EVIDENCE THAT METHYLATION OF THE FMR-I LOCUS IS RESPONSIBLE FOR VARIABLE PHENOTYPIC-EXPRESSION OF THE FRAGILE-X SYNDROME

DNA at the FMR-1 locus was analyzed by Southern blot using probe StB12 .3 in an unusual fragile X family with six brothers, three of whom are affected with fragile X to varying degrees, two of whom are nonpenetr ant carriers, and one of whom is unaffected. Fragile X chromosome stud ies, detailed physical examinations, and psychological testing were co mpleted on all six. Two of the affected brothers and the two nonpenetr ant brothers were found to be methylation mosaics. The three affected males spanned the phenotypic and cognitive spectrum of the fragile X s yndrome. A correlation was seen between the degree of methylation and the phenotypic expression identified in the three affected males. The two males initially classified as nonpenetrant were found to have mild phenotypic expression which consisted of minor cognitive deficits and a partial physical phenotype. These two, who were negative on fragile X chromosome studies, were found on DNA analysis to have large broad smears, with approximately 97% of the DNA unmethylated. The results de scribed here indicate that some ''nonpenetrant'' carrier males may hav e varying amounts of methylation of the FMR-1 region, which can result in mild expression of the fragile X syndrome. The apparently mild phe notypic and cognitive expression of the fragile X syndrome in the two males, initially classified as nonpenetrant, who are mosaic for hyperm ethylation of an expansion of the CGG repeat in the premutation range, indicates that expression of the syndrome is not confined to males wi th large, hypermethylated expansions (full mutation) but has instead a gradient effect with a threshold for the full expression of the pheno type.