三篇Nature文章聚焦癌症表观基因组靶向疗法
生物通 · 2016/01/11
与去年发表在《自然》(Nature)杂志上的两篇研究论文一起,在线发表于本周Nature杂志上的一篇论文提供了有关表观基因组靶向药物BET抑制剂潜力的一些新见解,并描述了肿瘤对这些药物产生耐药的一些机制。


许多现代的抗癌药物都是靶向突变激活的蛋白,但这种治疗策略有一定的局限性。在人类肿瘤中只能看到相对少量的频发突变,且耐药形成非常地迅速。靶向表观基因组是正在探索的替代疗法之一。与去年发表在《自然》(Nature)杂志上的两篇研究论文一起,在线发表于本周Nature杂志上的一篇论文提供了有关表观基因组靶向药物BET抑制剂潜力的一些新见解,并描述了肿瘤对这些药物产生耐药的一些机制。


人们早就知道肿瘤细胞具有一些独特的表观基因组特征,可导致过量生成诸如MYC一类的促癌转录因子。转录因子是一些具有挑战性的治疗靶点,因为它们缺乏药物能够轻易靶向的结构。但随着我们对影响基因表达的表观基因组调控因子认识的不断深入,许多似乎是“可以用药物控制”,从而为绕开这一障碍提供了一条潜在的途径。

Bromodomain蛋白家族,包括BET亚家族(BRD2, 3, 4 and T)便是其中的一些因子。BET蛋白包含两个溴结构域(Bromodomain),每个结构域有一些小口袋。这些口袋可以结合标记乙酰基的组蛋白,使得BET蛋白能够招募细胞的转录机器至基因组中的特异位点调控基因表达。BET亚家族成员如BRD4可以调控MYC基因转录,其与多种肿瘤(尤其是血癌)有关联,因此成为了治疗的候选靶点。

几年前,人们发现了第一个小分子BET抑制剂(JQ1),并证实其能够有效破坏癌细胞增殖。这一效应似乎反映出了对BET介导的MYC表达调控的抑制作用。早期在白血病和淋巴瘤中的BET抑制剂临床试验取得了一些令人鼓舞的结果。研究人员现正在探究这些抑制剂有可能发挥作用的其他疾病状况,并预测将不可避免地出现的获得性耐药机制。

2015年的两项研究将焦点放在了急性髓细胞性白血病(AML)中BET抑制的潜在耐药机制上。在第一项研究中,Rathert等筛查了小鼠的AML细胞寻找AML细胞存活必需的染色质修饰因子。他们证实AML细胞需要Brd4,并鉴别出了赋予AML细胞对JQ1耐药性的其他几个因子。在JQ1耐药AML细胞中,作者们观察发现在DNA增强子区域一些特异性表观基因组特征发生了改变。这些改变意味着可以在没有Brd4的情况下激活MYC基因表达。

Rathert等还发现,与已知的肿瘤形成驱动子——Wnt信号通路相关的一些基因在耐药细胞中上调。Wnt激活有可能通过在耐药细胞中特异生成的启动子处驱动MYC转录,成为了促进JQ1耐药的充足条件。最后,作者们在一些其他类型的癌症及来自白血病患者的血细胞中发现了相同的JQ1耐药机制。这些数据表明,结合一些Wnt信号通路抑制剂可以扩大BET抑制剂的作用。

采取一种不同的方法,在第二项研究中Fong等通过让小鼠AML细胞持续暴露于另一种BET抑制剂下,赋予了这些细胞对BET抑制的耐药性,最终生成了一些耐药克隆群。这项研究也证实,Wnt激活在耐药中起作用。并且耐药细胞具有一些干细胞特征,表明AML癌症干细胞群对BET抑制不产生反应。

以往的研究证实,Wnt信号通路与AML癌症干细胞耐药有关。并且,很好地确立了癌症干细胞的耐药特性。但Rathert和同事们并未找到耐药AML细胞具有干细胞特征的证据——这是两份研究报道之间的区别。

在本周最新的研究中,Shu等在人类乳腺癌中探讨了BET抑制。通过分析一组乳腺癌细胞系,他们观察到一种癌症亚型:“三阴性”乳腺癌对BET抑制敏感。像Fong等一样,作者们通过在JQ1中培养敏感的三阴性细胞模拟出了对BET抑制的获得性耐药,随后确定了出现的耐药细胞的特征。发现耐药细胞仍然依赖于BRD4,但这种依赖与蛋白质的bromodomain无关。

相比于敏感细胞,在耐药细胞中一种广泛激活的转录调控蛋白MED1更紧密地结合了BRD4。作者们将这种更紧密地结合归因于CK2酶介导促进了BRD4磷酸化。这种结合导致了bromodomain非依赖性、BRD4介导的MYC转录激活。这些数据表明组合CK2和BET抑制剂治疗三阴性乳腺癌或许可防止耐药。

尽管以往许多研究已在动物及细胞模型中证实了药物对三阴性乳腺癌的疗效,值得注意的是这些药物至今都未能对抗人体的肿瘤。因此,应对此持精神乐观的态度。

总而言之,这三篇研究论文表明了BET抑制剂或许具有比以往认识到的要更广泛的潜力。它们还强调了联合其他药物来使用BET抑制剂克服先天与获得性耐药的可能性。
查看更多
  • Response and resistance to BET bromodomain inhibitors in triple-negative breast cancer

    Triple-negative breast cancer (TNBC) is a heterogeneous and clinically aggressive disease for which there is no targeted therapy1, 2, 3. BET bromodomain inhibitors, which have shown efficacy in several models of cancer4, 5, 6, have not been evaluated in TNBC. These inhibitors displace BET bromodomain proteins such as BRD4 from chromatin by competing with their acetyl-lysine recognition modules, leading to inhibition of oncogenic transcriptional programs7, 8, 9. Here we report the preferential sensitivity of TNBCs to BET bromodomain inhibition in vitro and in vivo, establishing a rationale for clinical investigation and further motivation to understand mechanisms of resistance. In paired cell lines selected for acquired resistance to BET inhibition from previously sensitive TNBCs, we failed to identify gatekeeper mutations, new driver events or drug pump activation. BET-resistant TNBC cells remain dependent on wild-type BRD4, which supports transcription and cell proliferation in a bromodomain-independent manner. Proteomic studies of resistant TNBC identify strong association with MED1 and hyper-phosphorylation of BRD4 attributable to decreased activity of PP2A, identified here as a principal BRD4 serine phosphatase. Together, these studies provide a rationale for BET inhibition in TNBC and present mechanism-based combination strategies to anticipate clinical drug resistance.

    展开 收起
  • BET inhibitor resistance emerges from leukaemia stem cells

    Bromodomain and extra terminal protein (BET) inhibitors are first-in-class targeted therapies that deliver a new therapeutic opportunity by directly targeting bromodomain proteins that bind acetylated chromatin marks1, 2. Early clinical trials have shown promise, especially in acute myeloid leukaemia3, and therefore the evaluation of resistance mechanisms is crucial to optimize the clinical efficacy of these drugs. Here we use primary mouse haematopoietic stem and progenitor cells immortalized with the fusion protein MLL–AF9 to generate several single-cell clones that demonstrate resistance, in vitro and in vivo, to the prototypical BET inhibitor, I-BET. Resistance to I-BET confers cross-resistance to chemically distinct BET inhibitors such as JQ1, as well as resistance to genetic knockdown of BET proteins. Resistance is not mediated through increased drug efflux or metabolism, but is shown to emerge from leukaemia stem cells both ex vivo and in vivo. Chromatin-bound BRD4 is globally reduced in resistant cells, whereas the expression of key target genes such as Myc remains unaltered, highlighting the existence of alternative mechanisms to regulate transcription. We demonstrate that resistance to BET inhibitors, in human and mouse leukaemia cells, is in part a consequence of increased Wnt/β-catenin signalling, and negative regulation of this pathway results in restoration of sensitivity to I-BET in vitro and in vivo. Together, these findings provide new insights into the biology of acute myeloid leukaemia, highlight potential therapeutic limitations of BET inhibitors, and identify strategies that may enhance the clinical utility of these unique targeted therapies.

    展开 收起
  • Transcriptional plasticity promotes primary and acquired resistance to BET inhibition

    Following the discovery of BRD4 as a non-oncogene addiction target in acute myeloid leukaemia (AML)1, 2, bromodomain and extra terminal protein (BET) inhibitors are being explored as a promising therapeutic avenue in numerous cancers3, 4, 5. While clinical trials have reported single-agent activity in advanced haematological malignancies6, mechanisms determining the response to BET inhibition remain poorly understood. To identify factors involved in primary and acquired BET resistance in leukaemia, here we perform a chromatin-focused RNAi screen in a sensitive MLL–AF9;NrasG12D-driven AML mouse model, and investigate dynamic transcriptional profiles in sensitive and resistant mouse and human leukaemias. Our screen shows that suppression of the PRC2 complex, contrary to effects in other contexts, promotes BET inhibitor resistance in AML. PRC2 suppression does not directly affect the regulation of Brd4-dependent transcripts, but facilitates the remodelling of regulatory pathways that restore the transcription of key targets such as Myc. Similarly, while BET inhibition triggers acute MYC repression in human leukaemias regardless of their sensitivity, resistant leukaemias are uniformly characterized by their ability to rapidly restore MYC transcription. This process involves the activation and recruitment of WNT signalling components, which compensate for the loss of BRD4 and drive resistance in various cancer models. Dynamic chromatin immunoprecipitation sequencing and self-transcribing active regulatory region sequencing of enhancer profiles reveal that BET-resistant states are characterized by remodelled regulatory landscapes, involving the activation of a focal MYC enhancer that recruits WNT machinery in response to BET inhibition. Together, our results identify and validate WNT signalling as a driver and candidate biomarker of primary and acquired BET resistance in leukaemia, and implicate the rewiring of transcriptional programs as an important mechanism promoting resistance to BET inhibitors and, potentially, other chromatin-targeted therapies.

    展开 收起
发表评论 我在frontend\modules\comment\widgets\views\文件夹下面 test