A heretical theory about the origins of cancer could explain a long-standing medical mystery - why cancers often become resistant to the drugs used to treat them.
The controversial theory, if correct, also suggests a possible way to treat cancer to restore its sensitivity to drugs.
The theory, espoused by Peter Duesberg, professor of molecular and cell biology in the College of Letters & Science at the University of California, Berkeley, disputes the common assumption that cancer results from a series of genetic mutations that kick a cell into uncontrolled growth.
Instead, Duesberg argues, cancer results from a wrench thrown into the machinery - the spindle apparatus - controlling chromosome splitting when a cell divides. This leads to an abnormal number of chromosomes that unbalances hundreds to thousands of genes and proteins in the cell and generates the many odd characteristics of cancer cells.
Cells with the wrong number of chromosomes are called aneuploid cells, and scientists have known for a long time that essentially all solid tumors are aneuploid. This was thought to be the result of the genetic mutations leading to cancer, however, not the cause of cancer.
One consequence of this aneuploidy is that the sensitive balance of proteins in the spindle apparatus also is upset, destabilizing the machinery that normally divides the chromosomes evenly between two daughter cells. As a result, each time a cell with an abnormal balance of chromosomes divides, it makes new mistakes in chromosome division, creating a snowball effect that keeps changing cellular properties, particularly in cancer cells with highly abnormal chromosome numbers.
In the Dec. 19, 2000, issue of the Proceedings of the National Academy of Sciences, Duesberg and his colleagues, UC Berkeley researcher Reinhard Stindl and Ruediger Hehlmann of the University of Heidelberg at Mannheim, Germany, reported the results of a simple experiment that bolsters their argument that aneuploidy, not genetic mutation, causes cancer.
The experiment showed that cells with an abnormal number of chromosomes respond very differently to chemotherapeutic drugs than do cells with a normal number. Aneuploid cells with 50 percent more chromosomes than normal developed drug resistance after as few as nine days of drug treatment, whereas no normal cells developed resistance. Thus, aneuploid cells respond to chemotherapeutic drugs just like cancer cells.
The researchers concluded that continuous chromosome reassortment facilitated by aneuploidy is a likely mechanism to explain the high mutation rates of cancer cells and the frequent development of drug resistance.
"People have been working on drug resistance for 60 years, and the question still remains - why the huge discrepancy between mutation to drug resistance in cancer cells and everything that is known about mutations in normal cells," Duesberg said. "Our cancer hypothesis offers a unifying explanation for the high mutation rates in aneuploid cancer cells and the generation of drug resistance."
An analysis of Duesberg's paper appearing in the January 2001 issue of Nature Biotechnology concluded that "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."
The author, Harvey Bialy, resident scholar in the Institute of Biotechnology at the National Autonomous University of Mexico in Cuernavaca, also endorsed Duesberg's theory that aneupolidy is the basis of cancer.
"The best tests of any theory are its accuracy at making experimental predictions ... and its ability to explain previously inexplicable findings," Bialy wrote. "In the latest PNAS paper, 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."
Previous researchers looking at the development of drug resistance have failed to consider chromosome reassortments catalyzed by aneuploidy as a mechanism of mutation to drug resistance, Duesberg said. In his experiments, he compared mutation rates to drug resistance of aneuploid cells that typically develop into tumors with mutation rates of normal diploid cells.
Every one of the cultures of highly aneuploid cells developed drug resistance after repeated treatment with one of a variety of cancer drugs, such as methotrexate. None of the normal cell cultures developed drug resistance. The mutation rates of aneuploid cancer cells to drug resistance fell between one in a thousand and one in a million cells. By contrast, the mutation rates of normal, diploid cells to drug resistance were too low to be detected under the conditions used.
Duesberg, Stindl and Hehlmann found that cells with highly abnormal chromosome numbers have a one to two percent chance of gaining or losing a chromosome each time they divide. In a colony of a million cells, that provides lots of opportunity for one or more cells to mutate and gain the ability to disarm a drug, probably through activation of a new biochemical pathway. By contrast, the odds for a normal cell to lose or gain a chromosome during mitosis are undetectably low under most conditions.
Duesberg also noticed that his aneuploid cells exhibited something else common in cancer cells - multidrug resistance. Frequently, when a cancer becomes resistant to one drug, it proves to be resistant to others that are structurally different. Scientists have had a hard time explaining this as the result of genetic mutation and have suggested various hypothetical scenarios, including mutator genes and multidrug resistance (MDR) genes that somehow become mutated in cancer cells.
Duesberg dismisses this as improbable. "If there are mutator or multidrug resistance genes, why do only cancer cells, but not normal cells, benefit from such genes in the face of chemotherapeutic drugs?" Duesberg said. "Resistance of normal cells is never seen in chemotherapy."
Based on their experiment, Duesberg and his colleagues concluded that aneuploidy best explains the development of multidrug resistance. Chromosome reassortment is capable of activating numerous biochemical pathways that could disarm a variety of drugs. The results suggest, however, that the cause of drug resistance is not the loss of drug-sensitive genes when a chromosome is lost.
One possible way to force a cancer to revert from drug-resistant to drug-sensitive is to exploit the instability of aneuploid cells. By treating cancer cells with an agent such as a phorbol ester, which promotes the reassortment of chromosomes without causing cancer by itself, it may be possible to force aneuploid cancer cells to rearrange their chromosomes enough to lose the ability to resist chemotherapeutic drugs.
"Phorbol esters catalyze the reassortment of chromosomes and could cause the phenotype to change and lose drug resistance," Duesberg suggested.
Cancerous tumors that have become drug resistant sometimes spontaneously revert and again become sensitive to drugs, something expected from the unstable nature of aneuploidy, he said.
The research, most of it conducted at the University of Heidelberg in Mannheim, was supported by grants from various sources, including San Francisco philanthropist Robert Leppo.
Note: Peter Duesberg can be reached at 510-642-6549 or duesberg@uclink4.berkeley.edu.
Journal
Proceedings of the National Academy of Sciences