Prion Diseases (Transmissible Spongiform Encephalopathies)

Prion diseases are rare, uniformly fatal and currently untreatable neurodegenerative disorders. They are caused not by a virus or any other conventional microbe but by a normal host protein that has misfolded into an abnormal, self-propagating form. They matter to the clinical virologist out of proportion to their rarity for three reasons: they enter the differential diagnosis of any rapidly progressive dementia; they are transmissible, both between people through contaminated instruments and tissues and, in the case of variant Creutzfeldt-Jakob disease, from cattle through the food chain; and the agent resists the heat, formalin and radiation that inactivate conventional pathogens, which creates real laboratory and surgical biosafety problems.

Historical background

Scrapie, a fatal disease of sheep and goats described in 1750 and shown to be transmissible in the 1930s, is the prototype. The human story began with kuru, an epidemic of progressive ataxia and dementia among the Fore people of Papua New Guinea, spread by mortuary endocannibalism; Carleton Gajdusek transmitted it to primates and received the Nobel Prize in 1976. Stanley Prusiner then coined the term prion, from proteinaceous infectious particle, and showed that the agent was an aggregated protein containing no nucleic acid, work recognised by the Nobel Prize in 1997. In 1986 a new transmissible spongiform encephalopathy, bovine spongiform encephalopathy or mad cow disease, appeared in British cattle and was spread through meat-and-bone-meal feed. From 1995 a new human disease, variant Creutzfeldt-Jakob disease, emerged from dietary exposure to bovine spongiform encephalopathy and has caused more than two hundred deaths, most of them in the United Kingdom.

The prion protein and how prions replicate

The cellular prion protein (PrP-C) is a normal cell-surface glycoprotein, encoded by the prion gene PRNP on chromosome 20, expressed by most cells and most abundantly by neurons, and predominantly alpha-helical in structure. Disease arises when PrP-C refolds into a beta-sheet-rich, aggregation-prone and protease-resistant form, the scrapie-type protein (PrP-Sc). The misfolded protein acts as a template, recruiting and converting further molecules of normal PrP-C in a self-amplifying chain. The protein is therefore both the product and the infectious agent, which is the protein-only hypothesis.

Several properties follow that are clinically important. Infectivity titres in the brain and spinal cord are extremely high; the agent is not inactivated by formalin, by ionising or ultraviolet radiation, or by nucleases, consistent with its lack of nucleic acid; and it provokes no immune response and no inflammation, so the usual markers of infection are absent.

Different conformations of PrP-Sc behave as distinct strains: stably transmissible phenotypes, each with its own incubation period and neuropathological lesion profile, that persist even among patients with an identical PRNP sequence. Because the agent carries no genome, this strain information must reside in the shape of the protein itself, encoded by differences in folding, in the ratio of the glycosylated bands, and in the size of the protease-resistant core fragment. Serial transmission into inbred mice of identical PRNP genotype reproduces each strain’s incubation time and lesion pattern, showing the phenotype to be a property of the agent rather than the host. Strain identity also governs the species barrier: the bovine spongiform encephalopathy agent and variant Creutzfeldt-Jakob disease share the same strain signature, which established the link between them. Western blot typing of the glycoform pattern, combined with the codon 129 genotype, defines the molecular subtypes of sporadic Creutzfeldt-Jakob disease.

A common polymorphism at PRNP codon 129, encoding either methionine or valine, strongly modifies both susceptibility and phenotype. Homozygosity, methionine-methionine or valine-valine, predisposes to disease, and all but one of the variant Creutzfeldt-Jakob disease cases identified to date have occurred in methionine homozygotes.

The forms of human prion disease

Human prion disease takes three forms. Sporadic disease, almost all of it sporadic Creutzfeldt-Jakob disease, accounts for about eighty-five per cent of cases and is thought to arise from spontaneous misfolding or a somatic PRNP mutation. Genetic disease accounts for about fifteen per cent and follows autosomal dominant, highly penetrant mutations in PRNP; it is divided by its clinical picture into familial Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease and fatal familial insomnia, the commonest mutation worldwide being E200K. Acquired disease is now rare and comprises kuru, iatrogenic Creutzfeldt-Jakob disease, and variant Creutzfeldt-Jakob disease. Overall incidence is roughly one to one and a half cases per million people per year, fairly constant between countries.

The human prion diseases

Disease	Cause and relative frequency	Typical onset	Hallmark features	Median duration
Sporadic Creutzfeldt-Jakob disease	Sporadic; about 85 per cent of cases (most common)	About 65 years	Rapidly progressive dementia, myoclonus, ataxia	About 6 months
Familial Creutzfeldt-Jakob disease	Inherited PRNP mutation; commonest genetic form (genetic forms about 15 per cent together)	40 to 60 years	Dementia and ataxia, course varies with mutation	Months to years
Gerstmann-Sträussler-Scheinker disease	Inherited PRNP mutation; rare	40 to 60 years	Slowly progressive ataxia, later dementia	3 to 8 years
Fatal familial insomnia	Inherited PRNP mutation (D178N); rare	About 50 years	Intractable insomnia, dysautonomia, later motor and cognitive decline	About 18 months
Iatrogenic Creutzfeldt-Jakob disease	Contaminated grafts, instruments or pituitary hormone; rare, declining	All ages	Cerebellar ataxia, then dementia	About 12 months
Variant Creutzfeldt-Jakob disease	Dietary exposure to bovine spongiform encephalopathy; rare (about 230 cases, epidemic over)	Late twenties	Psychiatric onset, then ataxia, painful sensory symptoms, dementia	About 14 months
Kuru	Mortuary endocannibalism; historical, near-extinct	All ages	Cerebellar ataxia and tremor, later behavioural change	About 12 months

Clinical features

Sporadic Creutzfeldt-Jakob disease is the prototype and the one a clinician is most likely to meet. It presents in the seventh decade as a rapidly progressive dementia, with cognitive decline accompanied by ataxia, extrapyramidal and pyramidal motor signs, and visual disturbance; myoclonus appears as the disease advances, and the terminal stage is one of akinetic mutism. Progression is relentless and rapid, with a mean survival of about six months. This tempo, a dementia that worsens over weeks rather than years, is the single most useful clinical clue.

The other forms are distinguished by their demographics and their leading symptom. Variant Creutzfeldt-Jakob disease occurs in much younger patients, with a mean onset in the late twenties, and begins as a psychiatric illness before progressing months later to ataxia, involuntary movements and persistent painful sensory symptoms. Fatal familial insomnia opens with intractable insomnia and autonomic instability. Gerstmann-Sträussler-Scheinker disease runs a slower course dominated by progressive cerebellar ataxia. Kuru, now all but extinct, presented with cerebellar ataxia and tremor. Whatever the form, almost all patients become bedbound and die, most often of aspiration pneumonia.

Diagnosis

The diagnosis is built from the clinical picture supported by imaging, the electroencephalogram and cerebrospinal fluid testing, with definitive confirmation still requiring neuropathology. Magnetic resonance imaging is the most useful single investigation: diffusion-weighted and fluid-attenuated sequences show cortical ribboning and high signal in the deep grey nuclei in sporadic disease, and a hyperintense posterior thalamus, the pulvinar sign, in variant disease. The electroencephalogram may show periodic sharp-wave complexes in sporadic Creutzfeldt-Jakob disease, though usually only late.

Cerebrospinal fluid is acellular with a normal or mildly raised protein; the older markers of neuronal injury, the 14-3-3 protein, total tau and neuron- specific enolase, are supportive but neither sensitive nor specific. The important advance is the real-time quaking-induced conversion assay, which amplifies and detects PrP-Sc directly in cerebrospinal fluid with a specificity around ninety-eight per cent, giving a positive test real diagnostic weight. Genetic testing of PRNP, after counselling, identifies the inherited forms and the codon 129 genotype. Where tissue is examined, spongiform change, neuronal loss and astrogliosis with immunostaining for PrP-Sc confirm the disease, and Western blotting distinguishes the molecular subtypes.

The differential diagnosis is that of a rapidly progressive dementia. It matters because several of the alternatives are treatable: the autoimmune and paraneoplastic limbic encephalitides above all, alongside other neurodegenerative dementias, central nervous system infection, vascular disease and tumours.

Prevention and laboratory biosafety

Because prions resist routine sterilisation, the handling of potentially contaminated material is a real laboratory and surgical concern. Prion infectivity is highest in the brain, spinal cord and eye and is also present in lymphoid tissue. Standard methods, including formalin fixation, alcohol and ordinary autoclave cycles, do not reliably inactivate it, so protein-denaturing methods and disposable instruments are used instead.

Decontamination of prion-contaminated material

Does not inactivate prions	Effective against prions
Formalin fixation; alcohol; ionising and ultraviolet radiation; nucleases; standard autoclave cycles	1 N sodium hydroxide for 1 hour followed by autoclaving; sodium hypochlorite (2 per cent); porous-load autoclaving at 134 °C for at least 30 minutes; concentrated formic acid for already-fixed tissue; single-use instruments, then incineration

Iatrogenic transmission, historically from human pituitary growth hormone, dura mater grafts, corneal grafts and neurosurgical instruments, is prevented by using single-use equipment for high-risk procedures and incinerating contaminated instruments. The risk of transmission through blood is the basis for the leucoreduction of donated blood and for donor-deferral policies, since variant Creutzfeldt-Jakob disease, unlike the sporadic form, is lymphotropic and has been transmitted by transfusion. Control of bovine spongiform encephalopathy through bans on feeding ruminant-derived protein to ruminants removed the source of the variant disease.

Treatment

There is no disease-modifying treatment for any prion disease. Numerous candidate compounds have reached trials without proven benefit, and care is therefore supportive and symptomatic: agents for myoclonus, and treatment of the depression, agitation and psychiatric symptoms that arise during the illness.

South African context

Prion disease in South Africa, as elsewhere, is overwhelmingly sporadic Creutzfeldt-Jakob disease, which belongs in the differential of any patient with a rapidly progressive dementia. The practical local task is twofold: to recognise that picture and investigate it with magnetic resonance imaging and the appropriate cerebrospinal fluid tests, and to handle suspected cases with the biosafety precautions above, since neurosurgical instruments and post-mortem tissue from an unrecognised case cannot be made safe by routine sterilisation. Transfusion services apply leucoreduction and donor-deferral measures aligned with international practice.

References and recommended reading

• Sigurdson CJ, Kim M-O, Geschwind MD. Prion Diseases. In: Richman DD, Whitley RJ, Hayden FG, editors. Clinical Virology, 4th edition, Chapter 59. ASM Press; 2017. The principal source for the biology of the prion protein and its conversion, the classification and epidemiology of human prion disease, the clinical features and diagnostic tests including real-time quaking-induced conversion, and the principles of decontamination and prevention.
• Burrell CJ, Howard CR, Murphy FA. Prions. In: Fenner and White’s Medical Virology, 5th edition, Chapter 38. Academic Press / Elsevier; 2017. The source for the terminology of the transmissible spongiform encephalopathies, the spectrum of human and animal prion diseases, and the codon 129 polymorphism.