Proteomic profiling identifies candidate biomarkers for small intestinal neuroendocrine tumours G1 and G2

Although small intestinal neuroendocrine tumours (SI-NET) Grade 1 (G1) and 2 (G2) are often studied together, clinical course and response to treatment may differ. We hypothesized that these tumour groups express different markers in blood that may inform future treatment strategies. Blood from 107 SI-NET patients (G1 n = 64, G2 n = 43) and 50 healthy controls was analysed with proximity extension assay (Olink, Uppsala, Sweden). Two oncology protein panels were used. Outcomes were analysed with regression analysis and machine learning algorithms (Random Forest, Boruta). In age-adjusted analyses, 188 proteins differed between patients and healthy controls, with most being elevated in patients. Six proteins were elevated in G2 vs. G1 patients. After adjusting for age, Retbindin (RTBDN), Cryptic family protein 1 (CFC1), and Calbindin 1 (CALB1) remained significant, while adjusting for age and stage left only RTBDN significant. In a stage IV-restricted analysis, RTBDN was higher in G2 than in G1 (estimate 1.688; 95% CI 0.796-2.580; p <0.001). RTBDN and Midkine (MDK) were associated with poor survival, whereas the opposite was seen for Integrin subunit alpha V (ITGAV). RTBDN emerged as the only candidate biomarker associated with tumour grade after adjustment for age and stage in the G1/G2 analysis. In the stage IV-restricted analysis, RTBDN remained the top-ranked protein and showed a similar direction of association without reaching the strict Bonferroni-adjusted significance threshold. Higher RTBDN levels were also associated with increased mortality risk. These findings are exploratory and require independent validation before clinical application.