MYC proteins are critical drivers of a wide range of cancers, and as transcription factors are generally considered undruggable. MYCN is the neuroblastoma-derived isoform of MYC and is a driving oncogene in neuroblastoma and medulloblastoma, two deadly solid tumors of childhood. Here we describe, in three chapters, (1) MYC proteins and their role in neuroblastoma and medulloblastoma, and (2, 3) two strategies for MYC-directed therapeutics using ATP-competitive small-molecule inhibitors of kinases.
This first strategy entails targeting MYCN for proteolysis through inhibition of Aurora Kinase A (Aurora A). MYCN is ubiquitinated and degraded by the proteasome, and Aurora A physically interacts with ubiquitinated MYCN to prevent its degradation. However, this MYCN-stabilizing function is completely independent of its kinase activity, as conventional Aurora A inhibitors have no effect on MYCN degradation. In chapter 2 we describe a new class of conformation-disrupting (CD) inhibitors of Aurora A that bind the ATP binding pocket and sufficiently alter the conformation of Aurora A as to prevent this physical interaction with ubiquitinated MYCN. This new class of compounds effects potent and rapid loss of MYCN protein in MYCN-expressing neuroblastoma and medulloblastoma, both in cell lines as well as in vivo, and reduces tumor burden and extends survival in an animal model. Furthermore, cell line profiling demonstrates that this new strategy for targeting MYCN is both a MYC- and MYCN-directed therapy, as MYC-like expression signature and MYC family overexpression predicts sensitivity to Aurora A conformation-disrupting inhibitors. These CD compounds represent a new strategy for targeting MYCN by way of inducing its degradation.
The second strategy for targeting MYCN, described in Chapter 3, is through the reliance of MYCN on mTOR signaling through 4EBP, the latter a main regulator of cap-dependent translation. 4EBP and c-MYC have been shown to cooperate in oncogenesis, through c-MYC inhibition of the senescence induced by 4EBP and 4EBP inhibition of the apoptosis promoted by c-MYC. Here we show that rapamycin, an allosteric inhibitor of mTOR, blocks signaling through rpS6 but not 4EBP, whereas MLN0128, an ATP-competitive inhibitor of mTOR, blocks signaling through both rpS6 and 4EBP. Accordingly, in MYCN-driven medulloblastoma, MLN0128 but not rapamycin promotes apoptosis, and this apoptosis is consistent with loss of 4EBP phosphorylation but not loss of phosphorylation of other mTOR targets. We also show that the efficacy of mTOR kinase inhibitors is unrelated to any change in MYCN protein levels, and that MLN0128 can reduce mTOR signaling in a mouse model of MYCN-driven medulloblastoma.