MDB-60. CDK7 INHIBITION DISRUPTS ACQUIRED RESISTANCE MECHANISMS IN MYC-MB

Abstract

Abstract Medulloblastoma (MB) is the most common malignant brain tumor in children, accounting for 20% of all pediatric brain tumors. Medulloblastoma manifests as a clinically heterogeneous tumor with varying prognoses depending on the molecular subgroup. Group 3 tumors have the worst overall prognosis, and are associated with MYC gene amplification, higher rates of metastasis, and increased likelihood of recurrence. Our prior work demonstrated that WEE1 is upregulated in all MB subtypes and is a critical mediator of MB cell viability. We have established that Myc-driven MB is particularly sensitive to the WEE1 inhibitor AZD1775. Although AZD1775 showed promising results in combination with other genotoxic agents, tumor cells frequently acquire resistance to small molecule inhibitors. Thus, to identify regulators of resistance in Myc-MB we performed a Kinome scale RNAi screen in isogenic parental and resistant cells and identified CDK7 as a key mediator of AZD1775 resistance that restored sensitivity to WEE1 inhibition when depleted. CDK7 controls transcriptional initiation by phosphorylating the c-terminal domain of RNA Pol II. CDK7 has been implicated in several Myc-associated malignancies and is frequently enriched at Super Enhancers. Enhancer-promoter interactions (EPIs) are dynamic and can facilitate resistance mechanisms by inducing additional enhancer elements to promote transcription. Here, we investigated 3D chromatin interactions by HiChIP and further assessed CTCF and H3K27ac patterns after treatment with the CDK7 inhibitor, THZ1, by Cut and Run. Our analysis suggests that AZD1775-resistant cells are reorganized to establish alternate cis-acting elements that reinforce transcription of DNA repair and proliferation genes, circumventing AZD1775 treatment. We further examine cisplatin-resistant MB cells to determine if the mechanism is consistent in Myc-MB cell lines or therapy specific. Overall, we suggest that reorganized chromatin structure aids resistance development and that inhibition of CDK7 intervenes in this process, restoring sensitivity to standard therapies.