Synergistic combined molecular treatment targeting the HBP1 tumor suppressor gene in neuroblastoma

Neuroblastoma (NB) is a devastating childhood tumor of the peripheral nervous system. Poor survival rates warrant development of more efficient and less toxic treatment. In search for opportunities for combination molecular therapy in ALK-mutated NB, we established a 77-gene signature that recapitulates the transcriptional response upon ALK inhibition. Functional annotation analysis revealed predominantly MAPK/ERK, PI3K/AKT/mTOR, MYC/MYCN and neuronal differentiation signaling components. Amongst these, we identified HBP1, a transcriptional repressor and tumor suppressor gene. Based on a study describing HBP1 as a target of the PI3K/FOXO pathway and our observation of PI3K/AKT activation by ALK, we established an ALK-PI3K/AKT-FOXO3-HBP1 regulatory pathway in NB. In keeping with this, treatment with BEZ-235, an AKT/mTOR inhibitor, leads to loss of AKT and FOXO3 phosphorylation and subsequent HBP1 upregulation. Interestingly, we also found a functional connection between MYCN and HBP1. Given this interconnection, we hypothesized that combination treatment directly targeting both proteins could offer novel opportunities for chemotherapy-resistant tumors. To test this, we analysed effects on mouse xenografts with EGCG (epigallocatechin gallate) as tool compound for upregulating HBP1 levels in combination with the BRD4 inhibitor JQ1, effective for MYCN-amplified NB. Tumor growth was significantly delayed in the combination-treated group. Next, we reasoned that combining JQ1 with an histone deacetylase (HDAC) inhibitor could be effective, given the role of HDAC in the HBP1 repressive function. In a first proof-of-principle test, we observed strong synergistic effects with complete growth arrest in several cell lines. In conclusion, we identified HBP1 as a novel potent drugable target in NB