Background:

The most common cytogenetic lesions in the embryonal neural tumors medulloblastoma (MB) and neuroblastoma (NB) affect chromosome 17, with 17q+ or isochromosome 17q, in approximately one-third of MB with these aberrations being a significant indicator of poor clinical outcome. Similarly, in NB gain of 17q is the most powerful genetic predictor of adverse clinical outcome. 17q+ correlates with poor survival in our population-based material where we found aberrations of chromosome 17 in 85% of primary neuroblastomas, specifically, gain of PPM1D/Wip1 at 17q23. Wip1 is a serine/threonine phosphatase encoded by the gene PPM1D, described as a gatekeeper in the Mdm2-p53 regulatory loop involved in genetic stability, inflammation and a potential oncogene contributing to carcinogenesis.

Methods:

Comparative genomic hybridization (CGH), immunostaining, mRNA arrays, qPCR, exome- and RNA-sequencing was used to examine PPM1D/Wip1 in neuroblastoma and medulloblastoma. Genetic and pharmacological inhibition was used to analyze the function of Wip1 in preclinical neuroblastoma and medulloblastoma models.

Results:

CGH-array analysis detected PPM1D/Wip1 extra copies in all tumors and cell lines containing 17q-gain. Expression arrays and immunostaining showed high expression of Wip1 in neuroblastoma corresponding to poor survival. RNA sequencing confirmed PPM1D-gain and revealed truncated isoforms with oncogenic potential. Exome-sequencing detected a mutation leading to constitutive PPM1D/Wip1 activation in an aggressive metastatic infant neuroblastoma. Wip1 knockdown experiments showed significant decrease of cell viability, proliferation and colony formation as well as substantial increase of DNA-damage response in neuroblastoma and medulloblastoma cells. Tumor neuroblastoma xenograft development was significantly delayed showing median tumor development (0.10 mL) to be more than doubled (median 15 days, vs. 33 days, p<0.001) after Wip1 downregulation compared to scrambled controls. A novel Wip1 inhibitor was highly potent in cytotoxic/cytostatic effect in neuroblastoma and medulloblastoma cell lines. Furthermore, this Wip1 inhibitor significantly inhibited growth of established human neuroblastomas and medulloblastomas in nude mice after treatment (P<0.01).

Conclusions:

Our results show that PPM1D/Wip1 is oncogenic in neuroblastoma and medulloblastoma development. We propose three different ways on how PPM1D/Wip1 is activated: due to chromosomal gain, alternative RNA-isoforms and/or DNA-mutation. PPM1D/Wip1 provides a novel therapeutic target in neuroblastoma and medulloblastoma.

Citation Format: Jelena Milosevic, Nina Eissler, Diana Treis, Malin Wicktröm, Susanne Fransson, Baldur Sveinbjornsson, Ninib Baryawno, Subazini Kosalai, Chandrasekhar Kanduri, Kazuyasu Sakaguchi, Tommy Martinsson, John Inge Johnsen, Per Kogner. PPM1D/Wip1, promising new target in childhood cancers neuroblastoma and medulloblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1945. doi:10.1158/1538-7445.AM2017-1945