Abstract PR03: Prmt1 and Prmt1-dependent translation initiation are critical vulnerabilities of osteosarcoma
Authors: Hsu Jessie Hao-Ru , Hubbell-Engler Benjamin , Adelmant Guillaume , Perry Jennifer , Cowley Glenn , Marto Jarrod , Orkin Stuart H. 
Published: May 2016

Abstract
Osteosarcoma (OS) remains a challenging clinical entity for which targeted therapy is lacking. The frequent mutations of p53 and Rb in OS would be anticipated to create a genetic context in which specific vulnerabilities might exist. To discover genetic vulnerabilities in OS, we conducted high throughput shRNA-based screens in vitro and in vivo using p53/Rb-null OS cells and identified Prmt1, a protein arginine methyltransferase, among other factors as critical to growth/survival of OS cells from genetically engineered mice or human tumors. Indeed, depletion of Prmt1 in OS cells using shRNAs and a Prmt1-specific inhibitor leads to growth arrest and death in vitro. In vivo, Prmt1 inhibition impairs xenograft engraftment and proliferation. Moreover, deletion of Prmt1 in p53/Rb-null osteoblast progenitors using Cre/Lox-based technology significantly inhibits OS initiation and progression in mice, while normal bone development is largely unaffected. In rescue experiments, we find that enzymatically inactive Prmt1 cannot restore proliferation of Prmt1-depleted cells, indicating that the enzymatic activity of Prmt1 is essential for tumorigenicity. To gain mechanistic insights into the molecular functions of Prmt1, we characterized the Prmt1-associated arginine-methylome and downstream targets of Prmt1 using a SILAC-based quantitative proteomics approach. This innovative technique identified many candidate Prmt1-methylated substrates representing various molecular pathways including RNA processing, transcription and translation. In particular, we have shown that loss of Prmt1 leads to a decrease in arginine methylation of members of the eIF4F translation initiation complex, thereby disrupting their physical association and inhibiting translation. Consistent with these findings, we observed that OS cells are sensitive to inhibition of eIF4G, a major component of the eIF4F translation initiation complex, further exposing an additional OS vulnerability to translation inhibition that could be exploited therapeutically. Taken together, our findings implicate a role of Prmt1 in initiation and maintenance of OS and suggest that Prmt1-mediated effects on translation initiation are responsible for tumor proliferation and survival. Based on our findings, we propose that targeted therapy directed to inhibition of Prmt1 and its associated pathways represents a novel and promising therapeutic strategy for OS.
This abstract is also presented as Poster A17.
Citation Format: Jessie Hao-Ru Hsu, Benjamin Hubbell-Engler, Guillaume Adelmant, Jennifer Perry, Glenn Cowley, Jarrod Marto, Stuart H. Orkin. Prmt1 and Prmt1-dependent translation initiation are critical vulnerabilities of osteosarcoma. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr PR03.