Abstract 3804: Linear PVT1 isoforms and circPVT1 regulate DNA damage response and immunological pathways in acute myeloid leukemia and have a potential role in the crosstalk between leukemic cells and the tumor microenvironment
Authors: Ghetti Martina , Ledda Lorenzo , Vannini Ivan , Fonzi Eugenio , Bochicchio Maria Teresa , Cardon Tristan , Luserna di Rorà Andrea Ghelli , Paganelli Matteo , Servili Chiara , Fabbri Francesco , Marianini Sabina , Marconi Giovanni , Rondoni Michela , Chicchi Roberta , Biguzzi Rino , Lanza Francesco , Salzet Michel , Martinelli Giovanni , Simonetti Giorgia 
Published: June 2023

Abstract
The human Plasmacytoma Variant Translocation 1 (PVT1) gene encodes for 176 linear (PVT1, lncipedia.org) and 29 circular isoforms (circPVT1, www.circbase.org). The most common isoform of circPVT1 is 410bp and is a product of back-splicing containing the whole exon 2 of PVT1 in a closed loop-like structure (hsa_circ_0001821). PVT1 and circPVT1 have oncogenic roles in several tumor types and can contribute to the suppression of anti-tumor immune responses. Our study aims to investigate whether and how PVT1 isoforms and circPVT1 promote an aggressive phenotype in acute myeloid leukemia (AML) and are involved in the crosstalk between leukemic and immune cells. Six out of the 14 PVT1 isoforms expressed by white blood cells and lymphnodes (noncode.org) were detected in a panel of AML cell lines along with circPVT1. Downregulation (KD) of groups of linear isoforms (based on sequence homology) or of circPVT1 by antisense-oligonucleotides (ASOs) in the t(8;21) KASUMI-1 and the NPM1-mut OCI-AML3 cells induced an impairment of leukemia cell growth especially when all the 6 linear isoforms were silenced by ASO combination, in both cell lines, under normoxia and hypoxia mimicking the bone-marrow microenvironment. Interestingly circPVT1-KD induced apoptosis in OCI-AML3 cells. Moreover, MYC protein expression decreased by ASO combination or circPVT1_KD in OCI-AML3 under normoxia and in KASUMI-1 under hypoxia. To further investigate the biological consequences of PVT1/circPVT1-KD, we performed RNAseq. Analysis on AML cell lines revealed that downregulation of PVT1 and, especially circPVT1, altered the expression of genes involved in RNA transport and degradation, as expected for a long non-coding genes, but also in DNA damage response (DDR) and immunological pathways, as also confirmed by transcriptomic data of primary AML cases and proteomic analysis of both cell lines. Based on these novel findings, we tested the downregulation of PVT1/circPVT1 in combination with inhibition of ATM or ATR, key DDR transducers. Interestingly, circPVT1-KD sensitized OCI-AML3 to ATM (71% vs 95% live cells) or ATR (75% vs 90% live cells) inhibitors. PVT1 and circPVT1 expression was also detected at the level of cell-free RNA in primary AML samples, the latter showing higher levels. We then analyzed extracellular vesicles (EVs) from AML peripheral blood and healthy subjects. EVs from patients were larger, increased in number and expressed higher levels of myeloid lineage markers. Finally AML EVs had more circPVT1 cargo. In conclusion, we uncovered novel potential roles of PVT1 isoforms and circPVT1 in the DDR and in the tumor microenvironment that pave the way for novel therapeutic combinations.
Citation Format: Martina Ghetti, Lorenzo Ledda, Ivan Vannini, Eugenio Fonzi, Maria Teresa Bochicchio, Tristan Cardon, Andrea Ghelli Luserna di Rorà, Matteo Paganelli, Chiara Servili, Francesco Fabbri, Sabina Marianini, Giovanni Marconi, Michela Rondoni, Roberta Chicchi, Rino Biguzzi, Francesco Lanza, Michel Salzet, Giovanni Martinelli, Giorgia Simonetti. Linear PVT1 isoforms and circPVT1 regulate DNA damage response and immunological pathways in acute myeloid leukemia and have a potential role in the crosstalk between leukemic cells and the tumor microenvironment. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3804.