PRC2-Mediated Apoptosis Evasion Is a Therapeutic Target of MDS/AML Harboring Inv(3)/t(3;3) and -7

Myelodysplastic syndromes and acute myeloid leukemia (MDS/AML) with both inversion/translocation of chromosome 3 (inv(3)/t(3;3)) and monosomy 7 (-7) is an extremely poor prognostic entity. To explore potential therapeutic target of MDS/AML harboring both inv(3)/t(3;3) and -7, we performed drug screen using YCU-AML1, a high-risk MDS/AML cell line harboring t(3;3) and -7 (Kunimoto et al. Hemasphere 2020), as well as OCI-AML20, another AML cell line with inv(3) and -7, and found that both YCU-AML1 and OCI-AML20 showed high response to EZH2 inhibitors valemetostat and tazemetostat. Previous study has shown that EVI1, an oncogenic transcription factor highly expressed in MDS/AML with inv(3)/t(3;3), directly binds to EZH2 and thereby recruits PRC2 complex to PTEN locus, leading to epigenetic silencing of PTEN expression and activation of PI3K/AKT/mTOR pathway in leukemia with 3q rearrangement (Yoshimi et al. Blood 2011). Together with the fact that EZH2 locus is on chromosome 7q, we hypothesized that the survival of MDS/AML cells with inv(3)/t(3;3) and -7 may be highly dependent on residual allelic EZH2-mediated silencing of specific targets which drive cell death. We further validated in colony-forming unit and cell growth assays that YCU-AML1 and OCI-AML20 are both highly sensitive to valemetostat and tazemetostat, whereas FKH-1, Kasumi-3 and SKM-1, MDS/AML cell line with -7, 3q rearrangement, and complex karyotype without chromosome 3 and 7 abnormalities respectively, are resistant to these drugs. Apoptosis analysis revealed that valemetostat and tazemetostat efficiently induced apoptosis in YCU-AML1 and OCI-AML20 but not in FKH-1, Kasumi-3 and SKM-1. To seek molecular basis of EZH2 inhibitor-mediated apoptosis induction, we performed CUT&Tag sequence for H3K27me3 using vehicle or valemetostat-treated cells. Strikingly, promoter region of GADD45 γ was the most robustly and significantly decreased annotated peak locus of H3K27me3 in valemetostat-treated OCI-AML20. H3K27me3 peak in GADD45 γ locus was also significantly decreased in valemetostat-treated YCU-AML1 but not in FKH-1 and Kasumi-3. As expected, valemetostat treatment induced increased expression of GADD45 γ in OCI-AML20. Moreover, transcriptomic analysis also demonstrated GADD45 γ as the most robustly upregulated gene in valemetostat-treated YCU-AML1 compared to vehicle-treated cells. GADD45γ is known to be an upstream regulator of stress-activated protein kinases such as p38 and JNK in which signalin