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Background: Children with diffuse intrinsic pontine glioma (DIPG) succumb to disease within 2 years of diagnosis despite treatment with ionizing radiation (IR) and/or chemotherapy. Our aim was to determine the role of protein phosphatase, magnesium-dependent 1, delta (PPM1D) mutation, present in up to 25% of cases, in DIPG pathogenesis and treatment., Methods: Using genetic and pharmacologic approaches, we assayed effects of PPM1D mutation on DIPG growth and murine survival. We assayed effects of targeting mutated PPM1D alone or with IR on signaling, cell cycle, proliferation, and apoptosis in patient-derived DIPG cells in vitro, in organotypic brain slices, and in vivo., Results: PPM1D-mutated DIPG cell lines exhibited increased proliferation in vitro and in vivo, conferring reduced survival in orthotopically xenografted mice, through stabilization of truncated PPM1D protein and inactivation of DNA damage response (DDR) effectors p53 and H2A.X. PPM1D knockdown or treatment with PPM1D inhibitors suppressed growth of PPM1D-mutated DIPGs in vitro. Orthotopic xenografting of PPM1D short hairpin RNA-transduced or PPM1D inhibitor-treated, PPM1D-mutated DIPG cells into immunodeficient mice resulted in reduced tumor proliferation, increased apoptosis, and extended mouse survival. PPM1D inhibition had similar effects to IR alone on DIPG growth inhibition and augmented the anti-proliferative and pro-apoptotic effects of IR in PPM1D-mutated DIPG models., Conclusions: PPM1D mutations inactivate DDR and promote DIPG growth. Treatment with PPM1D inhibitors activated DDR pathways and enhanced the anti-proliferative and pro-apoptotic effects of IR in DIPG models. Our results support continued development of PPM1D inhibitors for phase I/II trials in children with DIPG., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Neuro-Oncology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The protein phosphatase PPM1D (Wip1) was originally identified as a p53 target product. Activation of PPM1D through various mechanism promotes the tumorigenic potential of various cancers by suppressing p53 and other DNA damage response proteins. New functions of PPM1D have recently been revealed in physiological processes such as cell differentiation. However, the regulatory mechanisms of signalling pathway to maintain stemness and induce cell differentiation are still unclear. Here we report that PPM1D modulates retinoic acid (RA) signalling. PPM1D knockdown resulted in decreased alkaline phosphatase activity of the human teratocarcinoma cell line NT2/D1. Inhibition of PPM1D-induced cell differentiation and decreased gene expression of the stem cell marker Oct-4 (POU5F1). RA-induced cell differentiation was promoted by reducing PPM1D activity. RA treatment elicited activation of the MEK-ERK pathway and induced rapid and transient activation of the extracellular signal-regulated kinase 1/2 (ERK-1/2). PPM1D dephosphorylated a phosphopeptide with the TEY motif in ERK-1/2 in vitro. Moreover, phosphorylation of ERK-1/2 was facilitated by PPM1D inhibition. Our study shows that PPM1D plays an important role in maintaining the undifferentiation state and a new function in RA-induced ERK regulation and cell differentiation., (© The Author(s) 2018. Published by Oxford University Press on behalf of the Japanese Biochemical Society. All rights reserved.)

Maternal DNA damage during meiosis causes genetic abnormalities that can lead to infertility, birth defects, and abortion. While DNA damage can rapidly halt cell cycle progression and promote DNA repair in somatic cells, mammalian oocytes are unable to mount a robust G2/prophase arrest in response to DNA damage unless damage levels are severe. Here, we show that inhibition of WIP1 phosphatase enhances the ability of oocytes to respond to DNA damage. We found that WIP1 was expressed constantly during meiotic maturation, and that inhibition of WIP1 activity did not impair meiotic maturation. However, oocytes in G2/prophase were sensitized to DNA damage following WIP1 inhibition, not only increasing γ-H2AX level and ATM phosphorylation, but also decreasing entry into meiosis. Moreover, WIP1 inhibition significantly promoted the repair of damaged DNA during G2/prophase arrest, suggesting that WIP1 suppresses DNA repair in oocytes. Therefore, our results suggest that WIP1 is a key suppressor of the DNA damage response during G2/prophase arrest in mouse oocytes.

Protein phosphatase Magnesium-dependent 1, Delta (PPM1D) is a wild-type p53-inducible Ser/Thr phosphatase that acts as a negative regulator of the p53 tumor suppressor. Gene amplification and overexpression of PPM1D have been reported in various cancers including leukemia and neuroblastoma. Therefore, PPM1D is a promising target in cancer therapy. It has been reported that PPM1D knockout mice exhibit neutrophilia in blood and show a defective immune response. Here, we found that inhibition of PPM1D induced neutrophil differentiation of human promyelocytic leukemia cell line HL-60. The combination of a PPM1D inhibitor and all-trans retinoic acid significantly increased their differentiation efficiency. The PPM1D inhibitor also induced G1 arrest in HL-60 cells. Our results suggest that PPM1D may be a potential therapeutic target for blood cell diseases including leukemia., (© The Authors 2017. Published by Oxford University Press on behalf of the Japanese Biochemical Society. All rights reserved.)