Abstract LB563: Integrated multiomic profiling identifies the epigenetic regulator PRC2 as a therapeutic target to counteract leukemia immune escape and relapse
Authors: Gambacorta Valentina , Beretta Stefano , Ciccimarra Martina , Zito Laura , Giannetti Kety , Andrisani Angela , Gnani Daniela , Zanotti Lucia , Oliveira Giacomo , Carrabba Matteo G. , Cittaro Davide , Merelli Ivan , Ciceri Fabio , Di Micco Raffaella , Vago Luca 
Published: June 2022

Abstract
Background: Evasion from immune control represents one of the main drivers of acute myeloid leukemia (AML) relapse after allogeneic hematopoietic cell transplantation (allo-HCT). In particular, up to 40% of AML relapses display complete loss of surface expression of HLA class II molecules without any genetic lesion explaining this phenotype (Christopher et al, N Engl J Med, 2018; Toffalori et al, Nat Med, 2019). This led us to investigate the links between epigenetic changes, immune evasion and post-transplantation relapse.
Methods: Starting from primary AML samples pairwise collected from five patients at diagnosis and relapse with non-genomic loss of HLA class II expression, we generated Patients-Derived Xenografts (PDXs) into NOD-SCID γ-chain null mice. Leukemic cells expanded in the mice and their original human counterparts were characterized for changes in gene expression (by RNA-Seq), DNA methylation profile (by RRBS), and chromatin accessibility (by ATAC-Seq). The results obtained by all these approaches and in the different patients were integrated by Multi-Omics Factor Analysis (MOFA) and Gene Set Enrichment Analysis (GSEA). Finally, we tested the immunological effects of epigenetic drugs on AML cells and on their recognition by T cells in ex-vivo short-term cultures and in PDXs.
Results: We verified that PDXs faithfully recapitulate immune-related differences between AML diagnosis and post-transplantation relapse, including loss of expression of HLA class II molecules. Differences between diagnosis and post-transplantation relapse samples were mostly explained by changes in chromatin accessibility, and largely unrelated to the DNA methylation profile. In particular, in all five patients analyzed, we documented genomewide chromatin compaction at time of relapse, that was particularly evident for HLA class II genes and their master regulator CIITA, and not detected in relapses after sole chemotherapy. Integration of all the high-throughput technologies by MOFA, and of results from different patients by GSEA, pointed to the Polycomb Repressive Complex 2 (PRC2) as the main candidate mediator of HLA class II silencing.Pharmacological inhibition of PRC2 subunits rescued HLA class II expression in AML relapses ex vivo and in vivo, with consequent recovery of leukemia recognition by CD4+ T cells.
Conclusions: Our results uncover a novel link between epigenetics and leukemia immune escape, which may rapidly translate into innovative strategies to cure or prevent AML post-transplantation relapse.
Citation Format: Valentina Gambacorta, Stefano Beretta, Martina Ciccimarra, Laura Zito, Kety Giannetti, Angela Andrisani, Daniela Gnani, Lucia Zanotti, Giacomo Oliveira, Matteo G. Carrabba, Davide Cittaro, Ivan Merelli, Fabio Ciceri, Raffaella Di Micco, Luca Vago. Integrated multiomic profiling identifies the epigenetic regulator PRC2 as a therapeutic target to counteract leukemia immune escape and relapse [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB563.