Oncogenic Ppm1d mutations deregulate the p53 pathway in primary mouse gliomas

Diffuse midline gliomas (DMGs) are incurable brain tumors with limited treatment options. Approximately 20% of DMGs harbor truncating mutations in exon 6 of phosphatase PPM1D, which stabilize the protein and deregulate p53 signaling. However, the consequences of these mutations for tumor initiation, progression, and therapy remain unclear.

We developed a conditional Ppm1d-loxP-exon6-loxP-exon6-E518X-tag mouse allele (Ppm1d-flex-6) that enables lineage-, spatial-, and temporal-specific expression of a DMG-derived truncated Ppm1d protein from its endogenous locus in the presence of Cre-recombinase. Ubiquitous activation of mutant Ppm1d was modeled using the Meox2-Cre driver, and primary gliomas were modeled using the RCAS/tv-a system to introduce Cre and PDGFB co-drivers into Nestin-positive neural stem cells. Complementary studies were performed in mouse embryonic fibroblasts (MEFs) expressing truncated Ppm1d following Cre recombination.

While Meox2-Cre-driven ubiquitous recombination of Ppm1d-flex-6 produced muted phenotypes, Ppm1d-flex-6 recombination in Nestin+ neural stem cells accelerated gliomagenesis. Its oncogenic effect was weaker than complete p53 loss, and it did not accelerate tumorigenesis further in p53-null tumors. Single-cell RNA-sequencing revealed that Ppm1d-flex-6 gliomas adopt more progenitor-like transcriptional states and upregulate p53- and cell cycle associated pathways. In MEFs, Ppm1d-flex-6 enhanced proliferation and shifted transcriptomic programs toward MAPK and PI3K-Akt signaling, while impairing DNA damage responses, including reduced γ-H2AX induction after irradiation. These defects sensitized cells to radiation and decreased clonogenic survival after ionizing radiation and PARP inhibition.

Ppm1d mutations confer intermediate suppression of the p53 pathway, consistent with the clinical features of PPM1D-mutant DMGs and are associated with radiosensitivity and PARP inhibitor vulnerability.