Wöhrle D, Steinbach P
Abteilung Klinische Genetik der Universität, Ulm, Federal Republic of Germany.
Hum Genet. 1991 Aug;87(4):421-4. doi: 10.1007/BF00197160.
The major concept of fragile X pathogenesis postulates that the fragile site at band Xq27.3 [fra(X)] represents the primary defect. The expression of fra(X) is predicted to be an intrinsic property of the mutated chromosome and, hence, should not be suppressed by X inactivation in females or induced by X-linked trans-acting factors. We made fibroblast clones of a fra(X)-positive female. Monoclonality was demonstrated using the DNA methylation assay at DXS255. The mutated X chromosomes and their states of genetic activity in the different clones were also defined by molecular methods. Five clones were selected to induce expression of fra(X) by 10(-7) M FUdR; two carried an active mutated X chromosome, in the other three the mutated X chromosome was inactivated. Fra(X) was found expressed in both types of clones. The percentages of positive cells were as high as 7-10%, regardless of the genetic activity of the mutated X chromosomes. DNA replicating patterns, obtained by BUdR labelling, demonstrated that expression occurred only on the mutated X chromosomes previously identified by molecular methods. The concept that the fragile site represents the primary mutation is now strongly supported by experimental evidence. The expression of fra (X) in females is independent of X inactivation and other trans-acting factors.
脆性X综合征发病机制的主要概念假定位于Xq27.3带的脆性位点[fra(X)]代表主要缺陷。预计fra(X)的表达是突变染色体的固有特性，因此，不应被女性的X染色体失活所抑制，也不应被X连锁反式作用因子所诱导。我们构建了一名fra(X)阳性女性的成纤维细胞克隆。使用DXS255处的DNA甲基化检测证明了单克隆性。还通过分子方法确定了不同克隆中突变的X染色体及其遗传活性状态。选择五个克隆用10(-7)M氟尿嘧啶脱氧核苷（FUdR）诱导fra(X)的表达；两个携带活跃的突变X染色体，另外三个中的突变X染色体失活。在两种类型的克隆中均发现fra(X)表达。无论突变X染色体的遗传活性如何，阳性细胞的百分比高达7-10%。通过5-溴脱氧尿苷（BUdR）标记获得的DNA复制模式表明，表达仅发生在先前通过分子方法鉴定的突变X染色体上。现在实验证据有力地支持了脆性位点代表主要突变的概念。女性中fra(X)的表达独立于X染色体失活和其他反式作用因子。
