Introduction
Brain tumor, together with leukaemia, remains the leading cause of disease-related death in children [
1]. According to a population-based study by Kaatsch and colleagues [
2], approximately 60% of pediatric brain tumors are gliomas. Glioma is the most common type of primary brain tumor. Brain glioma can cause headaches, nausea and vomiting, seizures, and cranial nerve disorders as a result of increased intracranial pressure. Based on the observations that different gliomas share morphological similarities in different lineages of glial cells, the respective tumors have been classified as astrocytomas, oligodendrogliomas, and ependymomas [
3]. However, because the pathogenesis of these tumors is unclear, treatment is particularly complex. Many children who have been treated for brain tumors experience significant long-term problems, such as changes in intellectual and motor function [
4]. A better understanding of pediatric brain tumor pathogenesis is necessary to provide better markers for disease stratification and to assist in the development of less toxic therapies.
Aberrant microRNA expression has been found to be associated with a wide variety of human tumors, including lung cancer [
5], cervical cancer [
6], bladder cancer [
7], esophageal adenocarcinoma [
8] and pituitary adenomas [
9] et al. MicroRNAs also play a significant role in brain tumor pathology by regulating target gene expression, apoptosis and autophagy [
10]. The expression of miR-21 has been found to be increased between 5 and 100-fold in human glioblastoma tissues compared to control non-neoplastic brains [
11]. A set of brain-enriched microRNAs, miR-128, miR-181a, miR-181b and miR-181c have been found to be down-regulated in glioblastoma [
12]. Bottoni and his colleagues, using Northern blot, found that two microRNAs, miR-15a and miR-16-1 had a reduced expression in pituitary adenomas as compared to normal pituitary tissue [
13]. Upregulation of mir-372 has also been found to be related with poor prognosis in glioma [
14].
Large profiling studies using solid tissue and hematological tumors have established the usefulness of microRNA profiling for diagnosis and prognosis [
15]. We therefore sought to determine the expression profiles of microRNAs in pediatric gliomas and matched normal tissues using microRNAs microarrays. We also performed Gene Ontology (GO) and pathway analysis to investigate the changed biological processes and signaling pathways involved in pediatric gliomas.
Discussion
In the present study we used a microRNA microarray assay to study the differential expression pattern of microRNA in pediatric gliomas and the matched adjacent noncancerous tissues. Among the microRNAs detected in the microarray, we identified 40 microRNAs showing significantly higher or lower expression levels in tumors compared to the matched normal tissues. Results of GO analysis and KEGG pathway analysis suggested that target genes were closely associated with nervous system-related and tumor-related biological processes and signaling pathways.
Some of the microRNAs that showed differential expression between brain tumors and normal tissues in our study had previously been studied or found to be associated with gliomas, including pediatric gliomas. MiR-21, which was discovered to be upregulated by us, was also found and validated to be with increased expression in pediatric pilocytic astrocytoma (PA), a World Health Organization grade I pediatric glioma, in the study of Ho CY et al. [
25]. Overexpression of miR-222 in gliomas was also observed by the research of Li Q et al. [
26]. They proved that miR-222 could regulate Wnt/β-catenin signaling pathway and promote glioma genesis by using RNA interference and western blot technology. Furthermore, downregulation of miR-204, which was found to be associated with gliomas in our study, was also proved to contribute to glioma migration by targeting the migration-promoting receptor EphB2 [
27]. In the present study miR-218 was discovered to be with decreased expression in pediatric gliomas, indicating the reverse relationship between miR-218 expression and development of gliomas. This was consistent with the findings of Tu Y et al. [
28]. They found that upregulation of miR-218 reduced the migration, invasion and proliferation of glioma cells dramatically by regulating a wide range of genes and pathways. A new target gene of miR-218, which was noticed to be down-regulated in glioma in our study, has recently been found in the study of Shi ZM et al. [
29]. They identified p70S6K1 as a novel direct target of miR-128 and overexpression of p70S6K1 can partly rescue the inhibitory effect of miR-128 in the glioma cells. This result further confirms our findings.
As expected, some of the microRNAs that showed differential expression in gliomas in our data were novel. There is little evidence about the relationship between miR-1321, miR-513b, miR-769-3p and cancer genesis. In addition, the upregulated microRNAs including miR-424*, miR-760, miR-513b, miR-361-5p, miR-1259, miR-363, miR-199a, miR-1827, miR-423-5p, miR-1308, miR-1274, miR-1224, miR-513b and the downregulated microRNAs including miR-885, miR-769, miR-1296, miR-192, miR-331-3p, miR-484, miR-99b were found to be related with gliomas for the first time. This may provide new clues for gliomas research.
Aberrant expression of microRNAs may contribute to the induction of pediatric gliomas by regulating genes involved in nervous system-related and tumor-related biological processes and signaling pathways. According to the result of GO analysis, the target genes were involved in negative regulation of neuron apoptosis, axonogenesis, regulation of synaptic transmission, neurotransmitter secretion and synaptic vesicle transport et al.. These biological processes are crucial for maintaining normal functioning of the nervous system. Alteration of the processes induced by aberrant expression of certain microRNAs may favor glioma-genesis. As for biological pathways, several cancer-related pathways, including glioma, endometrial cancer, the TGF-beta signaling pathway, the MAPK signaling pathway, the wnt signaling pathway and the notch signaling pathway et al. were among the enriched pathways of the target genes. TGF-beta signaling has long been considered to contribute to glioma pathogenesis by direct support of tumor growth, self-renewal of glioma initiating stem cells and the inhibiting of anti-tumor immunity [
30]. Wnt signaling and MAPK signaling also play significant roles in almost all kinds of tumors, including glioma [
31]. In recent studies, notch signaling has been proved to be dysregulated in brain tumors and to contribute to the malignant potential of these tumors [
32]. Peng Xu et al. found out the different roles of Notch1 and Notch2. They proved that both upregulating of Notch1 and knocking down Notch2 had the effect of suppressing glioma cell growth and invasion as well as inducing apoptosis [
33].
In conclusion, we investigated the changed expression patterns of microRNAs in pediatric gliomas and identified the possible role of microRNA and corresponding target genes in brain tumor pathology. However, due to the limitation of the microarray assay, only 866 known human microRNAs were detected. The role of more microRNAs in brain tumors needs to be investigated using more advanced techniques, such as second-generation sequencing and higher capacity microarrays. The present study does at least provide new insights into pediatric brain tumor biology and may assist in finding new diagnostic and therapeutic tools for these tumors.
Acknowledgements
This work was supported by 973 Program (2011CB504000, 2010CB945203, 2010CB529600, 2010CB945203), the National Key Technology R & D Program (2012BAI01B09), the National Natural Science Foundation (81121001, 81271382, 81302005), the Shanghai Leading Academic Discipline Project (B205), the Shanghai Municipality Science & Technology Commission (09DJ1400601, 2008ZD001, 07DJ14005, 09XD1403300, 13ZR1438500) and the Chinese Academy of Sciences (2009KIP305).
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
JM, SYQ and YFY: conceived of the study and helped to draft the manuscript. FTL, YYX, YZ, LMT and HZ carried out part of the experiments. FTL, LMT, ZZ and YFY performed the statistical analysis. FTL wrote the manuscript. YL, GYF, BJL and LH participated in the design and coordination of the study. All authors read and approved the final manuscript.