Post-Translationally Modified Proteoforms as Biomarkers: From Discovery to Clinical Use.

Protein biomarkers are routinely measured for disease diagnosis and prognosis in clinical laboratories. Since most assays focus on protein quantity, information about proteoforms is often not acquired. Proteoforms of a protein represent the complex integration of genetic polymorphism, alternative splicing of RNA transcripts, and post-translational modifications (PTMs) on the amino-acid backbone. A detailed analysis of the post-translationally modified proteoforms (PTMPs), which are influenced by pathophysiological conditions, may lead to more precise diagnosis and prognosis. This article first discusses the methodologies used to accurately detect and characterize PTMPs, i.e., immunoassays, electrophoresis, chromatography, and intact and proteolysis-aided mass spectrometry techniques. Then it reviews specific examples of PTMP biomarkers that have been successfully translated from biomarker discovery to clinical use. The examples include β2-transferrin for cerebrospinal fluid leak diagnosis, phosphorylated tau proteoforms for Alzheimer disease diagnosis, and fucosylated alpha-fetoprotein for hepatocellular carcinoma prognosis. In addition, the article provides prospective views of novel analytical technologies and promising new PTMP biomarkers entering clinical practice. In summary, PTMs are controlled by biochemical processes to modulate the functions of proteins by expanding their chemical diversity. PTM alterations in proteins can be indicators for pathophysiological conditions. Advances in analytical technologies are deepening our understanding of PTMPs and paving the way for their translation to clinical use. As research continues to discover the clinical meaning of PTMP biomarkers, they are poised to become valuable additions to the clinical testing menu for precision medicine.