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This week at the AAAS : Mummies' curse - Codebreakers hold the key to ancient diseases

点击量:   时间:2017-09-19 08:02:09

By Charles Seife A BRANCH of mathematics used by code-breakers could help to solve a 500-year-old mystery—whether syphilis was brought to Europe by Christopher Columbus’s crew. Syphilis appeared in Naples soon after Columbus returned from the New World. Scientists have speculated that some of his sailors were infected in America with Treponema pallidum, the spiral-shaped bacterium that causes the disease. But there is little evidence to indicate that T. pallidum did indeed come from the Americas or whether it spread to both Europe and America from some other region, such as Africa. A group of 3000-year-old Chilean mummies held a tantalising clue suggesting that syphilis had been present in the Americas for centuries before Columbus arrived. Some of the bodies had unusual bony growths on their shins and skulls similar to deformities seen in advanced syphilis. Peter Rogan, a geneticist at Allegheny University of the Health Sciences in Pittsburgh, told the AAAS that he tried to retrieve the bacterium’s DNA from the mummies using probes that bind to known sequences from the T. pallidum genome. He failed, perhaps because the organism responsible for the mummies’ disease was not T. pallidum, but a related species. To test this possibility, Rogan turned to information theory to design genetic probes that would hunt down unknown species of Treponema even if their DNA was shattered into pieces after thousands of years of mummification. Information theory is widely used by cryptographers. It provides a mathematical basis for assessing the likelihood of making a correct guess about a given piece of data—for instance whether a coin will come down heads or tails. By comparing genetic sequences from known species of Treponema, Rogan’s team analysed how well they were able to predict which DNA base appeared in each position. For instance, if every known treponeme has an adenine at position 12 on a particular gene, they could predict with confidence that an unknown species would also have an adenine in that position. Between the predictable regions lie sections of DNA that vary between species. The researchers designed their probes to detect small segments of treponeme DNA known to consist of two highly predictable portions with a more variable stretch in between. If fragments of treponeme DNA are present in a body, these probes will fish them out by their ends, bringing the variable section with them. Comparing this section with sequences from known species will either produce a match or show that it is something different. Using this method, Rogan found that the Chilean mummies were indeed infected with a species of Treponema, but not T. pallidum. “It tells us that at least this group of pre-Columbian Americans did not have syphilis,” says Rogan. So the question of whether Europe’s syphilis epidemic came from the Americas is still open. But using the technique on other ancient corpses might reveal more about the global spread of the disease. “It’s very interesting in terms of trying to reconstruct the history of syphilis,” says Charles Merbs,