[Fatal familial insomnia: phenotypic changes determined by polymorphism of the codon 129].

We report a new case of fatal familial insomnia, characterized by mutation of codon 178 of prion protein gene and by methionine homozygosity at codon 129. This homozygotic form is revealed by severe insomnia and dysautonomia. Microscopic lesions, neuronal loss and gliosis, are limited to a part of the thalamus (dorso-median and anterior nuclei). From genetic analysis of a blood stain, retrospective diagnosis of fatal familial insomnia was made on the patient's mother who died three years earlier with clinical features suggestive of Creutzfeldt Jakob disease. In her case the mutation of the codon 178 was associated with the methionine/valine heterozygosity at codon 129. This genotypic polymorphism may account for conformation variation of prion protein isoform that could explain clinical and pathological differences.