Correlation of microrna-372 upregulation with poor prognosis in human glioma

Human gliomas are a heterogeneous group of primary intracranial tumors for both children and adults [1]. The entities are distinguished based on morphological criteria by histological analysis and presumed cell of origin. According to the World Health Organization (WHO) classification which is based on histomorphological criteria, human gliomas includes well-differentiated low grade astrocytomas [World Health Organization (WHO) grade I~II], anaplastic astrocytomas (WHO grade III) and glioblastoma multiforme (GBM, WHO grade IV) [2]. Despite great progress in therapeutic technologies, such as surgery, radiotherapy, photodynamic therapy, and chemotherapy, the clinical outcome of patients with gliomas remains poor, with a lower than 3% 5-year survival rate for patients with GBM [3]. Although the WHO classification can reflect the anticipated malignancy of the tumor and serve as a criterion to predict the clinical outcome of patients, recent studies have indicated that histomorphological criteria alone may not be sufficient to estimate the prognosis [2, 4‐8]. For example, Curran et al. [9] demonstrated that the median survival time of patients with high-grade gliomas range from 5 to 59 months and some patients with low-grade tumors also present poor outcome. Therefore, to investigate the molecular genetics of gliomas may help to overcome some of these limitations.

MicroRNAs (miRNAs) are a recently discovered class of short non-coding endogenous RNA molecules that have a wide impact on the regulation of multiple target genes’ expression post-transcriptionally [10]. At first, miRNAs are transcribed by RNA polymerase II to yield long transcripts known as pri-miRNAs, which are processed to pre-miRNAs by the RNase III enzyme Drosha in the nucleus; then, the pre-miRNAs are exported to the cytoplasm by exportin-5 and subsequently converted to mature duplex miRNAs by another RNase III enzyme, Dicer; after that, mature miRNAs regulate their targets by direct cleavage of the mRNA or by inhibition of protein synthesis, according to the degree of complementarities with their targets’ 3^′UTR regions [11, 12]. With the use of sophisticated techniques and screening tools, miRNAs have been demonstrated to be involved in multiple cellular processes, including development, cell proliferation and differentiation, stem cell maintenance, epithelial- mesenchymal transition, apoptosis and metabolism [13, 14]. miRNAs also play important roles in a wide variety of physiological and pathological processes involved in tumorigenesis and tumor progression. Depending on their target genes, miRNAs can function either as oncogenes or tumor suppressors. Accumulating findings have demonstrated that miRNAs are associated with glioma formation and growth [15, 16]. In the present study, we focus on miR-372, which has been demonstrated to act as either an oncogenic miRNA or an anti-oncomiR in various human malignancies [7, 17, 18]. However, its roles in gliomas have not been elucidated. To address this problem, miR-372 expression in human gliomas and non-neoplastic brain tissues was measured by real-time quantitative RT-PCR assay. The association of miR-372 with clinicopathological factors or prognosis of glioma patients was also statistically analyzed.

Biomarker screening is an emerging field for neurooncology. Especially for gliomas, considerable progresses have been made in identifying, characterizing, and applying molecular markers. In the present study, we initially found that miR-372 was upregulated in human glioma tissues compared with non-neoplastic brain tissues. Then, the increased expression of miR-372 in glioma tissues was significantly correlated with advanced tumor progression and aggressive clinicopathological features. Next, the Kaplan-Meier analysis revealed that glioma patients with high miR-372 expression tend to have poorer overall survival. In addition, the multivariate analysis clearly demonstrated that high miR-372 expression was a statistically significant risk factor affecting overall survival in glioma patients, suggesting that miR-372 upregulation in gliomas is not only in a grade-dependent fashion, it is also a predictor of overall survival. Finally, subgroup analyses showed the significant prognostic value of miR-372 upregulation for glioma patients especially for those with advanced pathological grade.

MiR-372, together with miR-371a, miR-371b and miR-373, belongs to miR-371~373 cluster which has been demonstrated to play important roles in tumorigenesis and tumor progression [20]. Among these members, miR-372 may act as either an oncogenic miRNA or an anti-oncomiR in various human malignancies. It can enhance cell proliferation, stimulate cell cycle progression, and decrease apoptosis of tumor cells in many types of cancer. For example, Cho et al. [17] revealed that miR-372 plays an oncogenic role through down-regulation of the tumor suppressor gene LATS2, which accelerated growth and survival of gastric cancer cells; Yamashita et al. [21] indicated that the increased expression of miR-372 in colon cancer was an independent prognostic factor and was associated with synchronous liver metastasis; Voorhoeve et al. [14] demonstrated that miR-372 could enhance cell proliferation, stimulate cell cycle progression, or decrease apoptosis in testicular germ cell tumors. Consistent with these previous studies, our data also found the upregulation of miR-372 in glioma tissues compared with paired adjacent non-neoplastic brain tissues. In addition, the aberrant expression of miR-372 was associated with advanced pathological grades and low KPS of glioma patients, indicating that this miRNA may be involved in the development of human gliomas. By contrast, accumulating studies showed the tumor suppressive roles of miR-372 in many cancers. For example, Tian et al. [22] found the downregulation of miR-372 in cervical carcinoma tissues as compared with adjacent normal cervical tissues. The authors demonstrated that its anti-oncogenic role might be through control of cell growth and cell cycle progression by down-regulating the cell cycle genes CDK2 and cyclin A1. These findings suggest that the different expression patterns and involvement of miR-372 in various cancers may depend on the roles of its target genes.

Our findings that miR-372 upregulation was associated with aggressive tumor progression mentioned above prompt us to investigate its possible prognostic value in glioma patients. According to the univariate and multivariate analyses, we identified miR-372 upregulation as an independent predictor for short overall survival of glioma patients, which was consistent with the findings of Yamashita et al. [21] in colon cancer. Interestingly, our subgroup analyses further suggested that miR-372 may act as a significant prognostic factor for glioma patients with high pathological grades (III~IV), but not for those with low pathological grades (I~II).

In conclusion, our data offer the convincing evidence for the first time that miR-372 may act as an oncogenic miRNA in gliomas and represent a potential regulator of aggressive development and a candidate prognostic marker for this malignancy, especially for advanced tumors with high pathological grades. Further elucidation of the mechanism by which the oncogenic roles of miR-372 in gliomas are thwarted is worth to be done.