Abstract 4465: Targeted disruption of HDAC3 induces terminal myogenic differentiation of embryonal rhabdomyosarcoma

Embryonal rhabdomyosarcoma (ERMS), one of the most common and lethal pediatric sarcomas, is characterized by an arrest in myogenic differentiation. We have previously shown that treatment of ERMS cells with pan histone deacetylase (HDAC) inhibitors promotes myogenic differentiation. However, the underlying mechanism specific to each HDAC in ERMS remains unknown. We have completed a high-efficiency CRISPR/Cas9 gene knockout screen of class I and II HDAC genes, revealing HDAC3 as a major driver of myogenic suppression in ERMS. HDAC3-deficient ERMS cells showed robust terminal myogenic differentiation with the formation of multinucleated myotubes. Expanding on our in vitro studies, we developed a novel tamoxifen-inducible CRISPR gene-targeting system in ERMS xenografts established in immune-compromised mice. Disruption of HDAC3 in vivo significantly suppressed tumor growth by inducing myogenic differentiation of ERMS cells. By combining inducible CRISPR gene targeting with HDAC3 structure-function analysis, we showed that HDAC3 functions in concert with the nuclear receptor co-repressor (NCOR1 and NCOR2) complex to regulate gene transcription. HDAC3/NCOR binding and HDAC3 deacetylase activity are both required for suppressing myogenic differentiation in ERMS. RNA sequencing analysis of differentially expressed genes in HDAC3-deficient ERMS cells highlighted key pathways dysregulated in ERMS. Our results suggest that epigenetic silencing of the myogenic program is a major mechanism required for ERMS tumor growth. Identification of the essential interaction between HDAC3 and the NCOR transcriptional repressor complex as well as downstream genes and pathways provides important insights into the biology of ERMS tumorigenesis. Our findings highlight potential new drug targets specific to the activity of HDAC3 in order to improve treatment of ERMS patients.

Citation Format: Michael Phelps, Jenna Bailey, Terra Vleeshouwer-Neumann, Eleanor Chen. Targeted disruption of HDAC3 induces terminal myogenic differentiation of embryonal rhabdomyosarcoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4465.