Harvey Bialy is resident scholar at the Institute of
Biotechnology, National Autonomous University of Mexico (IBT/UNAM),
Cuernavaca, Mexico (bialy@ibt.unam.mx).
Twenty-five years ago, Peter Duesberg isolated and characterized the first
viral oncogene—the principal base on which the predominant, current
gene mutation theory of cancer rests. During the past five years, Duesberg
has been applying a similar determination investigating an alternative genetic
explanation of cancer that derives from a century-old observation concerning
the chromosomal abnormalities (aneuploidies) associated with essentially every
solid human tumor1. Now in a paper published in the most recent
issue of PNAS2, he and his co-workers have provided compelling
evidence that the high mutation rates of cancer cells are due to an aneuploidy-based
continuous chromosome reassortment.
In a series of papers between 1997 and 2000, Duesberg and his colleagues
at the University of California, Berkeley and the University of Heidelberg
at Mannheim showed that aneuploidy, an imbalance in chromosome number, is
diagnostic in the earliest stages of cell culture of malignant transformation
induced by non-genotoxic carcinogens, such as polycyclic aromatic hydrocarbons
(PAHs)3; that aneuploidy perfectly explains the notorious genetic
instability of cancer cells4; and that aneuploidy is a transformation-related
event, because it precedes malignant transformation, and because 14 out of
14 cancers (100%) of inbred Chinese hamsters (CH) were aneuploid compared
with about 30% of PHA-treated CH cells5. (The chance that aneuploidy
of the 14 cancers was unrelated to transformation is only 0.3014,
or 5 10-8.) In addition to these experimental
supports, the aneuploidy explanation of cancer was given a rigorous formalization
based on the mathematical and logical underpinnings of metabolic control analysis6, until relatively recently an arcane biochemical discipline that
has nonetheless replaced the textbook concept of a rate-limiting enzyme with
the real-world idea of distributed control7.
But the best tests of any theory are its accuracy at making experimental
predictions in settings not directly related to those in which it was formulated,
and its ability to explain previously inexplicable findings. In the latest
PNAS paper2, aneuploidy as the functional genetic basis
of cancer passes both these tests in ways that should be of interest to biotechnologists
of a variety of stripes.
Of all the peculiarities of cancer cells, the extraordinarily high rate
at which they become resistant to chemotoxic agents (one in one thousand to
one in one million per mitosis) has for almost 50 years remained one of the
more difficult for theoreticians to explain and clinicians to confront. As
recently as 1995, Henry Harris considered it to be a "major conceptual
difficulty to reconcile the very high mutational frequency with genetic theory
if two functional alleles are present in the same cells."8
In light of this, numerous investigators have proposed that "epigenetic
mechanisms" might be responsible for drug and multidrug resistance in
cancer cells, although before now no specific mechanism had been suggested.
Because aneuploidy simultaneously imbalances, through effects in gene dosage,
large numbers of balance-sensitive proteins, including those involved in the
mitotic spindle apparatus, its occurrence in a cell results in a self-perpetuating
chromosomal instability. The average cell in a typically aneuploid tumor,
for example, is at a 46% risk to gain or lose one chromosome per mitosis3. Continuous chromosome reassortment, catalyzed by aneuploidy, as
Duesberg and his colleagues argue in their most recent paper, is a likely
mechanism to explain the high mutation rates of cancer cells. It may reflect
more than simple coincidence that this model has an exact precedent in the
mechanistic explanation of the influenza virus' exceptionally high mutation
rate through reassortment of subgenomic RNA segments that Duesberg provided
in the pages of a 1968 number of the PNAS9.
In the present work, the idea of aneuploidy-catalyzed reassortment as the
epigenetic "mutator" is directly tested by comparing the mutation
rates of aneuploid, tumorigenic cells with those of diploid, normal cells
from the same inbred line of Chinese hamsters. As all of the cells studied
have an otherwise identical genetic background, any differences in mutation
rates must result from their different chromosome numbers. The mutations investigated
were to resistance against the anticancer drugs puromycin, cytosine arabinoside,
colcemid, and methotrexate. Exactly in accord with a chromosomal reassortment
model, the mutation frequencies of aneuploid cells were high, between 10
-4 and 10-6, whereas the frequency of resistance
in the diploid cells was undetectable.
In addition to the quantitative dilemma that cancer cells pose for gene
mutation theories by their high mutation rates to single-drug resistance,
it is completely bewildering to these theories that cancer cells often become
simultaneously resistant to a number of functionally and structurally unrelated
drugs10. But such a result is precisely what one would expect
if a chromosomal reassortment model were applicable, as large numbers of genes
are being affected simultaneously. Experimentally, this means that selection
for one phenotype should produce cells possessing novel, unselected phenotypes.
As succinctly put in the paper's abstract, "Mutants selected from cloned
(aneuploid) cells for resistance against one drug displayed different unselected
phenotypes, e.g., polygonal or fusiform cellular morphology, flat or three-dimensional
colonies, and resistances against other unrelated drugs."
One final point in the support of the reassortment model is that it immediately
explains the otherwise puzzling finding that the drug resistance mutations
of cancer cells are significantly less stable than conventional mutations,
some reverting to the original phenotype at the same rate with which they
were generated10. The constantly shifting karyotype responsible
for the drug resistance can also lead to its loss. Indeed, preliminary evidence
for this interpretation is presented by the observation that "in many
cultures of drug-resistant cells growing in the presence of a selective cytotoxic
drug (there are) an excess of unattached, dead cells, compared with parallel
cultures grown in the absence of the drug."
Beyond providing the theoretician with a unifying explanation of the high
mutation rates of aneuploid cancer cells, and the frequent occurrence of multidrug
resistance, the chromosomal reassortment model offers the more practical minded
a potential way to determine effective chemotherapeutic regimes, as it predicts
the existence of phenotype-specific karyotypes still to be identified. And
perhaps with immediate applicability, it suggests using the aneuploidy-based,
high frequency of drug resistance in cancer cells as a functional alternative
to direct determinations of aneuploidy in order to diagnose the presence of
pre-neoplastic cells in benign lesions.
5 
10-8.) In addition to these experimental
supports, the aneuploidy explanation of cancer was given a rigorous formalization
based on the mathematical and logical underpinnings of metabolic control analysis
