Basic Neurosciences · Paper A
Classification of human prion diseases
Every human prion disease is the same accident — one protein folded the wrong way, forcing its correctly folded neighbours to copy it — so the only useful classification is by where that first wrong molecule came from. Three origins exist and no more. It can arise spontaneously in a person with an ordinary gene, which is sporadic disease and accounts for around 85 per cent of cases. It can be written into the person's own PRNP gene and inherited in an autosomal dominant pattern, giving genetic Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker syndrome and fatal familial insomnia, at roughly 10 to 15 per cent. Or it can be introduced from outside, in kuru, in iatrogenic cases from medical tissue and instruments, and in variant Creutzfeldt-Jakob disease from cattle with bovine spongiform encephalopathyA general term for brain dysfunction, often causing confusion, that has a physical cause.Read more →, which together make up well under 5 per cent. Sort a vignette by origin first and the syndrome names stop being a list to memorise.
Prion protein is a normal cell-surface protein present in everyone, encoded by PRNP on chromosome 20. Its healthy shape is harmless; a misfolded shape is not, because it behaves as a template rather than as a poison. When a misfolded molecule meets a normally folded one it forces the second into the same abnormal shape, so one becomes two, two become four, and the process accelerates without any organism, any genetic material of its own, and any immune response. The misfolded form resists the enzymes that would normally break a protein down and aggregates in the tissue, and neurones die around it, leaving the microscopic holes that gave the group its older name of transmissible spongiform encephalopathyA general term for brain dysfunction, often causing confusion, that has a physical cause.Read more →. That single templating mechanism explains why the three origins converge on one disease. Spontaneous misfolding is a rare accident that becomes far likelier over a long life, which is why sporadic disease peaks in the sixties. An inherited PRNP mutation makes the protein less stable from birth, so misfolding becomes near-inevitable at some point in adult life, which is why genetic forms run in families with a characteristic age of onset. Introduced material simply supplies the first template ready-made, and because seeding takes years to reach clinical disease, incubation periods after exposure can run for decades. The templating mechanism also explains why the abnormal protein is not destroyed by ordinary decontamination: there is no membrane or genome to disrupt, only an extremely stable protein shape, which survives routine autoclaving and formalin fixation and forces the quarantine of instruments.
Three patients reach a neurology clinic with the same end state: a brain filling with holes, no treatment, and months to live. One had no risk factor of any kind and no relative ever affected. One is the fourth generation of a family to develop it, always in their fifties. One received a hormone injection as a child, thirty years earlier. Under the microscope their brains look alike. What differs is how the disease got started, and that is the only sensible way to file this group of illnesses in your head: not by symptom, but by origin.
Human prion diseases are rare, invariably fatal degenerative brain diseases caused by a single protein adopting the wrong shape. That protein is prion protein (a normal protein sitting on the surface of nerve cells, present in everyone, whose day job is still not fully settled). The healthy folded version is harmless. A misfolded version is not, because it acts as a template: when it meets a correctly folded molecule, it forces that molecule into the wrong shape too, and now there are two templates, then four.
The older name for the group, transmissible spongiform , records two features — the diseases can in principle be passed on, and the affected brain develops microscopic holes that make the tissue look like a sponge.
The word transmissible does real damage here, because it suggests that prion disease is contagious in the everyday sense. It is not. You cannot catch sporadic Creutzfeldt-Jakob disease from a patient by nursing them, sharing a room, coughing distance, or ordinary contact, and standard infection precautions are enough for routine care.
What transmissible actually means is narrow and specific: the misfolded protein can seed the disease if it is introduced into nervous tissue — by a graft, by an instrument that touched a brain, by injected material of human origin, or in one historical case by eating infected tissue. Prions also survive procedures that destroy ordinary infection risk, including routine autoclaving and formalin fixation, which is why surgical instruments used on a suspected case are quarantined rather than reprocessed.
Ask a simple question: where did the first misfolded molecule come from? There are only three possible answers, and they generate the whole classification.
It can appear spontaneously, by a rare misfolding accident in a person with an ordinary gene. That is sporadic disease, and it is by far the commonest, accounting for roughly 85 per cent of cases.
It can be encoded in the person's own DNA. Mutations in PRNP (the gene on chromosome 20 that carries the instructions for prion protein) make misfolding far more likely, and they are inherited in an autosomal dominant pattern, so half the children of an affected parent carry the mutation. This route accounts for something like 10 to 15 per cent.
Or it can be introduced from outside, in tissue or on an instrument. That is acquired disease, and it is the rarest group, well under 5 per cent — but it is the group that generates public health action and most of the examination questions.
Through the sporadic door comes sporadic Creutzfeldt-Jakob disease, occurring worldwide at about one to two cases per million people per year, usually beginning in the sixties, and progressing so fast that most patients die within six months. Two rarer sporadic forms exist: sporadic fatal insomnia and variably protease-sensitive prionopathy.
Through the genetic door come three named syndromes, and knowing which mutation goes with which is worth the effort. Genetic Creutzfeldt-Jakob disease looks like the sporadic form but arrives earlier and runs in families, most often with the E200K mutation. Gerstmann-Straussler-Scheinker syndrome, usually the P102L mutation, leads with progressive cerebellar (unsteadiness of gait and limbs from cerebellar damage) and runs for years rather than months, with dementia arriving late. Fatal familial insomnia, from the D178N mutation, begins with untreatable insomnia and disturbance of the autonomic nervous system — sweating, fever, blood pressure swings — with degeneration concentrated in the .
Through the acquired door come three quite different stories. Kuru appeared among the Fore people of Papua New Guinea and spread through mortuary feasts in which relatives ate the tissue of the dead; it presented with cerebellar and tremor long before dementia, and incubation periods stretched beyond fifty years. Iatrogenic disease came from medicine itself: growth hormone extracted from cadaveric pituitary glands, dura mater grafts, corneal grafts, and neurosurgical instruments and depth electrodes. Variant Creutzfeldt-Jakob disease came from cattle with bovine spongiform entering the human food chain, overwhelmingly in the United Kingdom.
For example, take the route of entry seriously in iatrogenic cases, because it predicts the picture. Material injected peripherally, as growth hormone was, tends to produce a slower illness that starts with . Material placed against the brain, as a dura mater graft is, tends to produce something much closer to the sporadic dementing picture.
| Presents as rapid cognitive decline | What separates it from prion disease |
|---|---|
| Autoimmune or paraneoplastic encephalitis | Seizures, movement disorder and psychiatric change with a treatable antibody in serum or cerebrospinal fluid — the one you must not miss |
| Rapidly progressive Alzheimer's or Lewy body disease | Still measured in years, and lacks early (sudden, brief, shock-like muscle jerks) with the characteristic imaging changes |
| Confusion with eye movement abnormality and in a person at nutritional risk, reversing with thiamine | |
| Cerebral vasculitis or multiple infarcts | Focal deficits and vascular changes on imaging rather than diffuse cortical restriction |
| Late neurosyphilis or HIV-associated disease | Serology and a risk history give the answer, and both have treatment |
A definite diagnosis of any human prion disease requires brain tissue, showing spongiform change, neuronal loss and (scarring laid down by the brain's supporting glial cells) without inflammation — and in the variant form, the distinctive florid plaques. In life, the diagnosis is probable, built from the clinical picture plus supporting tests.
For sporadic disease, magnetic resonance imaging showing restricted diffusion in the cortex and is the most useful test; electroencephalography shows periodic sharp wave complexes in around two-thirds of cases, more often later in the illness; cerebrospinal fluid 14-3-3 protein is supportive but non-specific, and real-time quaking-induced conversion, an assay that detects seeding activity directly, has largely displaced it where available. For variant disease, the pulvinar sign — high signal in the back of the — is characteristic, and tonsil biopsy can show the abnormal protein because it accumulates in lymphoid tissue, which it does not do in the sporadic form. Where a genetic form is suspected, PRNP sequencing is the definitive test and requires genetic counselling first.
No drug alters the course of any human prion disease, and no trial has shown benefit; care is entirely supportive and palliative, with early involvement of specialist palliative services, treated symptomatically, and intensive practical and emotional support for families facing a fast, disfiguring illness in a relative who is often young.
Around that sit three obligations. Suspected cases in the United Kingdom are reported to the National CJD Research and Surveillance Unit in Edinburgh, which runs national surveillance and supports diagnosis. Infection control follows the Advisory Committee on Dangerous Pathogens transmissible spongiform guidance, published and maintained by UKHSA, which governs instrument handling and quarantine. And families with a genetic form need formal genetic counselling, because predictive testing in a healthy relative for an untreatable illness is a decision requiring proper support rather than a blood test.
Two details usually decide the answer. A patient in their sixties with a dementia moving over weeks and prominent is the sporadic form. A patient in their twenties whose illness opened with months of depression, anxiety and withdrawal, followed by painful sensory symptoms in the limbs and then , is the variant form — and that psychiatric opening is why these patients reach psychiatry first. Add the family history for the genetic forms, and the exposure history for the acquired ones. Before you move on, test yourself: a man of 45 develops slowly worsening unsteadiness over two years, and his father died of something similar — which syndrome and which mutation should you name?
File it this way: every human prion disease is the same misfolded protein recruiting its neighbours, so classify by origin — sporadic, which is most of it, genetic through autosomal dominant PRNP mutations giving genetic Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker syndrome and fatal familial insomnia, and acquired through kuru, medical procedures, or beef, with variant disease standing apart by its young patients and its psychiatric opening.