Selective targeting of MYC by antisense oligonucleotides

Abstract

MYC is one of the most commonly dysregulated genes across all cancers. As a master transcription factor with greater than 10,000 binding sites throughout the genome, the MYC oncoprotein coordinates a transcriptional regulatory network consisting of approximately 15% of all genes, controlling cancer hallmark expression programs responsible for cellular proliferation, growth, metabolism, and evasion from apoptosis. MYC dysregulation occurs genetically, epigenetically, and post-transcriptionally through a wide variety of mechanisms. Despite its well-characterized properties as a proto-oncogene, direct potent and selective inhibition of MYC remains a significant challenge. Models of systemic MYC inhibition utilizing inducible genetic constructs in mice have revealed that inhibition of MYC activity leads to potent tumor regression with an evident therapeutic window, suggesting that pharmacologic MYC inhibition may be a viable cancer therapeutic strategy. Small molecule inhibitors designed to block MYC protein activity exhibit low potency, display poor selectivity, and lack antitumor efficacy, which has led MYC to be historically classified as 'undruggable.' Efforts aimed at indirectly targeting MYC transcription often lead to development of resistance characterized by reinforced expression of MYC. Clearly, alternate strategies are needed to achieve selective and potent inhibition of MYC. The goals of this research were to develop antisense oligonucleotides specifically targeted against the MYC mRNA to achieve potent inhibition of MYC translation, and to characterize the activity of these molecules as specific modulators of MYC expression and as prototypical MYC-directed therapeutics. We designed and synthesized a library of MYC-targeting antisense oligonucleotides (MYCASOs) containing several chemical synthetic features to increase target affinity and stability. Treatment of MYC-expressing cancer cells with MYCASOs leads to RNase H-mediated cleavage of MYC mRNA and a potent decrease in MYC protein levels. MYC knockdown is accompanied by significant effects on cellular viability and inhibition of cellular proliferation. Furthermore, MYCASO treatment specifically perturbs MYC-driven gene expression signatures. In a MYC-induced murine model of hepatocellular carcinoma, MYCASO treatment leads to cleavage of the MYC transcript, decreased MYC protein levels within tumors, and reduced tumor burden. MYCASOs represent a new chemical tool for in vitro and in vivo modulation of MYC activity, and promising therapeutic agents for MYC-addicted tumors.