PDTM-06. TARGETED CATALYTIC INHIBITION OF EZH2 SYNERGIZES WITH LOW-DOSE PANOBINOSTAT IN MALIGNANT RHABDOID TUMOR

Malignant Rhabdoid Tumor (MRT) is a rare pediatric cancer predominant in the kidney and CNS that is resistant to current treatment regimes. MRT is genetically characterized by homozygous inactivation of SMARCB1, a core subunit of the SWI/SNF chromatin-remodeling complex. Next-generation sequencing data demonstrates SMARCB1 loss is the primary driver mutation, implicating epigenetic dysregulation in pathogenesis of MRT. Recently, we showed that sustained treatment of MRT cell lines with low-dose Panobinostat (LBH589), inhibited tumor growth through driving multi-lineage differentiation in vitro and in vivo. Furthermore, physiological re-expression of SMARCB1 in G401 MRT cells phenocopied the low-dose LBH589 treatment and led to growth inhibition, senescence and terminal differentiation in vitro and in vivo. EZH2, a core subunit of the Polycomb Repressive Complex 2, confers transcriptional silencing via methylation at Lysine 27 of Histone 3 (H3K27me3), and is a transcriptional target of SMARCB1. Interestingly, EZH2 expression and H3K27me3 were drastically reduced following sustained low-dose LBH589 treatment and re-expression of SMARCB1 in G401 MRT cells. Sustained siRNA knockdown of EZH2 in G401 cells demonstrated similar cellular growth reduction and changes in mRNA expression as those observed following low-dose LBH589 treatment and SMARCB1 re-expression. However, MRT cells treated with EZH2-catalytic inhibitor GSK-126, had no effect on EZH2 expression and moderately reduced cell growth and H3K27me3 at doses 1nM-10μM suggesting important non-catalytic EZH2 function. Interestingly, MRT cells treated in combination with low-dose LBH589 and GSK-126, had diminished EZH2 and H3K27me3 expression and exhibited reduced cell growth in vitro compared to single agent controls, revealing a synergistic relationship. In vivo xenograft models of low-dose LBH589 and GSK-126 treatment produced a marked reduction in tumor growth, not observed with single agent treatments. This data positions EZH2 as an important mediator of MRT proliferation and provides evidence for dual therapeutic targeting of EZH2 with low-dose HDACi in MRT.