Abstract 1809: Engineered destabilized ARE on the 3UTR of MYC degrades oncogenic c-MYC transcript and protein and induces apoptosis in multiple cancer types

The c-MYC oncogene is overexpressed in more than 18 human cancers, including triple negative breast cancer (TNBC), prostate cancer, and osteosarcoma, yet there is no clinically approved drug that targets this major oncogenic driver. We discovered that the 3’UTR of c-MYC in many cancers is enriched with stabilizing poly U tract sequences that can be destabilized with our novel engineered stabilizing AU Rich Elements (ARE), leading to degradation of c-MYC. We hypothesize that c-MYC destabilizing constructs will degrade c-MYC transcript and proteins across a range of cancers that overexpress this oncogene. We used heterogeneous panels of TNBC, prostate cancer, and osteosarcoma cell lines that are reflective of the heterogeneity of human cancers. We introduced our c-MYC destabilizing construct and assessed cell viability. We then performed immunofluorescence and western blot for assessment of c-MYC protein expression. Vector constructs were used as controls. Introduction of destabilizing elements achieved the degradation of both c-MYC transcript and proteins within four days, as well as a subsequent loss in cell viability. We observed that treated cells disintegrated from their nucleus, leading to increased active caspase 3/7 and cell death. Taken together, we have developed a novel technology that reliably targets and degrades the c-MYC oncogene across a range of highly aggressive and difficult to treat cancers

Citation Format: Chidiebere Awah, Fu Dong, Yana Glemaud, Fayola Levine, Daniel Weiser, Olorunseun Ogunwobi. Engineered destabilized ARE on the 3UTR of MYC degrades oncogenic c-MYC transcript and protein and induces apoptosis in multiple cancer types [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1809.

Therapeutic Targeting of Exportin-1 in Childhood Cancer

Overexpression of Exportin-1 (XPO1), a key regulator of nuclear-to-cytoplasmic transport, is associated with inferior patient outcomes across a range of adult malignancies. Targeting XPO1 with selinexor has demonstrated promising results in clinical trials, leading to FDA approval of its use for multiple relapsed/refractory cancers. However, XPO1 biology and selinexor sensitivity in childhood cancer is only recently being explored. In this review, we will focus on the differential biology of childhood and adult cancers as it relates to XPO1 and key cargo proteins. We will further explore the current state of pre-clinical and clinical development of XPO1 inhibitors in childhood cancers. Finally, we will outline potentially promising future therapeutic strategies for, as well as potential challenges to, integrating XPO1 inhibition to improve outcomes for children with cancer.

Abstract 1328: Using whole-genome CRISPR-Cas9 screening to identify resistance networks in glioblastoma

Glioblastoma (GBM) is the most aggressive and common type of adult malignant brain tumor, with 12,000 new diagnoses each year. Even with the current standard of care—surgical resection, radiation, and temozolomide (TMZ)-based chemotherapy—the median survival is about 20 months. This is partly due to the high rate of resistance to conventional therapy, including TMZ, leading to recurrence rates close to 100%. It remains largely unknown what drives the development of this resistance. Many studies have shown differences between primary and recurrent tumors, but a deeper understanding of resistance mechanisms is needed. CRISPR-Cas9 screening is a powerful tool for systematic and unbiased genetic analysis, which we applied to understand TMZ resistance. We performed a genome-wide CRISPR knockout screen in H4 human GBM cells, encompassing over 17,000 genes. A DMSO-treated population was compared with a TMZ-treated population over 14 days. In this drug sensitivity screen, depletion of guides corresponds to a TMZ-resistance gene, whereas enrichment of guides corresponds to a TMZ-sensitivity gene. Analysis showed that there was significant enrichment in guides for known TMZ-sensitivity genes that have been highly cited—ATG14, MSH6, MLH1, and PMS2—thus validating our screen results. However, more importantly, we were able to identify a list of 200 novel genes implicated in TMZ resistance. Pathway analysis revealed that these genes were enriched in Hippo and Notch signaling, both known to play a role in chemoresistance. From this list of novel genes, we identified 4 previously unstudied genes. These genes showed significant elevations in RNA expression (p<0.05) in recurrent tumors when compared to primary tumors in patient datasets, along with significant survival benefits corresponding to low gene expression (p<0.05). To validate the identified genes, we assessed RNA expression in multiple patient-derived xenograft (PDX) lines and found that multiple exposures of TMZ were required to generate a resistant phenotype with gene expression elevation. We validated this at the protein level and showed that multiple exposures of TMZ resulted in target expression elevations compared to the control, thus confirming that the effect of TMZ-resistance gene upregulation is only noted when cells are forced into resistance. Further validation experiments revealed that knocking out these genes in vitro resulted in increased TMZ sensitivity. In summary, a whole-genome CRISPR-Cas9 knockout screen was performed to identify a novel set of genes that contribute to therapeutic resistance in GBM, as validated by in vitro experiments performed on a set of these genes. We have also identified a specific network of enriched pathways that represent novel genetic vulnerabilities. Ultimately, we believe this work will provide critical insight into mechanisms of resistance in GBM—a disease desperately in need of new therapeutic approaches.

Citation Format: Shreya Budhiraja, Shivani Baisiwala, Ella Perrault, Li Chen, Cheol Park, Chidiebere Awah, Crismita Dmello, Andrew Zolp, Adam Sonabend, Atique Ahmed. Using whole-genome CRISPR-Cas9 screening to identify resistance networks in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1328.