A subtype of sporadic prion disease mimicking fatal familial insomnia

Objective: To establish a variant of sporadic prion disease as the sporadic form of fatal familial insomnia (FFI). Background: FFI is a recently described prion disease characterized clinically by severe sleep impairment, dysautonomia, and motor signs, and pathologically by atrophy of thalamic nuclei, especially the medial dorsal and anterior ventral, and of the inferior olive. FFI is linked to the D178N mutation coupled with the methionine codon at position 129 in the prion protein gene (PRNP). It is also identified by the properties of the abnormal prion protein (PrPSc), which has the relative molecular mass of 19 kDa, corresponding to the so-called type 2, and a marked underrepresentation of the unglycosylated form relative to the diglycosylated and monoglycosylated forms. Methods: Clinical, pathologic, PrPSc, and PRNP data from 5 subjects with a sporadic prion disease phenotypically similar to FFI were collected and analyzed. Results: All 5 subjects had a disease clinically similar and histopathologically virtually identical to FFI. PrPSc type 2 was present in all subjects in amount and distribution similar to those of FFI. However, the PrPSc did not show the striking underrepresentation of the unglycosylated isoform of the protein that is characteristic of FFI. Moreover, none of the subjects had the D178N PRNP mutation but all were homozygous for methionine at codon 129. Conclusion: This condition is likely to represent the sporadic form of FFI and the term "sporadic fatal insomnia" is proposed.