Gene may signal ancient prion-disease outbreaks - Cannibalism's DNA Trail

Science News, April 12, 2003 by B. Bower

Cannibalism among prehistoric humans may have left lasting genetic marks, a team of scientists contends. Their controversial argument hinges on a link between specific DNA mutations and a disease that afflicted South Pacific villagers who practiced cannibalism as late as 1950.

Gene variations that protect against prion diseases--deadly neurological illnesses caused by proteins known as prions (SN: 10/11/97 p. 229)--frequently occur in modern human populations, say John Collinge of University College London and his colleagues. These variations occur in the gene that encodes a protein that's usually harmless but causes prion disease when it's in a mutant form. The protective variations originated around 500,000 years ago and spread via natural selection because they protect against epidemics of prion diseases, Collinge's team contends in an upcoming issue of Science.

Prion diseases include Creutzfeldt-Jakob disease in people and mad cow disease. These maladies unfold when prions, presumably from meat consumption, induce their harmless versions to assume the prions' warped shapes and then to clump together. This process leads to brain degeneration and death.

The potential link of protective genes to cannibalism stems from studies of a prion ailment known as kuru, which Fore villagers in Papua New Guinea contracted from 1920 to 1950 as a result of eating human brains and other tissue during funeral rituals. Enough prehistoric cannibalism occurred to have instigated kurulike epidemics in ancient human populations (SN: 9/9/00, p. 164), Collinge asserts. "Selection for [the protective genes] has been widespread and began very early in the evolution of modern humans," Collinge says.

A previous study led by Collinge determined that people with one regular copy and one mutated copy of what he calls "the prion-protein gene" were protected against Creutzfeldt-Jakob disease. The scientists did not find anyone with two mutated copies; those with two regular copies were not protected. The mutation consists of a single amino acid substitution at a specific location. Other researchers studying Japanese populations found that a different mutation on the same gene had a comparably protective effect.

Collinge's team studied DNA sections containing the critical gene obtained from more than 2,000 people worldwide. All the groups examined contained a substantial proportion of people with a single copy of one of the two protective mutations. People in Japan and Taiwan exhibited the lowest population rate of the variations, 6 to 8 percent. Fore villagers had the highest rate--55 percent.

These genetic patterns indicate that "balancing selection" took place, Collinge proposes. In this rare phenomenon, also seen with malaria, individuals with different versions of the same gene receive more protection against infectious diseases than those with two copies of the common version do.

A prion disease acquired in prehistoric populations through cannibalism represents the most likely explanation of the high prevalence of people with one mutated and one regular prion protein gene, in Collinge's view.

Although the new data are suggestive of early cannibalism and kurulike diseases, they're also consistent with the presence of prion diseases that result from eating meat of nonhuman animals, comments anthropologist Henry Harpending of the University of Utah in Salt Lake City.