Introduction:

The H3K27 histone methyltransferase EZH2 is the catalytic component of the polycomb repressive complex 2 (PRC2), and is amplified, overexpressed, or mutated in multiple cancer types, supporting its function as an oncogene. In addition to genetic alterations in EZH2 itself, distal genetic changes in other proteins can lead to oncogenic dependency on EZH2 activity. For example, we have previously established that cell lines and xenografts deficient in INI1 (SNF5/SMARCB1), a core component of the SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeling complex, display profound sensitivity and durable regressions in the presence of the selective EZH2 inhibitor tazemetostat (EPZ-6438). Intriguingly, a complete response was observed in a patient with an INI1-negative rhabdoid tumor who participated in the tazemetostat Ph1 dose escalation study. This suggests that these tumors are addicted to dysregulated PRC2 activity, and confirms the previously proposed antagonistic relationship of SWI/SNF with PRC2, which is perturbed in INI1-deficient tumors. The loss of INI1 induces inappropriate SWI/SNF function, abrogating the repression of PRC2 activity, resulting in Polycomb target genes, such as those involved in differentiation and tumor suppression, to become aberrantly repressed. In addition to deletion of INI1, there are numerous reports describing genetic alterations in other SWI/SNF complex members. Given the oncogenic dependency of INI1-deficient tumors on PRC2 activity, we sought to investigate the sensitivity of other SWI/SNF mutated cancer types to EZH2 inhibition. Specifically, we investigated the effects of EZH2 inhibition in ovarian cancers carrying somatic mutations in the SWI/SNF complex members ARID1A and SMARCA4.

Methods and results:

A panel of ovarian cancer cell lines of different histologies was subjected to proliferation assays in 2-D tissue culture for 14 days in the presence of increasing concentrations of an EZH2 inhibitor. Selected cell lines were also tested in 3-D cultures, as it has been suggested in the literature that this context is necessary to observe anti-proliferative effects with EZH2 inhibitors. We found that ovarian cancer cell lines deficient in the SWI/SNF component SMARCA4 (also known as BRG1) are among the most sensitive in response to EZH2 inhibition, as demonstrated by decreased proliferation and/or morphology changes, at concentrations that are clinically achievable. In contrast, mutations in ARID1A, another SWI/SNF component, do not broadly confer sensitivity to EZH2 inhibition in ovarian cancer cell lines in either 2-D or 3-D in vitro assays. Furthermore, the effects of EZH2 inhibition on SMARCA4-negative ovarian cancer cells are context specific, since other cell types with SMARCA4 deletion, such as lung carcinoma cell lines, do not exhibit anti-proliferative affects with EZH2 inhibitor treatment.

Conclusions:

These data suggest that tazemetostat may have therapeutic benefit in SMARCA4-deleted ovarian cancer, such as small cell cancer of the ovary of the hypercalcemic type (SCCOHT), which shows a high degree of loss of SMARCA4 expression. Further in vitro and in vivo studies are underway to interrogate these initial results further.

Citation Format: Sarah K. Knutson, Allison E. Drew, Christopher Plescia, Robert A. Copeland, Jesse J. Smith, Heike Keilhack, Scott Ribich. EZH2 inhibition leads to decreased proliferation in SMARCA4-deleted ovarian cancer cell lines. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C87.