Targeting MYB for Degradation using Microtubule-Destabilizing Agents - Project Summary Microtubule-targeting agents, a class of drugs comprised of microtubule destabilizing agents (MDAs) and microtubule stabilizing agents, have long been used to treat oncologic and inflammatory disease. While their direct action on tubulin is well-established, the basis of their therapeutic activity in specific disease contexts and tissue types is less understood. Intriguingly, microtubule stabilizing agents are almost exclusively used for the treatment of solid tumors, while MDAs are predominantly used in the treatment of hematologic malignancies. Both classes of tubulin targeting agents are antimitotic, yet the basis for their largely non-overlapping clinical use is not formalized. We have determined that treatment with structurally diverse MDAs results in rapid, proteasome- mediated degradation of MYB, a hematopoietic transcription factor that has been characterized as essential in many hematologic malignancy models. Additionally, concurrent administration of microtubule stabilizing taxanes counteracts MDA-induced MYB degradation, indicating that MYB degradation is an on-target effect of microtubule destabilization. Importantly, overexpression of MYB in AML cells confers resistance to MDAs, suggesting that MYB degradation may play a role in the antileukemic activity of MDAs. These observations suggest that MDA-mediated degradation of MYB may account for the particular efficacy of MDAs in the treatment of hematologic malignancies, which could inform future clinical use and development of microtubule targeting agents. The overarching objective of the proposed work is to unravel the molecular mechanisms of MYB degradation in response microtubule disruption, thereby defining a strategy to target a central leukemic dependency with clinically established microtubule targeting agents. This will be pursued from three angles: defining the UPS machinery mediating MYB degradation in response to MDA treatment (Specific Aim 1), characterizing the molecular, biochemical and structural features of MYB that facilitate its degradation by MDAs (Specific Aim 2), and elucidating the mechanism by which microtubule destabilization engages the MYB degradation pathway (Specific Aim 3). These aims seek to map the protein players, molecular features and cellular events that associate microtubule dynamics with the stability of MYB. In particular, the role of MAP3K1, a dual E3 ligase and kinase protein we found to mediate MDA-induced MYB degradation, will be investigated. This study will leverage unbiased functional genomics and proteomics approaches to guide focused, molecular studies. The mechanistic insights gleaned from this work will lay the foundation for future investigations into MYB and its MDA-engaged regulators as potential biomarkers to guide the clinical application of microtubule-targeting agents.
