A phrase like “mad cow” is sure to whip up a media frenzy. When the USDA confirmed last week the first case of bovine spongiform encephalopathy (BSE) in six years, news headlines were splashed with reports of “mad cow disease,” the informal and scarily evocative term for BSE. What got lost in these initial reports is that this case of BSE involves a different protein than previous epidemics in Europe, and there’s no evidence that this type is transmissible to humans.
Nature News has a solid and thorough explanation of the science behind this case of BSE, known as L-type. As it happens in nature, mutations arise spontaneously, and L-type BSE is caused by a spontaneous mutation in a particular protein. A lot is still unknown about L-type, but we have never seen it spread through cow populations (or jump to humans) through ingestion. Previous BSE epidemics in Europe were spread by the admittedly gruesome practice of grinding up leftover cow parts and feeding them back to cows, but this has been long banned in the United States because of BSE. Critics have argued that there may still be indirect sources of cow protein in feed (cow protein is fed to chickens, and poultry leftovers go back into cow feed), so more stringent regulations are needed.
It sounds like the start of a science fiction movie: a lethal brain disease that goes airborne. But while scientists have indeed found that the prions responsible for mad cow disease and other neurological ailments can float on the breeze and infect those who inhale, they say there’s no reason to barricade your gas-masked family inside your house.
Prions are misfolded proteins that cause brain degeneration; in mad cow disease they’ve typically been transmitted when one cow eats the infected brain or spinal cord tissue of another (something that agricultural institutions now agree shouldn’t happen in the first place). Other prion diseases, including the human variant Creutzfeldt-Jakob disease, are also passed along through body fluids and tissue. But for a new study published in PLoS Pathogens, researchers decided to find out if airborne prions could serve as infectious agents.
The short answer: Yes.
For years, prions have been known only as a serious danger to animal and human health. These misfolded brain proteins have been linked to fatal diseases–like mad cow disease in cattle and the deadly variant Creutzfeldt-Jakob disease in humans. But apart from causing these diseases, scientists puzzled for years about the exact function of a properly folded prion protein.
A new study in Nature Neuroscience may have some answers. After 20 years of research, an international team of neuroscientists reports that, in mammals, the mysterious proteins help to maintain the myelin sheath that protects the body’s nerves [Nature News]. A healthy sheath is necessary for nerve cells to transmit impulses rapidly.
For evolution to take place, you need DNA or RNA to change through mutation, providing the variations for natural selection to select. Right? Well, it may be more complicated than that. A new study suggests an exception: prions, the infectious protein bits that can cause degenerative brain diseases like mad cow disease. In a paper in Science, researchers document these lifeless structures evolving, despite the fact that they lack any DNA or RNA.
Study leader Charles Weissmann and his team transferred prions from brain cells to other kinds of cells and watched as certain members of the prion population adapted to the new environment and took over, out-competing their brethren. When he transferred the prions back to brain cells, the ones most adapted to brain living got the upper hand and increased in number as they out-competed the prions that had adapted to other cells. Weissmann argues that this shows Darwinian evolution can go even further than we thought: “In viruses, mutation is linked to changes in nucleic acid sequence that leads to resistance. Now, this adaptability has moved one level down- to prions and protein folding – and it’s clear that you do not need nucleic acid (DNA or RNA) for the process of evolution” [BBC News].
Members of a tribe in Papua New Guinea has evolved resistance to a affliction similar to mad cow disease (called Creutzfeldt Jakob disease, or CJD, in people). How did they do it? Cannibalism, according to a study in the New England Journal of Medicine.
The Papua New Guinea variant is called kuru, and it was a disaster there. When members of the Fore people in Papua New Guinea died, others would eat the dead person’s brain during funeral rituals as a mark of respect. Kuru passed on in this way killed at least 2500 Fore in the 20th century until the cause was identified in the late 1950s and the practice halted [New Scientist].
The scientists compared DNA samples of about 3,000 living Fore people, some of whom had participated in the old rituals, to 152 samples of stored DNA from Fore that kuru killed. They looked at the genes for prions, ordinary brain proteins that take on a misfolded shape in prion disease such as CJD and kuru. They found a mutation called G127V that protected people from kuru. Only people who ate brains and survived have it, they found [Reuters].
The discovery excited scientists with the possibility of understanding and even treating other prion diseases, like CJD. And British neurologist John Hardy exemplified the scientific glee at seeing human evolution happen in such a short time. “It’s fantastic demonstration of natural selection… In Papua New Guinea kuru became the major cause of death, so there was a clear survival advantage and the selection pressure was enormous” [BBC News].
Discoblog: For Early Europeans, Cannibalism Was One Perk of Victory
Discoblog: Mad Cow Fears Keep Euro Sperm Out of U.S.
80beats: Female Tarantulas Devour Extra Suitors to Benefit Their Young
Image: D. Carleton Gajdusek