[X-linked mental retardation: variations in the fragile X mutations and genetic counseling]

The fragile X syndrome is one of the most common forms of genetic mental retardation and is caused by elongation of a small target DNA fragment containing a repeat of the trinucleotide CGG located in a 5' exon of the FMR-1 gene on the X chromosome. The genetic mutations were classified as two main types, premutation and full mutation, according to the size of the elongation. Because clinical findings are varied in patients with the fragile X syndrome, diagnosis of the disease is very difficult when based only on clinical symptoms. Molecular findings in three families with the fragile X syndrome showed that individuals with the full mutation were moderately mentally retarded, but individuals with the premutation had normal intelligence. The intellectual quotient levels in the families related to the sizes of the mutation which were unstable from generation to generation. These findings suggested that molecular studies in patients with mental retardation are very usefull to diagnose the fragile X syndrome, and that genetic counseling should be carried out based on the DNA analyses of the FMR-1 gene in the family members.