Circulating microRNAs as emerging non-invasive biomarkers for gliomas.

No single circulating biomarker has been put to practice for malignant gliomas so far, the most lethal primary brain tumors. Many promising protein biomarkers such as the mutant EGFRvIII or glial fibrillary acidic protein (GFAP) have already been detected in the blood and cerebrospinal fluid (CSF) of patients with gliomas, but their clinical value is still pending validation. Furthermore, these and other proteins seem to lack sufficient sensitivity and specificity required for a successful biomarker in this clinical setting. The expression profiling of microRNAs (miRNAs) has already entered cancer clinics as diagnostic and prognostic biomarkers, for assessing tumor initiation, progression and response to treatment. Large-scale miRNA expression analyses reported both up-regulation and down-regulation of several miRNAs in tumour tissues from patients with gliomas compared to normal brain tissue, thus supporting the development of miRNA-based biomarkers. Using comprehensive high-throughput approaches, such as microarrays, different circulating miRNAs were proposed as potential biomarkers of gliomas. This review is aimed to summarize the clinical evidence about circulating miRNA biomarkers discovered to date. Mandatory issues to develop clinically validated biomarkers to improve time of diagnosis, predicting response to treatment and prognosis of patients with gliomas are also herein addresses.