Abstract
Inherited prion diseases (IPDs) are phenotypically diverse neurodegenerative conditions caused by mutations in the prion protein gene (PRNP). We describe IPD due to a novel PRNP E146G mutation in a 50-year-old man presenting with slowly progressive dysarthria, prominent myoclonus especially in the lower limbs, and less prominent gait ataxia, pyramidal and extrapyramidal signs. Cognitive impairment was not overt at disease onset. MRI revealed cerebellar atrophy and white matter hyperintensities. His 46-year-old sister carries the mutation and has subtle gait ataxia and dysarthria. Both patients exhibit a distinctive fluid biomarker profile: in CSF S100B is > twofold upper limit of normal, total tau is moderately elevated, and neurofilament light chain, 14-3-3 and RT-QuIC are negative; in plasma there is marked elevation of GFAP but repeatedly normal neurofilament light chain. The proband's father died aged 55 following an 8-year dementing illness with similar presentation. Post-mortem revealed cerebellar cortical atrophy and profuse large PrP amyloid plaques across cerebral and cerebellar grey matter. Immunoblotting identified low molecular weight protease-resistant PrP fragments. E146G mutation IPD broadly fits into the historical Gerstmann-Sträussler-Scheinker disease spectrum but, based on deep clinical phenotyping of this initial pedigree, we highlight some distinctive features, which may aid in identification of this disease.