Last summer a team of researchers in Sweden and Finland got a lot of publicity when they concluded, from data obtained from large Scandinavian twin studies, that inherited factors make a "minor contribution" to most cancers. But they were using the wrong analytic techniques, says genetic epidemiologist Neil Risch of Stanford University. Risch has just done his own analysis, which concludes the opposite: that genes play a strong role in who gets cancer.
Risch looked at the same twin data as the earlier study, headed by Paul Lichtenstein of Sweden's Karolinska Institute and published in The New England Journal of Medicine (ScienceNOW, 12 July 2000). Scientists try to detect genetic influence by comparing cancer rates in pairs of identical twins--who share all the same genes--with occurrence in fraternal twins, who share, on average, 50% of their genes. Lichtenstein found that the environment was more important than genes for almost all kinds of cancers.
But Risch says the methodology Lichtenstein used was not appropriate: One problem with it, for example, is that it doesn't have the power to produce meaningful calculations for rare cancers. So instead, Risch looked at data from both twin and family studies to estimate the probability that a first-degree relative of a cancer victim--that is, a parent, child, or sibling--would develop the same cancer.
He found that in "the great majority of cancers," an immediate family member was about twice as likely as a member of the general population also to develop the cancer. If anything--contrary to Lichtenstein's conclusions--the genetic risk was higher for rarer cancers, Risch reports in the July issue of Cancer Epidemiology, Biomarkers & Prevention. Prostate, colorectal, and breast cancers are usually seen as having the strongest genetic components. But the top three on Risch's list are thyroid, testicular, and multiple myeloma. His exercise means that "we should be looking for susceptibility genes for all cancers," Risch says.
Lichtenstein was on vacation and unavailable for comment. Whether or not Risch's conclusions are more accurate, his paper "helps clarify the underlying assumptions" of the earlier study, says cancer epidemiologist Sholom Wacholder of the National Cancer Institute in Bethesda, Maryland. It is "a reminder of the need to be cautious about interpreting studies that attempt to distinguish genetic and environmental factors."