Circulating small extracellular vesicle-derived splicing factor 3b subunit 4 as a non-invasive diagnostic biomarker of early hepatocellular carcinoma

Background Hepatocellular carcinoma (HCC) accounts for a majority of primary liver cancer cases and related deaths. The purpose of this study was to assess the diagnostic value of splicing factor 3b subunit 4 (SF3B4) as a novel non-invasive biomarker for HCC and determine the association between SF3B4 expression and immune cell infiltration. Methods An enzyme-linked immunosorbent assay (ELISA) was used to detect SF3B4 levels in plasma samples obtained from healthy controls (HCs) and patients with chronic hepatitis, liver cirrhosis, and HCC. The expression levels of autoantibodies that detect SF3B4 in the plasma samples of each group of patients were measured. Small extracellular vesicles (EVs) were isolated from patient sera, and the expression levels of EV- SF3B4 were measured using quantitative reverse transcription PCR. Results ELISA results confirmed that the expression levels of SF3B4 proteins and autoantibodies in the plasma of patients with HCC were higher than those in HCs. However, their diagnostic performance was not better than that of alpha-fetoprotein (AFP). The mRNA expression of SF3B4 in serum EV increased but not in the buffy coat or serum of patients with HCC. Serum EV- SF3B4 displayed better diagnostic power than AFP for all stages of HCC (AUC = 0.968 vs. 0.816), including early-stage HCC (AUC = 0.960 vs. 0.842), and this was consistent in the external cohort. Single-cell RNA sequencing indicated that SF3B4 expression was correlated with myeloid-derived suppressor cells. The Tumor Immune Estimation Resource database reconfirmed the correlation between SF3B4 expression and immune cell infiltration in HCC. Conclusions SF3B4 may be associated with tumor immune infiltration in HCC, and EV- SF3B4 shows potential as a novel non-invasive diagnostic biomarker of HCC.