Abstract A050: Truncated PPM1D cooperates with the PI3K pathway to drive Diffuse Midline Gliomas

Abstract Diffuse Midline Gliomas H3K27-altered (DMGs) represent a subtype of pediatric high-grade brain tumors that are almost universally fatal with an overall survival of 8 – 12 months post-diagnosis. Because surgery is often not an option and radiation remains the only therapy available to patients, there is an enormous unmet need for targeted therapeutics that is based on an understanding of the unique biology of these tumors and the pathways that interact to drive tumorigenesis. Here we investigate the cooperation between the phosphatase PPM1D and the PI3K pathway in driving DMGs. Mutations in PPM1D are present in 8 -12% of DMGs – mutually exclusive with TP53 mutations - that mainly lead to a C-terminally truncated protein that is thought to drive tumorigenesis at least in part due to its enhanced phosphatase activity on targets such at p53, ATM and CHK1/2. However, because p53 is still intact in PPM1Dtr tumors, we find that that its activation is suppressed but not completely ablated. Leveraging a CRISPR screen performed in isogenic mouse neural stem cell (mNSC) system, we find that activation of the PI3K/mTOR pathway provides a selective fitness advantage to PPM1Dtr-expressing cells. Consistent with that, we show that PPM1D mutations significantly cooccur with alterations in the PI3K/mTOR pathway in an analysis of ∼180 pediatric high-grade brain tumors. Using an in utero electroporation (IUE) murine DMG model, we also demonstrate that PI3K pathway activation indeed accelerates tumor formation and reduces survival of neonatal mice. To explore the mechanism behind the PPM1Dtr – PI3K pathway cooperation, we investigated the response to irradiation-induced DNA damage and replication stress. Interestingly, PI3K pathway activation further suppressed p53 activation in addition to PPM1Dtr. Moreover, in patient-derived PPM1D-mutant cell lines, CRISPR activation of PI3K/MAPK genes rescued cells from small molecule inhibition of PPM1D and allowed cells to progress through the cell cycle despite wildtype p53 activation. Taken together, our results suggest that PPM1D mutant DMGs evolve differently from TP53-mutant ones and acquire PI3K pathway alterations to enhance gliomagenesis. Future work will aim to unravel genetic dependencies of PPM1D-PI3K co-altered tumors and identify therapeutic strategies to target them. Citation Format: Adam Fiseha Kebede, Eric Morin, Ipsita Kundu, Apichaya Sethaudom, Nicolas Poux, Daren Zhang, Leslie Lupien, Veronica Rendo, Timothy N. Phoenix, Pratiti Bandopadhayay. Truncated PPM1D cooperates with the PI3K pathway to drive Diffuse Midline Gliomas abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Discovery and Innovation in Pediatric Cancer— From Biology to Breakthrough Therapies; 2025 Sep 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85 (18Suppl₂): Abstract nr A050.