[Growing genes cause neurological diseases]

A growing number of hereditary neurodegenerative disorders have been found to be caused by expansion of trinucleotide repeats. A smaller number of diseases such as fragile X syndrome, myotonic dystrophy, and Friedreich's ataxia, have been found to be due to expansions in non-coding DNA. In a large group of diseases, the expansion consists of CAG repeats in the coding region of the gene, producing an expanded polyglutamine sequence in the protein. Nine diseases have so far been identified as belonging to this group: Huntington's disease, spinobulbar muscular atrophy (SBMA), dentatorubral pallidoluysian atrophy (DRPLA), autosomal dominant "pure" spastic paraplegia (ADPSP), and five forms of spinocerebellar ataxia (SCA 1,2,3,6 and 7). Except for SBMA, all of the CAG repeat disorders are characterised by autosomal dominant heredity and anticipation (i.e., earlier onset age and increasing severity in successive generations). The mutated protein causes disease via an as yet unidentified gain-of-function mechanism in specific subsets of neurones. Today, DNA analysis permits the diagnosis of a trinucleotide disease in individual cases.