Humans and cows linked by genes

A genetic similarity between cows and humans may mean that we can catch mad cow disease by eating infected beef, yet be at no risk of the equivalent disease, scrapie, from sheep, according to new research.

A team of British scientists in Oxford has discovered a surprising relation in one aspect of a particular gene in cows and hominoids (such as humans, gorillas and chimpanzees). Called the PrP gene, it makes the "prion" protein, which is known to become misshapen and then builds up in the brains and spinal cords of cows with bovine spongiform encephalopathy (BSE), or in humans with Creutzfeldt-Jakob Disease (CJD). Both diseases cause loss of coordination, have no known treatment, and are always fatal.

But the genetic similarity in the PrP gene does not occur between hominoids and sheep, according to the scientists, from the University of Oxford's department of zoology and the National Environment Research Council's Institute of Virology. This difference may explain why humans have never developed CJD from eating sheep infected with scrapie, the equivalent of BSE or CJD, although it has existed for 200 years.

But the similarity between cows and humans might also explain the cause of 10 recent cases of CJD in young adults. The suggestion by expert advisers that the cases might have been caused by exposure to infected beef products sparked off the BSE crisis a month ago.

The scientists analysed the PrP genes by looking at their "sequence". Every protein is made up of long chains of shorter molecules, called amino acids. The gene determines the content and sequence of amino acids in the eventual protein.

At two points in the PrP protein sequence, cows and humans have substituted particular amino acids for others found in other mammals. The probability of this being a chance event is less than 0.5 per cent, according to Mark Pagel, who led the project.

"It doesn't mean that cows and humans have a common ancestor," said Dr Pagel yesterday. "When we reconstruct the likely evolutionary events, the only explanation is that humans and cows have independently evolved these substitutions. There must be a positive reason for it - which means this substitution gave our human ancestors, and cows, an advantage in natural selection. That would explain how it has spread throughout the species."

He does not know what the advantage might be, though the team suggests in the science journal Nature that it could in the past have given some resistance against "prion diseases" such as CJD and BSE by making it harder to catch prion diseases from other species.

Equally, the substitutions could have had some positive effect on the function of PrP, which recent research suggests helps keep vital nerve cells alive.