Xeroderma pigmentosum (XP) is a rare hereditary human disorder clinically associated with severe sun sensitivity and predisposition to skin cancer. Some XP patients also show clinical characteristics of Cockayne syndrome (CS), a disorder associated with defective preferential repair of DNA lesions in transcriptionally active genes. Cells from the two XP-patients who belong to complementation group D and exhibit additional clinical symptoms of CS are strikingly more sensitive to the cytotoxic effects of UV-light than cells from classical XP-D patients. To explain the severe UV-sensitivity it was suggested that XP-D-CS cells have a defect in preferential repair of UV-induced 6-4 photoproducts (6-4PP) in active genes. We investigated the capacity of XP-D and XP-D-CS cells to repair UV-induced DNA lesions in the active adenosine deaminase gene (ADA) and in the inactive 754 gene by determining (i) the removal of specific lesions, i.e. cyclobutane pyrimidine dimers (CPD) and 6-4PP, or (ii) the formation of BrdUrd-labeled repair patches. No differences in repair capacity were observed between XP-D and XP-D-CS cells. In both cell types repair of CPD was completely absent whereas 6-4PP were inefficiently removed from the ADA gene and the 754 gene with similar kinetics. However, whereas XP-D cells were able to restore UV-inhibited RNA synthesis after a UV-dose of 2 J/m2, RNA synthesis in XP-D-CS cells remained repressed up to 24 h after irradiation. Our results are inconsistent with the hypothesis that differences in the capacity to perform preferential repair of UV-induced photolesions in active genes between XP-D and XP-D-CS cells are the cause of the extreme UV-sensitivity of XP-D-CS cells. Rather, the enhanced sensitivity of XP-D-CS cells may be associated with a defect in transcription regulation superimposed on the repair defect.

译文

着色性干皮病 (XP) 是一种罕见的遗传性人类疾病,临床上与严重的阳光敏感性和皮肤癌易感性相关。一些XP患者还表现出Cockayne综合征 (CS) 的临床特征,这是一种与转录活性基因中DNA损伤的优先修复缺陷相关的疾病。来自两名XP患者的细胞属于补充D组,并表现出CS的其他临床症状,与经典xp-d患者的细胞相比,对紫外线的细胞毒性作用更为敏感。为了解释严重的紫外线敏感性,建议XP-d-cs细胞在活性基因中优先修复紫外线诱导的6-4光产物 (6-4PP) 方面存在缺陷。我们通过确定 (i) 去除特定病变,即环丁烷嘧啶二聚体 (CPD) 和6-4PP,研究了XP-D和XP-D-CS细胞修复活性腺苷脱氨酶基因 (ADA) 和非活性754基因中紫外线诱导的DNA损伤的能力,或 (ii) 形成BrdUrd标记的修复斑块。在XP-D和XP-D-CS细胞之间未观察到修复能力的差异。在两种细胞类型中,完全没有CPD的修复,而以相似的动力学从ADA基因和754基因中无效地去除6-4PP。然而,尽管xp-d细胞在2 J/m2的紫外线剂量后能够恢复受紫外线抑制的RNA合成,但在照射后24小时内,xp-d-cs细胞中的RNA合成仍受到抑制。我们的结果与以下假设不一致: xp-d和xp-d-cs细胞之间活性基因中紫外线诱导的光损伤的优先修复能力差异是xp-d-cs细胞极度紫外线敏感性的原因。相反,xp-d-cs细胞的敏感性增强可能与叠加在修复缺陷上的转录调控缺陷有关。

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