Currently, the etiological factors and treatment strategies for PSC remain contentious (Dyson et al. 2018). In this study, we conducted a comprehensive exploration of the correlation between 91 inflammation-related proteins, 1400 plasma metabolites, and PSC using MR analysis based on existing GWAS. Our findings suggest a correlation between the genetically predicted TWEAK and an elevated risk of PSC (a 53.11% increase in PSC risk for each 1 standard deviation increase in TWEAK), with 6.34% of this effect mediated through 3-MT. To date, we are pioneers in employing MR analysis to thoroughly explore the correlation between 91 inflammation-related proteins and PSC. Concurrently, we've confirmed that 3-MT, observed among the spectrum of 1400 metabolites, mediates their causal relationship.
As a member of the tumor necrosis factor (TNF) ligand family, TWEAK is a multifaceted and versatile cytokine that regulates inflammatory pathways by inducing diverse cellular responses, contingent upon cell types and their microenvironment (Bertin et al. 2013). TWEAK has a dual role: promoting tissue regeneration after acute injury but hindering tissue repair in chronic diseases with persistent activation, particularly by inhibiting progenitor cell differentiation (Burkly et al. 2007). As an autoimmune disease, PSC is commonly associated with inflammatory bowel disease, with 70% of patients experiencing both conditions simultaneously (Sabino et al. 2016). In the case of inflammatory bowel disease, such as patients with ulcerative colitis, Kawashima et al. observed a significant upregulation of TWEAK expression in the intestinal mucosa compared to normal mucosal tissues (Kawashima et al. 2011). Similarly, elevated expression of TWEAK has been observed in autoimmune diseases such as systemic lupus erythematosus (Wang et al. 2012), lupus nephritis (Zhi-Chun et al. 2012), rheumatoid arthritis (Park et al. 2013), when compared to healthy control groups, and this expression has been correlated with the severity of the diseases. Moreover, Tumor Necrosis Factor Receptor Superfamily Member-12A (TNFRSF12A/Fibroblast-Inducible 14, FN14) acts as the specific binding receptor for TWEAK (Dohi et al. 2012). Observational studies have noted a significant increase in the expression of both TWEAK and FN14 in patients with primary biliary cholangitis and obstructive cholestatic disorders (Liao et al. 2023). Research confirms that bile acids can activate the Fn14-NFκB/Caspase-1/GSDMD signaling pathway, aggravating cholestatic liver injury, and TWEAK further amplifies this effect (Liao et al. 2023). Given the pivotal role of bile acids in the onset of PSC (Liwinski et al. 2020), coupled with our research findings, we hypothesize that TWEAK may play a significant role in the development of PSC.
Through MR, we have identified 3-MT as a potential mediator of PSC development induced by TWEAK. 3-MT is an O-methylated product of L-dopa catalyzed by catechol-O-methyltransferase (Goldstein et al. 2003), potentially involved in this process. In most cells, O-methylated compounds containing an amine moiety undergo further metabolic breakdown. Consequently, 3-MT may be converted into its corresponding glucuronide or sulfate forms, facilitating its excretion from the body, potentially via bile (Goldstein et al. 2003). This suggests a connection between 3-MT and the hepato-biliary system. Importantly, studies indicate a correlation between the expression of dopamine receptor D2 and oxidative damage and inflammation in the liver, possibly through the TGF-β1/Smads and NFκB pathways (Zhao et al. 2021). As a metabolite of dopamine, 3-MT may have a reasonable association with inflammation-related proteins and liver-related diseases. However, further research is needed to elucidate the exact nature of this relationship.
This study faces several limitations. Firstly, despite conducting MR analysis on published GWAS datasets, the conclusions lack experimental support, as they rely solely on genetic IVs and causal inference through multiple MR analytical approaches. Secondly, the scarcity of research on 3-MT raises concerns about its relationship with TWEAK and PSC, as the association is established only at the MR analysis level and lacks robust literature support. Thirdly, heterogeneity is observed in the MR analysis of 3-MT and PSC. Lastly, our study reveals a notably low genetic prediction (6.38%) of TWEAK mediated by 3-MT levels.