Mass Spectrometry-Based N-Glycomics of Colorectal Cancer

Abstract

Colorectal cancer (CRC) is one of the most prevalent cancers worldwide. An increased molecular understanding of the CRC pathology is warranted to gain insights into the underlying molecular and cellular mechanisms of the disease. Altered protein glycosylation patterns are associated with most diseases including malignant transformation. Recent advances in mass spectrometry and bioinformatics have accelerated glycomics research and present a new paradigm for cancer biomarker discovery. Mass spectrometry (MS)-based glycoproteomics and glycomics, therefore, hold considerable promise to improve the discovery of novel biomarkers with utility in disease diagnosis and therapy. This review focuses on the emerging field of glycomics to present a comprehensive review of advances in technologies and their application in studies aimed at discovering novel glycan-based biomarkers. We will also discuss some of the challenges associated with using glycans as biomarkers.

Keywords: N-glycomics; cancer; colorectal cancer; glycome; glycosylation.

Figure 1

Types of N-glycans. Three main N-glycan structures in a mature glycoprotein include high mannose, complex and hybrid type. All classes share a common core, which may receive α1,6-fucosylation, bisecting β1,4-GlcNAcylation or other glycol determinants of the complex and hybrid type. Paucimannosidic type structures belong to an unusual (fourth) type of N-glycans in humans that may be truncated from the N-glycan core.

Figure 2

Sample preparation for N-glycan analysis by LC-MS/MS.

Figure 3

Extracted Ion Chromatograms (EIC) for mono- and di-sialylated biantennary complex type N-glycans (A) m/z 1038.9²⁻ and (B) m/z 1184.4²⁻ showing the separation power of PGC for α2,3 and α2,6-sialylation. Different ratios for α2,3 and α2,6-sialylation were observed between EGFR⁺ CRC (green, T1,T4) and EGFR⁻ CRC tissues (red, T2, T3 and T5) [57]. *, a low abundant glycan isomer.

Figure 4

Nomenclature for the fragmentation of glycans; A and X ions represent cross-ring fragments and B, C, Y and Z are glycosidic fragment ions. (Adapted from Domon and Costello) [136].

Figure 5

(A) Positive ion MS/MS fragmentation pattern of core fucosylated high mannose N-glycan type structure at m/z 751.9 [M + 2H]²⁺ [35]; (B) Negative ion MS/MS fragmentation pattern of core fucosylated bisecting N-glycan type structure at m/z 832.6 [33].