Two researchers at Johns Hopkins this week reported their development of a statistical model for cancerous cell mutations. In the Jan. 2, 2015 issue of the journal Science, biomathmatician Cristian Tomasetti and oncologist Bert Vogelstein reported that “two-thirds of cancer incidence across tissues is explained by random DNA mutations that occur when stem cells divide.” The study authors further said, “These results suggest that only a third of the variation in cancer risk among tissues is attributable to environmental factors or inherited predispositions.” Their abstract even mentions “bad luck” as the culprit accounting for two-thirds of cancer risk.
The authors of this retrospective study looked at earlier studies which documented the rate of stem cell division in 31 types of tissue, and then plotted those numbers against average lifetime incidences of various cancers. They determined the correlation between stem cell divisions and cancer risk was 0.804. The closer this value is to one, the more correlation exists.
The authors even went so far as to classify which cancers they believed were due to “bad luck” and which were the result of “bad luck plus environmental and inherited factors.” However, copy errors during cell division have been proven to be affected by many environmental and hereditary factors, not the least of which are cigarette smoking, pesticide exposure, oncoviruses, gene expression, radiation, and diet. All cancers, even those identified as being caused by “bad luck” in the study, have correlations with other causes. Vogelstein concedes, “All cancers are caused by a combination of bad luck, the environment and heredity, and we’ve created a model that may help quantify how much of these three factors contribute to cancer development.”
It remains to be seen what conclusions other researchers will draw from this study, and whether the bold statements about the limited effects of environmental issues stand up to scrutiny.
UPDATE Jan 10, 2015: A few days after the study in Science appeared, the study authors have clarified widely-quoted statements about the simplistic “bad luck” reported involved in two-thirds of cancer cases. The Economist wrote, “None of this, though, is reason for fatalism. Copying errors during cell division are by no means the only source of cancer-causing mutations. Chemicals that damage DNA, ultraviolet light, ionising radiation and viral infections are all culprits too—and culprits that can often be avoided by thoughtful behavior.” Study author Cristian Tomasetti further clarified, “We have not showed that two-thirds of cancer cases are about bad luck. Cancer is in general a combination of bad luck, bad environment and bad inherited genes.”