BRCA1 at a branch point.
BRCA1 at a branch point.
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BRCA1 位于分支点。
DOI：
10.1073/pnas.121184998
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发表时间：
2001
影响因子：
11.1
通讯作者：
Parvin,JD
中科院分区：
文献类型：
--
作者：
Parvin,JD
乳腺和卵巢特异性肿瘤抑制蛋白BRCA 1参与调节受损DNA修复、染色质重塑和转录的核过程。尽管许多人已经寻找了BRCA 1结合的特异性DNA序列，但没有报道过这样的序列。在本期PNAS（1）中描述的新研究中，BRCA 1蛋白以序列非特异性方式与DNA高亲和力结合。纯化的重组BRCA 1作为多聚体与DNA结合，BRCA 1蛋白通过抓取多个长度的DNA产生DNA环（图1，转载自参考文献1）。长DNA分子与BRCA 1的结合亲和力高于短DNA分子。对于给定长度的DNA，分支DNA分子比线性分子以更高的亲和力结合。对分支DNA的较高亲和力和序列特异性的缺乏支持BRCA 1在修复受损DNA中的直接作用。有了这一新发现，BRCA 1领域可能更接近了解这种蛋白质的突变如何导致乳腺癌和卵巢癌。BRCA 1在基因组维持中的作用首先由使用BRCA 1特异性抗体对细胞进行染色的实验提出。已经注意到BRCA 1在细胞周期的S期期间在细胞核中以及在邻近活跃的减数分裂重组的DNA束上形成病灶（2），尽管这些病灶如何与修复过程相对应还不清楚。BRCA 1病灶与Rad 51或Rad 50以及其他已知的修复因子（如PCNA）共定位（2-4）。这些发现和当前的数据符合Paull，Gellert及其合作者开发的BRCA 1修复功能的框架（1）。对基因组的损伤导致组蛋白H2 A家族成员H2 AX非常快速地磷酸化为-H2 AX（5），然后将BRCA 1募集到损伤部位（3）。考虑到Paull等人的新发现，(1)BRCA 1可以在DNA断裂的位点与DNA结合，并募集Mre 11 Rad 50 Nbs 1（MRN）复合物或Rad 51。在同一组织培养皿中的一些细胞中，MRN被募集到BRCA 1的病灶，而在一些细胞中，Rad 51被募集，但从未同时被募集。因此，BRCA 1可能在修复途径的选择中作用于分支点。病灶形成的时间表明，MRN或Rad 51在2-5小时后与这些受损DNA病灶相关（3，4），但大多数双链断裂修复在第一个小时内完成（6）。辐照细胞原位分离后收集的数据表明，MRN复合体与早期时间点（10分钟）的DNA损伤相关，未经分离而变得明显的病灶代表难以修复的DNA损伤（7）。MRN复合物通过两种机制参与DNA双链断裂的修复，即非同源末端连接（NHEJ）和同源重组（HR）（8）。BRCA 1存在于受损DNA的病灶中，被认为与MRN或Rad 51相互作用以刺激损伤的修复。由MRN介导的修复过程已被证明需要其亚基之一Mre 11的核酸外切酶活性。然而，Paull et al. (1)报道，BRCA 1与DNA的结合抑制了MRN的核酸外切酶活性，这是通过在BRCA 1存在下DNA片段化量的减少来测量的。因此，BRCA 1可能会抑制由MRN介导的修复过程。与BRCA 1抑制NHEJ一致，其他研究表明，细胞中BRCA 1活性的缺乏导致HR过程几乎完全丧失，并增强MRN介导的NHEJ。
The breast-and ovarian-specific tumor suppressor protein, BRCA1, has been implicated in regulating the nuclear processes of repair of damaged DNA, chromatin remodeling, and transcription. Although many have sought specific DNA sequences bound by BRCA1, no such sequences have been reported. In new research described in this issue of PNAS (1), BRCA1 protein is shown to bind to DNA with high affinity in a sequence-nonspecific fashion. Purified recombinant BRCA1 bound to DNA as a multimer, and the BRCA1 protein generated DNA loops by grabbing multiple lengths of DNA (Fig. 1, reprinted from ref. 1). Long DNA molecules were bound by BRCA1 with higher affinity than shorter molecules. For a given length of DNA, branched DNA molecules bound with higher affinity than did the linear molecules. The higher affinity for branched DNAs and the absence of sequence specificity support a direct role for BRCA1 in the repair of damaged DNA. With this new finding, the BRCA1 field is perhaps a step closer toward learning how mutation of this protein can lead to breast and ovarian cancer. The role for BRCA1 in the maintenance in the genome was first suggested by experiments in which cells were stained by using BRCA1-specific antibodies. BRCA1 had been noted to form foci in nuclei during the S phase of the cell cycle and on tracts of DNA neighboring active meiotic recombination (2), although exactly how these foci correspond with the repair process is not well understood. The BRCA1 foci colocalize with Rad51 or with Rad50 as well as with other known repair factors, such as PCNA (2–4). These findings and the current data fit in the framework of BRCA1 function in repair being developed by Paull, Gellert, and collaborators (1). Damage to the genome results in the very rapid phosphorylation of histone H2A family member H2AX to-H2AX (5), which then recruits BRCA1 to the site of damage (3). Taking into account the new findings of Paull et al.(1), BRCA1 then may bind to DNA at the site of the DNA break and recruit either the Mre11Rad50Nbs1 (MRN) complex or Rad51. In some cells in the same tissue culture dish, MRN is recruited to the foci of BRCA1, and in some cells Rad51 is recruited, but never both. Thus, BRCA1 may act at a branch point in the choice of repair pathway. The timing of focus formation suggests that the MRN or Rad51 associate with these foci of damaged DNA after 2–5 hours (3, 4), but most double-strand break repair is completed in the first hour (6). Data collected after in situ fractionation of irradiated cells suggest that the MRN complex associates with the DNA lesions at earlier time points (10 min), and the foci that become apparent without fractionation represent DNA lesions that were refractory to repair (7). The MRN complex participates in the repair of double-stranded breaks in the DNA via two mechanisms, nonhomologous end joining (NHEJ) and homologous recombination (HR)(8). BRCA1, present in the foci of damaged DNA, is presumed to interact with either MRN or Rad51 to stimulate the repair of the lesion. The repair process, as mediated by MRN, has been shown to require the exonuclease activity of one of its subunits, Mre11. However, Paull et al.(1) report that the BRCA1 binding to DNA inhibited the exonuclease activity of MRN as measured by decreased amounts of DNA fragmentation in the presence of BRCA1. Thus, BRCA1 may be expected to inhibit the repair process mediated by MRN. Consistent with BRCA1 inhibition of NHEJ, other studies have shown that the absence of BRCA1 activity in a cell results in a nearly complete loss in the HR process and an enhancement of the MRN-mediated NHEJ …