Molecular mechanisms of GBM radioresistance and strategies for radiosensitization

描述（由申请人提供）：多形性胶质母细胞瘤（GBM）肿瘤是30 - 50岁成人癌症相关死亡的第三大原因，尽管它们占美国每年报告的所有新发癌症病例的不到1.5%。GBM的非常高的死亡率（2年时>90%）在过去40年中保持相对不变，尽管进行了积极的治疗，包括手术切除、放疗和化疗。然而，随机试验已经确定了切除后放射的明显生存优势，表明GBM患者的中位生存期得到改善，在接近最大脑耐受剂量的电离辐射后，从约6个月提高到10-12个月。具有里程碑意义的Stupp研究也说明了放射治疗在治疗GBM中的核心作用，该研究表明，同时进行放射治疗和替莫唑胺可以进一步将中位生存期从12.1个月增加到14.6个月。因此，放射治疗仍然是GBM治疗的主要手段，Stupp研究的成功虽然不大，但为GBM的放射治疗提供了进一步改善的希望。治疗的任何改进都需要对GBM辐射抗性的基础有更深入的了解。GBM治疗缺乏改善的部分原因是缺乏适当的遗传模型，可用于描述GBM中发生的遗传变化对辐射抗性的影响。最近由癌症基因组图谱网络绘制的GBM基因组图谱显示，这些肿瘤已经从根本上改变了基因组，具有许多突变，基因拷贝数的增加和丢失以及甲基化变化。在构成GBM基因组景观的无数遗传改变中，5个关键的遗传改变占主导地位：Ink 4a、Arf、p53或PTEN的丢失和EGFR（特别是组成型活性EGFRvIII）的扩增。这些关键的遗传畸变中的哪一个可能赋予治疗抗性仍不清楚。了解这些病变的贡献，单独和组合，GBM辐射抗性沿着的潜在机制（S）将是至关重要的，在开发更有效的治疗方式。为了实现这一目标，我们计划使用条件性基因敲除小鼠模型，其中这些关键的遗传病变可以引入培养物中的星形胶质细胞和神经干细胞（NSC）中，并使用Cre-ERT 2系统引入小鼠大脑中。我们建议研究这些遗传变化如何影响辐射诱导的DNA双链断裂（DSB）的修复机制。基于我们的初步结果，我们假设EGFRvIII-PI 3 K-Akt轴和DNA修复酶DNA-PKcs之间的串扰是GBM的辐射抗性的基础，并且这种连接可以针对有效的放射治疗。具体而言，我们建议：1。证实GBM放射抗性是由关键GBM特异性遗传病变之间的相互作用赋予的。2.验证Akt磷酸化DNA-PKcs促进辐射损伤DNA的修复。3.为了测试Akt-DNA-PKcs是否是一个脆弱的节点，可以靶向改善GBM的放射治疗。 公共卫生相关性：多形性胶质母细胞瘤是成人中最常见和最具侵袭性的原发性脑肿瘤，尽管采用了包括手术切除、化疗和放疗在内的侵袭性治疗方案，但仍普遍致命。我们建议了解胶质母细胞瘤的辐射抗性的基础，并假设这是由于熟练修复辐射诱导的DNA损伤，由于GBM相关的遗传变化激活特定的信号通路。这将有助于确定脆弱的节点，可以针对这些肿瘤的有效放射增敏。

DESCRIPTION (provided by applicant): Glioblastoma Multiforme (GBM) tumors are the third leading cause of cancer-related death among adults aged 30 - 50 years, even though they account for less than 1.5% of all new cancer cases reported in the United States each year. The very high mortality rate of GBMs (>90% at 2 years) has remained relatively unchanged over the past 40 years despite aggressive therapy that includes surgical resection, radiotherapy and chemotherapy. However, a clear survival advantage of post-resection radiation has been established by randomized trials, showing that the median survival of GBM patients was improved, from approximately 6 months to 10-12 months, following near-maximal brain-tolerated doses of ionizing radiation. The central role that radiation plays in treating GBM is also illustrated by the landmark Stupp study showing that concurrent radiation and Temozolomide can further increase median survival from 12.1 to 14.6 months. Thus, radiation remains the mainstay of GBM therapy, and the success of the Stupp study, though modest, offers hope that radiotherapy of GBMs can be further improved. Any improvement in therapy would require a more mechanistic understanding of the basis of GBM radioresistance. The lack of improvement in GBM treatment is partly due to a paucity of appropriate genetic models that can be used to delineate the effects of genetic changes that occur in GBMs on radioresistance. The recent mapping of the GBM genome by the Cancer Genome Atlas Network revealed that these tumors have radically altered genomes with many mutations, gene copy number gains and losses, and methylation changes. Amongst the myriad genetic alterations that populate the GBM genomic landscape, 5 key genetic alterations dominate: loss of Ink4a, Arf, p53, or PTEN and amplification of EGFR (especially, the constitutively active EGFRvIII). Which of these key genetic aberrations may confer therapeutic resistance remains unclear. Understanding the contribution of these lesions, singly and in combination, to GBM radioresistance along with the underlying mechanism(s) will be of paramount importance in developing more effective therapeutic modalities. Towards this goal, we plan to use conditional knock out mouse models wherein these key genetic lesions can be introduced in astrocytes and neural stem cells (NSCs) both in culture and in the mouse brain using the Cre-ERT2 system. We propose to examine how these genetic changes impact on mechanisms for the repair of radiation-induced DNA double-strand breaks (DSBs). Based upon our preliminary results, we hypothesize that cross talk between the EGFRvIII-PI3K-Akt axis and the DNA repair enzyme, DNA-PKcs, underlies the radioresistance of GBMs and that this connection can be targeted for effective radiotherapy. Specifically, we propose: 1. To confirm that GBM radioresistance is conferred by interactions between key GBM-specific genetic lesions. 2. To validate that phosphorylation of DNA-PKcs by Akt promotes the repair of radiation-induced DNA breaks. 3. To test whether Akt-DNA-PKcs is a vulnerable node that can be targeted to improve radiotherapy of GBMs. PUBLIC HEALTH RELEVANCE: Narrative Glioblastoma multiforme is the most common and aggressive primary brain tumor in adults and is universally fatal despite aggressive treatment regimens including surgical resection, chemotherapy, and radiotherapy. We propose to understand the basis of radioresistance in glioblastomas and postulate that this is due to proficient repair of radiation-induced DNA damage due to activation of specific signaling pathways by GBM-relevant genetic changes. This would help to identify vulnerable nodes that could be targeted for effective radiosensitization of these tumors.