Abstract 649: Site-specific phospho-regulatory collapse of PML governs RTK hyperactivation in high-risk pediatric malignancy.

Abstract High-risk neuroblastoma (HR-NBL), a deadly pediatric malignancy of neural crest origin is characterized by persistent hyperactivation of receptor tyrosine kinases (RTKs) such as ALK, EGFR that orchestrates oncogenic trajectory and perpetuates aggressive, treatment refractory phenotype. Despite extensive characterization of the RTK signaling, the upstream regulatory mechanism that constrain RTK activity and restore lineage commitment remain elusive, limiting the efficacies of differentiation-based therapies. Stability and functional integrity of promyelocytic leukemia protein (PML) and its nuclear body architecture are essential for sustaining differentiation and therapeutic sensitivity. Our recent studies identified a mechanism that derange PML stability, defining a site-specific (serine 518, S518) post-translational modification that dictates PML functional integrity. Herein, we delineate a S518 phosphorylation-centric signaling architecture driving oncogenic RTK signaling. To this end, we employed a dual modality framework integrating RD3 expression modulation (knockout, RD3-/- or overexpression, RD3+/+) that dictates S518 phosphorylation, and stable site-specific mutagenesis of PML S518 to phospho-dead (A518) or phosphomimic (E518), in patient-derived NBL cells representing distinct clinical stages, diagnostic (Dx) and progressive disease (PD). Functionally, RD3-loss dependent PML destabilization (RD3-/- and RD3+/+PML E518) led to a hyperphosphorylated cellular state, disrupting post-translational signaling control and reinforcing an aggressive, therapy defiant phenotype. Quantitative profiling of RTKs phosphorylation (curated panel of 71 human RTKs, phospho-RTK array) was performed to assess pro-oncogenic transformation signaling. RD3-/- and RD3+/+PML E518 clones exhibited a pronounced increase in phospho-RTK signals, consistent with hyperactivated receptor landscape. Notably, RD3 loss driven PML S518 site specific phosphorylation included robust activation of tightly integrated oncogenic RTKs (e.g., Lyn, SYK, Tie-1), that are decisively documented in HR-NBL pathogenesis. In contrast, RD3 overexpression and PML S518 phospho-dead mutants attenuated RTK phosphorylation profiles, with minimal activation of canonical oncogenic RTKs. These findings critically demonstrate that RD3 loss driven S518 site-specific phosphorylation of PML reprograms the cellular signaling landscape towards a programmed RTK hyperactivation, mechanistically bridging nuclear architectural disruption with membrane-proximal oncogenic signaling. Targeting the RD3-PML-RTK axis offers a mechanistically anchored strategy restoring differentiation competence and attenuate the malignant trajectory of HR-NBL. Funding: Department of Defense CA-210339; OCAST-HR19-04; NIH-P20GM103639; and supported by P30CA225520 and P30GM154635. Citation Format: Sreenidhi Mohanvelu, Poorvi Subramanian, Natarajan Aravindan. Site-specific phospho-regulatory collapse of PML governs RTK hyperactivation in high-risk pediatric malignancy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 649.