Systematic screening for fragile X syndrome in a cohort of 574 mentally retarded children.

In this study, we evaluated the prevalence of the fragile X syndrome in a cohort of 574 mentally retarded children. The only inclusion criterion was the diagnosis of mental retardation according to the DSM-IIIR classification. We used a PCR-based strategy for the diagnosis of fragile X syndrome to facilitate systematic screening. This diagnostic scheme is based on an initial PCR to eliminate most fragile X-negative patients followed by Southern blotting for fragile X syndrome diagnosis. Altogether, 403 boys and 171 girls were tested. The prevalence of this genetic disorder was 1.9% (11/574) in the whole cohort and 2.5% (10/403) in boys. Only one case of fragile X syndrome was detected among the 171 girls tested (0.6%). Clinical examination, especially in the youngest children, was often unremarkable, and the only reason for suspecting fragile X syndrome was the presence of mental retardation. Thus, a systematic screening for the fragile X syndrome in mentally retarded children seems justified because of the importance of a precise diagnosis in genetic counseling.