PRNP V210I

Why does it look like sporadic disease?

Because on every measure that has been compared, it is similar. Schelzke and colleagues, reporting from German surveillance, compared 16 patients carrying V210I with sporadic Creutzfeldt-Jakob disease patients matched for age, sex and codon 129 genotype. The clinical syndrome and the test findings were similar in the two groups: spinal fluid 14-3-3 was positive in all 16, MRI was positive in all 16, and the EEG pattern of periodic sharp waves appeared in 44 percent, none of which differed significantly from the matched comparison group.

Mean age at onset was 63 years, with a range from 45 to 80. Mean duration was 6 months, with a range from 1 to 20 months. Early symptoms were often unspecific, including dizziness, personality change and headache, followed by dementia and unsteadiness.

The consequence is that the diagnosis of a genetic form depends entirely on whether PRNP is sequenced. Nothing in the bedside picture prompts it.

Why is there usually no family history?

None of the 16 patients in that series had a family history of prion disease. This fits the penetrance estimate rather than contradicting it. The 2022 review by Appleby and colleagues describes V210I as a low penetrance variant at about 10 percent, alongside V180I at about 1 percent. Penetrance means the proportion of carriers who become ill during a lifetime.

At that level most carriers in a family never develop symptoms, so most affected people appear as isolated cases, and a relative who carries the change may have parents and grandparents who stayed well. Appleby and colleagues note that a missing family history in genetic prion disease can be a consequence of low penetrance rather than evidence against an inherited cause.

Three cautions belong with the 10 percent figure. It comes from comparing how often the change appears in patients with how often it appears in large population databases, a method its authors describe as imperfect because populations are hard to match. It is an average across many people. And it is not a statement about anyone’s own chances, which is a question for a genetic counsellor and the laboratory that issued the report. Counselling comes first describes those appointments.

What happens to relatives once the result is known?

They become people who can ask for a predictive test and who may not want one. This is where the counselling process is most important, because the family has usually had no warning and is often in the first weeks after a death.

Goldman and Vallabh, writing on genetic counselling for prion disease, set out what a pre-test conversation should cover, including the range of possible results, which are positive, negative and variant of uncertain significance, and what each would mean for other relatives. They describe options that sit between testing and not testing: banking DNA from the affected person, which can be done at the time of the diagnosis or after a death, and holding a result in reserve rather than receiving it. For a variant with low penetrance, where a positive result would describe a possibility rather than a certainty, those middle options come up often.

Testing is voluntary everywhere this site has checked. The Australian national registry says so directly and recommends consultation with a genetic service. The right not to know covers choosing not to be tested, which is an ordinary outcome of counselling rather than a failure to decide.

Does the codon 129 genotype change anything here?

Less than in some other variants. In the German series the variant sat on the methionine allele in all 16 patients, 13 of whom were MM and 3 MV at codon 129. The authors found no significant codon 129 difference in most measures, with one exception: cerebellar signs appeared later in the MM patients. With only three heterozygous patients, that comparison is fragile.

For some other variants the codon 129 allele decides which named syndrome follows or how long the illness lasts. For V210I the published data do not show such an effect, so a report listing a codon 129 genotype alongside this variant adds little for a family.

What is not settled

The clinical description above comes from one surveillance unit with 16 patients, 3 of them heterozygous at codon 129, and the paper is a short report rather than a full cohort study. Its authors note that the prion protein type could not be analysed because it was unavailable for most cases, that the E200K and V210I groups were not matched to each other, and the paper’s text and tables disagree on several values, including some spinal fluid tau medians and MRI positivity in the comparison group. A clinician should treat the comparisons between variants in it as provisional and rely on the matched comparison with sporadic disease.

The open question is practical rather than scientific. Because the illness looks sporadic, how many V210I cases are found depends on how readily PRNP is sequenced in rapidly progressive dementia, and the published series do not establish what that threshold should be. Families only learn that an inherited change was present if the test was done.

Neighbouring pages are the variants hub, A117V and V180I.