Abstract 4140: The dual role of MDM2-ALT1 as both a suppressor and driver of oncogenesis is highlighted in a new RMS mouse model
Authors: Comiskey Daniel F. , Jacob Aishwarya G. , Montes Matias , Perle Krista La , Boyaka Prosper N. , Chandler Dawn S. 
Published: August 2018

MDM2 is an important negative regulator of protein stability and transcriptional activity of tumor suppressor p53. We have previously shown that the alternatively spliced variant of MDM2, MDM2-ALT1, is induced specifically after exposure to ultraviolet light (UVC) and cisplatin treatment and is associated with various soft tissue sarcomas, including rhabdomyosarcoma (RMS). We have also shown that MDM2-ALT1 interferes with the normal heterodimer formation between MDM2 and MDM4 that is required for efficient p53 degradation by MDM2-mediated ubiquitination. Consistently, we have shown upregulation of p53 activity as a result of transient transfection of this isoform in wild-type p53 background cells. Furthermore, we observed increased levels of p53 and its downstream targets, p21 and Bax, when MDM2-ALT1 is expressed in cells, as well as the expected cell-cycle arrest. In contrast, we have also shown the MDM2-ALT1 can promote cell proliferation and metastatic behavior when transfected into tumor cells lacking an intact p53 pathway. These seemingly opposing roles of MDM2-ALT1 are poorly characterized in vivo, and models to better understand them are not available. To investigate the effects of MDM2-ALT1 on cell cycle and the progression of tumorigenesis in the context of the whole animal, we generated an inducible MDM2-ALT1 mouse model. When MDM2-ALT1 is ubiquitously expressed in mice lacking p53 expression, we see an increased incidence of spindle cell sarcomas, including RMS. Hence, constitutive MDM2-ALT1 expression is oncogenic and exacerbates the tumorigenesis induced by p53 loss. We next assessed the role of MDM2-ALT1 in the presence of wild-type p53. Surprisingly, when MDM2-ALT1 expression was limited to B cells in the presence of wild-type p53 expression, it also gave rise to increased tumorigenesis, albeit with an extended latency (after 18 months). Interestingly, flow cytometric analyses for B-cell markers prior to the onset of tumorigenesis revealed an MDM2-ALT1-associated decrease in the B-cell population of the spleens of these animals. Our data suggest that the B-cell loss is a p53-dependent response to persistent MDM2-ALT1 expression. In conclusion, the MDM2 ALT1 splice variant is a modifier of both p53-dependent and p53-independent tumorigenesis in vivo. Our findings underscore the importance of MDM2 post-transcriptional regulation in controlling the p53 pathway and in tumorigenesis. Furthermore, the MDM2-ALT1-expressing p53 null mice represent a novel mouse model of fusion-negative RMS, a cancer type with few animal models available for therapy design and testing.

Citation Format: Daniel F. Comiskey, Aishwarya G. Jacob, Matias Montes, Krista La Perle, Prosper N. Boyaka, Dawn S. Chandler. The dual role of MDM2-ALT1 as both a suppressor and driver of oncogenesis is highlighted in a new RMS mouse model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4140.