Histone deacetylase activity limits the response to EZH2 inhibition-based therapy in epithelioid sarcoma and is targetable by epigenetic combination

Epithelioid sarcoma (EpS) is an ultra-rare, aggressive soft tissue sarcoma (STS) driven by INI1 loss and consequent hyperactivation of the chromatin-modifying enzyme EZH2. Although the EZH2 inhibitor tazemetostat has shown clinical activity, responses remain limited, highlighting the need for improved treatment strategies. Here, using two in-house generated patient-derived xenograft models and matched cell lines derived from INI-1 deficient EpS, we investigated EZH2 inhibition in combination with doxorubicin, the first-line standard for advanced STSs, identifying distinct patterns of response and resistance. Integrated transcriptomic and functional analyses revealed that response to EZH2 inhibition-based therapy was associated with chromatin remodeling, characterized by increased H3K27 acetylation and downregulation of histone deacetylase (HDAC)-related transcriptional programs. Conversely, the intrinsically resistant model failed to undergo this epigenetic transition despite EZH2 inhibition. Pharmacological HDAC inhibition restored H3K27 acetylation, promoted apoptosis, and enhanced the activity of EZH2 inhibition-based therapy. These findings identify failure to accumulate H3K27 acetylation as a hallmark of resistance to EZH2 inhibition-based treatment, and show that pharmacological HDAC inhibition can restore this chromatin transition and re-sensitize resistant tumors, providing a rationale for combined epigenetic targeting strategies in INI1-deficient malignancies. Translational relevanceProspective trials are challenging in rare tumors such as epithelioid sarcoma (EpS), limiting the level of evidence for existing therapies and the development of new agents. This is particularly relevant for EpS, where drug regimens are those used for all soft tissue sarcomas (STSs), and the specific mechanisms of drug response remain poorly understood. This preclinical study of tazemetostat in combination with doxorubicin shows differential outcomes in two INI1-deficient proximal-type EpS models, providing evidence of the heterogeneity that exists even within the same tumor subtype and fostering the need to better understand the molecular mechanisms driving drug sensitivity/resistance in this disease. In addition, we demonstrated the potential to treat EpS models through modulation of epigenetic mechanisms, showing that HDAC inhibition may restore sensitivity to EZH2-targeted therapy. These findings support the rationale for developing combination strategies incorporating epigenetic modulators and provide a preclinical framework for overcoming resistance to current therapies in EpS.