Point and octapeptide repeat (24 bp) insertional mutations in the prion protein gene (PRNP) cause a dominantly transmitted dementia, associated with spongiform degeneration of the brain, astrocytic gliosis and neuronal loss due to cell accumulation of mutated protease resistant prion protein. The octapeptide repeat region lies between codon 51 and 91, and comprises a nonapeptide followed by a tandem repeat containing four copies of an octapeptide. The normal tandem length in healthy individuals is five repeats R1-R2-R2-R3-R4, but mutations can contain up to nine additional extra repeats. Some insight into this genetic mechanism comes from the de novo meiotic insertional extra repeat mutation in PRNP we detected in a patient whose parents had a normal phenotype and a wild-type sequence in the same gene. To our knowledge, this is the first time this condition has been described.
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