Prion diseases

is characterized by degeneration of neurons in the basal ganglia leading to choreiform movements, psychiatric complications, and progressive dementia. The HD gene huntingtin (IT15) has been mapped to chromosome 4p16.3 and the underlying mutation was identified as a CAG repeat expansion within exon 1 of the gene (22). While in the normal population the number of CAG repeats ranges from 6 to 35, in individuals affected by HD it ranges from 40 to 121, and the age of onset of HD is inversely correlated with CAG repeat length. Trinucleotide repeat expansions can be observed also in noncod-ing regions of the causative gene leading to altered mRNA levels, as it happens in the first intron of the frataxin gene causative of Friedreich's ataxia, a disease characterized by neurodegeneration involving the spinocerebellar pathways as well as cardiomyopathy (23).

Creutzfeld-Jakob disease (CJD), fatal familial insomnia (FFI), and Gerstmann-Straussler-Scheinker syndrome (GSS) represent another group of neurodegenerative conditions, better known as prion diseases, including sporadic, inherited, and transmitted forms. The central event in the pathogenesis of prion diseases is thought to be a change in protein conformation that results in the conversion of a normal protein, identified as cellular prion protein, into an isoform that is partially resistant to proteases. Genetic factors, such as several amino acid changing mutations in polymorphic sites of the prion protein gene (PRNP) have been identified to be causative of familial autosomal dominant forms of prion diseases (24). Causative genes for neurodegenerative diseases are listed in Table 1.

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