Varjosalo, M.; Bong, Y. T.; Liu, X.; Chowdhury, I.; Kaasinen, E.; Tan, Z.; Malaymar Pinar, D.; Wang, Z.; Karhu, A.; Välimäki, N.; Ohman, T.; Wei, G.-H.; Keskitalo, S.; Aaltonen, L. A.

Recurrent somatic MED12 mutations occur in about 70% of uterine leiomyomas (ULs), defining their dominant molecular subtype, yet the downstream mechanisms remain poorly understood. Using an integrated multi-omics approach, including DDA and DIA proteomics, phosphoproteomics, immunohistochemistry, and interactomics across genetically stratified UL cohorts, we identified prostate-associated gene 4 (PAGE4) as an effector of MED12-mutant UL pathogenesis. PAGE4 was strongly upregulated in MED12-mutant ULs and carried the most prominent hyperphosphorylation sites in the tumor phosphoproteome. Kinase-substrate enrichment analysis identified HIPK2 as the main upstream kinase, with additional support from the CMGC family. PAGE4 phosphorylation acted as a molecular switch, reshaping its interactome toward the Mediator complex and RNA Pol II transcriptional machinery. This rewiring was supported by ChIP-seq and luciferase assays, which showed corresponding changes in transcription factor binding and pathway activity. Together, these findings establish phospho-PAGE4 as a key mechanistic node downstream of MED12 mutation, expand PAGE4 biology into female reproductive tumors, and nominate it as a biomarker and potential therapeutic target in the most common molecular subtype of ULs.