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Life, 56(2): 65–81, February 2004
Critical Review
Aneuploidy, the Primary Cause of the Multilateral Genomic Instability
of Neoplastic and Preneoplastic Cells
Peter Duesberg1,2, Alice Fabarius2 and Ruediger Hehlmann2
1
Dept. Mol. and Cell Biol., Donner Laboratory, UC Berkeley, Berkeley, CA 94720, USA
III. Medizinische Klinik Mannheim of the University of Heidelberg at Mannheim, Wiesbadener Str.7-11, 68305 Mannheim,
Germany
2
Summary
Cancers have a clonal origin, yet their chromosomes and genes
are non-clonal or heterogeneous due to an inherent genomic
instability. However, the cause of this genomic instability is still
debated. One theory postulates that mutations in genes that are
involved in DNA repair and in chromosome segregation are the
primary causes of this instability. But there are neither consistent
correlations nor is there functional proof for the mutation theory.
Here we propose aneuploidy, an abnormal number of chromosomes,
as the primary cause of the genomic instability of neoplastic and
preneoplastic cells. Aneuploidy destabilizes the karyotype and thus
the species, independent of mutation, because it corrupts highly
conserved teams of proteins that segregate, synthesize and repair
chromosomes. Likewise it destabilizes genes. The theory explains 12
of 12 specific features of genomic instability: (1) Mutagenic and nonmutagenic carcinogens induce genomic instability via aneuploidy. (2)
Aneuploidy coincides and segregates with preneoplastic and
neoplastic genomic instability. (3) Phenotypes of genomically
unstable cells change and even revert at high rates, compared to
those of diploid cells, via aneuploidy-catalyzed chromosome
rearrangements. (4) Idiosyncratic features of cancers, like
immortality and drug-resistance, derive from subspecies within the
‘polyphyletic’ diversity of individual cancers. (5) Instability is
proportional to the degree of aneuploidy. (6) Multilateral
chromosomal and genetic instabilities typically coincide, because
aneuploidy corrupts multiple targets simultaneously. (7) Gene
mutation is common, but neither consistent nor clonal in cancer
cells as predicted by the aneuploidy theory. (8) Cancers fall into a
near-diploid (2 N) class of low instability, a near 1.5 N class of high
instability, or a near 3 N class of very high instability, because
aneuploid fitness is maximized either by minimally unstable
karyotypes or by maximally unstable, but adaptable karyotypes.
(9) Dominant phenotypes, because of aneuploid genotypes. (10)
Uncertain developmental phenotypes of Down and other aneuploidy
syndromes, because supply-sensitive, diploid programs are
Received 10 October 2003; accepted 8 December 2004
Address correspondence to: Peter Duesberg, Department of
Molecular and Cell Biology, Donner Laboratory, UC Berkeley,
Berkeley, CA 94720, USA. Tel: 510-642-6549. Fax: 643-6455. E-mail:
[email protected]
ISSN 1521-6543 print/ISSN 1521-6551 online # 2004 IUBMB
DOI: 10.1080/15216540410001667902
destabilized by products from aneuploid genes supplied at
abnormal concentrations; the maternal age-bias for Down’s would
reflect age-dependent defects of the spindle apparatus of oocytes.
(11) Non-selective phenotypes, e.g., metastasis, because of linkage
with selective phenotypes on the same chromosomes. (12) The target,
induction of genomic instability, is several 1000-fold bigger than gene
mutation, because it is entire chromosomes. The mutation theory
explains only a few of these features. We conclude that the transition
of stable diploid to unstable aneuploid cell species is the primary
cause of preneoplastic and neoplastic genomic instability and of
cancer, and that mutations are secondary.
IUBMB Life, 56: 65–81, 2004
Keywords
Autocatalytic
karyotype
evolution;
speciation;
aneuploidy-dependent gene mutation; drug-resistance;
immortality; metastasis; developmental uncertainty.
INTRODUCTION
Cancers are at once clonal (1 – 3) and non-clonal (3 – 11) –
and thus a geneticist’s nightmare. Even though cancers are
derived from a single cell, their cells are, more or less,
heterogeneous with regard to DNA content (12), chromosome
numbers and abnormal structures (3, 6, 13, 14), gene
mutations (15 – 17), ‘immortality’ in vitro and on transplantation (18 – 21), abnormal expression of genes (11, 16, 22),
abnormal numbers of spindle poles (23, 24), abnormal
metabolism (25), resistance to cytotoxic drugs (26, 27), ability
to metastasize (28), a risk for ‘delayed reproductive death’
(29 – 32), and even carcinogenicity itself (see below), (4, 5, 8).
There is but one solution to this apparent paradox: Cancers
are clones, which are derived from single, genomically
unstable cells. Owing to this cellular genomic instability
cancers are heterogeneous, consisting of subclones with
subclone-specific karyotypes, genes and phenotypes.
The rates at which cancer cells and tumorigenic cell lines
spontaneously change their karyotypes range from lows of a
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DUESBERG ET AL.
few percent to highs approaching 100% per mitosis (33 – 35).
At the same time such cells spontaneously change phenotypes,
such as drug-resistance and cell morphology, at the abnormally high rates of 1073 to 1076 per mitosis (26, 27). As a
result of this genomic instability ‘No single tumor is composed
of genetically identical cells’ (36), and cancers are said to be
‘chock-full of mutations and chromosomal abnormalities’
(15). By contrast, the rates at which normal, diploid cells
spontaneously lose, gain or rearrange chromosomes are only
1073 to 1075 per mitosis (37, 38). Spontaneous phenotype
alterations occur in normal cells only at rates of 1076 to 1077
based on mutations of haploid genes and at rates of 10712 to
10714 based on mutation of diploid genes (21, 26, 27, 39).
However, genomic instability is not limited to cancer cells.
It is also observed in previously normal, but as yet
untransformed cells soon after treatment with carcinogens as
well as in congenital aneuploidy syndromes (39, 179). For
example, in the 1940s Charlotte Auerbach et al. first observed
‘unstable genes’ with ‘a tendency to mutate, which remains
latent until a later cell division’ in Drosophila cells treated with
alkylating carcinogens (40). Mammalian cells are also
rendered genomically unstable by experimental carcinogens
applied in vivo or in vitro (2, 25, 31, 41), or just by propagating
normal cells in vitro (42 – 44). In addition to a persistent
‘tendency to mutate’, genomically unstable mammalian cells
also have a tendency to undergo ‘a delayed reproductive
death’ (2, 29, 30, 45, 46) and an even more delayed tendency to
transform themselves into cancer cells. In vivo, the delay from
carcinogen to cancer takes a minimum of many months in
carcinogen-treated rodents, and takes years to decades in
humans accidentally or occupationally exposed to carcinogens
(2, 47 – 50). Because of their inherent risk to become
neoplastic, genomically unstable cells are considered ‘initiated’
or ‘preneoplastic’ (2, 45, 47, 51).
To account for the heterogeneous karyotypes and phenotypes of cancers, researchers have long postulated that
carcinogenesis must be initiated by genomic destabilization
(3, 5, 7, 45, 50, 52). For example, von Hansemann wrote in
1890, ‘The anaplastic cell is one in which, through some
unknown agency, a progressive disorganization of the mitotic
process occurs’ (53, 54). Nowell proposed in 1976, ‘Acquired
genetic instability permits stepwise selection of variant
sublines and underlies tumor progression’ (3). However,
Nowell, as well as his predecessors, left open the question,
whether gene mutations or chromosomal alteration are the
primary cause of the underlying genomic instability, which he
called ‘the genetic-versus-epigenetic debate’ (3).
This defines the challenge: which and how many of the
multiplicity of chromosomal alterations and genetic mutations
of cancer cells and their preneoplastic precursor cells are
responsible for genomic instability? Despite over 100 years of
cancer research, it is still debated, whether aneuploidy or gene
mutation is the primary cause of genomic instability (15, 17,
49, 50, 55 – 58). Aneuploidy is defined as an abnormal balance
of chromosomes or ‘by loss or duplication of chromosomes or
chromosomal segments’ by the textbook, Genes (21), which
defines mutation as ‘any change in the sequence of genomic
DNA’ (21).
Here we briefly review the mutation and aneuploidy
theories of genomic instability. Then we determine which
theory is better suited to explain 12 specific features of the
genomic instability of neoplastic and preneoplastic cells.
The Theory that Gene Mutation is the Primary Cause of
the Genomic Instability of Neoplastic and Preneoplastic
Cells
Several gene mutation theories are currently advanced to
explain the genomic instability of cancer cells (15, 56 – 62).
According to a recent review, ‘In general, these theories
assume that genomic instability is derived from mutations in
genes that are involved in processes such as DNA repair and
chromosomal segregation. The mutations of the ‘‘mutator
genes’’ have no direct selective advantage or disadvantage,
only an effect on the mutation rates of other genes.’ (58).
However, the necessity of such ‘‘mutator genes’’ (63, 64) for
carcinogenesis has recently been questioned for four different
reasons:
(1)
(2)
(3)
Mutator genes are only detected in a small minority of
cancers (15, 29, 33, 36, 49, 56, 58, 60, 65 – 69).
Likewise, the mutations that are thought to create them
are neither consistent nor clonal in cancers (see Feature
#7 below). In view of this Rajagopalan et al. proposed,
‘epigenetic events that do not involve mutational changes
in nucleotides could certainly have a significant role.’
(57).
Among the mutator genes that have been found, some, as
for example the adenomatous polyposis coli (Apc) and
the p53 tumor suppressor genes, are now considered
consequences rather than causes of genomic instability
(70 – 73). According to a review by Lengauer et al., ‘p53
mutations do not usually occur until much later [after
initiation of carcinogenesis]’ and thus ‘are unlikely to be
its primary cause’ (36).
There is as yet no functional proof for a direct role of
mutator genes in carcinogenesis (28, 32, 49, 74). For
example, in experimental mice with a transgenic lambda
phage as indicator of mutagenesis, ‘the frequencies of
lambda cII-mutants were not significantly different in
normal mammary epithelium, primary mammary adenocarcinomas, and pulmonary metastases’ (75). Moreover,
mice with null mutations of p53 and with deletion
mutants of Apc proved to be ‘surprisingly’ procreative,
and thus not genomically unstable (76, 77). In view of
this, the study on p53 concluded, ‘an oncogenic mutant
form of p53 is not obligatory for the genesis of many
types of tumours’ (76), and the study on Apc concluded,
‘Most importantly, Apc1638T/1638T animals that sur-
ANEUPLOIDY, THE PRIMARY CAUSE
vive to adulthood are tumor free’ (77). ‘Surprisingly’
encore, mice with null mutations of the cyclin-dependent
kinase 2, another hypothetical tumor suppressor and cell
cycle control gene (78), were recently also found to be
sufficiently stable to survive to adulthood – again cancer
free (79).
Some studies of mice with mutated tumor suppressor
genes point out that such mice have higher risks of cancer
than untreated controls owing to their artificial mutator
genes (16), as for example the study on mice without p53
(76). However, carcinogenesis in such animals is both
age- and ‘strain-dependent’ (76), indicating that their
artificial genes are not sufficient for carcinogenesis. A
calculation of the cellular cancer risk of these mice makes
this point even more obvious. Since cancers originate
from single cells (2, 6, 80), and since mice consist of
about 5 6 1010 cells and have renewed many of their cells
by the time they develop cancers, their cellular cancer risk
is less than 5 6 10710. This extremely low cellular cancer
risk undermines the argument for a direct role of such
genes in carcinogenesis.
(4) The hypothesis, that an elevated rate of mutation is
necessary for carcinogenesis, is also burdened by an
inherent paradox, which was described in a recent review
as follows, ‘an elevation of mutation rate must generally
be regarded as a growth disadvantage to the cell. Models
which disregard such mechanisms are particularly incompatible with the growing amount of data indicating
that practically all neoplastic cells express some form of
genomic instability’ (50). Tomlinson et al. expressed the
same reservations about the mutator gene hypothesis,
‘The scenarios for a role of a raised mutation rate assume
that there is no selective disadvantage to a cell in having
an increased number of mutations. This may not be the
case: for example, a deleterious or lethal mutation may be
much more likely than an advantageous mutation. More
subtly, an accumulated mutational load might induce
apoptosis’ (67).
Thus, there is as yet no consistent correlative or functional
proof for any of the mutation theories. Moreover the suicidal
consequences of persistent and autocatalytically escalating
numbers of mutator genes are hard to reconcile with the long
latency and many cell generations between initiation of
preneoplastic genomic instability and cancer, and even harder
with the immortality of cancer cells (49). In view of this we
advance here the theory that aneuploidy is the primary cause
of genomic instability.
The Theory that Aneuploidy is the Primary Cause of the
Genomic Instability of Neoplastic and Preneoplastic Cells
The aneuploidy theory proposes that corruption of the
highly conserved chromosome balance of a diploid species,
alias aneuploidy, is the cause of genomic instability (Fig. 1).
67
Aneuploidy destabilizes the numbers and structures of
chromosomes because it unbalances the highly conserved
teams of proteins, which segregate, synthesize and repair
chromosomes. Since unbalanced teams of mitosis proteins are
error-prone, they are likely to change the numbers and
structures of chromosomes, and thus the cell’s species, every
time the cell divides (35). In other words, aneuploidization is
analogous to phylogenesis, which also generates new species
by rearranging old genes into new sets of chromosomes (81).
Aneuploidy also increases the spontaneous rates of gene
mutations by corrupting protein teams that repair DNA and
synthesize nucleotide pools (82, 83). Thus the aneuploidy
theory postulates that the genomic and phenotypic heterogeneity of preneoplastic and neoplastic cells is a direct
consequence of the inherent instability of aneuploidy. In
contrast to the mutation theory, the aneuploidy theory
proposes that genomically unstable preneoplastic and neoplastic cells are new, albeit unstable species of their own,
rather than mutations of their precursors.
Further, the aneuploidy theory proposes, that the inherent
instability of aneuploidy is sufficient for the somatic evolution
of neoplastic cells from randomly aneuploid precursors.
According to the theory, carcinogenesis is initiated by a
random aneuploidy, which is either induced by a carcinogen or
arises spontaneously (49) (Fig. 1). The inherent instability of
aneuploidy then initiates and catalyzes a chain reaction of
aneuploidizations, which would provide ever-more aneuploid
species for selection for autonomous growth. However the
theory also predicts that carcinogenesis is not a necessary
consequence of aneuploidy, because the probability that a
random karyotype variation generates a cell species that is
more viable than its diploid precursor, with a 3 billion-year
advantage of evolution, is much lower than the probability of
generating a less viable or non-viable species. Such less viable
and non-viable species would derive either from nullisomies,
or other non-viable or less viable chromosome combinations
or from gene mutations (Fig. 1).
Specific Features of Neoplastic and Preneoplastic
Genomic Instability
In the following we first describe 12 specific features, which
define the genomic instability of neoplastic and preneoplastic
cells, and then test the aneuploidy and mutation theories for
their ability to explain these features.
(1) Mutagenic and non-mutagenic carcinogens induce
genomic instability. Both mutagenic and non-mutagenic
carcinogens induce genomic instability. The mutagenic carcinogens include ionizing radiation (31, 84 – 86), alkylating
agents (20, 87, 88), DNA chain-terminators (41) and others
(89). The non-mutagenic carcinogens include polycyclic
aromatic hydrocarbons (41, 89 – 91), mineral oil (92, 93),
butter yellow (94), bisphenol (95), colcemid (96), hormones
(97), inert plastic and metallic ‘solid bodies’ (30, 98), nonmutagenic tumor promoters (99), nickel and cadmium ions
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DUESBERG ET AL.
Aneuploidy-catalyzed
cell death
nullisomies
non-viable
chromosome
assortments
lethal mutations
Chromosomes:
*
normal,
collateral mutations,
large-scale rearrangement,
substantial deletion
Figure 1. Genomic instability and carcinogenesis via aneuploidization. The aneuploidy theory predicts that almost any nonlethal aneuploidy generates numerical and structural instability of chromosomes by unbalancing teams of proteins that
segregate, synthesize and repair chromosomes and maintain the integrity of genes – independent of gene mutation. Thus
aneuploid cells undergo chromosome non-disjunctions and gene mutations due to error-prone chromosome segregation and
error-prone DNA repair and synthesis. This inherent instability of aneuploidy initiates and then catalyzes a chain reaction of
aneuploidizations and mutations. The initiating aneuploidy is generated either by a carcinogen or spontaneously. Most products
of this aneuploidy-catalyzed karyotype evolution will be cells, which are non-viable due to nullisomies, non-viable chromosome
combinations or lethal mutations, or cells, which are less viable than their diploid progenitors with a 3 billion-year advantage of
evolution. However, there is a low probability of chromosome rearrangements with selective advantages for semi-autonomous
growth in the permissive habitat of the host organism. These will be the basis for the somatic evolution of cancer cells. Thus
carcinogenesis is analogous to phylogenesis, which generates new species with new phenotypes by regrouping old,
phylogenetically conserved genes into new sets of chromosome (see text). Accordingly genomically unstable, aneuploid cells
and cancer cells are new cell species, rather than mutants of diploid precursor cells. However, in contrast to autonomous
phylogenic species, cancer cells contain unbalanced, aneuploid and thus genomically unstable karyotypes.
ANEUPLOIDY, THE PRIMARY CAUSE
(18, 30, 100), and asbestos (89, 101). Even ‘extranuclear’ (86),
or ‘cytoplasmic’ (31), or ‘non-DNA damage’ (102) radiation
induces chromosomal instability in animal and human cells.
(2) Aneuploidy coincides and segregates with preneoplastic
and neoplastic genomic instability. Since aneuploidy is
ubiquitous in human cancer cells (6, 13, 14, 103 – 105, 205),
numerous investigators have asked whether aneuploidy precedes carcinogenesis. All of these have found aneuploidy at ‘an
early stage’ (106) or as an ‘early event’ (114, 205) in human
precancerous neoplasias of the cervix, throat, colon (with and
without microsatellite mutations (205, 206)), lung, skin,
pancreas, gonads, esophagus and acute leukemias (6, 56, 68,
103, 106 – 121, 207).
Animal experiments with carcinogens, undertaken to study
the origin of aneuploidy in carcinogenesis, have also found
aneuploidy prior to carcinogenesis in the liver, skin and
subcutaneous tissues of carcinogen-treated rodents (90, 94, 98,
122 – 125), (unpublished observations). It is relevant in this
context to point out that carcinogens can induce two different
types of lesions, namely aneuploidies with or without
neoplastic phenotypes, and diploid hyperplasias, possibly via
gene mutation (80, 90, 126).
Treatments of diploid human and animal cells in vitro with
carcinogens, at carcinogenic doses, also generate aneuploidy
and persistent or ‘delayed’ genomic instability (18, 41, 87, 88,
127 – 135). In all of these experiments aneuploidy always
segregates with preneoplastic genomic instability and subsequently with morphological transformation and tumorigenicity (41, 88). For example, Holmberg et al. report in 1993 ‘an
increased frequency of sporadic chromosome aberrations was
only observed in irradiated cells with aberrant karyotypes and
not in irradiated cells with normal karyotypes, which suggests
that the ‘‘genomic instability’’ in these clones is associated with
the abnormal karyotype rather than with the radiation
exposure as such’ (133). Aneuploidy also precedes and
subsequently segregates with transformation of human and
animal cells by Simian Virus 40 and other DNA tumor viruses
(136 – 138). Even spontaneous transformation of cells in vitro
is preceded by, and inevitably associated with aneuploidy
(42 – 44).
By contrast, events that induce balanced chromosome
translocations are not expected to induce genomic instability
according to the aneuploidy theory. The Philadelphia chromosome and its reciprocal counterpart, which are associated
with most chronic myelocytic leukemias (CML), are a classical
example. The two reciprocally rearranged chromosomes are
thought to induce a hyperplasia of genomically stable,
terminally differentiating myeloblasts, probably via gene
mutation involving the genes at the breakpoints of the two
chromosomes (14, 28, 139). Indeed, the cellular risk of CML
cells to become aneuploid is extremely low. Since the chronic
phase of CML lasts on average 3 years and generates about
5 6 109 cells per patient, which have an approximate turn over
time of 2 months (139, 140), the cellular risk of aneuploidiza-
69
tion is only about 10711 (1:3[yrs] 6 6[months] 6 5 6 109)
(141). However, it is the occurrence of this rare cellular
aneuploidy, which initiates the terminal, cancerous blast crisis
of CML (14, 139).
(3) Phenotypes of genomically unstable cells change and
even revert at high rates, compared to those of diploid
cells. Genomically unstable cells generate and even lose
new, abnormal phenotypes at very high rates compared to
normal diploid cells. For example, cancer cells may mutate to
drug and multidrug-resistance at rates of up to 1073 per
mitosis and many can mutate back at similarly high rates (26,
27). By contrast, the phenotypes of normal diploid cells, which
are controlled by haploid genes, mutate spontaneously only at
rates of 1076 to 1077 and those controlled by diploid genes
only at rates of 10712 to 10714 (26, 27, 29). Even malignancy is
reversible at high rates in the non-selective condition of cell
culture. For example, extraction of ‘less virulent clones’ from
highly malignant ascites tumors was achieved by Hauschka et
al. (45). Hsu derived even benign cultures from a virulent rat
Novikoff hepatoma (142). Eagle et al. obtained ‘loss of
neoplastic properties’ in clonal derivatives of a human
carcinoma cell line in parallel with ‘characteristic’ karyotype
alteration (4). Simi et al. and Sachs et al. also observed loss of
the transformed phenotypes together with specific chromosomal alterations at high rates in transformed Chinese hamster
cells, as well as subsequent reversion to malignancy (143 –
145). And Fidler et al. observed reversibility and backreversibility of metastatic phenotypes of mouse melanoma
cells at relatively high rates (8).
(4) Idiosyncratic phenotypes of cancer and other genomically
unstable cells, such as immortality and drug-resistance, are
never observed in diploid biology. A given cancer as well as
cultures of immortal cell lines survive many adverse conditions, such as toxic drugs, otherwise lethal mutations,
metastasis to heterologous sites, transplantation to heterologous hosts and even to heterologous species, indefinite
generations in cell culture or in transplants from animal to
animal, as if they were immortal (see Introduction). By
contrast, none of these phenotypes have been observed in
diploid cell populations of the same size, or in diploid biology,
despite 3 billion years of evolution and mutation.
(5) Chromosomal and phenotypic instabilities are proportional to the degrees of aneuploidy. Based on analyses of
clonal cultures of cells with distinct degrees of aneuploidy, it
was found that the rates of chromosomal alterations are
proportional, even ‘exponentially’ (35) to the degrees of
aneuploidy (34, 35, 146). In addition, karyotype and
phenotype heterogeneity was reported to correlate with the
degree of aneuploidy in primary human cervical, lung, colon,
pancreatic, breast and gastric cancers (147 – 152). This
karyotypic and phenotypic heterogeneity of most clonal
cancers is an inevitable consequence of the inherent chromosomal instability of aneuploid cells. Structural heterogeneity of
the chromosomes of human cancers has also been reported to
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DUESBERG ET AL.
correlate with degrees of malignancy, without data on
aneuploidy (153). However, since malignancy is directly
proportional to the degree of aneuploidy (3, 49, 68, 103,
105, 154 – 157), degrees of malignancy serve as surrogate
markers for degrees of aneuploidy. Transplantability of rodent
cancers to heterologous hosts is also proportional to the
degree of aneuploidy, because the range of variant sub-species
with distinct host ranges depends on the degree of aneuploidy
(158). As expected heterogeneity of centrosome numbers and
structures is also proportional to the degrees of aneuploidy
(24, 159).
(6) Multilateral chromosomal and genetic instabilities
typically coincide in the same cells. Most genomically
unstable cells, above all cancer cells, differ from normal cells
not only in abnormal chromosome numbers, but also in
abnormal chromosome structures and gene mutations (13, 15,
29, 32, 36, 50, 55, 56, 160). Thus there is a coincidence of
multiple mechanisms of genomic instability in most genomically unstable cells, particularly in cancer cells: one that alters
chromosome numbers, one that rearranges chromosome
structures and simultaneously some genes at the respective
breakpoints, and one that mutates individual genes.
(7) Gene mutation is common, but neither consistent nor
clonal in cancer cells. About 50% of all cancers of a given
kind contain various gene mutations of hypothetical oncogenes and tumor suppressor genes (24, 66, 68, 161 – 163).
However, consistent correlations between such mutations and
specific kinds of cancers have not been found (28, 66, 74, 141).
For example, Little reports ‘While radiation-induced cancers
show multiple unbalanced chromosomal rearrangements, few
show specific translocations or deletions as would be
associated with the activation of known oncogenes or tumor
suppressor genes’ (31). Grosovsky et al. also find, ‘no
consistent elevation of specific locus mutation rate has been
reported’ (29). Further Gisselson et al. note, ‘the correlation
coefficient between breakpoint frequency and telomere length
[a potential mutator] was low in both osteosarcomas and
pancreatic sarcomas’ (208). A recent statistical analysis of the
relation between gene mutations and cancer arrived at the
conclusion that ‘no tumor has the same spectrum of
mutations’ (208). A survey of genomic instability by Lengauer
et al., in addition to discounting a role for p53 (see above),
states in 1998 (a) ‘no consistent pattern of defects in
polymerases has been found in tumors’ and (b) mismatch
repair deficiencies are only in ‘13% of colorectal, . . .
endometrial and gastric cancers . . . other types are rarely
(5 2%) MMR-deficient’ (36).
Moreover, mutations of oncogenes and tumor-suppressor
genes of many clonal cancers are non-clonal, and thus not
necessary for carcinogenesis (16). In the words of a recent
survey by Scientific American, ‘A few cancer-related genes,
such as p53, do seem to be mutated in the majority of tumors.
But many other cancer genes are changed in only a small
fraction of cancer types, a minority of patients, or a sprinkling
of cells within a tumor’ (17). However, if gene mutations are
found, they are proportional to the degree of aneuploidy (165)
or of malignancy (163), which is also proportional to
aneuploidy (49, 154 – 157, 165).
The following two studies on the occurrence of gene
mutations in colon cancers illustrate the resulting confusions.
After finding at least 11,000 random mutations in human
colon cancers, Stoler et al. concluded, ‘Together these results
support the model of genomic instability being a cause rather
than an effect of malignancy, facilitating vastly accelerated
somatic cell evolution . . .’ (166). But, Wang et al., who found
about 3000 mutations, concluded that, ‘The accumulation of
approximately one nonsynonymous somatic change per Mb of
tumor DNA is consistent with a rate of mutation in tumor
cells that is similar to that of normal cells. These data suggest
that most sporadic colorectal cancers do not display a mutator
phenotype at the nucleotide level’ (65).
(8) Cancer cells fall into a near-diploid class of low and two
most aneuploid classes of high genomic instability. The theory
that cancer results from selections of random mutations
among genomically unstable cells predicts a continuum of
cancers with increasing degrees of genomic abnormalities.
However, known cancers and even tumorigenic cell lines fall
into a near diploid (2 N) class of low instability and two highly
aneuploid classes of high instability – a relatively rare, near
1.5 N class and a very common, near 3 N class (14, 33, 34,
152, 167 – 170).
(9) The phenotypes of genomically unstable, neo- and
preneoplastic cells are dominant. In contrast to the recessive
phenotypes of hereditary instability syndromes such as
xeroderma pigmentosum, Fanconi’s anemia, hereditary nonpolyposis colon cancer, hereditary hyperplastic polyposis and
Bloom syndrome (32, 58, 62, 69, 80, 171), the phenotypes of
genomically unstable neoplastic and preneoplastic cells are
dominant either directly or after a delay of several cell
generations. The evidence for this dominance was gained
from the following combinations of experimental fusions
between cells with and without certain instability-specific
phenotypes:
(i)
(ii)
(iii)
(iv)
Instant or delayed tumorigenicity is dominant in hybrids
consisting of normal and cancer cells (74). The delayed
tumorigenicity typically follows a spontaneous loss and
reassortments of various chromosomes (28, 74, 172).
Delayed ‘reproductive death’ of irradiated Chinese
hamster cells is dominant in hybrids with un-irradiated
counterparts (173).
Immortality is dominant in all ‘hybridomas’ made from
normal, antibody producing immune cells and aneuploid, immortal mouse myeloma cells (74).
Chromosomal instability of highly aneuploid colon
cancer cells is dominant in cell hybrids made with
near-diploid, chromosomally relatively stable counterparts (33, 34).
ANEUPLOIDY, THE PRIMARY CAUSE
(10) Uncertain developmental phenotypes and maternal
age bias of Down and other congenital aneuploidy
syndromes. Congenital aneuploidy syndromes such as Down
syndrome are characterized not only by standard, but also by
uncertain, developmental phenotypes (39, 179, 209). For
example, a Down patient has a ‘300 times risk increase . . .
for annular pancreas, cataracts and duodenal atresia, an about
100 times risk increase for megacolon and small choanal
atresia, [and a] 10 and 30 times [risk increase for] esophageal,
anal and small bowel atresia, preaxial polydactyly, and
omphalocele’ compared to non-Down controls (210). In
addition Down patients have 20-fold increased risks of
leukemias compared to non-Down cases, which appear ‘at
an age about 3 years younger than in the general population’
(179). Moreover, they have an increased risk (less than-20fold) of other cancers.
The occurrence of Down syndrome is sporadic, but its risk
is known to increase exponentially with the age of the mother
(6, 39, 211, 212). However, the mechanism of this maternal age
effect is as yet unknown. For example, Hassold et al.
acknowledged in 1993 that ‘we know almost nothing about
its basis’ (211), while Pellestor et al. proposed ‘an agedependent deterioration of some cellular factors’ in 2003 based
on the analysis of the karyotypes of 1397 oocytes from women
aged 19 – 46 (212).
(11) Phenotypes without selective advantages are common in
cancer cells. The theory that cancers evolve via a genetic
variation-selection scheme, which conserves advantageous
and discards deleterious mutations, predicts that all
phenotypes of cancer cells are selected to contribute to
malignancy (3, 42, 45, 60, 64, 174). However, contrary to
this theory, the following non-selective phenotypes are
common in cancers:
(i) Immortality. Although many cancer cells are immortal, immortality is not required for carcinogenesis. Even a
tumor of 1% of the mass of an adult animal or human would
require only 1/6 of the cell generations, which the descendants
of a fertilized egg normally achieve in ontogenesis. In culture
normal, diploid human cells can grow about 50 generations
(175), and thus about 18 generations more than a genomically
unstable, preneoplastic cell needs to generate a potentially
fatal tumor of 100 ml or 1010 cells (60).
(ii) Resistance to cytotoxic drugs and even to multiple,
unrelated cytotoxic drugs. Resistance to cytotoxic drugs is
inherent in a minority of cells of practically all cancers and
preneoplastic cell lines. But, drug-resistance is not a selective
advantage for natural carcinogenesis (26, 27). In addition
other non-selective, new morphological phenotypes co-segregate with some drug-resistant variants from transformed
Chinese hamster, mouse and human cells (26, 27), (unpublished observations).
(iii) The ability to metastasize. A minority of cells of
nearly all cancers is able to metastasize (7, 8). However, even
proponents of the mutation theory acknowledge that growth
71
at a heterologous site ‘would not seem to confer increased
proliferative benefit at the native site’, as for example Bernards
and Weinberg in a recent article entitled, ‘A progression
puzzle’ (174).
(iv) Transplantability to heterologous hosts even to heterologous host species. Transplantability is one of the definitions
of cancer (7, 158). But, transplantability does not confer any
selective advantages to natural carcinogenesis.
(v) Susceptiblity to viruses whose host range does not
include normal cell counterparts. This phenotype is common
in cancer cells (45), but is of no selective value for
carcinogenesis.
(vi) ‘Silent’ or non-coding gene mutations. Some cancer
cells accumulate non-selective mutations at abnormally high
rates. But such silent or ‘selectively neutral’ mutations are
irrelevant for the competitive survival of cancer cells (176).
(12) The target, induction of genomic instability, is several
1,000-times bigger than the target, gene mutation. Since
human cells contain 35,000 genes, at most a few of
35,000 cells irradiated with a dose that inactivates one gene
per cell are likely to acquire a mutator gene that may
cause genomic instability (assuming haploid genes). But, all
strategies developed to aim at the target, mutator gene, or
the ‘target genes’ of genomic instability have obtained an
unexpectedly ‘high frequency’ of hits in view of the gene
mutation theory (18). The following examples illustrate this
point:
(i)
(ii)
(iii)
(iv)
In 1992 Kadhim et al. observed ‘chromosomal instability’ at ‘high frequency’, namely in about 50% of mouse
bone marrow cells that had survived alpha-irradiation.
Since 10% of the cells had survived the treatment, the
size of the target, genomic instability, was similar to that
of cell lethality. Sabatier et al. also reported in 1992 that
50% of dermal fibroblasts from a normal human donor
that had survived ionic neon radiation developed
‘chromosomal instability’ (85).
In 1993, Holmberg et al. observed in irradiated cultures
of human T-lymphocytes ‘unstable karyotypes . . . in
about half of the X-irradiated clones’ (133).
Bols et al. observed in 1992 that a single treatment with
non-mutagenic benzpyrene was enough to render 3 –
10% of Syrian hamster embryo cells ‘immortal’,
compared to none of the untreated controls (19).
Trott et al. found that single treatments with ionizing
alpha, gamma and neutron radiation at about half lethal
doses, or treatments with non-mutagenic nickel chloride,
at non-toxic doses, converted over 5% of surviving
Syrian hamster embryo cells to immortality and
chromosomal heterogeneity, compared to none of the
controls (18). And 30% of the immortalized cultures
generated morphologically transformed sub-clones
spontaneously, soon after selection for clonogenicity or
‘immortality’ (18).
72
DUESBERG ET AL.
Since mutagenic carcinogens, like radiation, mutate, at
half-lethal doses, a specific gene in only 1 out of 10576 cells
(29, 177), but induce genomic instability in 1 out of 2 – 30
surviving cells, the target, genomic instability, is about 3 – 6
orders bigger than the target, gene mutation.
The features of genomic instability according to the
aneuploidy hypothesis
In the following we show how the aneuploidy theory
explains the 12 features of genomic instability (see also
Table 1).
Feature #1. Non-mutagenic carcinogens can cause aneuploidy by disrupting the spindle apparatus (28, 89). For
example, polycyclic aromatic hydrocarbons and colcemid can
cause aneuploidy by intercalating into, and thus depolymerizing microtubules. Likewise ‘extranuclear’ (86) or ‘cytoplasmic’
(31) or ‘non-DNA damage’ (102) radiation can target the
spindle apparatus at various non-chromosomal sites, as for
example centrosomes. The non-DNA damage-study has
indeed advanced ‘the hypothesis that . . . the mitotic spindle
apparatus, may be involved in aberrations in chromosome
segregation after X irradiation’ (102).
Feature #2. Neoplastic and preneoplastic aneuploidy is
self evident according to the aneuploidy theory.
Feature #3. The high rates of variation and even reversion
of cancer-specific phenotypes, such as drug-resistance, tumorigenicity, and metastasis, reflect the high rates of aneuploidycatalyzed chromosome reassortments and rearrangements of
aneuploid cells (26, 27, 35), (Fig. 1).
Feature #4. According to the aneuploidy theory, most
cancers and cell lines are veritable zoos of chromosomally
distinct, cellular species, despite their clonality. Such ‘polyphyletic’ cell populations, as Hauschka and Levan have called
them previously (5), include species that can survive many
adverse conditions, such as toxic drugs, otherwise lethal
mutations, transplantation to heterologous sites and species
etc., which are lethal to normal diploid cells and even to most
cancer cells. As a result a given cancer survives many adverse
conditions via preexisting, resistant subspecies from within its
diverse, ‘polyphyletic’ population. By contrast, a given
population of the same number of homogeneous, diploid cells
would be unable to survive the same lethal conditions, because
the normal rates of gene mutations would be many orders too
low to save the population via pre-existing resistant variants
(see Feature #3, above). Take, for example, the consistent
failures of the normal cells of cancer patients to survive toxic
chemotherapy, or the fact that despite 3 billion years of
mutations (!) no diploid organism has ever achieved immortality, which all cancers do in matter of months or years.
Feature #5. Aneuploidy-dependent chromosome instability is self evident according to the aneuploidy theory.
Feature #6. The coincidence of different hypothetical
mutator genes, generated by multiple, independent mutations
in most cancers, is improbable. Therefore, several investigators have previously postulated a common underlying
mechanism. For example, Grosovsky et al. postulated,
‘Elevated rates of chromosomal-scale and intragenic mutations may be attributable to independent processes, but their
Table 1
Compatibility of 12 specific features of genomic instability with the aneuploidy and the gene mutation theories
Feature of genomic instability
1
2
3
4
5
6
7
8
9
10
11
12
Induction by mutagenic and non-mutagenic carcinogens
Aneuploidy coincides and segregates with preneoplastic and neoplastic genomic
instability
Phenotypes of unstable cells change and even revert at high rates, compared to gene
mutation
Idiosyncratic phenotypes of unstable cells, e.g. immortality and drug-resistance, are
never observed in diploid biology
Instability is proportional to the degree of aneuploidy
Coincidence of multilateral chromosomal and genetic instabilities in the same cells
Gene mutations are common, but neither consistent nor clonal in cancer cells
Cancers fall into a near-diploid (2 N) class of low instability, a near 1.5 N class of high
instability, and a near 3 N class of very high instability
Dominant phenotypes
Uncertain phenotypes and maternal age-bias of congenital aneuploidy syndromes like
Down Syndrome
Phenotypes without selective advantages
The target, genomic instability, is several 1000-times bigger than the target, gene
Mutation
theory
Aneuploidy
theory
+/7
7
+
+
7/+
+
7
+
7
7/+
7
7
+
+
+
+
7/+
7
+
+
7
7
+
+
ANEUPLOIDY, THE PRIMARY CAUSE
coincident appearance in the same clones indicates that they
may each be a manifestation of a single underlying mechanism’ (29). Kobayashi et al. proposed, ‘the same molecular
backgrounds’ because ‘MSI and LOH [microsatellite instability and loss of heterozygosity] tend to be coincident (72).
Further, Masuda and Takahashi proposed ‘some overlap in
their underlying mechanisms’ (149), which Matzke et al.
confirmed with reference to them, ‘Chromosome numerical
and structural alterations usually coexist in cancer cells,
however, suggesting that they are inextricably linked’ (55).
Indeed, aneuploidy provides a simple explanation for the
coincidence of different mechanisms of genomic instability in
most cancer cells (35), because it simultaneously unbalances
thousands of functionally linked proteins (see Fig. 1).
Feature #7. Gene mutations, including mutator genes,
would only be generated in those genomically unstable cells, in
which teams of enzymes involved in the synthesis and
maintenance of DNA are corrupted by aneuploidy (35),
(Fig. 1). Clonal mutations would derive from preneoplastic
aneuploidy and non-clonal mutations would derive from
neoplastic aneuploidy. This explains not only why gene
mutations are common, but also why they are not consistently
associated with genomically unstable cells and why they tend
to be late and non-clonal (see Mutation theory).
Feature #8. The distribution of cancers into different
ploidy classes reflects two alternatives to maximize aneuploid
fitness: a near-diploid class with a minimally unbalanced
karyotype approaching the stability of normal karyotypes,
and two highly aneuploid classes, a relatively rare, near 1.5 N
and a common, near 3 N class, which are maximally unstable
but also maximally adaptable (28, 178). This latter principle is
analogous to the immune system, which is maximally effective
if it consists of maximally recombinogenic stem cells. The
high risk of lethal nullisomies also explains why the near
1.5 N aneuploidy is rare compared to the near 3 N aneuploidy
(Fig. 1).
Feature #9. Aneuploidy-generated genomic instability is
inevitably dominant, because the aneuploid dosages of
thousands of genes dominate gene expressions and phenotypes
(22, 179 – 182).
Feature #10. According to the aneuploidy theory, the
standard phenotypes of Down syndrome are direct consequences of the abnormal gene dosage of the trisomic
chromosome 21. By contrast the uncertain developmental
phenotypes of Down syndrome, like endocardial defects and
polydactyly, would be indirect consequences of trisomy 21.
For example, supply-sensitive diploid developmental programs could be rendered error-prone by abnormal dosages
of chromosome 21 products. This particular kind of genomic
instability has been termed, ‘amplified developmental instability’ by Shapiro (179,209). Indeed, a high risk of ‘nonconcordant’ phenotypes is typical for all aneuploidy syndromes (39,179). Likewise, certain phenotypes of cancer cells
may result from diploid genes whose functions are rendered
73
error-prone by abnormal dosages of products from aneuploid
genes. Thus aneuploidy is able to generate not only certain
phenotypic abnormalities, but also uncertain developmental
abnormalities.
The higher-than-normal leukemia and cancer risks of
Down syndrome are also predicted by the aneuploidy theory
based on the modestly enhanced degree of chromosomal
instability (Fig. 1).
According to the aneuploidy theory the maternal age-bias
of Down’s would reflect age-dependent defects of the spindle
apparatus of oocytes, which increase the risk of nondisjunction either during meiosis or an early mitosis. Indeed,
rare Down syndromes, which are generated by translocations
of chromosome #21 to other chromosomes, are not agedependent (6,39). And about 2% of the age-dependent
trisomies 21 are mosaics derived from non-disjunction during
the first or second mitosis of the fertilized egg (39).
Feature #11. The non-selective phenotypes of cancer cells
are those which are linked to selective phenotypes on the same
aneuploid chromosome (26, 27). They are common in cancers
because any chromosome with selective genes also carries a
large excess of non-selective genes.
Feature #12. The large target, genomic instability, has
intrigued many investigators in the past, particularly in view of
the prevailing gene mutation hypothesis. For example, Bols et
al. deduced in 1992 from the high yields of immortalization of
Syrian hamster embryo cells by benzpy-rene, ‘according to the
mutation hypothesis immortalization is hardly to be expected.
. . . However, immortalization was frequently observed. Therefore the induction of immortalization appears indirect’ (19).
Studying the same system, Trott et al. concluded in 1995 that
the ‘high frequency change in the treated population’ was ‘an
indirect consequence of carcinogen exposure’, ‘rather than . . .
direct targeted mutagenesis’ (18). Murnane found in 1996,
‘The high percentage of cells that develop induced genetic
instability after exposure . . . to ionizing radiation or alkylating
agents . . . and the prolonged period over which the instability
occurs, indicates that the instability is not in response to
residual damage in the DNA or mutations in specific genes.
Instead, changes affecting most of the exposed cells, such as
epigenetic alterations in gene expression or chain reactions of
chromosome rearrangements, are a more likely explanation’
(32). Hall asked in 1997, ‘At least one aspect of the model [of
carcinogenesis] must cause consternation to the radiation
biologist, namely how can a single brief exposure to a low dose
of radiation result in six or seven mutations at different loci?’
[A reference to the hypothesis that 4 – 7 mutations cause
cancer (49, 183, 184)]. Holmberg et al. noted in 1998, ‘The
phenotype [genomic instability] is not likely caused by single
gene mutation, since it is expressed in a large fraction of
surviving cells . . . 10% or more of the surviving cells, a
frequency that is at least three orders of magnitude greater
than expected for a specific locus mutation. Thus one would
have to assume there are hundreds of genes . . . [or] epigenetic
74
DUESBERG ET AL.
perturbations . . . or some persistent impression of the
genotoxic [mutation] stress response . . .’ (185). Wright wrote
in 1999, ‘The instability phenotype is usually induced in a
relatively large proportion of the irradiated cell population
(generally *10 – 20%). This is orders of magnitude greater
than conventional gene mutation’ (86). Little made that same
point in a review in 2000, ‘It has been widely assumed that the
initial genetic events by radiation in mammalian cells occur as
a direct result of DNA damage . . . There is now increasing
evidence, however, that exposure of cell populations to
ionizing radiation may also produce non-targeted effects; . . .
The first was a frequent event which involved a large fraction
(20 – 30%) of the irradiated cell population, and enhanced the
probability of the occurrence of a second event.’ But, the
question of ‘how it is initiated and how it is maintained’ was
left open (31). Morgan noted in 2003, ‘This observed
frequency of instability is grossly in excess of the frequency
reported for gene mutations at similar doses, suggesting that it
is unlikely that mutation in a single gene or gene family is
responsible for the unstable phenotype in all unstable clones’
(186).
All these questions are basically answered by the theory
that the target of genomic instability is the chromosome,
rather than a gene. The aneuploidy theory predicts that the
target, induction of genomic instability, is the loss or gain of
a chromosome and thus much larger than the target, gene
(Fig. 1). Since humans contain 35,000 different genes (187)
on 24 different chromosomes, the average human chromosome is the equivalent of 1458 genes. Thus, in a first
approximation the target, genomic instability, is about 1458
times bigger than a human gene (and about 3000 times
bigger than a Chinese hamster gene). The higher discrepancies estimated above probably reflect the fact that there are at
least two alleles of most genes in diploid cells and even more
in aneuploid cells, and thus the likelihood to mutate a
corresponding function is only 1/35,000 2 or 4 2. Since the
loss of one or more chromosomes is also the cause of cell
death (Fig. 1), we have before us an explanation why the
targets, induction of genomic instability and lethality, have
about the same size.
The 12 Features of Genomic Instability According to the
Mutation Theory
Only 4 of the 12 features of genomic instability can be
partially explained by the mutation theory (Table 1). And 3 of
these 4 can only be explained via aneuploidy generated by as
yet unconfirmed chromosome instability genes (see Mutation
theory). In brief: Feature #1, Induction of genomic instability
by non-mutagenic carcinogens is un-explained; Feature #3,
Phenotypes, which are variable and reversible at high rates,
can only be explained via aneuploidy generated by hypothetical chromosome instability genes; Feature # 6, Coincidence
of multilateral genomic instabilities with aneuploidy can only
be explained as suggested for Feature #3; Feature #9,
Dominant phenotypes can only be explained via aneuploidy
generated as suggested for Feature #3.
Only the Aneuploidy Theory Offers a Coherent
Explanation of all Features of Genomic Instability
In sum, the aneuploidy theory can offer a coherent
explanation for all of 12 features that define genomic
instability, listed above and in Table 1. By contrast, the
original version of the mutation theory can only explain 1 out
of 12 (59), whereas modified versions, which postulate mutator
genes that generate aneuploidy (see above and (17, 56, 188)),
can explain 3 or more additional features (Table 1). Since
aneuploidy is the only theory that provides a coherent
explanation for all features analyzed, we conclude that
aneuploidy is the primary cause of genomic instability.
Aneuploidy Theory Resolves the Paradox that Carcinogens
and Hypothetical Cancer Genes Transform Aneuploid
Rodent Cell Lines, but not Diploid Cells
The convention, to use morphological transformations of
aneuploid rodent cell lines as an assay for carcinogens and
cancer genes (29, 31, 186, 189), has generated several
paradoxes. For example, Parodi and Brambilla have reviewed
dozens of studies, which arrived at the conclusion that ‘34
carcinogenic compounds’ are several 1000-times more efficient
in transformation of aneuploid rodent cell lines than in
mutating any specific gene: ‘The difference in frequency
between structural mutations and transformations was about
102–103 and it appears statistically extremely significant. . . .
The frequency of morphological transformations is so much
higher than the frequency of somatic mutations that it is
probably more reasonable to consider the morphological
transformation as an event of ‘‘epigenetic’’ type’ (191). Based
on morphological transformation of aneuploid mouse cell
lines with carcinogens Cairns also arrives at the conclusion,
‘The first step in the sequence seems to be too efficient; in
certain situations, even quite low doses of mutagen are capable
of initiating the process in almost every exposed cell’ (192). By
contrast, the efficiency of carcinogens to transform normal
diploid cells to cancer cells in vivo – via a multistep chain
reaction of aneuploidizations – is very low and the process is
very slow, particularly in humans (see also Cairns (2) and
Introduction).
Even the popular view that point mutations generate
human cancer, as for example mutations of the ras genes, is
functionally only based on ‘morphological transformation’ of
the highly aneuploid mouse 3T3 cell line (193 – 195). But, as
yet, no genes have been isolated from cancer cells that alone or
in combination with others are able to transform diploid
human or animal cells to cancer cells (31, 49, 138, 172, 196,
197). By contrast, a retroviral ras gene with a retroviral
promoter isolated from a virus-transformed cell, transforms
diploid cells under the same experimental conditions in which
its cellular relatives fail (198).
ANEUPLOIDY, THE PRIMARY CAUSE
According to the aneuploidy theory the high frequencies
at which carcinogens and hypothetical cancer genes cause
‘morphological transformations’ in aneuploid cell lines
simply reflect the low thresholds of phenotype alterations
of aneuploid cells. In aneuploid cell lines transforming
agents would function as exogenous catalysts that accelerate
spontaneous, aneuploidy-catalyzed karyotype variations. In
such cells carcinogens and hypothetical cancer genes do not
have to initiate de novo the lengthy and inefficient steps of
immortalization and transformation, because these steps
have already been taken by all aneuploid cell lines (see
Introduction and Feature #4). Accordingly, all cell lines are
immortal and practically all are tumorigenic, even if they
appear morphologically untransformed in vitro (190, 199 –
201). Even the specific-gene-mutation standards, to which
the transformation frequencies of aneuploid cell lines were
compared by Parodi and Brambilla (191), are elevated by
aneuploidy-catalyzed gene mutations and thus are also not
directly relevant to the natural mutation rates of diploid
cells (see Aneuploidy theory and Fig. 1). Instead of being
relevant to the causation of cancer in diploid cells, the
studies of morphological transformations of aneuploid cell
lines are only relevant to either the progression of
preneoplastic to neoplastic cells or to the progression of
carcinogenesis (7).
The Relationship Between the Genomic Instability of
Hereditary Syndromes that Predispose to Cancer and the
Genomic Instability of Aneuploid Cells
The cells of humans with hereditary genomic instability
syndromes that predispose to cancer, such as the Bloom’s
syndrome, Fanconi’s anemia, hereditary nonpolyposis colorectal cancer, hereditary hyperplastic polyposis and xeroderma pigmentosum carry a higher risk of spontaneous
carcinogenesis than normal cells (14, 36, 58, 62, 69, 80).
But, in contrast to the cancer-related phenotypes of
aneuploid cells (see Feature #9, above), the phenotypes of
the hereditary syndromes are recessive. Further, the
karyotypes of these syndromes are ‘generally not associated
with visible cytogenetic changes’, i.e. are diploid (14, 69).
Some of these hereditary syndromes generate hyperplastic
phenotypes, as for example diploid hyperplastic polyposis
(69). However, the associated cancers can originate either
from the hyperplastic or from normal tissues (51, 69).
According to the aneuploidy theory the elevated cancer-risk
of these syndromes is an elevated risk of spontaneous
aneuploidization. Indeed, Hoeijmakers points out that the
genetic ‘lesions frequently induce various sorts of chromosomal aberrations, including aneuploidy’ (62). The Bloom’s
syndrome is a classical point in case (202). Since the
hereditary, genomic instability syndromes increase the low,
normal risk of spontaneous aneuploidization, their genetic
defects are functionally equivalent to exogenous carcinogens.
Thus the cancer-relevant genetic defects of the hereditary
75
instability syndromes are risk factors for spontaneous
aneuploidization.
Aneuploidy, the Common Cause of Genomic Instability
and Cancer
Considering that carcinogens and spontaneous errors in
chromosome segregation each cause random aneuploidy, and
that aneuploidy is inherently unstable and thus able to
generate new species with new phenotypes – we have before
us the facts to explain the entire multistep sequence of
carcinogenesis (Fig. 1). This sequence begins with a random
aneuploidy, which is caused either by a carcinogen or arises
spontaneously. Since almost any aneuploidy unbalances at
least some conserved teams of proteins that segregate,
synthesize and repair chromosomes, the karyotypes of
aneuploid cells are perpetually at risk of autocatalytic
variations. The risk and rates of karyotype variation would
be proportional to the degree of aneuploidy. The basis for the
somatic evolution of cancer cells from randomly aneuploid
precursor cells is selection for advantages in growth. Thus
aneuploidy is necessary, but not sufficient for carcinogenesis.
Since cancer cells are generated from precursor cells by
rearranging old genes into new sets of chromosomes – just like
new species are generated in phylogenesis (81) – cancer cells
are new species of their own, rather than mutants of any
precursor cells. However, due to the inherent instability of
aneuploidy, aneuploid cell species are unable to achieve
phylogenetic autonomy, since they are too unstable to
maintain the many genetic investments that are necessary for
autonomy, but as parasites they are able to progress from bad
to worse autocatalytically, i.e. from phenotype-less preneoplastic to highly malignant cancer cells.
The many idiosyncratic phenotypes of cancer cells, such as
immortality, reproductive death, invasiveness, drug-resistance,
metastasis, abnormal gene expression, etc. (see Introduction
and Feature #4), represent distinct subspecies from within the
inherent ‘polyphyletic’ diversity (5) of individual cancers.
Since aneuploidy and genomic diversity are incompatible with
the identity and survival (203, 204) of an autonomous, diploid
species, the idiosyncratic phenotypes of cancer cells are never
observed in diploid biology.
We conclude that the transition of stable diploid to
unstable aneuploid cell species is the primary cause of
preneoplastic and neoplastic genomic instability and of cancer.
Accordingly the many heterogeneous and non-clonal gene
mutations of cancer cells are just inevitable, but probably nonselective consequences of aneuploidy.
ACKNOWLEDGMENTS
We thank Paul La Rosee (III Med. Klinik, Mannheim of
the University of Heidelberg), Rainer Sachs (Dept. Mathematics, UC Berkeley), Rudi Werner (University Miami,
Medical School) and Lynn Hlatky (Dana-Farber at Harvard,
76
DUESBERG ET AL.
Boston) for critical reviews of and helpful suggestions for the
manuscript, and Sachs also for the original design of Fig. 1.
Ruhong Li (UC Berkeley) is acknowledged for critical
discussions and numerous drafts of Fig. 1 and David
Rasnick (UC Berkeley) for valuable information. We are
indebted to the Abraham J. and Phyllis Katz Foundation
(New York), Robert Leppo (philanthropist, San Francisco),
an American foundation that prefers to be anonymous, other
private sources, and the Forschungsfonds der Fakultaet for
Klinische Medizin Mannheim for support. PD is grateful to
the Deutsche Krebshilfe for a guest professorship at
Mannheim.
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