[Hereditary neurological diseases caused by amplification of triplet repetitions]

NEW TYPE OF MUTATION: Repeated sequences of nucleotide triplets can cause two groups of diseases.

These diseases result from an expansion of a noncoding portion of a gene which may be repeated more than 1000 times. This group includes several multisystem diseases such as the fragile X syndrome and its variants, Steinert's disease and Friedreich's disease in which nervous system disorders are not always predominant. The molecular mechanism of the cellular disorder is probably related to a nonfunctional abnormal protein.

Huntington's disease, spinobulbar amyotrophy or Kennedy's disease, dentato-rubo-pallidolusian atrophy and spinocerebellar ataxias 1, 2, 3, 6, and 7 are characterized by local expansion of the coding part of a gene. This low-amplitude expansion always involves the CAG triplet and leads to expression of a protein with an abnormal number of glutamines, producing typical neurodegenerative disease almost exclusively limited to the nervous system. The underlying mechanism of the neuronal suffering is probably the production of an abnormal but functional protein. The causes of this type of mutation remain unclear.

Positive diagnosis is now possible with DNA sequencing. While antenatal diagnosis offers essential information for family genetic counselling there is no perspective of therapeutic propositions for the near future. The problems raised by presymptomatic diagnosis must not be underestimated.