Ledbetter D H, Ledbetter S A, Nussbaum R L
Nature. 1986;324(6093):161-3. doi: 10.1038/324161a0.
The fragile site at Xq27, associated with a common form of X-linked mental retardation (XLMR), is expressed in a variable proportion of the peripheral lymphocytes of affected males when the cells are cultured under thymidylate stress (Td stress) produced by folate or thymidylate deprivation. Some clinically normal males--transmitting males--are known to carry and transmit the fragile X mutation and yet show no cytogenetic expression in lymphocytes. Normal males with no family history of X-linked mental retardation express the site only rarely. When the fragile X chromosome from affected males is isolated in a rodent genetic background by somatic cell hybridization, the level of expression is similar to that seen in lymphocytes under Td stress. Here we show that X chromosomes from two transmitting males and two normal control males, all of which were fragile X negative in lymphocytes or lymphoblasts, could be made to express the fragile site in hybrids, although at levels that were below those seen in hybrids from affected males. Furthermore, transmitting males could be differentiated from normal males by their significantly higher expression rates when hybrids were exposed to caffeine before cytogenetic harvest. One male chimpanzee also showed low level expression in hybrid cells. These data suggest that the hybrid system lowers the threshold for fragile X expression, a fragile site at Xq27 may be present on all human and chimpanzee X chromosomes and constitutes a previously unrecognized common fragile site and the hybrid system with caffeine post-treatment can distinguish between the common Xq27 fragile site of control males, the occult mutant fragile site of a transmitting male, and the fully expressed fragile site of an affected male with XLMR. Thus the mutation producing XLMR may represent a multi-step alteration of a naturally occurring DNA sequence producing a continuum of cytogenetic expression and a threshold for clinical manifestation.
与一种常见的X连锁智力迟钝（XLMR）形式相关的Xq27脆性位点，当细胞在叶酸或胸苷酸缺乏所产生的胸苷酸应激（Td应激）条件下培养时，在受影响男性的外周淋巴细胞中以可变比例表达。已知一些临床正常的男性——传递男性——携带并传递脆性X突变，但在淋巴细胞中未表现出细胞遗传学表达。无X连锁智力迟钝家族史的正常男性很少表达该位点。当通过体细胞杂交在啮齿动物遗传背景中分离出受影响男性的脆性X染色体时，其表达水平与Td应激下淋巴细胞中的表达水平相似。我们在此表明，来自两名传递男性和两名正常对照男性的X染色体，尽管其表达水平低于受影响男性杂交细胞中的表达水平，但在淋巴细胞或淋巴母细胞中均为脆性X阴性的情况下，在杂交细胞中可使其表达脆性位点。此外，当杂交细胞在细胞遗传学收获前暴露于咖啡因时，传递男性可因其显著更高的表达率而与正常男性区分开来。一只雄性黑猩猩在杂交细胞中也表现出低水平表达。这些数据表明，杂交系统降低了脆性X表达的阈值，Xq27处的一个脆性位点可能存在于所有人类和黑猩猩的X染色体上，构成了一个先前未被认识的常见脆性位点，并且经咖啡因后处理的杂交系统能够区分对照男性的常见Xq27脆性位点、传递男性的隐匿突变脆性位点以及患有XLMR的受影响男性的完全表达的脆性位点。因此，产生XLMR的突变可能代表了自然发生的DNA序列的多步骤改变，产生了细胞遗传学表达的连续体以及临床表现的阈值。
