TRINUCLEOTIDE REPEAT EXPANSION IN THE FRAXE LOCUS IS NOT COMMON AMONGINSTITUTIONALIZED INDIVIDUALS WITH NONSPECIFIC DEVELOPMENTAL-DISABILITIES

Expansion of a polymorphic GCC-repeat at the FRAXE locus has been asso ciated with expression of chromosome fragility at this site and cognit ive impairment in some individuals previously testing negative for CGG -repeat expansion in the fragile X mental retardation-1 (FMR1) gene. T o determine the frequency of FRAXE triplet repeat expansion among pers ons with developmental disability, 396 individuals from two institutio ns were studied, all of whom were negative for FMR1 repeat expansion. Clinically, there was a wide range of mental impairment, with the majo rity (61.1%) being severely to profoundly affected, The distribution o f FRAXE GCC-repeat numbers in the study population was 5-38: 28 (5.6%) with 10-14 repeats; 366 (73.8%) with 15-19 repeats; 74 (14.9%) with 2 0-24 repeats; 20 (4.0%) with 25-29 repeats; and 5 (1.0%) with 30-38 re peats, with no individuals demonstrating repeat expansion. One profoun dly retarded male was found to have a deletion of about 40 bp. Souther n blots of HindIII-digested DNAs from individuals with greater than or equal to 26 repeats all showed normal patterns. These results suggest that FRAXE GCC-repeat expansion is not a common cause of developmenta l disability in institutionalized persons with mild to profound mental retardation. (C) 1996 Wiley-Liss, Inc.