Tuesday, May 19, 1998 Published at 19:17 GMT 20:17 UK
The prion: simply mad
The prion theory is not supported by all scientists
Scientists continue to argue about the cause of BSE and its human equivalent, CJD, but the culprit would seem to be the prion - short for proteinaceous infectious particle.
The prion is a misfit because it ignores the rules that are supposed to govern biological reproduction.
Unlike other agents that cause disease, like a virus, it contains none of the genetic information necessary to spread itself through its victim. Instead, the prion corrupts a perfectly normal protein, PrP, which usually sits on the external surface of brain cells.
It appears that the prion is able to alter the shape of the normal protein to produce a mirror image of itself - it recruits rather than creates. The result is insoluble deposits building up within cells, which then no longer function properly and die.
But the evidence that the prion carries no genetic information is strong. A simple test is to bombard it with wavelengths of ultra violet radation known to disrupt nucleic acids (DNA and RNA carry the information living organisms need to reproduce themselves). Do this and the prion is unaffected; it still seems to cause disease if injected into mice.
Prusiner's work received sufficient support for him to be awarded the Nobel Prize for medicine in 1997. But there are still many dissenting voices that are convinced a virus is responsible for corrupting the PrP protein. These scientists continue to pursue their own research.
Another more controversial theory to receive wide publicity is the one that tries to link BSE to the insecticides used on cattle to treat pests like warble fly. The chemicals are rubbed into the backs of the animals and can, under some circumstances, disrupt the nervous system.
Mark Purdey, a Somerset farmer, has invested a large sum of his own money to prove the theory. So far, all epidemiological studies have failed to find a link between the use of the chemicals and BSE-infected herds.
One of the great mysteries centres on the role of the normal protein, PrP. At first, it seemed the protein had no function at all. Mice engineered to lack the gene that codes for PrP appeared to show no ill effects.
This raised the possibility that if a way could be found to block the gene in humans, it might be possible to cure other spongiform diseases such as CJD and Gertmann-Staussler syndrome.
Sadly, the early work may have been misleading. More recent research indicates that PrP deficiency adversely affects the nervous system and disrupts sleep patterns.