Small de novo duplication in the repeat region of the TATA-box-binding protein gene manifest with a phenotype similar to variant Creutzfeldt-Jakob disease

A 20‐year‐old North American patient developed rapidly progressive cognitive decline and pronounced ataxia, a phenotype compatible with prion disease. No structural changes were found in the PRNP gene, which excludes genetic prion disease, but the patient's PRNP codon 129 Met/Met genotype is known to predispose to variant Creutzfeldt‐Jakob disease (vCJD). Further studies identified an expanded allele with 55 CAG/CAA repeats in the TBP gene. The increase of trinucleotide repeat number in the coding region of the TBP gene has previously been associated with spinocerebellar ataxia type 17 (SCA17). The patient's unaffected parents and siblings show normal‐size TBP alleles with 37–38 repeats. Haplotype and nucleotide sequence analyses clearly indicate that the mutation has occurred de novo on a paternal chromosome by insertion/duplication of a (CAA)(CAG)(CAA)(CAG)15 sequence. This report presents a second fully investigated sporadic case of SCA17 occurring as a result of a DNA rearrangement within the polymorphic TBP trinucleotide repeat region. Our findings suggest that patients suspected of vCJD should undergo testing for SCA17, Huntington's disease and other neurodegener‐ative disorders having phenotypic similarities with vCJD.