations and t r a n s m i t them to their daughter cells. Several now classical experiments have shown t h a t one could observe within eukaryotic cells in culture conditions the same type of selection phenomena t h a t have been observed among prokaryotes. In general the agent of selective pressure is a drug which has a lethal action on these cells. The only cells which survive and ultimately are detected are those in which random mutations are favourable for growth under these altered conditions (Siminovitch, 1976). These mutations in eukaryotes, as with prokaryotes, generally consist of molecular changes in the composition of nucleic acids due to mechanisms of base substitution, deletion or addition. In the light of these observations, it seems t h a t the notion of the somatic genome as an integral and invariable unit must be reconsidered. Rather surprising phenomena such as the amplification of a cooing sequence or a somatic r e a r r a n g e m e n t between distant genes have been described in higher organisms.
These mechanisms may either be of a random nature and are then revealed under selective pressure: for instance, when mouse sarcoma cells are incubated with methotrexate, one can detect a cellular population resistant to this drug; a biochemical analysis showed t h a t the gene coOing for the enzyme inhibited by the methotrexate is amplified up to two hundred times in these cells (Schimke et al., 1977), and t h a t this amplification occurs on a distinct chromosome (Nunberg et al., 1978). Or these mechanisms may be part of a sequence of development and therefore explain certain steps of the differentiation process, as the somatic rearrangement of constant and variable regions of immunoglobulin light chain (Brack et al., 1978), the amplification of DNA sequences during chicken cartilage and neural retina differentiation (Strom et al., 1978), or the translocation of inverted repeat sequences during sea urchin embryogenesis (Dickinson and Baker, 1978). Even more complex are the notions of insertion of exogen sequences into the genome. These insertions may occur in an apparently silent m a n n e r but have unforeseen consequences on the behaviour of the target population (insertion of viral sequences at some sites, preferential or not, insertion of coOing sequences from an eukaryotic species into the genome of an other, as the gene for thymidine kinase, transferred to deficient mouse cells using total h u m a n DNA as donor (Wigler et' al., 1978)). It is therefore possible now to consider the
24 genome as a variable and non-static entity, the mechanisms experimentally described which maintain this variability are diverse, from simple mutations and mitotic recombinations, to gene amplification and insertions. This variability should be able to be largely amplified by external agents. Mutagens may increase up to a hundred times the number of favourable mutations on cultured cells. The great instability of our ecosystem allows us to think that these mechanisms of amplification operate effectively on living organisms. It is highly probable that, as a result of these variations which may occur with a relatively high frequency, there will be occasionally some "deviant" cells which will offer an advantage over other members of the population. If a favourable variation becomes stabilized, it will have a mitotic advantage and may then repopulate a part or perhaps all of an organ undergoing an aggression. The case of the pulmonary epithelium lesions caused by NO2, where it has been shown that a total in vivo restoration of tissue is obtained while maintaining the chemical aggressor (Evans et al., 1971), could very well illustrate this repairing mechanism. This hypothesis which we formulate as "somatic darwinism" is not without cytological foundations. It has, in fact, largely been shown that the rhythm of cellular divisions is subjected to a high degree of variability. Mitosis m a y be modulated according to physiological conditions, a cell remaining quiescent until its biochemical activity reaches a certain stage from which it is able to enter the cycle (Radley et al., 1976). Such a model could explain how a small number of living cells being obliged to assume a large amount of biochemical functions, e.g., in the case of regenerating liver, may proliferate very rapidly. Thus, equally in physiology of cell growth, biological mechanisms are not rigid and may be replaced by a more flexible frame when the organism is faced with great environmental diversity. It seems quite clear that this variability of
the somatic genome may be of consequence from the point of view of evolution. In effect, regions of high mutability may have been selected in the genome (e.g., inheritance of methylation sites (Bird, 1978)), this variability which would be in some way "coded" would stabilize the species, each organism having through its own existence the possibility to survive changes of the ecosystem. If such a scheme would be experimentally confirmed, it would go in some w a y against the "disposable soma theory" of senescence (Kirkwood, 1977). In effect there would be less advantage for a species to maintain a certain level of error in the germ line cells in order to allow for evolution. Moreover, the lower level of precision for macromolecular synthesis and repair predicted by the author for somatic cells would not be an obligatory source of "catastrophe". In conclusion, we would like to point out that the problem of variability of the soma is now very topical in molecular biology. The resulting speculations are very important and already permit to reconsider the problem of mutagens. In effect it is probable that a certain level of mutations is beneficial for the organism, as well as the amplifications through mutagenic factor that it may undergo. This benefit is obviously a rare event: somatic mutations are in general silent or repaired, or the cell undergoing the DNA modification is eliminated. However in certain cases, these mutations might allow a population of cells to escape from an external lethal aggression. There is also a negative aspect--which has been largely developed and maybe slightly oversimplified (Barrett and Ts'o, 1978) these mutation events being able to participate sometimes in a complex process ending with the cancerisation of a cell. This negative side could be the cost of somatic variability. In this new optic a systematic assimilation mutagen = cancer should be considered with more and more care, for it might be the result of an incomplete picture of the role of somatic mutations in the individual.
25
References Barrett, J.C. and P.O.P. Ts'o, 1978, Relationship between somatic mutation and neoplastic transformation. Prec. :Natl. Acad. Sci. USA 75, 3217-3221. Bird, A.P., 1978, Use of restriction enzymes to study eukaryotic DNA methylation: II. The symmetry of methylated sites supports semi-conservative copying of the methylation pattern. J. Mol. Biol. 118, 49-60. Brack, C., M. Hirama, R. Lehnard-Schueller m.t S. Tonegawa, 1978, A complete immunoglobulin gene is created by somatic recombination. Cell 15, 1-14. Dickinson, D.G. and R.F. Baker, 1978, Evidence for translocation of DNA sequences during sea urchin embryogenesis. Prec. Natl. Acad. Sci. USA 75, 56275630. Evans, M.J., R.J. Stephens and G. Freeman, 1978, Effects of nitrogen dioxide on cell renewal in the rat lung. Arch. Int. Med. 128, 57-60. Kirkwood, T.B.L., 1977, Evolution of ageing. Nature 270, 301-304. Nunberg, J.H., R.J. Kaufman, R.T. Schimke, G. Urlaub and L.A. Chasin, 1978, Amplified dihydrofolate reduc-
tase genes are localized to a homogeneously staining region of a single chromosome in a methotrexate-resistant Chinese hamster ovary cell line. Proc. Natl. Acad. Sci. USA 75, 5553-5557. Radley, J.M., J.S. Hedgson and K.W. Koschel, 1976, Properties of Go cells: variations in the proliferative response following isoprenaline. Cell Tissue Kinet. 9, 371378. Schimke, R.T., F.W. Alt, R.E. Kellems, N. Kaufman and J.R. Bertino, 1977, Selective multiplication of dihydrofolate reductase gene in methotrexate-resistant variants of cultured murine cells. Cold Spring Harbor Syrup. Quant. Biol. 42, 649-657. Siminovitch, L., 1976, On the nature of hereditable variations in cultured somatic cells. Cell 7, 1-11. Strom, C.M., M. Moscona and A. Dorfman, 1978, Amplification of DNA sequences during chicken cartilage and neural retina differentiation. Proc. Natl. Acad. Sci. USA 75, 4459 A.!S2. Wigler, M., A. Pellicer, S. Silverstein and R. Axel, 1978, Biochemical transfer of single copy eukaryotic gene using total cellular DNA as donor. Cell 14, 725-731.