Fxs is mainly caused by a kind of dynamic mutation , the expansion of a cgg repeat located in the 5 ' - untranslated region ( 5 ' - utr ) of the fmr1 ( fragile x mental retardation ) gene 該綜合癥由患者細胞中可見xq27 . 3的脆性位點fraxa而得名。絕大多數(shù)脆性x綜合征患者的發(fā)病是由于fmr1基因5端非翻譯區(qū)的三核苷酸cgg重復(fù)的不穩(wěn)定擴增所致。
The absence of the fmr1 gene product , fragile x mental retardation protein ( fmrp ) , is believed to be responsible for the typical physical and mental characteristics of the fragile x syndrome . alleles with between 43 and 200 cgg repeats are called permutation . they are generally unmethylated with normal transcript and protein level , but are extremely unstable during transmission to next generation 帶有前突變( n = 43 200 )的個體其fmr1基因通常不會甲基化,可以正常轉(zhuǎn)錄和翻譯產(chǎn)生fmrp ,故沒有臨床表型出現(xiàn),但是,在向下一代傳遞的過程中卻是非常不穩(wěn)定的,會從前突變擴增成為全突變。