Cancer patients may view their tumors as parasites taking over their bodies, but this is more than a metaphor for Peter Duesberg, a molecular and cell biology professor at the University of California, Berkeley.
Cancerous tumors are parasitic organisms, he said. Each one is a new species that, like most parasites, depends on its host for food, but otherwise operates independently and often to the detriment of its host. Duesberg and colleagues describe their theory that carcinogenesis -- the generation of cancer -- is just another form of speciation, the evolution of new species.
A molecular biologists has long believed that cancer results from chromosome disruption rather than a handful of gene mutations, which is the dominant theory today. That idea has led him to propose that cancers have actually evolved new chromosomal karyotypes that qualify them as autonomous species, akin to parasites and much different from their human hosts.
"Cancer is comparable to a bacterial level of complexity, but still autonomous, that is, it doesn't depend on other cells for survival; it doesn't follow orders like other cells in the body, and it can grow where, when and how it likes," said Duesberg. "That's what species are all about."
This novel view of cancer could yield new insights into the growth and metastasis of cancer, Duesberg said, and perhaps new approaches to therapy or new drug targets. In addition, because the disrupted chromosomes of newly evolved cancers are visible in a microscope, it may be possible to detect cancers earlier, much as today's Pap smear relies on changes in the shapes of cervical cells as an indication of chromosomal problems that could lead to cervical cancer. The idea that cancer formation is akin to the evolution of a new species is not new, with various biologists hinting at it in the late 20th century. Evolutionary biologist Julian S. Huxley wrote in 1956 that "Once the neoplastic process has crossed the threshold of autonomy, the resultant tumor can be logically regarded as a new biologic species …."
Last year, Dr. Mark Vincent of the London Regional Cancer Program and University of Western Ontario argued in the journal Evolution that carcinogenesis and the clonal evolution of cancer cells are speciation events in the strict Darwinian sense. The evolution of cancer "seems to be different from the evolution of a grasshopper, for instance, in part because the cancer genome is not a stable genome like that of other species. The challenging question is, what has it become?" Vincent said in an interview. "Duesberg's argument from karyotype is different from my argument from the definition of a species, but it is consistent."
Picture: Staining chromosomes with different dyes highlights the orderly nature of the normal human karyotype (left), that is, humans have precisely two copies of each chromosome with no leftovers. A bladder cancer cell (right) has extra copies of some chromosomes, a few missing normal chromsomes, and a lot of hybrid or marker chromosomes, which characterize cancer cells.