Carlson, C. R.; Shen, Y.; He, H.; Gudenschwager, E. K.; Hou, C.; Ma, J. R.; Chiu, J. C.; Liu, A. C.

Smith-Kingsmore syndrome (SKS) is a rare neurodevelopmental disorder caused by gain-of-function mutations in MTOR, yet whether these mutations phenocopy TSC2 loss or establish a distinct signaling state remains unclear. Using quantitative proteomics, phosphoproteomics, and transcriptomics in isogenic cell models of SKS (MTOR{Delta}4aa), TSC2 loss (TSC2-/-), and wild-type controls under glucose depletion and refeeding, we find that MTOR{Delta}4aa and TSC2-/- cells occupy fundamentally distinct regulatory states. TSC2-/- cells exhibit broad anabolic remodeling and a transcriptional program dominated by NF-{kappa}B- and STAT-driven inflammatory responses. MTOR{Delta}4aa cells instead display enrichment of nuclear and RNA processing programs, E2F/MYC-driven transcription, and a constrained proteomic dynamic range across nutrient states. Phosphoproteomic analysis of MTOR{Delta}4aa reveals rerouting of nutrient-responsive signaling toward MAPK/ERK- and Ca2+/CaMK-dependent pathways with limited canonical mTORC1/S6K1 engagement. These findings establish SKS as a signaling rewiring disorder distinct from classical mTORC1 hyperactivation, with implications for therapeutic targeting.