Glycosylation has long been considered a clinically relevant posttranslational modification, particularly in the search for biomarkers for disease and in the development of innovative drug therapy protocols. Unfortunately, the level of complexity generally found in biological tissue samples brings marked challenges to the technical aspects of analysis.
Recently, Li et al. used an original high-throughput approach to monitor the quantities of both glycosylated and sialylated prostate-specific antigen (PSA) in cancerous and noncancerous tissues. By coupling glycopeptide extraction with selected reaction monitoring (SRM), the researchers were able to identify the sialylated isoform of PSA as a biomarker with potential for clinical relevance.
To do this, solid-phase extraction of N-linked glycopeptide (SPEG) was performed to isolate glycopeptides, which were then subjected to quantitative analysis with LC-MS/MS. This allowed researchers to simultaneously identify glycosylated proteins, the sites of glycosylation, and the relative quantity of the target glycopeptides. The specific benefits of performing SRM with a triple quadrupole mass spectrometer (Thermo Scientific) include increased sensitivity and accuracy in comparison to other approaches. The multi-step nature of the method limits the background noise while increasing the MS signals for the target ions.
In terms of clinical relevance, many potential prostate cancer patients fall into a so-called diagnostic gray zone that limits specific diagnoses and options for therapeutic intervention. It is possible that innovative approaches to glycoprotein analysis, like the one evaluated in this study, may offer enhanced diagnostic opportunity and improved clinical implications for therapeutic responses to prostate cancer. Li et al. found that sialylated isoforms of PSA may be particularly useful novel biomarkers for prostate cancer, a finding that is corroborated by earlier studies that highlight glycosylation as a PTM that differentially expresses with the different states and stages of disease.
The researchers call for further research in the area, particularly into the question of whether sialylated PSA can be distinguished in blood serum since this may increase diagnostic specificity. Additional studies using this innovative high-throughput approach for identifying and quantifying glycoproteins in biological samples may also prove useful.
Li, Y., et al (2011) ‘Simultaneous Analysis of Glycosylated and Sialylated PSA Reveals Differential Distribution of Glycosylated PSA Isoforms in Prostate Cancer Tissues‘, Analytical Chemistry, 83 (1), (pp. 240-245)