Biomarker

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A biomarker, or biological marker, generally refers to a measurable indicator of some biological state or condition. The term is also occasionally used to refer to a substance whose detection indicates the presence of a living organism.

Biomarkers are often measured and evaluated to examine normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention.^[1] Biomarkers are used in many scientific fields.

In medicine, a biomarker can be a traceable substance that is introduced into an organism as a means to examine organ function or other aspects of health. For example, rubidium chloride is used as a radioactive isotope to evaluate perfusion of heart muscle.

It can also be a substance whose detection indicates a particular disease state, for example, the presence of an antibody may indicate an infection. More specifically, a biomarker indicates a change in expression or state of a protein that correlates with the risk or progression of a disease, or with the susceptibility of the disease to a given treatment.

Other biomarkers can be based on measures of the electrical activity of the brain (using Electroencephalography (so-called Quantitative electroencephalography (qEEG)) or Magnetoencephalography), volumetric measures of certain brain regions (using Magnetic resonance imaging), saliva testing of natural metabolites, such as saliva nitrite, a surrogate marker for nitric oxide or analysis of urinary exosomes that carry biomarkers of disease^[2]. One example of a commonly used biomarker in medicine is prostate-specific antigen (PSA). This marker can be measured as a proxy of prostate size with rapid changes potentially indicating cancer. The most extreme case would be to detect mutant proteins as cancer specific biomarkers through Selected reaction monitoring (SRM), since mutant proteins can only come from an existing tumor, thus providing ultimately the best specificity for medical purposes.^[3]

Biomarkers used for personalized medicine are typically categorized as either prognostic or predictive. An example is KRAS, an oncogene that encodes a GTPase involved in several signal transduction pathways. Prognostic biomarkers indicate the likelihood of patient outcome regardless of a specific treatment. Predictive biomarkers are used to help optimize ideal treatments, and indicates the likelihood of benefiting from a specific therapy. Biomarkers for precision oncology are typically utilized in the molecular diagnostics of chronic myeloid leukemia, colon, breast, and lung cancer, and in melanoma.^[4]

Proof of concept

Previously used to identify the specific characteristics of the biomarker, this step is essential for doing an in situ validation of these benefits. A large number of candidates must be tested to select the most relevant ones.^[5]

Experimental validation

This step allows the development of the most adapted protocol for routine use of the biomarker. Simultaneously, it is possible to confirm the relevance of the protocol with various methods (histology, PCR, ELISA, ...) and to define strata based on the results.

Analytical performances validation

One of the most important steps, it serves to identify specific characteristics of the candidate biomarker before developing a routine test. Several parameters are considered including:

• sensitivity
• specificity
• robustness
• accuracy
• reproducibility

Protocol standardization

This optimizes the validated protocol for routine use, including analysis of the critical points by scanning the entire procedure to identify and control the potential risks.

In cell biology, a biomarker is a molecule that allows the detection and isolation of a particular cell type (for example, the protein Oct-4 is used as a biomarker to identify embryonic stem cells).^[6]

In genetics, a biomarker (identified as genetic marker) is a DNA sequence that causes disease or is associated with susceptibility to disease. They can be used to create genetic maps of whatever organism is being studied.

A biomarker can be any kind of molecule indicating the existence, past or present, of living organisms. In the fields of geology and astrobiology, biomarkers, versus geomarkers, are also known as biosignatures. The term biomarker is also used to describe biological involvement in the generation of petroleum.

Biomarkers are used to indicate an exposure to or the effect of xenobiotics which are present in the environment and in organisms. The biomarker may be an external substance itself (e.g. asbestos particles or NNK from tobacco), or a variant of the external substance processed by the body (a metabolite) that usually can be quantified.

The widespread use of the term "biomarker" dates back to as early as 1980.^[7] The term "biological marker" was introduced in 1950s.^[8][9] In 1998, the National Institutes of Health Biomarkers Definitions Working Group defined a biomarker as "a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention."^[10][11]

Look up biomarker in Wiktionary, the free dictionary.

• Geomarkers versus biomarkers: Paleoenvironmental and astrobiological significance
• Henry Stewart Talks Biomarkers Series The path forward to highly sensitive and specific molecular diagnostics.