Friday, December 10, 2010

Cannibals survived brain disease epidemic thanks to a mutation

Survivors of the deadly epidemic of prion disease kuru, which was transmitted through cannibalistic rituals, share a particular genetic mutation. Researchers call it a striking example of human evolution in action.

Kuru was a slow-developing epidemic among the Fore tribe in Papua New Guinea, which devastated the population in 19th and first half of the 20th Centuries. Victims suffered from progressively more severe neurological conditions as the disease developed, and it was dubbed “laughing sickness” by journalists for allegedly causing outbursts of unmotivated laughter.

Kuru was revealed to be transmitted through cannibalistic feasts practiced by the tribesmen upon their relatives who died from the disease. When such rituals were forbidden in the late 1950s, the epidemic stopped.

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It was not until 1982 when its true cause was discovered. A special kind of proteins called prions, which are normally present in human brain, can get misfolded into a highly-stable pathogenic form. Moreover, they can cause normal proteins to misfold too, spreading the disease.

This leads to the brain transforming into a sponge-like mass and ultimately kills the victim. So far, there is no cure for diseases caused by prions, which include the rare Creutzfeldt-Jakob disease in humans and the notorious BSE, better known as Mad Cow Disease.

A group of researchers at the British prion research centre at University College London have discovered that the surviving Fore share a common mutation in the PRNP, the gene expressing prions.

The team, led by Simon Mead, performed genetic and selected genealogic assessment of more than 3,000 persons from the area affected by kuru, including more than 700 cannibalistic rites participants, 152 of whom subsequently died of the disease.

In 51 survivors of the epidemic and their descendants, they’ve discovered a previously-unknown variant of a section of PRNP. None of the victims had the mutation. Moreover, the bloodlines of the mutation carriers suffered from kuru about six times less than bloodlines of those without it.

In their work, published in The New England Journal of Medicine, Mead and colleagues say that the mutation is an acquired prion disease resistance factor, which underwent positive selection during the decades of epidemic.

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