THE CLONING OF FRAXF - TRINUCLEOTIDE REPEAT EXPANSION AND METHYLATIONAT A 3RD FRAGILE SITE IN DISTAL XQTER

Three fragile sites, FRAXA, FRAXE and FRAXF lie in the Xq27 - 28 regio n of the human X chromosome, The expression of FRAXA is associated wit h the fragile X syndrome, the most prevalent form of inherited mental retardation whilst the expression of FRAXE is associated with a rarer and comparatively milder form of mental handicap, Both the FRAXA and F RAXE sites have been cloned and the fragile site expression found to b e due to the expansion of analogous CGG/GCC trinucleotide repeat array s, We describe here the cloning of the third fragile site, FRAXF, and demonstrate that it involves the expansion of a (GCCGIC)n(GCC)n compou nd array, PCR analyses across the repeat of normal individuals show th at the number of triplets in the array ranges from 12 - 26 and the mos t common allele consists of 14 triplet units, Sequencing analyses show that 95% of normal individuals have three copies of the GCCGTC motif and in these individuals, the size variation observed by PCR is due to copy number alterations in the GCC array, In a cytogenetically positi ve male with developmental delay, the array is expanded by > 900 tripl ets and the adjacent CpG-rich region is methylated, The array is also expanded in cytogenetically positive carrier females from the family o riginally used to define the FRAXF site, We conclude that the expanded array corresponds to the FRAXF fragile site.