The Role of BRCA1/BARD1 in Basal-Like Breast Cancer

目前仍不清楚BRCA1如何抑制正常乳腺上皮细胞的肿瘤形成，以及为什么BRCA1 突变携带者会患上基底样乳腺癌。在体内，BRCA1以异二聚体的形式存在 BARD1蛋白，其许多生物学特性是通过BRCA1/BARD1复合体介导的。 我们最近使用条件性突变技术证明可以在小鼠体内诱发基底细胞样乳腺癌。 通过特定于乳房的Bardl或Brcal的失活。共同的碱基样表型 Bard1和Brc1突变的乳腺癌意味着BRC A1介导的肿瘤抑制 由BRCA1/BARD1异源二聚体实现。与此同时，生化研究表明， 异源二聚体是一种有效的泛素E3连接酶。此外，我们还发现BRCA1/BARD 1诱导了 形成在结构上不同于传统的K48连接的K6连接的多泛素链 为蛋白酶体降解标记细胞蛋白质的链，最近的研究表明，蛋白质 轴承K6链条不是周转目标。此外，由于BRC A1/BARD 1的E3活性 被肿瘤相关的BRCA1错义突变消融，异二聚体的酶功能是 很可能是肿瘤抑制所必需的。因此，为了阐明BRCA1-1的分子机制。 我们将评估BRCA1/BARD1异源二聚体及其相关的E3 连接酶活性促进肿瘤抑制并研究BRCA1/BARD1功能中断为何导致 形成基底样乳腺癌。特别是，我们将1)确定E3连接酶的活性是否 BRCA1/BARD1是正常发育和BRCA1介导的肿瘤抑制所必需的，2)检查 E3连接酶活性在双链DNA断裂修复和细胞周期检查点控制中的作用 探讨BRCA1/Bard 1介导泛素信号转导的分子机制，以及4)检测 BRCA1和PTEN通路在基底细胞样乳腺癌发生发展中的相互作用。

It is still unclear how BRCA1 suppresses rumorigenesis in normal mammary epithelial cells and why BRCA1 mutation carriers develop basal-like breast carcinomas. In vivo, BRCA1 exists as a heterodimer with the BARD1 protein, and many of its biological properties are mediated through the BRCA1/BARD1 complex. We recently used conditional mutagenesis to show that basal-like breast carcinomas can be induced in mice by mammary-specific inactivation of either Bardl or Brcal. The common basal-like phenotype shared by the Bardl- and Brcal-mutant mammary carcinomas implies that BRC A1-mediated tumor suppression is implemented by the BRCA1/BARD1 heterodimer. Meanwhile, biochemical studies have show that the heterodimer is a potent ubiquitin E3 ligase. Moreover, we have found that BRCA1/BARD1 induces the formation of K6-linked polyubiquitin chains that are structurally distinct from the conventional K48-linked chains that mark cellular proteins for proteasomal degradation, and recent studies indicate that proteins bearing K6-linked chains are not targeted for turnover. Furthermore, since the E3 activity of BRC A1/BARD 1 is ablated by tumor-associated BRCA1 missense mutations, the enzymatic function of the heterodimer is likely to be essential for tumor suppression. Thus, to elucidate the molecular mechanisms of BRCA1- mediated tumor suppression, we will evaluate how the BRCA1/BARD1 heterodimer and its associated E3 ligase activity promote tumor suppression and investigate why disruption of BRCA1/BARD1 function leads to formation of basal-like breast cancer. In particular, we will 1) determine whether the E3 ligase activity of BRCA1/BARD1 is required for normal development and BRCA1-mediated tumor suppression, 2) examine the role of this E3 ligase activity in double-strand DNA break repair and cell cycle checkpoint control, 3) explore the molecular mechanisms of ubiquitin-mediated signaling by BRCA1 /BARD 1, and 4) examine interactions between the BRCA1 and PTEN pathways in the development of basal-like breast cancer.