MicroRNAs and cholangiocarcinoma: elucidating the effects of tiny giants

MicroRNAs (miRNAs) are small non-coding strands of RNA (approximately 18–22 nucleotides) found in plants, animals, and some viruses with functions in RNA silencing and post-transcriptional regulation of gene expression (1). MiRNAs have been considered as valuable tools for prognosis of disease as stable biomarkers and as therapeutic targets for disease amelioration (2,3). MiRNA processing begins with primary miRNA (pri-miRNA) transcribed from DNA by polymerase II. Pri-miRNA is then cleaved by RNase III endonuclease, Drosha, and is released as precursor miRNA (pre-miRNA). Upon export of the nucleus into the cytoplasm by exportin 5 (XPO5), dicer further cleaves pre-miRNA into a, now activated, miRNA dimer (2). MiRNAs can bind to the 3'-untranslated region (UTR) of messenger RNAs and interfere with their translation (1). The significant post-transcriptional regulatory step in gene expression by miRNAs makes understanding their mechanism and regulation important for the prevention and improvement of disease.