CBMT-32. EGFR SUPPRESSES p53 FUNCTION THROUGH DNA-PKcs BINDING TO p53: NOVEL CROSSTALK BETWEEN EGFR AND TP53 IN GLIOBLASTOMA

Comprehensive analysis of the TCGA Glioblastoma database has identified 3 prognostically relevant subgroups (Proneural, Classical and Mesenchymal). Interestingly, it has also revealed a near mutual exclusivity of EGFR amplification and TP53 mutations. There are only a few tumors with both amplified EGFR and mutant TP53. We decided to investigate the functional relationship between EGFR and TP53 relative to tumorigenesis and tumor proliferation. We selected from our molecularly characterized GSC bank a series of EGFR amplified+TP53wt/TP53mut lines for this study. Here we report that EGFR amplification overrides the effect of p53wt but co-regulates tumorigenesis with TP53R175H mutation. We show that CRISPR EGFR knockout in TP53wt cells leads to complete loss of cell growth suggesting that EGFR is essential for cell survival, providing growth signal and suppression of p53wt function. We further show that EGFR induces physical binding between p53wt and DNA-PKcs, which suppresses p53wt by inhibiting p53 phosphorylation at S15. The knockdown of DNA-PKcs restores p53wt anti-tumor function. In contrast, GSC262 harboring TP53R175H mutation is the only GSC that survives after EGFR knockout, suggesting that this specific mutation has a gain of function (GOF) to sustain growth without EGFR. This is confirmed by dual knockout of EGFR and TP53R175H showing no surviving cells. Further investigation shows both EGFR and p53R175H induce tumor glycolysis, the major energy metabolism and heightened growth signal for this group. We further show that EGFR mediated suppression of p53R175H is through DNA-PKcs binding to p53R175H and reduced S15 phosphorylation. Taken together, this study reveals 1) EGFR amplification is essential for tumor growth in TP53wt GSCs; 2) novel crosstalk between EGFR and p53 through DNA-PKcs. 3) EGFR suppresses p53wt function by providing the energy required for tumor growth through inducing glycolysis, and 4) TP53R175H is a GOF mutation that sustains life without EGFR by inducing glycolysis.

Abstract A10: The myc post-translational landscape: How novel gain-of-function mutants are revealing new stability and functional regulatory systems

The c-MYC (MYC) oncogene plays an important role in tumorigenesis and is implicated in >50% of all human cancers. Deregulation of MYC can occur through abnormally high expression levels, but also through oncogenic lesions in upstream signaling cascades. The study of these signaling pathways have provided an alternative approach for the development of MYC-targeted therapeutics. For example, the study of post-translational modifications (PTMs) of MYC, such as P-T58 and the T58A gain-of-function mutant, identified FBXW7 as a tumor suppressor and the deubiquitinating enzyme USP28 as a therapeutic target.

We considered that MYC is highly modified post-translationally and that unknown mechanistic pathways may be modifying residues, in addition to T58, in order to control MYC stability and/or function. These undiscovered pathways may therefore provide additional opportunities for the development of MYC-targeted therapeutics. These considerations led to recent work in the Penn lab that uncovered clusters of negatively regulating residues of MYC function. These residues include S71/S81, a cluster of residues referred to as MYC-4 (T343, S344, S347 and S348) and a cluster of 6 lysine residues (6K) at the C-terminal end of MYC (K298, K317, K323, K326, K341 and K355). These negatively regulating residues were characterized using alanine (S71/S81 and 340 cluster) and arginine (C-terminal lysines) substitution mutants in our established transformation assays. The S71/S81A and MYC-4A mutants scored with having gain-of-function activity in comparison to wild-type MYC in multiple transformation assays including growth in soft agar and the disruption of regular acini formation using a normal, immortalized MCF10A cell line. In addition, these mutants were shown to regulate additional genes compared to wild-type MYC using genome-wide mRNA expression analysis of MCF10A acini, suggesting that these MYC proteins have gained additional transcriptional targets. Additionally, substitution of the C-terminal lysine residues with arginine (6KR) also revealed gain-of-function activity. 6KR expressing MCF10A and SH-EP cells had increased anchorage-independent growth compared to cells expressing wild-type MYC and was also more potent in promoting xenograft tumor growth of Rat1A and SH-EP cells. Interestingly, all three mutants do not have extended half-lives as seen with T58A, suggesting that functional activity and not stability is contributing to these transformative phenotypes.

The above mutants reveal that each of S71/S81, MYC-4 and C-terminal 6K residues are critically important for the negative regulation of MYC-induced transformation. To further explore these regions of MYC, we used mass spectrometry to identify post-translational modifications that occurred on MYC in growing cells. These data confirm phosphorylation events on S71/81 as well as at MYC-4A. Strikingly, three modifications were directly observed on three of the six lysine residues; acetylation of lysine 323, ubiquitylation of lysine 355 and SUMOylation of lysine 326. The importance of these modifications and the roles that these modifications have in regulating MYC activity are currently under investigation using our established transformation assays. I now aim to understand the contribution of single or multiple modifications within the indicated clusters and how these modifications modulate MYC activity.

Citation Format: Corey Lourenco, Amanda Wasylishen, Michelle Chan-Seng-Yue, Christina Bros, Dharmendra Dingar, William Tu, Manpreet Kalkat, Pak-Kei Chan, Peter Mullen, Brian Raught, Paul Boutros, Linda Penn. The myc post-translational landscape: How novel gain-of-function mutants are revealing new stability and functional regulatory systems. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr A10.

Abstract 3410: A novel regulatory region of the MYC oncogene decreases Myc transcriptional activity

MYC is a transcription factor that contributes to over 50% of cancers. While deregulated protein expression of MYC is an important factor driving tumorigenesis, remarkably little is known about the post-translational modifications (PTMs) that are necessary for MYC to function as an active oncogene. Given the important role of PTMs in the regulation of protein activity and stability, our overall objective is to further characterize novel PTMs of MYC, and the functional contribution of these modifications to the ability of the MYC to transform cells. Using a structure-function approach, we have identified six C-terminal lysines that can modulate the ability of MYC to transform cells. By utilizing lysine to arginine mutations (6KR) to abrogate signaling through these residues, we show that MYC 6KR increases anchorage-independent colony growth compared with MYC-WT. Moreover, this mutant was also more potent than WT in promoting xenograft tumour growth. Through target gene expression analysis and luciferase reporter assays, we show that 6KR has enhanced transcriptional activity compared with MYC-WT. To characterize the modifications that can occur at these lysine residues, we have performed mass spectrometry analysis and have identified a number of PTMs that can occur in this region. Furthermore, we have conducted BioID to identify MYC interactors that change in association with 6KR MYC compared to MYC-WT. We theorize that identifying the signaling pathways leading to post-translational modification of this region will be an important first step in the development of inhibitors targeting MYC induced transformation.

Citation Format: Manpreet Kalkat, Amanda Wasylishen, Pak-Kei Michael Chan, Aleksandra Pandyra, Sam Sulgi Kim, Christina Bros, Brian Raught, Linda Z. Penn. A novel regulatory region of the MYC oncogene decreases Myc transcriptional activity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3410. doi:10.1158/1538-7445.AM2014-3410

Abstract A170: High-content screening for inhibitors of oncogenic transcription by c-Myc

There is evidence that Myc partners with another molecule called TRRAP to regulate genes required for transformation. Our hypothesis is that blocking the interaction of Myc with TRRAP will selectively kill breast cancer cells. Traditional drug screening approaches to identify compounds that will break apart two interacting proteins have rarely been successful. However, our approach is to screen compounds in live breast cancer cells for those that prevent Myc and TRRAP from binding to each other. Since the cell is constantly replacing Myc, a compound that prevents it from binding TRRAP should kill even established tumors. Importantly, we previously mapped the places on Myc and TRRAP that bind them together and showed they don't involve the parts of the protein involved in other important functions. Therefore, compounds we find that prevent Myc-TRRAP binding should not be toxic to normal cells.

Methods: We have established a new assay in which Myc and TRRAP are expressed as fusion proteins to Cerulean and Citrine fluorescence proteins, respectively. We are developing two different but related assays for the interaction between the two proteins. In one we rely on proximity resulting in complementation between non-fluorescent fragments of the Cerulean fluorescence protein. In this assay heterodimerization results in increased fluorescence. In the other assay proximity results in fluorescence resonance energy transfer (FRET) between the Cerulean and Citrine fluorescence proteins. We detect FRET by fluorescence lifetime imaging (FLIM).

Results: A novel robotic microscope that can automatically perform high speed FLIM and thereby detect Myc-TRRAP binding quickly and accurately has been assembled and tested. Our automated microscope has had an environmental stage for live cell imaging fitted and tested. Both FRET standards and constructs based on the Cerulean-Myc / TRRAP-Citrine pair have been measured to validate the assay. Cell lines are being optimized for use for screening.

Conclusions: High speed FLIM can be used to detect FRET between Cerulean-Myc and TRRAP-Citrine in live cells. Using our automated microscope we can test individually the effect of large numbers of small molecules in live breast cancer cells. This will let us identify compounds that are not only effective but that get into breast cancer cells and are not toxic to normal cells.

Relevance/Impact: To stimulate the development of new drugs effective against a wide spectrum of cancers, we are identifying small drug like molecules that disrupt the function of a particularly potent cell growth gene called Myc, which is often misregulated in breast cancer.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A170.