PRC2 Inactivation Induces Resistance to Chemotherapy-Induced Apoptosis By Upregulating the TRAP1 Mitochondrial Chaperone in T-ALL

• Published in: Blood 2018-11, Vol.132 (Supplement 1), p.889-889
• Main Authors: Bodaar, Kimberly, Aries, Ingrid M., Karim, Salmaan, Ni Chonghaile, Triona, Burns, Melissa A., Hinze, Laura, Pfirrmann, Maren, Degar, James, Landrigan, Jack, Balbach, Sebastian, Peirs, Sofie, Menten, Bjorn, Stevenson, Kristen E., Neuberg, Donna S, Devidas, Meenakshi, Loh, Mignon L., Hunger, Stephen P., Teachey, David T, Rabin, Karen R., Winter, Stuart S., Dunsmore, Kimberly P., Wood, Brent, Silverman, Lewis B., Sallan, Stephen E., Van Vlierberghe, Pieter, Orkin, Stuart H., Knoechel, Birgit, Letai, Anthony, Gutierrez, Alejandro
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2018-99-113518

The tendency of mitochondria to undergo or resist BCL2-controlled apoptosis (so-called mitochondrial priming) is a powerful predictor of response to cytotoxic chemotherapy. Fully exploiting this finding will require unraveling the molecular genetics underlying phenotypic variability in mitochondrial priming. We analyzed pre-treatment T-ALL clinical specimens from a cohort of 47 patients (enriched for treatment failure, but with sufficient controls) treated on the COG AALL0434 or DFCI 05001 clinical trials using BH3 profiling analysis to assess mitochondrial apoptotic priming. We found that there was a strong association between resistance to mitochondrial apoptosis and a poor response to induction chemotherapy (P = 0.008). Furthermore, mitochondrial apoptosis resistance predicted significantly inferior event-free survival (65% vs. 91% at 5 years; P = 0.0376). To define the molecular determinants of this mitochondrial apoptosis resistance, we performed targeted exon sequencing and array CGH copy number analysis. This revealed that loss-of-function mutations in the polycomb repressive complex 2 (PRC2) core subunits (EZH2, EED or SUZ12) were associated with mitochondrial apoptosis resistance (P = 0.007) in clinical specimens. PRC2 is a chromatin modifying complex best known for its role in transcriptional repression, which functions as a tumor suppressor in T-ALL, but whether PRC2 regulates mitochondrial apoptosis is unknown. Using shRNA knockdown in human T-ALL cells, we found that depletion of PRC2 subunits in T-ALL cells induced mitochondrial apoptosis resistance, as assessed by BH3 profiling analysis (P < 0.001). PRC2 inactivation also induced resistance to chemotherapy-induced apoptosis (P < 0.0001), and increased T-ALL fitness following treatment with the antileukemic drug vincristine (P = 0.0001). Apoptosis resistance upon inactivation of EZH2 (a PRC2 catalytic subunit) was reversed by transduction of wild-type EZH2, but not by an EZH2 point mutant with impaired methyltransferase activity, indicating that this effect is mediated by the enzymatic activity of PRC2. In normal mouse thymocytes, heterozygous deletion of the PRC2 subunits Ezh2 or Eed was sufficient to induce apoptosis resistance in non-transformed double-negative T-cell progenitors (P < 0.010), indicating that apoptosis resistance can arise prior to oncogenic transformation. The best-known regulators of mitochondrial apoptosis are BCL2-family genes, but RNA-seq analysis of shRNA knockdown of