Thursday, April 28, 2011

Curse of the Pharaoh's DNA

Mummies found in King Tutankhamun's tomb are at the centre of a dispute over DNA analysis.

Jo Marchant  - Some researchers claim to have analysed DNA from Egyptian mummies. Others say that's impossible. Could new sequencing methods bridge the divide?

Cameras roll as ancient-DNA experts Carsten Pusch and Albert Zink scrutinize a row of coloured peaks on their computer screen. There is a dramatic pause. "My god!" whispers Pusch, the words muffled by his surgical mask. Then the two hug and shake hands, accompanied by the laughter and applause of their Egyptian colleagues. They have every right to be pleased with themselves. After months of painstaking work, they have finally completed their analysis of 3,300-year-old DNA from the mummy of King Tutankhamun.

Featured in the Discovery Channel documentary King Tut Unwrapped last year and published in the Journal of the American Medical Association (JAMA)1, their analysis — of Tutankhamun and ten of his relatives — was the latest in a string of studies reporting the analysis of DNA from ancient Egyptian mummies. Apparently revealing the mummies' family relationships as well as their afflictions, such as tuberculosis and malaria, the work seems to be providing unprecedented insight into the lives and health of ancient Egyptians and is ushering in a new era of 'molecular Egyptology'. Except that half of the researchers in the field challenge every word of it.

Enter the world of ancient Egyptian DNA and you are asked to choose between two alternate realities: one in which DNA analysis is routine, and the other in which it is impossible. "The ancient-DNA field is split absolutely in half," says Tom Gilbert, who heads two research groups at the Center for GeoGenetics in Copenhagen, one of the world's foremost ancient-DNA labs.

Unable to resolve their differences, the two sides publish in different journals, attend different conferences and refer to each other as 'believers' and 'sceptics' — when, that is, they're not simply ignoring each other. The Tutankhamun study reignited long-standing tensions between the two camps, with sceptics claiming that in this study, as in most others, the results can be explained by contamination. Next-generation sequencing techniques, however, may soon be able to resolve the split once and for all by making it easier to sequence ancient, degraded DNA. But for now, Zink says, "It's like a religious thing. If our papers are reviewed by one of the other groups, you get revisions like 'I don't believe it's possible'. It's hard to argue with that." ...

The disagreement stems from the dawn of ancient-DNA research. In the 1980s, a young PhD student called Svante Pääbo worked behind his supervisor's back at the University of Uppsala in Sweden to claim he had done what no one else had thought was possible: clone nuclear DNA from a 2,400-year-old Egyptian mummy2. Soon researchers realized that they could use a new technique called polymerase chain reaction (PCR) to amplify tiny amounts of DNA from ancient samples. There was a burst of excitement as DNA was reported from a range of ancient sources, including insects preserved in amber and even an 80 million-year-old dinosaur3.

Then came the fall. It turned out that PCR, susceptible to contamination at the best of times, is particularly risky when working with tiny amounts of old, broken-up DNA. Just a trace of modern DNA — say from an archaeologist who had handled a sample — could scupper a result. The 'dinosaur' DNA belonged to a modern human, as did Pääbo's pioneering clone. Once researchers began to adopt rigorous precautions4, including replicating results in independent labs, attempts to retrieve DNA from Egyptian mummies met with little success5.

That's no surprise, say sceptics. DNA breaks up over time, at a rate that increases with temperature. After thousands of years in Egypt's hot climate, they say, mummies are extremely unlikely to contain DNA fragments large enough to be amplified by PCR. "Preservation in most Egyptian mummies is clearly bad," says Pääbo, now at the Max Planck Institute for Evolutionary Anthroplogy in Leipzig and a leader in the field. Ancient-DNA researcher Franco Rollo of the University of Camerino in Italy went so far as to test how long mummy DNA might survive. He checked a series of papyrus fragments of various ages, preserved in the similar conditions to the mummies. He estimated that DNA fragments large enough to be identified by PCR — around 90 base pairs long — would have vanished after only around 600 years6.

Yet all the while, rival researchers have published a steady stream of papers on DNA extracted from Egyptian mummies up to 5,000 years old. ...
via Ancient DNA: Curse of the Pharaoh's DNA : Nature News.

1 comment:

offshore company said...

It was very important to ancient Egyptian religious beliefs that the human body was preserved. A method of artificial preservation called mummification was developed by the ancient Egyptians.