Abstract 960: A translational investigation of piRNAs in glioblastoma multiforme

Background: PIWI-interacting RNAs (piRNAs) are highly abundant small noncoding RNAs that partner with PIWI proteins to protect germline tissues from destabilizing transposon activity. While aberrant expression of PIWI proteins has been linked with poor outcomes for many cancer types, far less is known about the expression or function of piRNAs in a cancer context. Here we provide evidence supporting a novel role of piRNAs in gliomagenesis that may be leveraged for therapeutic benefit. Methods: In order to characterize piRNA expression patterns in glioblastoma multiforme (GBM), we performed array-based piRNA expression profiling in seven pairs of human normal and tumor tissue specimens with validation by qPCR. In vitro (cellular proliferation, colony formation, cell cycle, apoptosis, transcriptional profiling) and in vivo (orthotopic xenograft mouse model) assays were performed to characterize the phenotypic impact of differentially expressed piRNAs using cell line models of GBM. For all functional experiments, synthetic piRNA mimics or negative control oligos were transiently delivered into cells via Lipofectamine RNAiMAX transfection reagent. Results: piRNA profiling revealed expression of over 350 piRNAs in both normal and tumor brain tissues including several with dysregulated expression in GBM tissue. Individual transfection of four down-regulated piRNA mimics revealed antiproliferative effects on glioma cells, consistent with tumor-suppressive roles. Notably, two other piRNAs that were expressed at equivalent levels in tumor and normal tissues did not affect cell proliferation when delivered into glioma cells, indicating specificity of piRNA functionality. Upregulation of the most underexpressed piRNA (∼10-fold lower expression in GBM tissue) was found to induce cell cycle arrest and apoptosis and to alter transcriptional levels of several genes involved in cell stress and survival pathways in glioma cell lines, yet it did not affect normal astrocyte proliferation. Finally, the volume of intracranial mouse xenograft tumors was significantly reduced for approximately two weeks after pre-implantation cell transfection with the piRNA as compared to negative control RNA, and median survival was extended by two days for piRNA-treated mice (n = 9/treatment group). Conclusions: Although piRNAs have not traditionally been thought to be expressed in healthy somatic tissues, our results are consistent with recent studies that have detected expression of a similar number of piRNAs in other normal tissues. Our results show that a subset of these piRNAs exhibit dysregulated expression in GBM and have cancer-relevant phenotypic properties in glioma cell line and mouse models. Taken together, our study reveals a previously unidentified functional role for piRNAs as tumor suppressors in gliomagenesis, and suggests that restoration of normal piRNA levels may be a viable GBM therapy. Citation Format: Daniel I. Jacobs, Qin Qin, Alan Fu, Zeming Chen, Jiangbing Zhou, Yong Zhu. A translational investigation of piRNAs in glioblastoma multiforme. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 960.