**13. Conclusions and future applications**

The expression profile analysis for miRNAs in tumour cells has revealed that the deregulation of these molecules is frequent in a wide array of tumours. MiRNAs may act as tumour suppressants or oncoproteins, which regulate key routes involved in cell growth and apoptosis. Each miRNA may have hundreds of target genes, and several genes are targeted by several miRNAs: this creates a highly complex regulatory network. As we could appreciate in this revision, studies that analyse the expression profile for miRNAs in the different degrees of astrocytomas are scarce; therefore, it is convenient to include a greater number of cases, which helps define the expression profile characteristic for each degree: pilocytic, diffuse, anaplastic and GBM. Within the classing of astrocytic tumours, GBM is the most widely studied tumour, given the fact that it is the most common brain neoplasm in adults and it is quickly disseminated in the adjacent brain tissue, which makes its surgical resection impossible. In GBM, miRNAs participate in several cell processes such as cell proliferation, invasion, angiogenesis and differentiation. Different studies regarding the expression profile of miRNAs in GBM point to overexpressed miRNAs such as miR-10b, miR-21, miR-221 and miR-26 and less expressed miRNAs such as miR-124, miR-128, miR-137, miR-181, miR-7, miR-34 and miR-451. miR-21, miR-221, miR-124, miR-128, miR-181, miR-7 and miR-34 are the best characterised miRNAs with a potential to be used as tumour markers. Nevertheless, it is necessary to correlate the expression profile of miRNAs with clinical and pathological data to answer the therapy or survival of patients.

It is also important to highlight the role that miRNAs undertake in the stem cell, in the differentiation and in cell identity. MiRNAs involved in neural development have also been found deregulated in GBM, which implies that certain miRs allow the growth of the tumour by suppressing the differentiation and maintaining the characteristics of stem cells. Several miRNAs have been identified as having a functional importance in neural development. In particular, miR-7 and miR-124 participate in neural differentiation and are little expressed in GBM. MiR-128 is also altered, but its function in normal cells is unknown. In GBM, the suppression of miR-128 may have severe effects because it may keep the self-renewal of glioma stem cells [42].

The determination and validation of target RNAm will help understand the development of the tumour and will provide potential targets to reduce its growth. In such manner, one of the goals to pursue is to identify a group of miRNAs, whose expression is significantly correlated with clinical parameters and which may be used to classify different degrees of

Little is known about the role of miRNAs as prognosticating indicators. Nevertheless, in astrocytomas, it has been observed that some miRNAs are expressed in a differential manner as miR-221 which is over-expressed in high-grade gliomas, and miR-124 has a lower level of expression in the anaplastic astrocytoma and in the GBM by comparison with low-grade astrocytomas such as the pilocytic and the diffuse astrocytoma. The low expression of miR-137 in astrocytomas is associated with a more advanced clinical phase. The low expression of miR-181b or the high expression of miR-21 was significantly associated with a poor survival of the patient [43].

The miRNAs may have important therapeutic implications, given that they may be functionally antagonised or restored.

MiRNAs are less susceptible to chemical modifications and degradement by ribonucleases by comparison with RNAm. These features of miRNAs allow their detection not only from frozen tissue but also in bodily fluids such as plasma and serum, and even in samples fixed in formol and included in paraffin. This allows for the development of retrospective studies, including a greater number of cases. Particularly speaking, the level of expression starting from bodily fluids represents the most promising advance for a non-invasive diagnosis and allows for their use as biomarkers to detect tumours in early stages and correlating them with clinical development.
