Polyglutamine (polyQ) tract expansion leads to proteotoxic misfolding and drives a family of nine diseases. We study spinal and bulbar muscular atrophy (SBMA), a progressive degenerative disorder of the neuromuscular system caused by the polyQ androgen receptor (AR). Using a knock-in mouse model of SBMA, AR113Q mice, we show that E3 ubiquitin ligases which are a hallmark of the canonical muscle atrophy machinery are not induced in AR113Q muscle. Similarly, we find no evidence to suggest dysfunction of signaling pathways that trigger muscle hypertrophy or impairment of the muscle stem cell niche. Instead, we find that skeletal muscle atrophy is characterized by diminished function of the transcriptional regulator Myocyte Enhancer Factor 2 (MEF2), a regulator of myofiber homeostasis. Decreased expression of MEF2 target genes is age-and glutamine tract length-dependent, occurs due to polyQ AR proteotoxicity, and is associated with sequestration of MEF2 into intranuclear inclusions in muscle. Skeletal muscle from R6/2 mice, a model of Huntington disease which develops progressive atrophy, also sequesters MEF2 into inclusions and displays age-dependent loss of MEF2 target genes. Similarly, SBMA patient muscle shows loss of MEF2 target gene expression, and restoring MEF2 activity in AR113Q muscle rescues fiber size and MEF2-regulated gene expression. This work establishes MEF2 impairment as a novel mechanism of skeletal muscle atrophy downstream of toxic polyglutamine proteins and as a therapeutic target for muscle atrophy in these disorders.

Effective practice of precision medicine in oncology relies on identifying and targeting unique characteristics of individual cancers. Coupling functional analysis with genomic screening will further enhance the selection of efficacious cancer therapies. Cancer is a heterogeneous disease that is defined by distinct capabilities, one of which is the evasion of apoptosis, controlled by the antiapoptotic protein members, Bcl-2, Bcl-xL, Mcl-1, and Bfl-1. These antiapoptotic proteins have become validated therapeutic targets, as manifested by the FDA-approved venetoclax, a selective Bcl-2 inhibitor, and several other molecules in clinical trials that target Mcl-1 and Bcl-xL. One of the translational challenges lies in predicting functional antiapoptotic dependence in a patient’s cancer and discriminating responders from nonresponders to Bcl-2 family targeted therapy. We have successfully characterized an updated array of Bcl-2 family antagonists, consisting of peptides and selective small-molecule BH3-mimetics via in vitro binding assays and functional analysis using model cell lymphoma cell lines. With this selective set of chemical tools, we employed the BH3 profiling assay across a wide spectrum of hematologic and solid tumor cell lines in order to functionally dissect survival dependence. To validate the results of BH3 profiling, we performed treatments with BH3-mimetics and analyzed their ability to induce apoptosis, which further demonstrated how cancers can be differentiated and targeted based on antiapoptotic dependence. The obtained data suggest that hematologic cancer cell lines mainly rely on Mcl-1 and Bcl-2 for survival and commonly undergo apoptosis in response to single-agent inhibitors of these proteins. Solid tumor cell lines are more dependent upon Bcl-xL and Mcl-1 and certain cell lines display increased involvement of Bfl-1. These cell lines undergo apoptosis when exposed to a combination of Bcl-xL/Mcl-1 inhibitors. Single agent-responsive solid tumor cell lines are less common but can be identified by BH3 profiling. This approach was successfully applied to primary patient samples to validate clinical utility. Further applications of dynamic BH3 profiling were explored to demonstrate how various FDA-approved therapies can alter antiapoptotic dependencies and aid in the design of rational combination therapies with BH3-mimetics. This work lays a translational foundation in the field of precision medicine by incorporating Bcl-2 protein family targeting into potential personalized cancer treatments. Citation Format: Karson J. Kump, Matthew Lieberman, Rita A. Avelar, Charles Foucar, Malathi Kandarpa, Antonio Di Cristofano, Russell J. Ryan, Sami N. Malek, Ryan A. Wilcox, Dale L. Bixby, Tycel J. Phillips, Moshe Talpaz, Analisa DiFeo, Zaneta Nikolovska-Coleska. Precision medicine strategy to elucidate and target Bcl-2 prosurvival proteins in a wide spectrum of cancers [abstract]. In: Proceedings of the AACR Special Conference on Advancing Precision Medicine Drug Development: Incorporation of Real-World Data and Other Novel Strategies; Jan 9-12, 2020; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(12_Suppl_1):Abstract nr 10.