Introduction

The detection of pancreatic cystic neoplasm (PCN) has markedly increased due to more frequent applications of cross-sectional imaging, leading to a challenging dilemma in diagnosis and treatment. It is estimated that PCNs are present in more than 40% of individuals aged 60 years and older, and thus have become a significant global disease burden.1,2 A diverse group of lesions, such as intraductal papillary mucinous neoplasm (IPMN), mucinous cystic neoplasm (MCN), serous cystic neoplasm (SCN), and others, constitute PCNs, encompassing a range of conditions from benign to malignant. The majority of PCNs exhibit indolent behavior, characterized by slow progression and low malignant potential. Only a small subset of PCNs can give rise to pancreatic cancer, mainly composed of high-grade and invasive IPMNs and MCNs.3 The identification of indolent PCNs could prevent unnecessary pancreatectomy, which is associated with a considerable rate of postoperative complications and increased medical costs.4 Conversely, the early detection of malignant PCNs enables timely management of pancreatic cancer and substantially improves prognosis.5 Thus, the accurate differentiation of malignant PCNs from other indolent lesions is continually highlighted.

Several international guidelines have been established and have improved the clinical management of PCN.6,7,8 However, recent studies have shown that risk stratification based on conventional clinical and radiological factors remains unsatisfactory.9,10 The primary concern lies in the relatively high proportion of benign PCN patients undergoing surgery following the current guidelines, highlighting the pressing need to enhance diagnostic efficacy and achieve greater specificity in detecting malignant PCNs.

Cyst fluid, primarily produced by neoplastic epithelial cells with PCN, offers a direct way to assess the biological characteristics of these neoplasms. Consequently, cyst fluid analysis has emerged as a crucial approach in assisting the molecular-based risk stratification of PCNs.11,12 There have been advances in cyst fluid analysis of PCN, particularly in the detection of genomic variants.13,14 However, the exploration of additional types of cyst fluid biomarkers is still in its nascent stage. Glycosylation is a prevalent posttranslational protein modification. Through glycosylation, a wide range of glycan structures are conjugated to proteins or lipids, forming glycoconjugates that participate in recognition and interaction between cells and their extracellular environment. Recently, alterations in protein glycosylation have been found to be involved in fundamental molecular and cellular processes of carcinogenesis, and thus added as a new hallmark of cancer.15,16 The identification of cancer-specific glycoproteins holds great potential for the development of novel diagnostic biomarkers with high accuracy. In the past, the complexity of protein glycosylation posed a challenge in simultaneously analyzing all relevant features of glycoproteins, including glyosites, glycoforms, and glycopeptides. With the advent of mass spectrometry (MS) in analyzing intact glycopeptides, high-throughput quantitative detection of both glycosite- and glycoform-specific glycoproteins has become possible.17,18,Full size image

Fig. 3
figure 3

Characteristics of altered glycoproteins and proteins in cyst fluid during malignant transformation of IPMN. a Summarizing N-glycoforms of differentially expressed glycoproteins between inv-IPMN and LG-IPMN cyst fluid samples based on the classifications of high-mannose, hybrid, and complex Glycans. b Summarizing N-glycoforms of differentially expressed glycoproteins between inv-IPMN and LG-IPMN cyst fluid samples based on the classifications of fucosylated glycans (Fuc), high-mannose (HM) and sialylated glycans (Sia). c Pathway enrichment analysis of differentially expressed proteins between inv-IPMN and LG-IPMN cyst fluid samples using the PANTHER classification system

Identification of potential cyst fluid glycoproteins as novel biomarkers in distinguishing malignant PCNs

We further incorporated SCN and LG-IPMN cases as benign PCN group, classified inv-IPMN cases as malignant PCN group, and implemented more strict criteria to screen cyst fluid biomarkers of clinical utility in distinguishing malignancies. Only candidates that were specifically detected in more than 4 cases of one group and not detected in any cases of the other group were retained. Among 642 differentially expressed glycoproteins, four N-glycoproteins with specific glycosites and glycoforms were identified as potential cyst fluid biomarkers that could accurately distinguish malignant PCNs: N-glycosylated ATP6V0A1 (Asn-273, H5N4F0S1), ATP6V0A4 (Asn-367, H6N4F0S0), CEACAM5 (Asn-197, H5N4F0S0) and PHKB (Asn-935, H5N2F0S0, H4N4F0S0 and H5N4F0S0) (Fig. 4a, b). Another three glycoproteins were additionally identified to be exclusively expressed in LG-IPMN: N-glycosylated CELA3B (Asn-114, H5N4F1S0), CEP85 (Asn-646, H6N2F0S0) and TCOF1 (Asn-649, H6N3F1S0) (Fig. 4a, b). Unlike glycoproteins, none of the differentially expressed proteins screened by proteomics between the inv-IPMN group and the LG-IPMN group fulfilled the criteria for ideal biomarkers that would be specifically expressed in inv-IPMN but not in LG-IPMN or SCN. Thus, the above seven glycoproteins, exhibiting specific glycoforms, were identified as potential cyst fluid biomarkers for differentiating PCNs with varying malignant potentials.

Fig. 4
figure 4

Identification of cyst fluid glycoproteins as potential biomarkers for distinguishing malignant PCNs through MS-based glyosite- and glycoform-specific glycoproteomics. a Potential glycoprotein biomarkers identified by glycoproteomics in the discovery cohort. b Schematic diagrams illustrating N-glycoform structures of given glycoproteins. c Annotated MS/MS spectrum and graphical fragmentation map of matched fragment ions for CEACAM5 (Asn-197, H5N4F0S0) as a representative glycoprotein detected by glycoproteomics and analyzed using GPSeeker

Validation of cyst fluid glycoprotein biomarkers in distinguishing malignant PCNs

To validate the above findings, we developed a PRM-based targeted glycoproteomic analysis method and applied it to detect and quantify selected glycoproteins in a second validation cohort. This cohort comprised 30 cyst fluid samples, representing the most common types of PCN (Fig. 5). It included malignant PCNs, such as HG-IPMN, inv-IPMN, and MCN with associated invasive carcinoma. The associated clinical information is provided in Table 1. All selected glycoproteins except for CEP85 were detected by targeted PRM analysis in the validation cohort (Fig. 6a). Notably, N-glycosylated ATP6V0A4 (Asn-367, H6N4F0S0), CEACAM5 (Asn-197, H5N4F0S0) and PHKB (Asn-935, H5N2F0S0; Asn-935, H4N4F0S0; Asn-935, H5N4F0S0) were validated to present high accuracy in distinguishing malignancies (HG-IPMN, inv-IPMN and MCN with associated invasive carcinoma), with sensitivities ranging from 66.7 to 100%, specificities ranging from 81.2% to 93.8%, and area under curve (AUC) values spanning from 0.771 to 0.948 (Fig. 6b, c). Additionally, N-glycosylated TCOF1 (Asn-649, H6N3F1S0) was shown to be a reliable biomarker of low-grade mucinous lesions, and its expression level significantly decreased during malignant transformation (Fig. 6a). N-glycosylated CELA3B (Asn-114, H5N4F1S0) and ATP6V0A1 (Asn-273, H5N4F0S1) were discarded from cyst fluid biomarkers due to their poor discriminative ability shown by PRM analysis.

Fig. 5
figure 5

Representative MS/MS spectra for PRM-based quantitation of targeted glycoproteins. CEACAM5 (Asn-197, H5N4F0S0) (a) and PHKB (Asn-935, H5N4F0S0) (b) were shown as examples. Left row: a serial precursor ions of given glycoproteins were detected by PRM. Middle row: A serial y (labeled in red) and b (labeled in blue) product ions of given glycoproteins were detected by PRM. Right row: a serial product ions of given glycoproteins were detected by PRM and used for quantitation

Fig. 6
figure 6

Validation of cyst fluid glycoprotein biomarkers in distinguishing malignant PCNs. a Validation of selected glycoprotein biomarkers by PRM-based targeted glycoproteomic analysis in the validation cohort. b The ROC curves of ATP6V0A4 (Asn-367, H6N4F0S0), CEACAM5 (Asn-197, H5N4F0S0), PHKB (Asn-935, H5N2F0S0; Asn-935, H4N4F0S0; Asn-935, H5N4F0S0) in differentiating malignant and benign PCNs. c Diagnostic efficacy of ATP6V0A4 (Asn-367, H6N4F0S0), CEACAM5 (Asn-197, H5N4F0S0) and PHKB (Asn-935, H5N2F0S0; Asn-935, H4N4F0S0; Asn-935, H5N4F0S0) in detecting malignancies of PCN. d Clinical implications of the study