Kahan B, DeMars R
Proc Natl Acad Sci U S A. 1975 Apr;72(4):1510-4. doi: 10.1073/pnas.72.4.1510.
Evidence for derepression of the gene for hypoxanthine phosphoribosyltransferase (HPRT; IMP: pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) on the human inactive X chromosome was obtained in hybrids of mouse and human cells. The mouse cells lacked HPRT and were also deficient in adenine phosphoribosyltransferase (APRT; AMP: pyrophosphate phosphoribosyltransferase; EC2.4.2.7). The human female fibroblasts were HPRT-deficient as a consequence of a mutation on the active X but contained a normal HPRT gene on the inactive X. The two human X chromosomes were further distinguished by differences in morphology: the inactive X was morphologically normal while the active X included most of the long arm of autosome no. 1 translocated to the distal end of the X long arm. Forty-one hybrid clones were first isolated by selection for the presence of APRT; when these clones were selected for HPRT, six of them yielded derivatives having human HPRT with incidences of about 1 in 10-6 APRT-selected hybrid cells. The HPRT-positive derivatives contained a normal-appearing X chromosome indistinguishable from the inactive X of the parental human fibroblasts. The active X with the translocation was not found in any of the HPRT-positive hybrid cells. Human phosphoglycerokinase (ATP:3-phospho-D-glycerate 1-phosphotransferase. EC 2.7.2.3) and glucose-6-phosphate dehydrogenase (D-glucose 6-phosphate: NADP 1-oxidoreductase, EC 1.1.1.49), which are specified by X-chromosomal loci, were not detected in the hybrids expressing HPRT even though they contained an apparently intact X chromosome. The observations are most simply explained by the infrequent, stable derepression of inactive X chromosome segments that include the HPRT locus but not the phosphoglycerokinase and glucose-6-phosphate dehydrogenase loci.
在小鼠和人类细胞的杂交体中，获得了人类失活X染色体上次黄嘌呤磷酸核糖基转移酶（HPRT；IMP：焦磷酸磷酸核糖基转移酶，EC 2.4.2.8）基因去抑制的证据。小鼠细胞缺乏HPRT，并且腺嘌呤磷酸核糖基转移酶（APRT；AMP：焦磷酸磷酸核糖基转移酶；EC2.4.2.7）也有缺陷。人类女性成纤维细胞由于活性X染色体上的突变而缺乏HPRT，但在失活X染色体上含有正常的HPRT基因。两条人类X染色体在形态上还有其他差异：失活X染色体形态正常，而活性X染色体包含了1号常染色体的大部分长臂，该长臂易位到了X染色体长臂的远端。首先通过选择具有APRT的细胞分离出了41个杂交克隆；当选择这些克隆中的HPRT时，其中6个产生了具有人类HPRT的衍生物，其发生率约为每10^6个APRT选择的杂交细胞中有1个。HPRT阳性衍生物含有一条外观正常的X染色体，与亲代人类成纤维细胞的失活X染色体无法区分。在任何HPRT阳性杂交细胞中都未发现带有易位的活性X染色体。即使杂交细胞含有一条明显完整的X染色体，但在表达HPRT的杂交细胞中未检测到由X染色体基因座指定的人类磷酸甘油激酶（ATP：3-磷酸-D-甘油酸1-磷酸转移酶，EC 2.7.2.3）和葡萄糖-6-磷酸脱氢酶（D-葡萄糖6-磷酸：NADP 1-氧化还原酶，EC 1.1.1.49）。这些观察结果最简单的解释是，包含HPRT基因座但不包含磷酸甘油激酶和葡萄糖-6-磷酸脱氢酶基因座的失活X染色体片段偶尔会发生稳定的去抑制。
