Gain-of-Function Chromatin Remodeling Activity of Oncogenic FOXL2-C134W Reprograms Glucocorticoid Receptor Occupancy to Drive Granulosa Cell Tumors

Adult type ovarian granulosa cell tumors (AGCTs) are rare malignancies with the near universal c.C402G (p.Cys134Trp) somatic mutation in FOXL2, a Forkhead box-family transcription factor important for ovarian function. Relapsed AGCT is incurable, but the mechanism of the unique FOXL2 mutation could confer therapeutic vulnerabilities. To identify FOXL2-C134W-dependent pharmacologic synergies, we created and characterized endogenous FOXL2 isogenic AGCT cells and an AGCT tumoroid biobank. A drug screen identified that glucocorticoids promote FOXL2-C134W-dependent AGCT growth. Epigenetic investigation revealed that the Cys134Trp mutation exposes latent DNA sequence-specific chromatin remodeling activity in FOXL2. FOXL2-C134W-dependent chromatin remodeling activity redirected glucocorticoid receptor chromatin occupancy to drive hyaluronan synthase 2 gene expression and increase extracellular hyaluronan secretion. Treatment of AGCT models with hyaluronidase reduced viability, and dexamethasone rescued this effect. Combinatorial drug-drug interaction experiments demonstrated that dexamethasone antagonizes the potency of paclitaxel, a chemotherapy agent frequently used in the treatment of AGCT. Thus, gain-of-function pioneering activity contributes to the oncogenic mechanism of FOXL2-C134W and creates a potentially targetable synergy with glucocorticoid signaling.