Circulating MicroRNAs as Cancer Biomarkers in Liquid Biopsies

Biological fluids such as blood, saliva, and urine offer a rich source of biomarkers that have the potential to change the paradigm of cancer management. By allowing routine noninvasive sampling that can offer new insights into cancer progression and response to treatment so-called liquid biopsies can play an important role in personalized medicine. MicroRNAs (miRNAs) are a particularly attractive class of biomarkers as they are not only resistant to the high levels of RNases found in biological fluids, but also able to confer clinically useful information about the disease relating to diagnosis, prognosis, and the response to treatment. Circulating miRNAs are either associated with proteins or extracellular vesicles (EV) and although their origin and implied functions as intracellular messengers remain somewhat controversial, they are implicated in the progression and the establishment of metastatic niches. In this chapter, we review the rapidly emerging field of circulating miRNA cancer biomarkers, their origin and function, techniques to detect these molecules, and the use of bioinformatic tools to derive implied regulatory function, as well as the challenges that lie ahead for their clinical implementation.