From Correcting the Blueprint of Life:
An Historical Account
of the Discovery of DNA Repair Mechanisms
by
Cold Spring Harbor Laboratory Press, 1997
? 1997 by Cold Spring Harbor Laboratory Press. Used with permission.
(Posted August 15, 1997 ? Issue 14; archived September 5, 1997)
Editor's Note: A profound discovery in genetic research was made in the late 1920s, when it was found that X rays greatly increased mutation frequency; later, mutations would also be induced by UV radiation and by chemicals. This discovery gave birth - eventually - to the field of inquiry known as DNA repair and mutagenesis. But as Dr. Errol Friedberg tells us in this fascinating history, "eventually" was a while in coming. Here, "In the Beginning," we learn about the fits and starts, the inspirations and prejudices that contributed to, or hindered, the development of the field. We also get an insider's look at the early Cold Spring Harbor Symposia on Quantitative Biology, and at the legendary scientists who made them famous.
In his
historical accounting of the emergence and efflorescence of molecular biology in the mid 20th
century entitled The
Eighth Day of Creation: Makers of the Revolution in Biology, Horace
Judson reminds us that "mutations obtained one way or another have . . . always been
the chief tool of experimental genetics." But beginning with Mendel, biologists engaged
in exploring the nature of the gene and of inheritance were heavily reliant on the
appearance of spontaneous alterations in gene function in their chosen
experimental subjects, be they sweet peas, humans, or, as popularized by Thomas Hunt Morgan,
the fruit fly Drosophila. Spontaneous mutations are of course infrequent events
and the ability to recognize them in the mirror of altered phenotypes was further
hindered by the prevalent predilection for organisms with diploid genomes as experimental
subjects. Hence, the discovery in 1927 by Hermann J. Muller that X rays greatly
enhanced mutation frequency in Drosophila (a finding that earned him the Nobel
Prize 19 years later) represented a major contribution to genetics. The enormous experimental
utility of the increased mutation frequency attendant on the exposure of organisms
to X rays was clearly underscored by Muller himself in a paper that he presented at
the 1941 Cold Spring Harbor Symposium, a scientific event that we shall shortly
revisit.
The inordinate effectiveness of X-rays and related radiation . . . has hardly been realized to the full. For when we say that a dose of 110,000 r-units results in about 100 times as many lethals per X-chromosome in Drosophila spermatozoa as ordinarily occur in untreated material, we do not take into consideration the fact that, in most experiments, all these mutations were produced by a treatment lasting an hour or less, whereas the "natural" mutations represent the accumulation of a whole Drosophila generation, that is, of two weeks or more. When this time difference is taken into the reckoning, we find that, during the time of treatment here in question, not 100 but at least 35,000 times as many mutations were produced in the treated as in the untreated material; moreover, by the use of the tubes of higher radiation output now available, this rate of production could be stepped up almost indefinitely.
The
ability to manipulate the function of genes with X rays was rapidly broadened to
include ultraviolet (UV) radiation and eventually chemicals. UV radiation ultimately proved
not only to be a more potent mutagen than X rays for many studies, but also a
highly selective probe of the gene itself in living cells. Had investigators paid
greater attention to the selectivity of short wavelength UV radiation for nucleic
acids relative to other chemical components in the cell, our understanding of the chemical
nature of the gene might have been significantly accelerated. Ultraviolet radiation
was discovered in 1801 by J.W. Ritter, who found that the blackening of silver
chloride by light was greater in the dark region beyond the violet end of
the light spectrum than anywhere else in the spectrum. Recognition of the
utility of UV radiation as a physical probe to explore the biology of
living cells dates back more than a century. As early as 1877, Downes and Blunt
reported to the Royal Society of London that sunlight killed bacteria, and
they showed that this effect was chiefly associated with the short wavelength component of
the radiation. But as the noted photobiologist John Jagger has pointed out,
although UV radiation was quickly recognized as safer and easier to use than ionizing
radiation:
Its very availability is deceiving, for it encouraged some researchers to do extensive experimentation with little knowledge of the tool at hand. As a result, there are papers in the literature, dealing with interesting problems, in which the techniques are so inadequate that many of the results are unacceptable. The proper conduct of ultraviolet experiments in biology is at least as difficult as for X-ray experiments, and the proper interpretation at least as subtle.
The impact
on genetic studies hastened by the discovery that X rays and UV radiation can interact
with and in some way alter the "hereditary material" of cells is beyond the
province of this book. Of singular interest to us in this treatise is the fact that
these discoveries spawned the development of a new and distinctive investigative
focus in the field, namely the perturbation of genes by exogenous physical agents,
and the scrutiny of the ways in which living cells respond to such perturbations. This
focus emerged fleetingly as early as the mid nineteen-thirties, but its maturation to
the intellectually comprehensive body of knowledge that we now refer to as DNA repair and
mutagenesis was sporadic and slow. The field of DNA repair did not attain full
experimental clarity until the late nineteen-forties, and only achieved formal recognition
as a distinctive biological phenomenon in the late nineteen-fifties. Why was
this era in the history of genetics, an era that was characterized by the extensive,
almost routine, use of exogenous agents known to alter genes, not immediately
followed by investigations about the nature of these alterations and their physiological
consequences? To pose the question another way, what cultural and intellectual
influences delayed the emergence and maturation of the notion that genes
can sustain repairable damage?
Before attempting to answer these questions, it is instructional to steep ourselves more deeply in the general intellectual climate that characterized the science of genetics in the late nineteen-thirties and early nineteen-forties. A fitting place to begin is the 1941 Cold Spring Harbor symposium on Genes and Chromosomes, Structure and Organization, where Muller so authoritatively anointed the impact of the mutagenic properties of X rays on experimental genetics.
The
Biological Laboratory at Cold Spring Harbor on Long Island, New York has
long been one of the major intellectual breeding grounds of modern genetics and
molecular biology. The famous Cold Spring Harbor Symposia on Quantitative Biology were
formally initiated in 1933 by the director of the Laboratory at that
time, Reginald G. Harris, and with a brief interruption during World War II, these
have continued on an annual basis, covering a wide array of topics of biological interest.
In his introductory remarks which opened the first symposium entitled Surface
Phenomena, Harris implored the 29 assembled conferees to:
Give special consideration to theoretical and controversial aspects, that the discussion may be both significant and creative, and that these conferences may be of the greatest possible value not only to those of us who take part in them, but also to those who will have occasion to refer to them.
Prior to 1941, the symposia lasted for as long as five weeks and the majority of the participants remained in residence in the Laboratory for at least part of that time. Thus began the tradition of convening at Cold Spring Harbor for the summer, a tradition that attracted geneticists and microbiologists from around the country and even abroad. These visiting scientists often brought their own critical pieces of equipment and favored experimental subjects.
The
published proceedings of the symposia are important historical archives of the progress
of various subdisciplines in biology. Aside from the written accounts of papers
delivered at the meetings, the symposium volumes are distinguished by a
comprehensive recounting of the discussions that followed each formal scientific presentation
and, in more recent years, by a series of candid photographs of "key" conferees.
I have never determined who decides which photographs are included in the
published volumes. But having attended several such meetings, I have on occasion
had cause for wry amusement in watching the subtle jockeying of some of
the younger conferees when the symposium photographer is in attendance,
presumably in the hope of being captured for posterity standing next to, or
better yet actually talking to, some scientific luminary. Nonetheless, paging through
some of the older volumes and putting faces to names now legendary
in the annals of biology is an enchanting exercise. It is equally interesting
to peruse the list of symposium participants included in each of the published
volumes, regardless of whether or not they presented formal talks at the meeting.
The
1941 symposium was graced by the presence, among others, of Max Delbruck and
Salvador Luria, whose direct and indirect influence on the field of DNA repair will
be recounted in several places in this book. Hermann Muller, Barbara
McClintock, Milislav Demerec (the director of the Laboratory at that time, about whom
we shall hear more in the next chapter), Alexander Hollaender, Hans Ris, Alfred Mirsky,
Curt Stern, Lewis Stadler, and Danicl Mazia were among other scientific notables in
attendance. The general stage on which the drama of genetics was unfolding in the
nineteen-forties was eloquently summarized by Neville Symonds in some of his
historical reflections written in 1988. Symonds reminds us that by 1940, studies
carried out principally in maize and Drosophila had established the essential
foundations of genetics. It was known that inheritance was determined by genes that
were present on chromosomes. It was known too that chromosomes were duplicated in
all cells by a process of mitosis, except those in the germ line, where they underwent
meiosis. Mutations were known to reflect occasional spontaneous changes in genes,
and their frequency could be impressively increased by exposure of cells to
radiation. Enzymes were known to exist in cells and the nucleus was known to be
rich in nucleic acids, but there was a very strong "feeling" that genes
were made of proteins. As Symonds put it, "the foundations of formal genetics were
well laid, but there were no real ideas about biochemical genetics."
According to Richard Kimball, a longtime contributor to the DNA repair field and one of the conferees at the 1941 meeting, the symposium was characterized by considerable controversy about the mutagenic properties of ionizing radiation. The prevailing model of genes as "beads on a string" lent itself to the notion that mutations and chromosomal aberrations could arise from breaks caused by ionizing radiation. However, the experimental limitations of distinguishing between mutations resulting from deletions and rearrangements of genes, and those possibly arising from chemical alterations of the gene itself, were profound. In his 1987 review entitled "The Development of Ideas about the Effect of DNA Repair on the Induction of Gene Mutations and Chromosomal Aberrations by Radiation and Chemicals," Kimball suggests:
The difficulty arose in distinguishing between true gene mutations, taken to mean an alteration in the structure of the gene itself, and such breakage-and-reunion events as losses of single genes and changes in gene action by movement to new positions (position effects). . . . Thus there was no real basis for any discussion of the role of repair of radiation damage to the gene itself.
Nonetheless, the observation that chromosomal breakage induced by X rays was sometimes accompanied by aberrant chromosomal rejoining, which resulted in translocations, did not go unnoticed. Though not documented in the formal paradigm of gene repair, these events were discussed and considered by some in the general context of restitution phenomena. Evelyn Witkin, whose venerable contributions to the DNA repair and mutagenesis field are discussed in later chapters, was a young investigator at Cold Spring Harbor in the early nineteen-forties. She avowed that, in retrospect, her comprehension of the observations on chromosome rejoining by Berwin Kauffman and Barbara McClintock at Cold Spring Harbor in the early nineteen-forties sensitized her to the notion of the rectification of genetic damage, though she did not translate this notion to that of "gene repair" until many years later.
But in
general, in the early nineteen-forties, insightful dialogue about mechanisms of
mutation induction and the nature of cellular responses to genetic insult was
suffocated primarily by the informational vacuum about the chemical nature of
the gene itself. For this was a time not yet illuminated by either the 1944 paper of
Oswald Avery, Colin MacLeod, and Maclyn McCarty showing that the genetic material
was in fact DNA, nor by Watson and Crick's elucidation of the structure of DNA.
The prevailing dogma was that genes were made of proteins. This dogma was so
entrenched in mainstream biology that according to Judson, when James Watson arrived
in England in 1951 to begin the fellowship that culminated in his revolutionary
observations with Francis Crick a full seven years after Avery, MacLeod, and McCarty's famous
paper, most biologists still firmly believed it. Interred in the complex history of
this dogma one can identify a powerful abstraction that was antithetical to the notion
of the repair of genes. Quite simply, proteins were believed to be highly stable biological
macromolecules. They were not considered as mediators of genetic alteration and were
certainly not thought of as substrates for biochemical reactions that mediated the
repair of genetic material.
Errol C. Friedberg, M.D., is Professor and Andujar Distinguished Chair of the Department of Pathology at the University of Texas Southwestern Medical Center. He is the author of several books on DNA repair, and is the managing editor of the journal Mutation Research - DNA Repair.
Endlinks
For more on genetics and mutation, see our Cutting Edge debate on optimum mutation rates in evolution and disease, and its endlinks.
Also see our previous Essay, "Biochemical Genetics: Some Recollections" by George W. Beadle.
Lawrence Livermore National Laboratory Human Genome Center - information on research programs in genomics in the Lawrence Livermore Biology and Biotechnology Research Program, including work on sequencing of genes involved in DNA repair, replication, recombination, xenobiotic metabolism, cell cycle control, etc.
The Drosophila Virtual Library - a page of links to a number of Drosophila-related resources, including FlyBase; the Drosophila genome project (which includes the Encyclopedia of Drosophila); databases; protocols; and other resources.
Cold Spring Harbor Laboratory - general information on Cold Spring Harbor Laboratory, with links to information on development, education, history, laboratory statistics, meetings and courses, research, the CSHL Press, and more.
Cold Spring Harbor Laboratory Related Links - a page of links to numerous sites related to Cold Spring Harbor Laboratory, including the National Cancer Institute, the NIH, and the Human Genome Project.
Genethon Human Genome Research Centre - includes links to Genethon projects (DNA and cell banks; genotyping), public data (genetic map, Genexpress data), other information, and other Web resources.The Laboratory of Molecular Genetics, National Institute on Aging - the Laboratory of Molecular Genetics studies DNA damage and repair as it relates to aging and age-related diseases. This page includes links to programs such as DNA Damage and Repair in Aging, ongoing research projects such as In Vitro Transcription-Coupled Repair, a DNA Interest Group, and more.
The Biotech Chronicles - the history of biotechnology, including material on genetics and DNA research; profiles of figures such as Gregor Mendel, Rosalind Franklin, James Watson; brief essays on the story of DNA; timelines; and more.
You may purchase this book (cloth, 210 pp.) directly from Amazon.com, or from the publisher.