Glycoproteomics of plasma immunoglobulin A in patients with colorectal cancer

Analysis of intact glycopeptide and glycoproteins remains challenging because of the complexity of sugar attachment and difficulties in data interpretation. Immunoglobulin A (IgA), the second most antibody in blood, has two subclasses (IgA1 and IgA2) which contains two (N144 and N340) and five (N47, N92, N131, N205, and N327) N-liked glycosylation sites, respectively. In this study, we development a strategy for differential analysis of the intact site-specific N-glycopeptides of serum IgA between colorectal cancer (CRC) and healthy controls (HC). IgA was purified using Peptide-M from individual samples of patients with non-metastatic (NM-CRC, n=10) and metastatic CRC (M-CRC, n=10), and HC (n=11), and followed by in-solution tryptic digestion. The digested peptide samples were subjected to a nano liquid chromatography coupled tadem mass spectrometer with Orbitrap Mass Spectrometer using higher-energy collisional dissociation (HCD) for MS/MS fragmentation. Identification and quantitation of site-specific of N-glycopeptides were performed by label-free analysis of Progenesis QI and Byonic software using an in-house human IgA and N-glycan database containing 148 glycan forms. In total, several glycoforms in each sites were identified including 45 (N144/N131), 12 (N340), 6 (N92 and N205), 18 (N131) glycoforms, while none was detected in N47 and N327. The relative N-linked glycopeptides dominantly existed at N144/N131 with the average of 80% of all observed N-linked sites. Among 45 glycoforms at N144/N131, differential quantitation analysis showed that the level of HexNAc(5)Hex(3) was increased in a disease-dependent manner with statically different between HC and M-CRC (p<0.01). This strategy enables us to discover the detailed profiles of N-glycopeptides and the relative quantitation to determine the level changes in site-specific N-glycopeptides of patients with CRC.