BSE: the role of the 'infectious chemical'
Prions have been in the news constantly this year, thanks to the series of 'mad cow' scares. If bovine spongiform encephalopathy (BSE) really can transfer to humans as Creutzfeldt-Jakob Disease (CJD), it is these shadowy agents that are probably to blame.
Prions are apparently unique in being infectious agents yet without having any of the genetic mechanisms of true organisms. They are, if you like, infectious chemicals. Almost three decades ago, radiobiologist Tikvah Alper at Hammersmith Hospital realised that the agents responsible for transmissible spongiform encephalopathies (TSEs) - scrapie in sheep, CJD and related diseases in humans and, later, BSE in cattle - contained no nucleic acids (Nature, 1967, 214, 764). The infectious agents were later dubbed 'prions' (proteinaceous infectious particles) by Stanley Prusiner of the University of California at San Francisco (UCSF) (Science, 1982, 216, 136). Even today the prion hypothesis is not totally proven, and their mode of action is only dimly understood.
Prions seem to work by subverting the body's own chemistry. The prion's target is a glycoprotein, known as PrPC, which is found on the surface of nerve cells. The prion contains an altered form of this protein, known as PrPSC. This has an identical amino acid sequence but is folded into a different conformation. According to molecular modelling calculations by Fred Cohen, also at UCSF, the structure of PrPC contains long a helices, while in PrPSc the same parts of the sequence are folded into b-pleated sheets. By all accounts, when a prion encounters PrPC, it changes its conformation to PrPSc and thus, apparently, turns the normal protein into an infectious toxic agent itself - rather like the mode of replication attributed to the werewolves of legend.
The normal function of PrPCis not known for certain, though it may play a role in regulating sleep (Nature, 1996, 380, 639). This is an interesting observation, because one of the rare inherited human TSEs is Fatal Familial Insomnia. Strains of mice that do not make PrPCcan be bred and these mice are resistant to the toxic effects of PrPSC. The main ill-effect of the lack of PrPC seems to be slower transfer of chemical messengers between nerve cells (Nature, 1994, 370, 297) - leading to progressive loss of coordination as they get older (Nature, 1996, 380, 528).
The protein-altering process proposed by Prusiner may be an autocatalytic chain reaction, capable of overcoming an energy barrier to the less favourable conformation (Nature, 1991, 349, 569), though a 'chaperonin' - a protein that helps other proteins fold - may also be involved. The malfolding process may need the combined effects of an aggregate of PrPSCmolecules. In some scientists' minds the chain reaction may even be a polymerisation. However, despite the growing evidence for 'flipping proteins', including some analogous results from yeast (Science, 1995, 270, 93), a number of researchers have still not ruled out the involvement of a conventional virus hiding behind this bizarre process.
Prion diseases are characterised by the brain turning spongy as nerve cells die off. PrPSC, unlike PrPC, forms insoluble fibrils and tangles in brain cells - the analogy with Alzheimer's disease has not gone unnoticed by brain researchers.
The PrP protein has a backbone of 230 amino acids. Sheep and bovine PrPs differ at only seven amino acid positions, whereas bovine and human PrPs differ at more than 30 positions, which should be enough to confer human immunity to BSE. But the critical region of PrP may be only a small part of the whole. Recent research (Nature, 1996, 380, 345) indicates that a 20-amino acid segment (PrP106-126) from PrPSCin mice is toxic to their nerve cells, but only in the presence of the connective tissues of the central nervous system known as microglia. Microglia respond to PrP106-126 by increasing their production of oxygen radicals. The combined effect of the toxic chemical and oxygen radicals is enough to kill the nerve cell.
The rare human prion disease, CJD, is genetically caused: the PrPCgene apparently produces PrPSCby mistake, due to a mutation at amino acid 129. However, 17 cases of infectious CJD are reported amongst the 2000 or so British patients who received human growth hormone (HGH) extracted from pituitary glands collected from mortuaries, where some donors had apparently carried undiagnosed CJD. This threat led to natural HGH being phased out in favour of a synthetic form made by biotechnological processes (Chem. Br., August 1985, p 703). Now that the threat has turned to reality, families of the victims have started legal action against the Department of Health and the Medical Research Council.
But as well as inheriting or developing the genetic trait for CJD, or accidentally injecting the CJD prion along with HGH, can humans catch CJD from eating meat? The scientific jury is still out on this question, despite the reports of cases of 'anomalous' CJD in younger people. The question revolves around firstly the ability of prions to cross the species barrier and secondly whether they can infect by ingestion.
Scrapie does not appear to transfer to humans eating lamb, but a human prion disease, kuru, used to be endemic amongst New Guinea cannibals who ritually ate human brains. Scrapie has apparently leapt across the species gap to cattle, thanks to the now notorious practice of including animal matter in cattle feed (indeed, reports now suggest that BSE has found its way back into sheep). In principle there seems no reason why scrapie/BSE could not jump the gap to humans.
Prions are remarkably robust molecules. They are resistant to many chemicals, radiation or heat that would inactivate most proteins. Alper first differentiated them from nucleic acids because she found that they were resistant to radiation. PrPSCis differentiated from normal PrPCby its resistance to protease enzymes.
Prior to the 1980s, when animal carcasses were rendered down for tallow, bonemeal and animal feed, the process involved organic solvents and high temperatures. Coincidentally, this process seems to have killed prions. However, the value of tallow fell, the costs of solvents and fuel rose and there were fears about workers exposure to solvents. New processes avoiding the use of solvents and high temperatures came in - and it appears that active scrapie prions from sheep carcasses got into animal feed for cattle. The outcome could have been predicted - as far back as the 1940s there had been a major outbreak of scrapie when sheep offal was fed back to sheep, when it was discovered that formalin was unable to inactivate the scrapie agent. These newer processes were banned in 1988-89 after BSE appeared in British cattle.
There is reasonable confidence that gelatin used in food and pharmaceuticals will have escaped contamination with BSE because the prions are not found in bones or hide, which are the raw material for gelatin. Furthermore, the processing involves treatment by aggressive acids or bases and high temperatures.
The urgent need now is for reliable diagnostic tests for TSEs in farm animals (and humans) and for prion contamination of meat products. To be any use, a prion probe must detect the shape, and not just the sequence of the protein. Are the slight differences between prions enough to explain why humans have never caught scrapie, but may have caught a mutated variant via cows? Better knowledge of prion structure might make it possible to design drugs that bind to PrPSC(but not normal PrPC) and inactivate or destroy it. Meanwhile, the ultimate test for the prion theory will be to make a synthetic prion in a totally nucleic acid-free environment and demonstrate that it is infectious on its own. That difficult experiment has not yet been successfully performed.
Source: Chemistry in Britain