Cell-Cycle Dependent Expression of a Translocation-Mediated Fusion Oncogene Mediates Checkpoint Adaptation in Rhabdomyosarcoma

Abstract

Rhabdomyosarcoma is the most commonly occurring soft-tissue sarcoma in childhood. Most rhabdomyosarcoma falls into one of two biologically distinct subgroups represented by alveolar or embryonal histology. The alveolar subtype harbors a translocation-mediated PAX3:FOXO1A fusion gene and has an extremely poor prognosis. However, tumor cells have heterogeneous expression for the fusion gene. Using a conditional genetic mouse model as well as human tumor cell lines, we show that that Pax3:Foxo1a expression is enriched in G2 and triggers a transcriptional program conducive to checkpoint adaptation under stress conditions such as irradiation in vitro and in vivo. Pax3:Foxo1a also tolerizes tumor cells to clinically-established chemotherapy agents and emerging molecularly-targeted agents. Thus, the surprisingly dynamic regulation of the Pax3:Foxo1a locus is a paradigm that has important implications for the way in which oncogenes are modeled in cancer cells.

PLK1 Phosphorylates PAX3-FOXO1, the Inhibition of Which Triggers Regression of Alveolar Rhabdomyosarcoma

“…S7), which might further sensitize aRMS as well as other NMYCexpressing tumors to PLK1 inhibitors. In accordance with the proposed function of PAX3-FOXO1 in checkpoint adaptation (50), PLK1 might prevent premature degradation of PAX3-FOXO1 during the transition phase. In summary, this suggests a model whereby PLK1 phosphorylation can stabilize PAX3-FOXO1, mainly during G 2 -M transition, whereas at the same time it would inhibit the wild-type tumor suppressor FOXO1 to ensure cell-cycle progression and survival.…”

“…This resulted in a dose-dependent reduction of normalized luciferase activity with an IC 50 Supplementary Fig. S1C).…”

Uncovering metabolism in rhabdomyosarcoma

“…54 For example, the Pax3-Foxo1-dependent transcription of genes like hepatocyte growth factor receptor (HGFR or c-MET), FGFR4, IGF-2 and C-X-C chemokine receptor type 4 (CXCR4) contributes to increase tumor aggressiveness and metastasis recurrence. 53 In addition, the Pax3-Foxo1 fusion protein drives the oncogenic transcription of N-MYC.…”

Lineage of origin in rhabdomyosarcoma informs pharmacological response

“…Murine primary tumor cell cultures with stable knockdown of Pax3:Foxo1 were generated as described earlier (Kikuchi et al 2014).…”

“…This result suggests that the reduction in Pax3:Foxo1 levels observed upon entinostat treatment is mediated at the transcriptional (or post-transcriptional) level and not at the post-translational level. Other studies from our laboratory have shown that expression of Pax3:Foxo1 is G 2 cell cycle phase-dependent (Kikuchi et al 2014). To investigate whether the reduction in Pax3:Foxo1 expression caused by entinostat treatment is cell cycle-dependent, we performed a series of studies suggesting that this was not necessarily the case (Supplemental Fig.…”