Sweetening the pot: adding glycosylation to the biomarker discovery equation

Abstract

Background: Cancer has profound effects on gene expression, including a cell's glycosylation machinery. Thus, tumors produce glycoproteins that carry oligosaccharides with structures that are markedly different from the same protein produced by a normal cell. A single protein can have many glycosylation sites that greatly amplify the signals they generate compared with their protein backbones.

Content: In this article, we survey clinical tests that target carbohydrate modifications for diagnosing and treating cancer. We present the biological relevance of glycosylation to disease progression by highlighting the role these structures play in adhesion, signaling, and metastasis and then address current methodological approaches to biomarker discovery that capitalize on selectively capturing tumor-associated glycoforms to enrich and identify disease-related candidate analytes. Finally, we discuss emerging technologies--multiple reaction monitoring and lectin-antibody arrays--as potential tools for biomarker validation studies in pursuit of clinically useful tests.

Summary: The future of carbohydrate-based biomarker studies has arrived. At all stages, from discovery through verification and deployment into clinics, glycosylation should be considered a primary readout or a way of increasing the sensitivity and specificity of protein-based analyses.

Figure 1. Malignant cells release glycoproteins carrying disease-related carbohydrate epitopes into the interstitial space, where they can reach the circulation

Cancer is associated with major changes in the glycan biosynthetic machinery, including (1) upregulation of fucosyltransferases (FucTs), sialyltransferases (SiaTs), and the MGAT5 gene product, which is involved in the elaboration of highly branched N-linked glycans. Disease-relevant proteinases such as MMPs and ADAM family members are also upregulated. Changes in the expression of glycosyltransferases result in altered glycan assembly, which occurs in the endoplasmic reticulum and Golgi (2). Accordingly, the glycoprotein products of tumor cells carry aberrant carbohydrate structures as compared to their normal counterparts. Typical changes include increased levels of fucose (red triangle), sialic acid (purple diamond), the addition of polylactosamine units (repeating sequences of galactose [yellow circle] and N-acetylglucosamine [blue square]), and higher-ordered branching of N-linked glycans. O-linked glycans are also affected in cancer, typically carrying incomplete or prematurely truncated structures relative to those found on normal cells. After secretion or proteolytic cleavage, glycosylated molecules and/or their cleavage products are released into the interstitial space (3), where they can enter the circulation (4). Since glycoproteins and mucins usually carry many carbohydrate chains, the signals they produce are highly amplified as compared to proteins, making them attractive candidates as biomarkers.