Abstract
The term liquid biopsy is used in contraposition to the traditional "solid" tissue biopsy. In the oncology field it has opened a new plethora of clinical opportunities as tumor-derived material is shedded into the different biofluids from where it can be isolated and analyzed. Common biofluids include blood, urine, saliva, cerebrospinal fluid (CSF), pleural effusion or bile. Starting from these biological specimens several analytes can be isolated, among which we will review the most widely used: circulating tumor cells (CTCs), circulating tumor DNA (ctDNA), circulating tumor RNA (ctRNA), proteins, metabolites, and exosomes. Regarding the nature of the biomarkers it will depend on the analyte, the type of tumor and the clinical application of the liquid biopsy and it includes, somatic point mutations, deletions, amplifications, gene-fusions, DNA-methylated marks, tumor-specific miRNAs, proteins or metabolites. Here we review the characteristics of the analytes and the methodologies used for their isolation. We also describe the applications of the liquid biopsy in the management of patients with cancer, from the early detection of cancers to treatment guidance in patients with advanced tumors. Finally, we also discuss some current limitations and still open questions.
Highlights
Even before the definition of the term biomarker by a consensus panel at the WHO in Geneva in 2001 as “any substance, structure, or process that can be measured in the body or its products and influence or predict the incidence of outcome or disease” [1], the diagnosis and prognosis of multiple diseases had relied on the identification of different biomarkers in blood tests
Well-known examples are for instance the determination of the levels of different metabolites such as glucose or cholesterol; the presence of enzymes such as transaminases coming from dying hepatocytes; or the levels of monoclonal immunoglobulins, carcinoembryonic antigen (CEA), alfa-fetoprotein (AFP), prostate specific antigen (PSA), cancer antigen 125 (CA 125) or human chorionic gonadotropin (HCG) in the bloodstream [2, 3]
Methods based on circulating tumor cells (CTCs) biological properties rely on specific biomarkers expressed on the cell surface that allow their capture from the blood sample
Summary
Even before the definition of the term biomarker by a consensus panel at the WHO in Geneva in 2001 as “any substance, structure, or process that can be measured in the body or its products and influence or predict the incidence of outcome or disease” [1], the diagnosis and prognosis of multiple diseases had relied on the identification of different biomarkers in blood tests. Well-known examples are for instance the determination of the levels of different metabolites such as glucose or cholesterol; the presence of enzymes such as transaminases coming from dying hepatocytes; or the levels of monoclonal immunoglobulins, carcinoembryonic antigen (CEA), alfa-fetoprotein (AFP), prostate specific antigen (PSA), cancer antigen 125 (CA 125) or human chorionic gonadotropin (HCG) in the bloodstream [2, 3]. All these biomarkers have been commonly used with variable sensitivity and specificity rates to diagnose diabetes, liver diseases or the presence of myeloma, colon, liver, prostate, ovarian or germ-cell tumors. It is known that the genetic landscape of tumors is spatially heterogeneous and temporally dynamic [6, 7]
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