MAPT V363I Variation in a Sporadic Case of Frontotemporal Dementia Variable Penetrant Mutation or Rare Polymorphism?

The V363I mutation of the microtubule-associated protein tau gene has previously been associated with a case of primary progressive nonfluent aphasia with variable penetrance. Herein, we report the finding of the V363I variation in a sporadic early onset frontotemporal dementia patient and in several members of her family. The V363I variation was associated with frontotemporal dementia only in the proband which was also homozygous for the A allele of the progranulin single-nucleotide polymorphism rs9897526 and for methionine at codon 129 of the prion protein gene. The microtubule-associated protein tau V363I variation could be considered either an incomplete penetrant mutation or a rare polymorphism; although its pathogenicity has yet to be clearly demonstrated, modifier genetic factors seem to contribute to the pathogenic effects observed in the patient underlining the great complexity existing in neurodegenerative diseases and questioning so-called sporadic cases that can potentially be caused by gene mutation.