PNPLA3 rs738409 and risk of fibrosis in NAFLD: Exploring mediation pathways through intermediate histological features

Background and Aims It is unclear whether rs738409 (p.I148M) missense variant in patatin-like phospholipase domain-containing 3 rs738409 promotes fibrosis development by triggering specific fibrogenic pathways or by creating an unfavorable microenvironment by promoting steatosis, inflammation, and ultimately fibrosis. We tested the hypothesis that intermediate histologic traits, including steatosis, lobular and portal inflammation, and ballooning may determine the effect of rs738409 on liver fibrosis among individuals with biopsy-proven NAFLD. Approach and Results Causal mediation models including multiple mediators in parallel or sequentially were performed to examine the effect of rs738409, by decomposing its total effect on fibrosis severity into direct and indirect effects, mediated by histology traits in 1153 non-Hispanic White patients. Total effect of rs738409 on fibrosis was beta = 0.19 (95% CI: 0.09-0.29). The direct effect of rs738409 on fibrosis after removing mediators' effects was beta = 0.09 (95% CI: 0.01-0.17) and the indirect effect of rs738409 on fibrosis through all mediators' effects were beta = 0.010 (95% CI: 0.04-0.15). Among all mediators, the greatest estimated effect size was displayed by portal inflammation (beta = 0.09, 95% CI: 0.05-0.12). Among different sequential combinations of histology traits, the path including lobular inflammation followed by ballooning degeneration displayed the most significant indirect effect (beta = 0.023, 95% CI: 0.011-0.037). Mediation analysis in a separate group of 404 individuals with biopsy-proven NAFLD from other races and ethnicity showed similar results. Conclusions In NAFLD, nearly half of the total effect of the rs738409 G allele on fibrosis severity could be explained by a direct pathway, suggesting that rs738409 may promote fibrosis development by activating specific fibrogenic pathways. A large proportion of the indirect effect of rs738409 on fibrosis severity is mediated through portal inflammation.