Liber H L, Call K M, Mascioli D A, Thilly W G
Mutat Res. 1985 Aug;151(1):95-108. doi: 10.1016/0027-5107(85)90188-5.
We have studied the effects of 5-bromodeoxyuridine (BrdUrd) at two genetic loci in diploid human lymphoblast cells. In thymidine kinase heterozygotes (tk +/-), a 2-h dose of BrdUrd induced a transient, non-heritable resistance to the thymidine analogue, trifluorothymidine (F3TdR). We have called this phenomenon pseudomutation and have shown that affected cells acquire the ability to survive in the presence of F3TdR and then, after degradation of F3TdR in the medium, return to an apparently normal wild-type state. Our data suggest that BrdUrd incorporation into DNA as a thymidine analogue is responsible for the effect, which we interpret as a temporary loss of thymidine kinase activity. This effect is not seen in tk +/+ homozygotes. In contrast, at the hypoxanthine-guanine phosphoribosyl transferase locus in tk +/- heterozygotes, BrdUrd did not induce a permanent, heritable resistance to 6-thioguanine (gene locus mutation). We detected such mutations only in the tk +/+ homozygote and only at external BrdUrd concentrations considerably higher than those which saturate the uptake of BrdUrd into DNA as a thymidine analogue. We postulate that the reduced TK enzyme levels (30%) in the heterozygote prevent the build-up of a sufficiently high intracellular BrdUrd triphosphate pool to promote the misincorporations as deoxycytidine triphosphate which may be responsible for gene locus mutation.
我们研究了5-溴脱氧尿苷（BrdUrd）对二倍体人类淋巴母细胞两个基因位点的影响。在胸苷激酶杂合子（tk+/-）中，2小时剂量的BrdUrd诱导了对胸苷类似物三氟胸苷（F3TdR）的短暂、非遗传性抗性。我们将这种现象称为假突变，并表明受影响的细胞获得了在F3TdR存在下存活的能力，然后，在培养基中F3TdR降解后，恢复到明显正常的野生型状态。我们的数据表明，作为胸苷类似物掺入DNA的BrdUrd是造成这种影响的原因，我们将其解释为胸苷激酶活性的暂时丧失。在tk+/ +纯合子中未观察到这种效应。相比之下，在tk+/-杂合子的次黄嘌呤-鸟嘌呤磷酸核糖转移酶位点，BrdUrd并未诱导对6-硫鸟嘌呤的永久性、遗传性抗性（基因位点突变）。我们仅在tk+/ +纯合子中检测到这种突变，并且仅在外部BrdUrd浓度远高于作为胸苷类似物使BrdUrd掺入DNA达到饱和的浓度时才检测到。我们推测，杂合子中降低的TK酶水平（30%）阻止了足够高的细胞内三磷酸BrdUrd池的积累，从而促进了可能导致基因位点突变的作为三磷酸脱氧胞苷的错误掺入。
