Abstract 5107: Targeting CTPS1 and nucleotide biosynthesis pathways as a novel therapeutic approach in MYC-amplified medulloblastoma
Authors: Hathaway Matthew , Jagana Hawa , Meechan Michael , Hemenway John , Gadek Kate E. , Miyaki Aya , Terrones Isabella , Slusher Barbara , Pattwell Siobhan , Vitanza Nicholas , Evans Myron K. 
Published: March 2024

Abstract
Although leukemia is the leading pediatric oncology diagnosis, brain tumors are the leading cause of pediatric cancer-related death. Medulloblastoma is the most common pediatric brain tumor, representing approximately 20% of all diagnoses. Of all four defined subgroups, MYC-amplified Group 3 medulloblastoma (G3 MB) tumors are the deadliest, with a &lt; 50% 5-year overall survival rate. High risk patients receive large doses of chemo- and radio- therapy (CT/RT) with most survivors experiencing debilitating neurological and cognitive sequelae making the identification of novel targets of the utmost importance. Utilizing whole-genome CRISPR screening from the Pediatric Cancer Dependency Map, we identified the enzyme CTPS1 as a strong G3 MB-specific vulnerability. Recent studies corroborate our findings and suggest a dependence on the de novo pyrimidine biosynthesis pathway in MB. Targeted knockdown of CTPS1 restricts sphere formation and reduces proliferation of G3 MB tumor lines. We confirmed this genetic approach using a specific CTPS1 inhibitor, which also reduced proliferation but did not induce cell death, suggesting a cytostatic mechanism. Strikingly, we demonstrate that G3 MB is only dependent upon de novo pyrimidine synthesis and not de novo purine synthesis, despite regulation of both pathways by oncogenic MYC signaling. Previous work has shown that triggering an imbalance in pyrimidine and purine levels leads to replication stress and activation of the ATR/CHK1 pathway. Both genetic and pharmacologic inhibition of CTPS1 leads to CHK phosphorylation, allowing for cell survival despite lack of dNTPs. Previously, the CHK1 inhibitor, prexasertib, demonstrated robust brain accumulation and targeting of G3 MB in pre-clinical models. Dual inhibition of CHK1 and CTPS1 synergizes to significantly reduce cell viability in vitro and tumor growth in vivo. We further demonstrated robust synergy of CHK1 inhibition with either glutamine antagonists or other inhibitors of the de novo pyrimidine synthesis pathway, prolonging in vivo survival in an orthotopic xenograft G3 MB model. The results of this investigation confirm CTPS1 as a novel target in G3 MB, pinpoint a dependency on de novo pyrimidine rather than purine biosynthesis, and identifies a novel rational combination for treatment in MYC-amplified MB patients.
Citation Format: Matthew Hathaway, Hawa Jagana, Michael Meechan, John Hemenway, Kate E. Gadek, Aya Miyaki, Isabella Terrones, Barbara Slusher, Siobhan Pattwell, Nicholas Vitanza, Myron K. Evans. Targeting CTPS1 and nucleotide biosynthesis pathways as a novel therapeutic approach in MYC-amplified medulloblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5107.