By David N. Leff
In 1920 and 1921, when neuropsychiatrists Hans Gerhard Creutzfeldt (1885-1964) and Alfons Jakob (1884-1931) described the rare disease that bears their name, discovery of prions lay 60 years into the future. Until then, ¿classical¿ Creutzfeldt-Jakob disease (CJD) was blamed vaguely on a ¿slow virus.¿ Its dementia and weird bodily contortions struck its victims in late middle age, and death intervened inevitably within a year or so.
When neurologist Stanley Prusiner, at the University of California, San Francisco, reported his isolation of prions (proteinaceous infectious particles) in 1982, he described them as proteins devoid of nucleic acid ¿ no DNA, no RNA ¿ and 100,000 times tinier than the smallest virus. After years of skepticism, Prusiner won the Nobel Prize in Physiology or Medicine in 1997.
By 1996, British authorities had slaughtered 4 million head of cattle from the country¿s total stock of 11 million, to curb the spread of mad cow disease ¿ bovine spongiform encephalopathy (BSE). Meanwhile, in 1995, the first three human victims of latter-day Creutzfeldt-Jakob disease had succumbed to the same malady. But it was CJD with a difference from the classical type: Their symptoms arose in months, not years, and in the prime of life ¿ not old age. But death took its time, often arriving after many months or years, not a neat one year. These discordant signs and symptoms led to this new version of CJD being labeled variant Creutzfeldt-Jakob disease ¿ vCJD. The number of human victims has since edged to 100: 97 in Britain, two in France, one in Ireland.
Most of these casualties had eaten British beef, or come in contact with farm animals. Hence the brains of bovine and humans alike are riddled with sponge-like perforations, or vacuoles. And both share the same etiology: invasion by infectious prions.
Classical and variant Creutzfeldt-Jakob victims have the same sudden, horrendous disease onset. Their dementia and disorientation are reminiscent of Huntington¿s or Alzheimer¿s disease. Then the physical symptoms kick in (literally), with myoclonic jerks ¿ shock-like muscle contractions due to their central nervous system lesion. Ataxia comes to mess up muscle activity during voluntary movements of limbs, head or trunk. Dysarthria causes speech defects due to dysfunction of the muscles used for speaking. At very long last, death terminates their unwitting travail.
Benign prions, which are ubiquitous, but of unknown function, bear the designation PrPc. When they turn pathogenic, the infectious particle takes on a sinister superscript to become disease-related PrPSc. This vicious prion protein is extremely stable; in fact, it can be recognized by its resistance to enzyme cleavage, high temperatures (including autoclave sterilization) and organic solvents, including detergents. Once produced or acquired by a suitable target mammal, it can trigger a chain reaction by which normal PrPc protein is converted into the more stable, pathogenic PrPSc. Infection, Inheritance, Chance Sow Prion Ills
Prion diseases are unique in being both infectious and hereditary. Inherited versions arise from transmitted mutations in the prion gene, which lurks on human chromosome 20. No virus, bacterium, parasite or fungus contributes to its infectivity; PrPSc is higher tech than that. It causes disease by misfolding its protein conformation, of which there are half a dozen varieties ¿ and counting. It can even arise spontaneously because of chance misfolding of the prion protein.
¿Since all patients with vCJD studied are infected with the same prion strain [that causing BSE in cattle], the neuropathogenesis is likely to be closely similar between patients,¿ said neurogeneticist John Collinge, director of the British Medical Research Council¿s prion unit in London.
In its current issue, The Lancet, dated July 21, 2001, carries a paper titled: ¿Tissue distribution of protease resistant prion protein in variant Creutzfeldt-Jakob disease using a highly sensitive immunoblotting assay.¿ Collinge is its senior author.
¿We have developed a highly sensitive immunoblot method,¿ he said, ¿for detection of PrPSc in vCJD tissues that can be used to provide an upper limit on PrPSc concentrations in peripheral tissues, including blood . . . to prevent the spread of vCJD.¿
The threat of that spread is no longer from affected cows, but from contaminated surgical scalpels.
With consent of relatives, the paper¿s co-authors obtained a range of vCJD bodily tissues from four patients at post-mortem who had neuropathologically confirmed vCJD, and from control individuals without the disease. Their highly sensitive analysis involved three techniques ¿ protein precipitation with sodium phosphotungstic acid, western blotting, enhanced chemiluminescence. All procedures were carried out in a microbiological containment Level 3 facility.
Their assays detected PrPSc in peripheral tissue homogenates at concentrations 104- to 105-fold lower than those reported in the spongiform brains. Tonsil tissue provided the highest concentration, with PrPSc readily detected as well in the retina and optic nerve of vCJD eyes.
¿Ophthalmic surgical instruments,¿ Collinge cautioned, ¿used in procedures involving optic nerve and the posterior segment of the eye, in particular the retina, might represent a potential risk for iatrogenic transmission of vCJD. Tonsil is the tissue of choice,¿ he added, ¿for diagnostic biopsy and for population screening of surgical tissues to assess prevalence of preclinical vCJD infection within the UK and other populations. Moreover, it is known that neurosurgical instruments can act as a vector for transmission of classical CJD. As yet, however, there has been no demonstration of the detection of PrPSc in peripheral tissues from classical CJD cases.¿
Prime Suspect: The Human Tonsil
¿A distinctive PrPSc type, designated type 4t, is consistently seen in tonsil from patients with vCJD,¿ Collinge observed, ¿and tonsil biopsy can be used for diagnosis of vCJD before death. It has been suggested,¿ he continued, ¿that large-scale screening of surgical tonsillectomy and appendectomy tissues for PrPSc could provide early warning of a high degree of vCJD prion infection. Several such studies are in progress, and might give early indication of a large epidemic of preclinical vCJD.¿
Collinge continued: ¿That substantial numbers in the population may be incubating vCJD . . . raises concerns that iatrogenic transmission of vCJD could be a major issue. This finding provides support for the decision by the UK Department of Health to recommend use of disposable instrument kits for tonsillectomy as a first step in risk-reduction strategies to limit any secondary transmission of vCJD.¿