Cognitive and behavioral profile of fragile X boys: correlations to molecular data

Fragile X syndrome (FXS) is the most common form of inherited mental retardation after Down syndrome. The expansion of a CGG repeat, located in the 5'-untranslated region (5'-UTR) of the FMR1 (fragile X mental retardation) gene, leads to the hypermethylation of the repeat and the upstream CpG island. Methylation is associated with transcriptional silencing of the FMR1 gene. The lack of FMR1 protein is believed to be responsible for the typical physical and mental characteristics of the syndrome. To analyze the specific phenotype of that syndrome as well as possible associations between the phenotype and the genotype, we examined a group of 49 fragile X boys and a control group of 16 patients with tuberous sclerosis. To determine the cognitive and behavioral phenotype, the Kaufman Assessment Battery for Children (K-ABC), the Child Behavior Checklist (4/18), and a structured psychiatric interview (Kinder DIPS) were used. The genotype was analyzed by the Southern blot method. The phenotype of boys with FXS is characterized by a specific cognitive profile with strengths in acquired knowledge and in simultaneous processing. The psychiatric comorbidity is high and ADHD (attention deficit hyperactivity disorder), oppositional defiant disorder, enuresis, and encopresis predominate. In a group of 24 fragile X boys, no significant correlations between the specific aspects of the phenotype and the genotype were found.