(Over)correction of FMR1 deficiency with YAC transgenics: behavioral and physical features

Fragile X syndrome is a common cause of mental retardation involving loss o f expression of the FMR1 gene. The role of FMR1 remains undetermined but th e protein appears to be involved in RNA metabolism. Fmr1 knock-out mice exh ibit a phenotype with some similarities to humans, such as macroorchidism a nd behavioral abnormalities. As a step toward understanding the function of FMR1 and the determination of the potential for therapeutic approaches to fragile X syndrome, yeast artificial chromosome (YAC) transgenic mice were generated in order to determine whether the Fmr1 knockout mouse phenotype c ould be rescued. Several transgenic lines were generated that carried the e ntire FMR1 locus with extensive amounts of flanking sequence. We observed t hat the YAC transgene supported production of the human protein (FMRP) whic h was present at levels 10 to 15 times that of endogenous protein and was e xpressed in a cell- and tissue-specific manner. Macroorchidism was absent i n knockout mice carrying the YAC transgene indicating functional rescue by the human protein. Given the complex behavioral phenotype in fragile X pati ents and the mild phenotype previously reported for the Fmr1 knockout mouse , we performed a more thorough evaluation of the Fmr1 knockout phenotype us ing additional behavioral assays that had not previously been reported for this animal model. The mouse displayed reduced anxiety-related responses wi th increased exploratory behavior. FMR1 YAC transgenic mice overexpressing the human protein did produce opposing behavioral responses and additional abnormal behaviors were also observed. These findings have significant impl ications for gene therapy for fragile X syndrome since overexpression of th e gene may harbor its own phenotype.