A report naming a PRNP variant says less on its own than families expect. Two variants can differ by orders of magnitude in the chance that a carrier ever becomes ill, and one variant can produce two differently named diseases depending on a polymorphism that is common in the general population. This page sets out the structure of that variability for counsellors, clinicians and relatives who are reading the literature alongside a clinic appointment.
How many variants are there, and how much disease do they cause?
More than 60 PRNP variants have been described in the literature. Genetic prion disease accounts for somewhere between 5 and 15 percent of all prion disease, with 10 to 15 percent the figure most often quoted. Four variants, E200K, D178N, P102L and A117V, together account for more than half of genetic cases. E200K is the most common cause of genetic CJD worldwide.
Inheritance is autosomal dominant. One altered copy is enough, it usually comes from an affected parent, and a minority of cases arise from a variant not present in either parent.
What does penetrance actually look like?
The older assumption of full penetrance has been replaced by variant-specific estimates, largely through comparison of case series with large population control cohorts. The 2016 analysis by Minikel and colleagues found allele frequencies for P102L, A117V, D178N and E200K consistent with up to 100 percent penetrance, while estimating lifetime risk near 0.1 percent for M232R, 1 percent for V180I and 10 percent for V210I. For E200K specifically, survival analyses that paper cites give estimates in the region of 60 to 90 percent. That work also identified some missense variants as likely benign, and stated that variants very rare in both cases and controls could not be reclassified either way.
A 2022 review of the genetics of human prion diseases gives the same ordering: D178N and P102L nearly completely penetrant, E200K incompletely and age-dependently penetrant with most carriers developing illness as they age, V180I around 1 percent and V210I around 10 percent. An international study of 217 E200K carriers did not confirm earlier reports of anticipation, which that review attributes to ascertainment bias.
Two cautions follow. A low penetrance estimate is not a reassurance for an individual, and a variant reported in a person who is already ill is in a different evidential position from the same variant found incidentally. Interpretation for a particular family belongs with a genetic counsellor and the laboratory that issued the report, which is why genetic counselling comes first.
Why does codon 129 keep appearing?
PRNP carries a common methionine or valine polymorphism at codon 129. The 2022 review describes it as the strongest genetic influence across all prion diseases, and D178N is the clearest case. With 129M on the same allele as the variant, the phenotype is usually fatal familial insomnia. With 129V on that allele, it is usually genetic CJD. The variant is identical; the named disease is not.
Codon 129 also acts outside the inherited forms. Homozygosity increases risk in iatrogenic CJD, and nearly all autopsy-confirmed variant CJD cases were methionine homozygotes.
For octapeptide repeat insertions the main determinant is repeat number rather than codon 129: fewer than eight repeats tends towards a CJD-like presentation, eight or more towards Gerstmann-Straussler-Scheinker syndrome, with penetrance generally rising as repeat number rises. A one-repeat deletion is considered benign and a two-repeat deletion pathogenic with probably low penetrance.
The main variants, in outline
Pages for individual variants are being written, and the summary below is the shared background to all of them.
E200K produces a genetic CJD phenotype, often clinically indistinguishable from sporadic CJD, with incomplete age-associated penetrance. See E200K.
D178N produces fatal familial insomnia or genetic CJD according to the codon 129 allele in cis, and is among the near fully penetrant variants. See D178N.
P102L is the classical Gerstmann-Straussler-Scheinker variant, nearly completely penetrant, typically with a slower course and early ataxia. See P102L.
A117V is the fourth of the common variants and also presents in the Gerstmann-Straussler-Scheinker range. See A117V.
V210I and V180I carry substantially lower estimated lifetime risk, around 10 percent and 1 percent, and are often found without a family history. See V210I and V180I.
M232R has an estimated lifetime risk near 0.1 percent. The 2016 analysis stated it could still be benign while judging low penetrance the more likely of the two readings. See M232R.
P105L is a rarer variant associated with Gerstmann-Straussler-Scheinker presentations. See P105L.
Does the variant affect how the disease is detected?
It does, which is relevant to families whose relative was tested while alive. A 2026 Annals of Neurology study from the Canadian national reference laboratory compared substrates in a real-time quaking-induced conversion assay across 116 diagnostic CSF samples: 61 sporadic CJD submitted between 2014 and 2024, 50 grouped as inherited prion disease and 5 variably protease-sensitive prionopathy. A modified assay using a deer mouse recombinant substrate reached 96.6 percent sensitivity overall, against 87.1 percent for the standard hamster substrate and 72.4 percent for the widely used IQ-CSF assay.
The improvement was concentrated in inherited cases. For fatal familial insomnia with D178N, 4 of 13 were detected with the standard substrate and 12 of 13 with the modified one; the paper describes D178N associated with fatal familial insomnia as particularly refractory to IQ-RT-QuIC. For Gerstmann-Straussler-Scheinker syndrome, 9 of 12 became 12 of 12. E200K, V210I, octapeptide repeat insertions and D178N presenting as genetic CJD were detected with both substrates, and one V180I case was detected only by the IQ assay. A negative seeding assay therefore carries different weight depending on the variant in the family and the assay the laboratory runs.
Is the inheritance figure itself settled?
The figure in current guidance is 50 percent per child. A 2025 European Journal of Neurology study of 24 pedigrees, 65 nuclear families and 151 offspring reported 67.1 percent transmission for D178N, with a 95 percent confidence interval of 56.3 to 77.9, and 70.5 percent for E200K, interval 60.4 to 80.6, proposing meiotic drive as the most plausible explanation and arguing for variant-specific and sex-specific counselling figures. The study is retrospective, 18 percent of genotypes were inferred rather than tested, and the families are mostly Basque, so it is a finding to be replicated rather than a new standard. Families who have read both figures deserve to hear that the question is open.
How is a variant found in the first place?
Through sequencing of the PRNP coding region, usually arranged by a clinical genetics service or a national prion centre. Sequencing does not reliably detect octapeptide repeat insertions, so a suspected repeat insertion needs a method chosen for it. National registries also recommend storing DNA from suspected cases; the Australian registry advises storing DNA from every suspected CJD patient so that relatives can consider testing later, and states that genetic testing is entirely voluntary.
Related pages
Genetics and family risk is the hub for this section. Inherited prion disease explained is the plain introduction, what a result can tell you covers interpreting a report, and talking with family the conversations that follow one.
This page summarises published literature. It is not clinical or genetic advice, and no figure here should be read as the risk facing any particular person.
Explore
- PRNP A117VPRNP A117V: the GSS phenotype, reported ages at onset from the 20s, disease duration measured in years, and why CSF seeding assays behave differently.
- PRNP D178NPRNP D178N: how codon 129 in cis decides between fatal familial insomnia and genetic CJD, the onset and duration figures for each, and the assay problem.
- PRNP E200KPRNP E200K: the commonest cause of genetic CJD, founder populations, why about half of patients have no family history, and what the counselling frameworks say.
- PRNP M232RPRNP M232R: a variant with an estimated lifetime risk near 0.1 percent, reported mainly from Japan, Korea and China, and why its significance remains contested.
- PRNP Octapeptide repeat insertionsPRNP octapeptide repeat insertions: how repeat number tracks with phenotype and tempo, why routine sequencing can miss them, and what the reported series show.
- PRNP P102LPRNP P102L: the classical GSS variant, reported onset from the late 20s with durations of months to years, near-complete penetrance, and substrate-dependent CSF testing.
- PRNP P105LWhat PRNP P105L means in a family: a slow form of genetic prion disease reported mainly in Japan, often first called a spastic paraparesis.
- PRNP Protective variantsSome PRNP changes appear to lower the risk of prion disease. What is known about codon 219 lysine, the kuru variant G127V and codon 129, and what they do not do.
- PRNP V180IPRNP V180I explained for families and clinicians: late onset, low penetrance, usually no affected relatives, and an illness often first called Alzheimer's disease.
- PRNP V210IPRNP V210I explained: a low-penetrance variant that looks like sporadic Creutzfeldt-Jakob disease and is usually found in people with no affected relatives.
Sources for this page
- Appleby BS, Shetty S, Elkasaby M. Genetic aspects of human prion diseases. Frontiers in Neurology 2022
- Minikel EV and others. Quantifying penetrance in a dominant disease gene using large population control cohorts. Science Translational Medicine 2016
- Myskiw J and others. Enhanced sensitivity of a modified quaking-induced conversion diagnostic test for the broad detection of sporadic and inherited prion diseases: a retrospective study. Annals of Neurology 2026
- Kortazar-Zubizarreta I and others. The risk of transmission of genetic prion diseases is greater than 50%. European Journal of Neurology 2025
- MedlinePlus Genetics: Prion disease
- Australian National CJD Registry (The Florey): testing specifications