Mamon H J, Dahlberg W, Azzam E I, Nagasawa H, Muto M G, Little J B
Laboratory of Radiobiology Harvard School of Public Health 655 Huntington Avenue Boston MA 02115 USA.
Int J Radiat Biol. 2003 Oct;79(10):817-29. doi: 10.1080/09553000310001610952.
The ataxia-telangiectasia mutated (ATM) gene encodes a protein kinase, the activation of which is an early event in the cellular response to ionizing radiation. One of the many substrates of ATM is BRCA1 (breast cancer 1, early onset gene), which has been associated with susceptibility to breast and ovarian cancer, and has been implicated in DNA repair processes. Various cellular responses to radiation were analysed in cells with mutations in ATM or BRCA1 in an attempt to clarify which effects of ATM can be mediated through BRCA1.
The response to radiation of cells with mutations in ATM or BRCA1 was examined, as were BRCA1-mutant tumour cells transfected with an exogenous wild-type BRCA1 allele. Assays included cell-survival curves, studies of potentially lethal damage repair, measurement of chromosomal aberrations and of G1 arrest, and Western blot analysis of lysates of irradiated cells to determine the phosphorylation of the product of the human Mdm2 gene (HDM2).
Both ATM and BRCA1 mutations were associated with sensitivity to ionizing radiation, deficient repair of potentially lethal damage and markedly increased chromosomal aberrations. A BRCA1-mutated tumour cell line HCC1937, like ATM mutant cells, did not exhibit a normal G1 arrest but, unlike ATM mutant cells, did exhibit phosphorylation of HDM2. Expression of wild-type BRCA1 in HCC1937 cells partially restored radioresistance, restored repair of potentially lethal damage and markedly reduced radiation-induced chromosomal aberrations. G1 arrest, however, was not restored by expression of BRCA1.
The results are consistent with a model in which ATM phosphorylation of BRCA1 regulates DNA repair functions, particularly those involved in potentially lethal damage repair and chromosomal integrity, but not other aspects of the cellular response to radiation such as G1 cell cycle arrest. To the authors' knowledge, this is the first demonstration of the ability of exogenously expressed BRCA1 to restore the ability to perform potentially lethal damage repair and maintain chromosomal integrity in irradiated cells.
共济失调毛细血管扩张症突变（ATM）基因编码一种蛋白激酶，其激活是细胞对电离辐射反应的早期事件。ATM的众多底物之一是BRCA1（乳腺癌1号，早发基因），它与乳腺癌和卵巢癌易感性相关，并参与DNA修复过程。为了阐明ATM的哪些效应可通过BRCA1介导，对ATM或BRCA1发生突变的细胞中的各种辐射细胞反应进行了分析。
检测了ATM或BRCA1发生突变的细胞对辐射的反应，以及转染了外源性野生型BRCA1等位基因的BRCA1突变肿瘤细胞。检测方法包括细胞存活曲线、潜在致死性损伤修复研究、染色体畸变和G1期阻滞的测量，以及对受辐射细胞裂解物进行蛋白质印迹分析以确定人Mdm2基因（HDM2）产物的磷酸化情况。
ATM和BRCA1突变均与对电离辐射敏感、潜在致死性损伤修复缺陷及染色体畸变显著增加相关。BRCA1突变的肿瘤细胞系HCC1937与ATM突变细胞一样，未表现出正常的G1期阻滞，但与ATM突变细胞不同的是，它表现出HDM2的磷酸化。野生型BRCA1在HCC1937细胞中的表达部分恢复了放射抗性，恢复了潜在致死性损伤的修复，并显著减少了辐射诱导的染色体畸变。然而，BRCA1的表达并未恢复G1期阻滞。
这些结果与一种模型一致，即BRCA1的ATM磷酸化调节DNA修复功能，特别是那些参与潜在致死性损伤修复和染色体完整性的功能，但不调节细胞对辐射反应的其他方面，如G1期细胞周期阻滞。据作者所知，这是首次证明外源性表达的BRCA1能够恢复受辐射细胞进行潜在致死性损伤修复和维持染色体完整性的能力。
