PRNP M232R

What is the estimated risk?

Low, and estimated rather than measured. The 2016 population-control analysis compared 16,025 prion disease cases with 60,706 ExAC exomes and 531,575 genotyped individuals, estimating lifetime risk by comparing case and control allele frequencies against a baseline lifetime risk of roughly 1 in 5,000, derived from an upper-bound incidence of about 2 cases per million per year.

The best estimate for M232R was about 0.08 percent, which the text rounds to about 0.1 percent. The authors placed M232R, with V180I at about 1 percent and V210I at about 7.8 percent, on a risk continuum, judging these variants likely neither benign nor fully penetrant. They also stated that their confidence intervals span more than an order of magnitude, and that they could not fully correct for population stratification because genome-wide SNP data were not available for the cases.

Population stratification is not an abstract caveat for this variant. M232R is reported mainly from Japan, Korea and China, and the control cohorts used in the analysis were not matched to those populations, so an allele that is simply commoner in East Asian populations can look like a low-penetrance risk variant. That is one of the reasons the interpretation remains contested.

Could it be benign?

The 2016 paper left that open. It judged low penetrance the more likely of the two readings while not excluding benignity, and a 2022 review of prion disease genetics does not mention M232R at all, which is itself informative about how settled its status is.

What is not in doubt is the broader finding from that analysis: reportedly pathogenic PRNP missense variants are at least 30 times more common in population controls than expected. The authors anticipated at most about 1.7 carriers of such variants among the ExAC exomes and found 52. A large part of that excess came from variants with weak prior evidence of pathogenicity, and the published list of pathogenic PRNP variants is likely to contain false positives.

What is the reported clinical picture?

The 2017 review describes M232R as reported mainly from Japan, China and Korea, with penetrance estimated at about 0.1 percent, and therefore as likely a risk variant. Mean age at onset is 64 years with a range of 15 to 81, and mean duration 8 months with a range of 0 to 32 months. About 75 percent of reported cases follow a rapid course and 25 percent a slow one.

Those figures come from people who were ascertained because they developed prion disease. They describe what the illness looked like in reported cases, and they say nothing about the probability that a carrier will develop it. Keeping those two questions separate is the central point of this page.

What does an incidental M232R result mean?

It is a question for the reporting laboratory and a clinical genetics service, not for a website, and the honest general statements are these.

A variant reported in a person who is already ill occupies a different evidential position from the same variant found incidentally in someone who is well, for example on a dementia gene panel or a research exome. Risk estimates derived from population allele frequencies describe groups and carry wide uncertainty. And a low penetrance estimate is not a reassurance about an individual, just as a high one is not a timetable.

Laboratories differ in how they classify M232R, and classifications are revised. Anyone holding a report should ask the issuing laboratory what classification it currently applies and whether it has been reviewed since the report was written. PRNP variants sets out how the variants compare, and what a result can tell you covers reading a report.

Does predictive testing of relatives apply here?

That question goes to a genetics service, and the published frameworks were written with the high-penetrance variants in mind.

The 2026 Israeli framework describes a sequence built around a known pathogenic variant in the family: counselling first, psychological assessment, at least a month before blood sampling, and disclosure in person by the clinician who gave the counselling, with testing not offered under 18. It does not address low-penetrance or uncertain variants separately. Where the variant’s significance is itself uncertain, what a relative’s result would mean is uncertain too, and that needs saying before anyone is tested rather than after.

Testing is voluntary everywhere this site has checked, and declining is a recognised choice. The right not to know and genetic counselling comes first cover that ground, and nothing on this page is a reason to be tested or not to be tested.

What about the diagnostic tests?

M232R did not appear among the genetic subtypes in the 2026 Canadian CSF assay comparison, whose inherited group comprised D178N, P102L, E200K, octapeptide repeat insertions, V210I, V180I, P105S and A117V. There is therefore no variant-specific seeding assay performance figure to quote for M232R, and the general prion disease figures should not be presented as if there were.

Diagnosis in a symptomatic patient follows the usual route: the clinical picture, MRI, CSF studies including a seeding assay, and PRNP sequencing, interpreted together. How prion disease is diagnosed sets that out.

Where next?

E200K and D178N are the high-penetrance variants, octapeptide repeat insertions the group where sequencing can miss the change altogether, and M232R for researchers the scientific page.

This page summarises published literature and is not clinical or genetic advice. No estimate here describes the position of any particular person, and the interpretation of a specific report belongs with the laboratory that issued it and a genetic counsellor.