Oncogenic Ppm1d mutations deregulate the p53 pathway in primary mouse gliomas
Vennesa Valentine1, Abigail J Groth2, Joshua Tolliver2
1Department of Pharmacology & Cancer Biology, Duke University School of Medicine, Durham, NC 27710, USA.
Background:
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.
Methods:
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.
Results:
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.
Conclusions:
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.
Insights
Mutant PPM1D protein accelerates gliomagenesis in Diffuse Midline Gliomas (DMGs) by deregulating p53 signaling. This finding suggests PPM1D-mutant DMGs may respond to radiation and PARP inhibitors.
Area of Science:
- Neuro-oncology
- Molecular Biology
- Cancer Genetics
Background:
- Diffuse midline gliomas (DMGs) are aggressive brain tumors with poor prognosis.
- Approximately 20% of DMGs feature truncating mutations in exon 6 of PPM1D, leading to protein stabilization and p53 pathway dysregulation.
- The precise role of these PPM1D mutations in DMG development and treatment response remains largely unknown.
Purpose of the Study:
- To investigate the functional consequences of PPM1D exon 6 mutations in DMG initiation and progression.
- To develop a conditional mouse model for studying the temporal and spatial effects of mutant PPM1D expression.
- To assess the impact of mutant PPM1D on cellular signaling pathways and therapeutic vulnerabilities.
Main Methods:
- Development of a conditional mouse allele (Ppm1d-flex-6) for inducible expression of truncated Ppm1d.
- Modeling gliomagenesis using Cre-lox recombination in neural stem cells and mouse embryonic fibroblasts (MEFs).
- Utilizing single-cell RNA-sequencing to analyze transcriptional changes in gliomas and MEFs.
Main Results:
- Recombination of Ppm1d-flex-6 in neural stem cells accelerated gliomagenesis, though less potently than complete p53 loss.
- Ppm1d-flex-6 gliomas exhibited progenitor-like transcriptional states and upregulated p53 and cell cycle pathways.
- In MEFs, mutant Ppm1d enhanced proliferation, altered signaling (MAPK, PI3K-Akt), impaired DNA damage response, and increased sensitivity to radiation and PARP inhibition.
Conclusions:
- PPM1D mutations in DMGs result in intermediate p53 pathway suppression.
- These findings align with clinical observations in PPM1D-mutant DMGs.
- PPM1D-mutant DMGs exhibit radiosensitivity and vulnerability to PARP inhibitors, suggesting potential therapeutic strategies.
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