Friday, August 08, 2008

Contagious cancer

The [Tasmanian] devil, known to science as Sarcophilus harrisii, lives mostly by scavenging and sometimes by predation. It will eat, in addition to kangaroo meat, chickens, fish, frogs, kelp maggots, lambs, rats, snakes, wallabies, and the occasional rubber boot. It can consume nearly half its own body weight in under an hour, and yet—with its black fur and its trundling gait—it looks like an underfed bear cub. Fossil evidence shows that devils inhabited all of Australia until about 500 years ago, when competition with dingoes and other factors caused them to die out everywhere but in Tasmania, which dingoes had yet to colonize. More recently, Tasmanian stockmen and farmers have persecuted devils with the same ferocity directed elsewhere at wolves and coyotes. The devils’ reproductive rate, opportunistic habits, and tolerance for human proximity, however, have allowed localized populations to persist or recover, and at the time of Baars’s 1996 visit, their total number was probably around 150,000.

On his earlier visits, Baars had seen at least ten devils every night, and they were quick to adjust to his presence. They would walk into his blind, into his tent, into his kitchen, and he could recognize returning individuals by the distinctively shaped white patches on their chests. This trip was different. On the first night, his bait failed to attract a single devil, and the second night was only a little better. He thought at first that maybe the stockmen and farmers had finally succeeded in wiping them out. Then he spotted a devil with a weird facial lump. It was an ugly mass, rounded and bulging, like a huge boil, or a tumor. Baars took photographs. More devils wandered in, at least one of them with a similar growth, and Baars took more pictures. This was no longer wildlife photography of the picturesque sort; it was, or anyway soon would become, forensic documentation.

Back in Hobart, Tasmania’s capital, Baars showed his pictures to Nick Mooney, a veteran officer of Tasmania’s Parks and Wildlife Service who has dealt with the devil and its enemies for decades. Mooney had never seen anything like this. The lumps looked tumorous, yes—but what sort of tumor? Mooney consulted a pathologist, who suggested that the devils might be afflicted with lymphosarcoma, a kind of lymphatic cancer, maybe caused by a virus passed to the devils from feral cats. Such a virus might also be passed from devil to devil, triggering cancer in each.

The phenomenon of transmissible tumors isn’t confined to canines, Tasmanian devils, and Syrian hamsters. There have been human cases, too. Forty years ago a team of physicians led by Edward F. Scanlon reported, in the journal Cancer, that they had “decided to transplant small pieces of tumor from a cancer patient into a healthy donor, on a well informed volunteer basis, in the hope of gaining a little better understanding of cancer immunity,” which they thought might help in treating the patient. The patient was a fifty-year-old woman with advanced melanoma; the “donor” was her healthy eighty-year-old mother, who had agreed to receive a bit of the tumor by surgical transplant. One day after the transplant procedure, the daughter died suddenly from a perforated bowel. Scanlon’s report neglects to explain why the experiment wasn’t promptly terminated—why they didn’t dive back in surgically to undo what had been done to the mother. Instead, three weeks were allowed to pass, at which point the mother had developed a tumor indistinguishable from her daughter’s.

Weinberg went on to explain that the process is a little more complicated than classic Darwinian selection. Darwin’s version works by selection among genetic variations that differentiate one organism from another, and in sexually reproducing species those variations are heritable. But evolution in tumor lineages occurs by that sort of selection plus another sort—selection among epigenetic modifications of DNA. Epigenetic means outside the line of genetic inheritance: acquired by experience, by accident, by circumstance. Such secondary chemical changes to the molecule affect behavior, affect shape, and pass from one cell to another but do not, contrary to the analogy, pass from parent to offspring in sexual reproduction. These changes are peeled away in the process of meiosis (the formation of sperm and egg cells for sexual reproduction) but preserved in mitosis (the process of simple cell replication in the body). So cancerous cell reproduction brings such changes forward into the new cells, along with the fundamental genetic changes.

Does that mean tumors don’t evolve? Certainly not. They do. “It’s still Darwin,” Weinberg said. “It’s Darwin revised.”


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