GENTECH archive


Ionizing Induce Genomic Instability

Confirmation that Ionizing Radiation Can Induce Genomic Instability:
What is Genomic Instability, and Why Is It So Important?

John W. Gofman, M.D., Ph.D., and Egan O'Connor, Executive Director,
CNR. Spring 1998.


         * Glossary
              + Genome
              + Genes and Chromosomes
              + The Code
              + The Mitochondrial DNA (mtDNA)
         * Part 1 -- A Deep Insight from 1914, Slowly Confirmed
         * Part 2 -- Ionizing Radiation as a Cause of Genomic Instability
         * Part 3 -- Implications:   Curing vs. Preventing Cancer
         * Part 4 -- Five Key Facts and Three Restrained Comments
         * References

*   Genomic instability --- also called "genetic instability" and
"chromosomal instability" --- refers to abnormally high rates
(possibly accelerating rates) of genetic change occurring serially
and spontaneously in cell-populations, as they descend from the same
ancestral cell. By contrast, normal cells maintain genomic STABILITY
by operation of elaborate systems which ensure accurate duplication
and distribution of DNA to progeny-cells (Cheng 1993, p.124), and
which prevent duplication of genetically abnormal cells. These
systems ("metabolic pathways") involve an estimated 100 genes (Cheng
1993, p.142).

*   Why is genomic instability so important? Many (not all) cancer
biologists now believe that genomic instability "not only initiates
carcinogenesis, but also allows the tumor cell to become metastatic
and evade drug toxicity" (Tlsty 1993, p.645), and "The loss of
stability of the genome is becoming accepted as one of the most
important aspects of carcinogenesis" (Morgan 1996, p.247), and "One
of the hallmarks of the cancer cell is the inherent instability of
its genome" (Morgan 1996, p.254).

*   Although these observations are far from new, they certainly did
not receive the attention they merit until recently.


>>>>>   GLOSSARY   <<<<<

                       * GENOME
                       * GENES AND CHROMOSOMES
                       * THE CODE
                       * THE MITOCHONDRIAL DNA (mtDNA)


*   GENOME.   A person's genome is one set of his (or her) genes.
The human genes, which control a cell's structure, operation, and
division, are located in the cell's nucleus. The full human genome
(estimated at 50,000 to 100,000 genes) is present in every
cell-nucleus, even though many genes are inactive in cells which
have some specialized functions (the "differentiated" cells).

*   GENES AND CHROMOSOMES.   Genes are composed of segments of DNA.
In normal cell-nuclei, the DNA is distributed among 46 chromosomes
(23 inherited at conception from a person's father, and 23 from the
mother). Each chromosome consists of one very long strand of DNA and
numerous proteins, which are required for successful management of
the long DNA molecule. The longest chromosomes each "carry"
thousands of genes. Every time a cell divides, the cell must
duplicate the 46 chromosomes and must distribute one copy of each to
the two resulting cells.

*   THE CODE.   The DNA of each chromosome is composed of units ---
"nucleotides" of four different types (A, T, G, C). These
nucleotides are linked to each other in linear fashion. The sequence
of the four types of nucleotides is critical, because the sequence
produces the "code" which (a) determines the function of each
particular gene, (b) identifies the gene's start-point and
stop-point along the DNA strand, and (c) permits certain regulatory
functions. The code of the human genome consists of more than a
billion nucleotides.

*   THE MITOCHONDRIAL DNA (mtDNA).   Outside the nucleus, human
cells also have some "foreign" DNA located in structures called the
mitochondria. This small and separate set of DNA does not
participate in the 46 human chromosomes, and is not part of "the
genomic DNA." The mitochondria are inherited from the mother.



Part 1   *   A Deep Insight from 1914, Slowly Confirmed

*   It was the year 1956 when the normal number of human chromosomes
per cell was firmly established as 46. Soon thereafter, it became
clear that cells of advanced cancers have often evolved an abnormal
number of chromosomes ("aneuploidy").

*   Such observations were consistent with the prediction of Theodor
Boveri (Boveri 1914), a great German embryologist who postulated
that malignancy is the result of inappropriate balance of
instructions (genetic information) in the tumor cells. Such
"imbalance" can result not only from numerical chromosome
aberrations, but also from structural alterations within the 46
chromosomes. As a leading cause of structural chromosome aberrations
(deletions, acentric fragments, translocations, inversions,
dicentrics, etc.), ionizing radiation is well-established.

*   When my colleagues and I (JWG) initiated a research program in
1963 (at the Atomic Energy Commission's Livermore National
Laboratory), to test Boveri's hypothesis, there was very little
interest in the concept. Although the techniques for detecting
structural chromosome aberrations were extremely crude then,
compared with current techniques, we were making gradual progress
(Minkler 1970, + Minkler 1971). However, the Atomic Energy
Commission became angry with me after a paper I presented at an IEEE
Symposium (Gofman 1969), and canceled our funding in the early 1970s
(Seaborg 1993, Chapter 8, "Challenge from Within," + Terkel 1995,

*   In October 1976, the journal Science published Peter C. Nowell's
classic paper entitled, "The Clonal Evolution of Tumor Cell
Populations" --- a paper almost always cited by today's analysts of
genomic instability. Among other things, Nowell's 1976 paper
discussed evidence, from various analysts, indicating that as tumor
cells become increasingly aneuploid, the malignancy becomes
increasingly aggressive (Nowell, p.25). Reasoning from the available
evidence at that time, Nowell proposed the following model of
multi-step carcinogenesis:

*   Tumor initiation occurs by an induced change in a single,
previously normal cell, which makes the cell "neoplastic" (partially
liberated from normal growth controls) and provides the cell with a
selective growth advantage over adjacent normal cells (Nowell,

*   "From time to time, as a result of genetic instability in the
expanding tumor population, mutant cells are produced ... Nearly all
of these variants are eliminated, because of metabolic disadvantage
or immunologic destruction ... but occasionally one has an
additional selective advantage with respect to the original tumor
cells as well as normal cells, and this mutant becomes the precursor
of a new predominant subpopulation" (Nowell, p.23). And:

*   "Over time, there is sequential selection by an evolutionary
process of sub-lines which are increasingly abnormal, both
genetically and biologically ... Ultimately, the fully developed
malignancy as it appears clinically has a unique, aneuploid
karyotype associated with aberrant metabolic behavior and specific
antigenic properties, and it also has the capability of continued
variation as long as the tumor persists" (Nowell, p.23). And:

*   "The major contention of this article is that the biological
events recognized in tumor progression represent (i) the effects of
acquired genetic instability in the neoplastic cells, and (ii) the
sequential selection of variant subpopulations produced as a result
of that genetic instability" (Nowell, p.25).

*   The recent surge of interest in genomic instability reflects the
recognition that the cancer process represents a trip (or set of
trips) from the stable genome to the genome with diverse deviations.
It has been a long wait for Boveri.


Part 2   *   Ionizing Radiation as a Cause of Genomic Instability

*   Today, laboratory researchers are performing reality-checks on
this logic: Genomic instability can be initiated and intensified by
any type of genetic mutation (including chromosome aberrations),
when such mutation alters some of the DNA which maintains genomic
STABILITY. Of course, such DNA includes the numerous DNA segments
which govern DNA synthesis, cell-division, and also the routine
REPAIR of the genome --- the "repair genes" (Cheng 1993, p.131;
Morgan 1996, p.248).

*   When a mutagen has induced genomic instability in a cell, some
of the cell's descendants will experience new and unrepaired genetic
abnormalities at an excessive rate, even though the descendants
themselves received no exposure to the mutagen used in the
experiment. This occurs because such cells have inherited a genome
which was injured with respect to maintaining genomic STABILITY.

*   Very recently, a technique has been developed for efficiently
detecting three of the types of chromosome aberrations which are
very prominent in genomic instability: Aneuploidy (wrong number of
chromosomes), deletions (permanent removal of DNA segments, long or
short), and gene-amplifications (extra copies of specific DNA
segments). This technique, called Comparative Genomic Hybridization,
was first described by Kallioniemi (1992, in Science). However, such
a technique does not detect many other kinds of mutations.

*   The nature of the genetic code is such that mutations need not
be gross in order to have gross biological consequences. For
instance, permanent removal of a single nucleotide (a
micro-deletion) can totally garble much of a gene's code, by causing
what is called a "frame-shift." Then this non-functional gene can be
the phenomenon which wrecks part of the system which would otherwise
maintain genetic STABILITY.

*   Amplification (instead of injury), of the crucial genes in the
stability-system, also can permit a cell to escape the controls
which otherwise prevent duplication of cells with injured genomes.
Evidence is developing that gene amplification is associated with
dicentric chromosomes and circular acentric fragments called "double
minutes" (DiLeonardo 1993, p.656) --- very well-known products among
the consequences of ionizing radiation.

*   The sequence, in which various mutations accumulate in tumor
cells, may or may not matter. "For example, one or more
pre-cancerous mutations might lie dormant until additional mutations
create an environment in which the prior changes confer a selective
advantage" (DiLeonardo 1993, p.655, citing Kemp 1993, + Fearon 1990,
+ Temin 1988).

*   The fact, that ionizing radiation is a mutagen capable of
causing all known types of genetic mutation --- from micro to gross,
at any DNA location along any chromosome --- made it utterly
predictable that ionizing radiation would be a cause of genomic
instability. Indeed, one of the last projects completed by our
research group at the Livermore Lab, before the Atomic Energy
Commission shut down our work, was a demonstration which showed that
ionizing radiation can induce genomic instability. Our experiments
used gamma rays and cultured human fibroblasts (Minkler 1971).

*   During recent years, multiple experiments have confirmed the
fact that ionizing radiation can cause genomic instability. Such
results have been observed after both low-LET radiation (such as
xrays and gamma rays) and high-LET radiation (such as alpha
particles). Among numerous papers, see, for instance:

             * Kadhim 1992;
             * Holmberg 1993 (who cites Minkler 1971);
             * Marder 1993 (especially p.6674);
             * Mendonca 1993;
             * Kadhim 1994;
             * Kronenberg 1994 (radiation dose-response, p.605);
             * Kadhim 1995;
             * Morgan 1996 (review).

*   In the mass media, some writers have expressed astonishment that
radiation-induced genomic instability is not detected until several
cell-divisions have occurred after the radiation exposure. They seem
to imagine that the delay reflects a mysterious discontinuity
between cause and effect. There is NO discontinuity, of course --- a
point made explicitly in Kadhim 1992 (p.739). With current
techniques, and with uncertainties about where to search closely
among a billion nucleotides, it is just not possible to detect every
intermediate step.


Part 3   *   Implications:   Curing vs. Preventing Cancer

*   The induction of genomic instability in a cell does not
guarantee that it will become malignant. Genomic instability
increases the RATE of mutation in that cell and its descendants, and
with this higher rate, the cells each have a higher PROBABILITY that
at least one of them will accumulate all the genetic powers of a
killer-cancer. These powers include the ability to thrive BETTER
than normal cells, to invade inappropriate tissue, to adapt to the
new conditions there, to recruit a blood supply, to fool the immune
system, and many other properties.

*   No one claims, yet, that genomic instability must precede every
case of cancer. However, genomic instability helps to explain why
cancer is sometimes called "at least a hundred different diseases."
Indeed, genomic instability means that each case of cancer may
develop a genome like no other case. Is it any wonder that
individual tumors often differ in behavior from each other?

*   Nowell's 1976 paper was certainly not the last one to observe
that cancers become increasingly deviant in their genomes, as they
"advance." Tlsty 1993 (p.645) cites several more recent papers. Near
the end of his paper, Nowell wrote (p.27):

*   "The fact that most human malignancies are aneuploid and
individual in their cytogenetic alterations is somewhat discouraging
with respect to therapeutic considerations ... With variants being
continually produced, and even increasing in frequency with tumor
progression, the neoplasm possesses a marked capacity for generating
mutant sub-lines, resistant to whatever therapeutic modality the
physician introduces ... The same capacity for variation and
selection which permitted the evolution of a malignant population
[of cells] from the original aberrant cell, also provides the
opportunity for the tumor to adapt successfully to the inimical
environment of therapy, to the detriment of the patient."

And Some Lessons: -----------------

(A)   *   Genomic instability will probably keep cancer hard to

(B)   *   The quickest path to less cancer-misery in the future
would be a policy of reducing exposure to carcinogens.

(C)   *   Ionizing radiation is almost certainly the most potent
carcinogen to which vast numbers of people are actually exposed (see
Part 4).


Part 4   *   Five Key Facts and Three Restrained Comments

(1)   *   Ionizing radiation is a mutagen having special properties
which make some radiation-induced genetic injuries complex and
impossible for a cell to repair correctly --- quite unlike the
routine damage from endogenous free radicals (Ward 1988, + Gofman
1990, Chapter 18, Part 2, + Ward 1991, + Baverstock 1991, + Ward
1995, + Gofman 1997).

(2)   *   Ionizing radiation is a mutagen which undeniably can cause
every known kind of mutation, at any DNA location along any
chromosome. The body does not always eliminate cells having harmful
mutations. If it did, there would be no cancer or inherited

(3)   *   Ionizing radiation is a mutagen known to induce genomic
instability (references provided in earlier sections).

(4)   *   Ionizing radiation is a human carcinogen at every
dose-level, not just at high doses; there is no threshold dose. A
single photon or a single high-speed particle can cause unrepairable
genetic damage. (See Gofman 1990, Chapters 18-21, + UNSCEAR 1993,
Annex F, especially p.636 para.84, p.680 para.323, + NRPB 1995,
especially pp.59-61, p.68, p.75, + Pierce 1996, p.9, + Gofman 1996,
Chapter 45, + Riches 1997, p.519, + Hei 1997).

(5)   *   Ionizing radiation is a mutagen observed to induce
virtually every kind of human cancer (Gofman 1969, p.4, + BEIR 1980,
Section 5, + UNSCEAR 1988, p.460 para.394).

And the Comments: -----------------

(1)   *   In view of all the five facts above, it would be
inappropriate to doubt the menace of low-dose ionizing radiation.

(2)   *   And in view of all the five facts, it is strange --- in
studies which attempt to explain a difference in cancer-rates
between two groups --- that the question is so seldom asked: How do
the radiation histories differ between the groups? In view of the
five facts above, it should be the FIRST question.

(3)   *   And in view of the five facts, it is sad that so many
members of the medical profession give only lip-service to the need
to reduce the unnecessarily high exposures to radiation administered
by their own profession (UNSCEAR 1993, Annex C, + Gofman 1996,
Chapter 48). Today, the two largest sources of voluntary radiation
exposure are (i) pre-cancer medical procedures, including CT scans
and fluoroscopy (NCRP 1987, p.59, + NCRP 1989, p.69) and (ii)
cigarette-smoking --- which delivers appreciable alpha-particle
radiation to the lungs (Martell 1974, 1975, 1983, + NCRP 1984, +
BEIR 1990, p.19). As for involuntary exposures accumulated from
nuclear pollution, they have been poorly ascertained --- to put it
in a kindly fashion.

# # # # #

>>>>>   Reference List   <<<<< ---------------------------------

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Errata: Vol.143: 355.

# # # # #


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