Prion Diseases (Transmissible Spongiform Encephalopathies) — Questions

A complete answer defines what a prion strain is, explains how distinct strains can exist in an agent that carries no genome, and notes the supporting evidence and clinical relevance.

Definition. A prion strain is a distinct, stably transmissible prion phenotype, recognised by a reproducible combination of incubation period, clinical signs and neuropathological lesion profile (the distribution and severity of spongiform change). Strains behave this way despite an identical prion protein (PrP) amino acid sequence in the host, which is the central puzzle they pose.

Structural basis. Because the agent has no nucleic acid, strain information is held in the conformation of the misfolded protein, the scrapie-type prion protein (PrP-Sc), itself. Strains differ in their folding, in the ratio of the di-, mono- and un-glycosylated bands, and in the size of the protease-resistant core fragment. These are read out by Western blot glycoform and fragment typing (for example type 1 versus type 2), which, combined with the codon 129 genotype, defines the molecular subtypes of sporadic Creutzfeldt-Jakob disease (such as MM1 and VV2).

Evidence. Serial transmission into inbred mice of identical prion gene (PRNP) genotype reproduces each strain’s incubation time and lesion profile, showing the phenotype is a property of the agent and not the host. Strain also governs the species barrier: the bovine spongiform encephalopathy agent and variant Creutzfeldt-Jakob disease share the same strain signature, which is how the cattle-to-human link was established.

Clinical relevance. Strain, together with the codon 129 polymorphism, predicts the incubation period, clinical picture and lesion distribution, and underlies the molecular classification of human prion disease.

The cellular prion protein is predominantly alpha-helical. Disease arises when it refolds into a beta-sheet-rich, aggregation-prone and protease-resistant form, which then templates the conversion of further normal protein.

The defining change is in secondary structure, not the amino acid sequence, so the glycosylation, membrane-anchoring and dimerisation options all miss the point.

The differential of a rapidly progressive dementia spans other neurodegenerative dementias, infection, vascular disease and tumours, but the priority is to identify the autoimmune and paraneoplastic limbic encephalitides, because they are treatable and must not be mistaken for an untreatable prion disease.

The other options are themselves untreatable neurodegenerative conditions.

A common PRNP polymorphism at codon 129 encodes either methionine or valine, and homozygosity predisposes to disease. All but one variant Creutzfeldt-Jakob disease case identified to date has occurred in methionine homozygotes, which is why the codon 129 genotype is determined in suspected cases.

E200K and octapeptide insertions cause genetic rather than variant disease, and heterozygosity is relatively protective.

On diffusion-weighted magnetic resonance imaging, sporadic disease shows cortical ribboning and high signal in the deep grey nuclei, whereas variant Creutzfeldt-Jakob disease shows a hyperintense posterior thalamus, the pulvinar sign.

This sign is uncommon in the other prion diseases listed.

Human prion disease is sporadic in about 85% of cases (almost all sporadic Creutzfeldt-Jakob disease), genetic in about 15% (autosomal dominant PRNP mutations), and acquired in a small and declining remainder (kuru, iatrogenic and variant disease).

Overall incidence is roughly one to one and a half cases per million people per year.

Sporadic Creutzfeldt-Jakob disease is a rapidly progressive dementia: cognitive decline over weeks to months, far faster than Alzheimer’s disease, with ataxia and, as it advances, myoclonus, ending in akinetic mutism. This tempo is the single most useful clinical clue.

The other options point to dementia with Lewy bodies, a parkinsonian disorder, normal-pressure hydrocephalus, and a relapsing demyelinating disease.

Prions are not viruses or any conventional microbe but a misfolded, self-propagating form of the cellular prion protein. The agent contains no nucleic acid, which is why it resists nucleases and radiation and provokes no immune response, the protein-only hypothesis.

The distractors describe a virus, a bacterial variant and an autoantibody, none of which fit an agent that carries no genome and triggers no inflammation.

Variant Creutzfeldt-Jakob disease is lymphotropic, with prion protein in tonsil and appendix as well as brain, and has been transmitted by blood transfusion; this underlies the leucoreduction and donor-deferral measures applied to the blood supply.

Sporadic disease, by contrast, has not been shown to transmit by transfusion.

Prions resist formalin, alcohol, ultraviolet and ionising radiation, and standard autoclave cycles. Reliable decontamination needs protein-denaturing methods: immersion in sodium hydroxide followed by autoclaving, sodium hypochlorite, or porous-load autoclaving at 134°C.

Formalin, 70% alcohol, a routine 121°C cycle and ultraviolet irradiation all leave infectivity intact. For high-risk procedures, single-use instruments that are then incinerated are preferred.

The real-time quaking-induced conversion assay amplifies and detects the misfolded prion protein directly in cerebrospinal fluid, with a specificity around 98%, so a positive result carries real diagnostic weight.

The 14-3-3 and total tau assays measure non-specific markers of neuronal injury, and the periodic sharp waves and cortical ribboning are supportive findings rather than detection of the protein itself.