Molecular rationale for the use of PI3K/AKT/mTOR pathway inhibitors in combination with crizotinib in ALK-mutated neuroblastoma

Mutations in the ALK tyrosine kinase receptor gene represent important therapeutic targets in neuroblastoma, yet their clinical translation has been challenging. The ALKF1174L mutation is sensitive to the ALK inhibitor crizotinib only at high doses and mediates acquired resistance to crizotinib in ALK-translocated cancers. We have shown that the combination of crizotinib and an inhibitor of downstream signaling induces a favorable response in transgenic mice bearing ALKF1174L/MYCN-positive neuroblastoma. Here, we investigated the molecular basis of this effect and assessed whether a similar strategy would be effective in ALK-mutated tumors lacking MYCN overexpression. We show that in ALK-mutated, MYCN-amplified neuroblastoma cells, crizotinib alone does not affect mTORC1 activity as indicated by persistent RPS6 phosphorylation. Combined treatment with crizotinib and an ATP-competitive mTOR inhibitor abrogated RPS6 phosphorylation, leading to reduced tumor growth and prolonged survival in ALKF1174L/MYCN-positive models compared to single agent treatment. By contrast, this combination, while inducing mTORC1 downregulation, caused reciprocal upregulation of PI3K activity in ALK-mutated cells expressing wild-type MYCN. Here, an inhibitor with potency against both mTOR and PI3K was more effective in promoting cytotoxicity when combined with crizotinib. Our findings should enable a more precise selection of molecularly targeted agents for patients with ALK-mutated tumors

Role of Topoisomerase IIβ in the Expression of Developmentally Regulated Genes

Mice lacking topoisomerase IIβ (TopIIβ) are known to exhibit a perinatal death phenotype. In the current study, transcription profiles of the brains of wild-type and top2β knockout mouse embryos were generated. Surprisingly, only a small number (1 to 4%) of genes were affected in top2β knockout embryos. However, the expression of nearly 30% of developmentally regulated genes was either up- or down-regulated. By contrast, the expression of genes encoding general cell growth functions and early differentiation markers was not affected, suggesting that TopIIβ is not required for early differentiation programming but is specifically required for the expression of developmentally regulated genes at later stages of differentiation. Consistent with this notion, immunohistochemical analysis of brain sections showed that TopIIβ and histone deacetylase 2, a known TopIIβ-interacting protein, were preferentially expressed in neurons which are in their later stages of differentiation. Chromatin immunoprecipitation analysis of the developing brains revealed TopIIβ binding to the 5′ region of a number of TopIIβ-sensitive genes. Further studies of a TopIIβ-sensitive gene, Kcnd2, revealed the presence of TopIIβ in the transcription unit with major binding near the promoter region. Together, these results support a role of TopIIβ in activation/repression of developmentally regulated genes at late stages of neuronal differentiation