The protein phosphatase 2A-B56α complex regulates N-Myc degradation in neuroblastoma.

High-risk neuroblastoma is one of the most common and deadliest pediatric solid tumors. Proliferation, differentiation, and treatment resistance have been linked to the amplification of MYCN. Although N-Myc has proven to be a difficult therapeutic target, our group and others have previously demonstrated that a small-molecule targeting PP2A, DT-061, drives c-Myc degradation in MYC-driven cancers. This results from its ability to bias PP2A toward heterotrimers that contain the B56α regulatory subunit, which dephosphorylates the S62 c-Myc residue, affecting protein stability and driving its proteasomal degradation. Interestingly, despite a high degree of sequence homology in the phosphodegron of c-Myc and N-Myc, the role of PP2A-B56α in regulating the analogous S62 residue on N-Myc is unknown. Here, we show how N-Myc protein expression is significantly reduced after PP2A reactivation in neuroblastoma cell lines. Treatment with DT-061 combined with its inactive competitive antagonist, DT-766, and the proteasome inhibitor, MG-132, reversed this effect on the loss of N-Myc protein expression, suggesting that PP2A-B56α modulation affects N-Myc stability via the proteasomal degradation pathway. A loss in cell viability and inhibition of the colony formation potential of neuroblastoma cells accompanied this loss in N-Myc expression. Conversely, these effects were abrogated when the N-Myc S62 phosphosite was mutated. In a xenograft model, we observed tumor growth inhibition upon DT-061 treatment, along with a reduction in N-Myc protein expression in vivo. Combined, these results highlight the importance of the PP2A tumor suppressor in regulating MYCN oncogenic signaling and open new potential treatment regimens for high-risk neuroblastoma patients.