MOLECULAR BASIS OF MED12 IN THE PATHOGENESIS OF UTERINE FIBROIDS

Uterine leiomyomas (LM; fibroids) are monoclonal neoplasms of the myometrium (MM) and represent themost frequent tumors in women worldwide. Although benign, they nonetheless account for significantgynecologic and reproductive dysfunction. As no long-term non-invasive treatment option exists for LM,deeper insight regarding tumor etiology is key to the development of newer targeted therapies.Accordingly, this proposal is impactful as it suggests an etiologic basis for the predominant LM subtypeand further offers proof of concept for therapeutic intervention involving new druggable targets in this specificgenetic setting. LM arise from the genetic transformation of a single MM stem cell (SC) into a tumorinitiating cell (LM SC) that seeds and sustains fibroid growth through asymmetric cell divisions.Heretofore, the dominant drivers of cell transformation have been largely identified. The most prevalentamong these, accounting for ~70% of LM, are recurrent somatic mutations in the gene encoding the MED12subunit of Mediator, a multiprotein signal processor through which regulatory information conveyed by gene-specific transcription factors is transduced to RNA polymerase II (Pol II). However, the impact of thesemutations on MED12 function and the molecular basis for their tumorigenic potential remain unknown. Herein,we show that LM-linked mutations in MED12 disrupt its ability to activate Cyclin C (CycC)-dependentkinase 8 (CDK8) in Mediator, leading to reduced site-specific RNA Pol II phosphorylation and global genedysregulation. We also identify genetic programs uniquely dysregulated in MED12-mutant fibroids,leading us to hypothesize that Mediator kinase disruption as a consequence of MED12 mutations elicitstranscriptional reprogramming and altered signaling sufficient to drive MM SC transformation. We furtherhypothesize that MED12-mutant LM are therapeutically susceptible to reactivation of CDK8 orpharmacologic modulation of uniquely dysregulated signaling pathways. To test these hypotheses we will:(1) Establish the pathogenic role of Mediator kinase disruption in MED12-mutant LM. We will ask if genetic orchemical disruption of CDK8 (or its paralog CDK19) in Mediator can induce fibrotic transformation of MM SCsand, conversely, if WT MED12 can restore CDK8/19 kinase activity and suppress the fibrotic phenotype ofMED12-mutant LM SCs; (2) Elucidate the pathogenic mechanism of Mediator kinase disruption in MED12-mutant LM. We will define the biochemical basis by which MED12 mutations disrupt CycC-CDK8/19 kinaseactivity and employ an integrated genome-scale approach to acquire the unique transcriptomic and epigenomicprofiles of MED12 WT and mutant LM SCs; (3) Examine the therapeutic implications of Mediator kinasedisruption in MED12-mutant LM. We will ask if reactivation of CDK8/19 or pharmacologic manipulation ofsignaling pathways uniquely dysregulated in MED12-mutant LM SCs can reverse their fibrotic phenotype. Weexpect these studies to significantly impact personalized treatment of women with LM.