Abstract B05: Mechanisms of PPM1D-mediated tumorigenesis in diffuse intrinsic pontine glioma (DIPG)

Abstract

Abstract Diffuse intrinsic pontine glioma (DIPG), the most common pediatric brainstem tumor, is associated with a dismal prognosis with almost no survivors within two years of diagnosis. It is unamenable to surgical resection, is not responsive to conventional chemotherapy, and responds only temporarily to radiation therapy (XRT). Recent publications identified carboxy (C)-terminal truncating mutations of PPM1D, which encodes the protein WIP1, in up to 25% of DIPGs. WIP1 is a serine/threonine protein phosphatase that plays an important role in DNA damage response (DDR). Prior publications show that most, but not all, PPM1D C-terminal mutants function in a dominant negative fashion to promote tumor growth in cancer models. We hypothesized that PPM1D mutation promotes DIPG growth, and WIP1 inhibition suppresses tumor proliferation. We further hypothesized that, due to its key role in DDR, WIP1 inhibition enhances the antitumor effects of XRT. Using Sanger sequencing, we identified de novo mutation of PPM1D in the C-terminus of multiple patient-derived DIPG cell lines. We also stably introduced clinically relevant PPM1D mutations, which we identified in DIPGs resected from patients, into a PPM1D wild-type DIPG cell line, DIPG VI, as well as into DIPG cells derived from a DIPG mouse model. Using trypan blue exclusion, bioluminescence, and immunofluorescence detection of markers of proliferation and cell death, in the presence or absence of PPM1D knockdown or pharmacologic WIP1 inhibition, and/or following XRT exposure, we measured proliferation of DIPG VI, DIPG VI L513X, a cell line with stable expression of a mutant PPM1D, and DIPG7, a cell line with a de novo PPM1D mutation. We also assayed proliferation in organotypic brain slice cultures derived from symptomatic immunocompromised mice orthotopically xenografted with DIPG cells and of DIPG cells embedded within the brainstem of organotypic brain slices derived from immunocompetent mice. Finally, we orthotopically xenografted DIPG cells into the early post-natal (P0-2) brainstem of immunodeficient mice, treated mice with vehicle or with a small-molecule WIP1 inhibitor, and measured effects on survival, as well as proliferation in tumor cells post-necropsy. Under cell culture conditions, expression of PPM1D mutants promoted tumor cell proliferation in wild-type PPM1D murine and human DIPG cells. Conversely, PPM1D knockdown significantly suppressed DIPG cell proliferation, especially in PPM1D mutated DIPG cells. Similarly, treatment with a small-molecule WIP1 inhibitor suppressed the growth of PPM1D-mutated, but not PPM1D wild-type DIPG cells, in neurosphere cultures, on immunocompetent organotypic brain slices, and on organotypic brain slices derived from symptomatic, orthotopic, xenografted mice. WIP1 inhibition had similar effects on growth inhibition to XRT alone and augmented the antiproliferative and proapoptotic effects of XRT in vitro. Mice bearing an orthotopic xenograft of PPM1D-mutated human DIPG cells, DIPG7, died of disease earlier than mice xenografted with PPM1D wild-type human DIPG cells, DIPG VI. Treatment of mice bearing orthotopically, xenografted DIPG7 cells with a WIP1 inhibitor via a continuous 14-day infusion into the lateral ventricles resulted in a significant reduction in proliferation of tumor cells. In summary, our results show that C-terminal mutated PPM1D is oncogenic in DIPG, promoting proliferation and reduced survival in vivo. WIP1 inhibition suppresses growth and increases the radiosensitivity of PPM1D-mutated DIPG. These results suggest that WIP1 is a druggable target alone and in combination with XRT in DIPG. Further studies will examine the PPM1D-mediated mechanisms of tumorigenesis and treatment responsiveness in DIPG in order to develop clinically viable treatment paradigms that target PPM1D-mutated tumors. Citation Format: Mwangala P. Akamandisa, Kai Nie, Jing Wen, Rita Nahta, Dolores Hambardzumyan, Robert C. Castellino. Mechanisms of PPM1D-mediated tumorigenesis in diffuse intrinsic pontine glioma (DIPG) [abstract]. In: Proceedings of the AACR Special Conference: Pediatric Cancer Research: From Basic Science to the Clinic; 2017 Dec 3-6; Atlanta, Georgia. Philadelphia (PA): AACR; Cancer Res 2018;78(19 Suppl):Abstract nr B05.