Abstract 1313: Mutually exclusive somatic mutations in WNT pathway medulloblastomas reveal a critical interaction between DDX3X and SMARCA4

DDX3X is an ATP-dependent RNA helicase that belongs to an extended family of D-E-A-D motif (DEAD-box) containing proteins whose functions have been implicated in key biological processes ranging from RNA transcription and splicing to the regulation of translation initiation. Through whole exome sequencing of 94 primary medulloblasotmas, we have identified DDX3X as one of the most common recurrently mutated genes in medulloblastoma. These mutations predominantly occur in WNT pathway medulloblastomas which concurrently harbor stabilized forms of mutant beta-catenin. Interestingly, we have also identified SMARCA4 mutations in WNT pathway tumors that are mutually exclusive of DDX3X mutations. In addition, two WNT pathway tumors that have neither DDX3X nor SMARCA4 gene mutations have mutations in genes known to encode for direct binding partners of DDX3X and SMARCA4 (EIF3E and ACTL6A, respectively). This mutual exclusivity of DDX3X and SMARCA4 mutations suggests either a direct interaction between the DDX3X and SMARCA4 gene products or an overlap of the biological processes regulated by each. Here we reveal the direct interaction between DDX3X and SMARCA4 in normal physiological conditions and further investigate the impact of somatic mutations in DDX3X and SMARCA4 on this interaction in WNT pathway associated medulloblastomas. Our results point to a critical link between RNA helicases and the SWI/SNF chromatin remodeling complex in WNT/beta-catenin signaling and disease.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1313. doi:1538-7445.AM2012-1313

miR-34a confers chemosensitivity through modulation of MAGE-A and p53 in medulloblastoma

Recent studies have established miR-34a as a key effector of the p53 signaling pathway and have implicated its role in multiple cancer types. Here, we establish that miR-34a induces apoptosis, G2 arrest, and senescence in medulloblastoma and renders these cells more sensitive to chemotherapeutic agents. These effects are mediated in part by the direct post-transcriptional repression of the oncogenic MAGE-A gene family. We demonstrate that miR-34a directly targets the 3' untranslated regions of MAGE-A genes and decreases MAGE-A protein levels. This decrease in MAGE-A results in a concomitant increase in p53 and its associated transcriptional targets, p21/WAF1/CIP1 and, importantly, miR-34a. This establishes a positive feedback mechanism where miR-34a is not only induced by p53 but increases p53 mRNA and protein levels through the modulation of MAGE-A genes. Additionally, the forced expression of miR-34a or the knockdown of MAGE-A genes by small interfering RNA similarly sensitizes medulloblastoma cells to several classes of chemotherapeutic agents, including mitomycin C and cisplatin. Finally, the analysis of mRNA and micro-RNA transcriptional profiles of a series of primary medulloblastomas identifies a subset of tumors with low miR-34a expression and correspondingly high MAGE-A expression, suggesting the coordinate regulation of these genes. Our work establishes a role for miR-34a in modulating responsiveness to chemotherapy in medulloblastoma and presents a novel positive feedback mechanism involving miR-34a and p53, via direct targeting of MAGE-A.

Role of plasma membrane calcium ATPases in calcium clearance from olfactory sensory neurons

Odorants cause Ca(2+) to rise in olfactory sensory neurons (OSNs) first within the ciliary compartment, then in the dendritic knob, and finally in the cell body. Ca(2+) not only excites but also produces negative feedback on the transduction pathway. To relieve this Ca(2+)-dependent adaptation, Ca(2+) must be cleared from the cilia and dendritic knob by mechanisms that are not well understood. This work focuses on the roles of plasma membrane calcium pumps (PMCAs) through the use of inhibitors and mice missing 1 of the 4 PMCA isoforms (PMCA2). We demonstrate a significant contribution of PMCAs in addition to contributions of the Na(+)/Ca(2+) exchanger and endoplasmic reticulum (ER) calcium pump to the rate of calcium clearance after OSN stimulation. PMCAs in neurons can shape the Ca(2+) signal. We discuss the contributions of the specific PMCA isoforms to the shape of the Ca(2+) transient that controls signaling and adaptation in OSNs.

Ziwi, the zebrafish homologue of the Drosophila piwi: co-localization with vasa at the embryonic genital ridge and gonad-specific expression in the adults

PIWI regulates the proliferation and maintenance of germline stem cells in diverse organisms. The full-length 3.26 kb ziwi cDNA, the zebrafish homologue of piwi of Drosophila, encodes a putative protein of 858 amino acids. ZIWI is 65% homologous with the mouse and human PIWI, but only 38 and 33% with Caenorhabditis elegans and Drosophila PIWI, respectively. In adult zebrafish, ziwi is expressed exclusively in the gonads. In embryos and fry, its expression is detectable initially during segmentation and persisted for at least 4 weeks post hatching. During neurogenesis and organogenesis, its expression was detected in the CNS and fin buds. Starting from 24 hpf and later on, ziwi transcripts were found in the genital ridge.

Ziwi, the zebrafish homologue of the Drosophila piwi: co-localization with vasa at the embryonic genital ridge and gonad-specific expression in the adults

PIWI regulates the proliferation and maintenance of germline stem cells in diverse organisms. The full-length 3.26 kb ziwi cDNA, the zebrafish homologue of piwi of Drosophila, encodes a putative protein of 858 amino acids. ZIWI is 65% homologous with the mouse and human PIWI, but only 38 and 33% with Caenorhabditis elegans and Drosophila PIWI, respectively. In adult zebrafish, ziwi is expressed exclusively in the gonads. In embryos and fry, its expression is detectable initially during segmentation and persisted for at least 4 weeks post hatching. During neurogenesis and organogenesis, its expression was detected in the CNS and fin buds. Starting from 24 hpf and later on, ziwi transcripts were found in the genital ridge.