Dual Inhibition of MAPK and TORC1 Signaling Retards Development of Radiation Resistance in Pediatric BRAFV600E Glioma Models.

Abstract

Background: MAPK pathway inhibitors (MAPKi) have shown significant efficacy in treating childhood BRAF-activated brain tumors. For tumors harboring BRAFV600E mutations, the drugs are rarely curative, and patients can become refractory to treatment. MAPKi combining X-radiation therapy (XRT) may improve cure rate, but development of XRT-resistance is a challenge.

Methods: XRT-resistance was induced by multiple XRT cycles in pediatric BRAFV600E glioma patient-derived xenograft (PDX) models. RNA sequencing was performed to identify differentially expressed genes and pathways potentially contributing to XRT-resistance. Cells isolated from PDXs were used to test the contribution of specific genes and pathways to XRT-resistance. PDX models were used to evaluate the efficacy of targeted treatments combined with XRT.

Results: Tumors developed resistance after multiple cycles of XRT. MEK inhibition combining XRT significantly improved tumor control, compared to XRT alone, but resistance to combined therapy developed rapidly. RNA-sequencing analysis revealed up-regulation of MAPK and PI3K-mTOR signaling in the XRT-resistant tumors. Isolated cells showed in vitro resistance to XRT, which was partially reversed by inhibiting PI3K-mTOR. Up-regulation of TORC1 signaling in XRT naïve tumor cells, via constitutively active AKT or TSC2 deletion, conferred in vitro XRT-resistance.The pro-survival gene BIRC5 (Survivin), a target of TORC1 signaling, contributed to XRT-resistance. Combining trametinib-rapamycin with XRT significantly enhanced therapeutic efficacy in PDX models and prevented or delayed resistance development.

Conclusion: PI3K-mTOR activation promotes the development of XRT-resistance in pediatric BRAFV600E glioma. Dual targeting of MAPK and TORC1 signaling significantly enhances the therapeutic efficacy of XRT, and can potentially prevent the development of XRT-resistance. (250 words).