Impaired transitioning of the FXR ligand binding domain to an active state underlies an FXR-associated PFIC phenotype

Nuclear receptor farnesoid X receptor (FXR) acts as a key regulator of bile acid pool homeostasis and metabolism. Within the enterohepatic circulation, reabsorbed bile acids act as FXR agonists, which transcriptionally controls the synthesis and transport of bile acids. Binding occurs in the ligand binding domain (LBD), favoring a conformational change to the active state in which helix 12 interacts with the LBD to form an interaction surface for nuclear co-activators. The homozygous missense variant T296I, identified in a PFIC patient, is located close to the critical helix 12 interaction. Here, we identified reduced transcriptional activity of the variant protein on the downstream targets bile salt export pump (BSEP) and small heterodimer partner (SHP) in vitro, within the patients liver, and in iPSC-derived hepatic organoids. BSEP-dependent Tauro-DBD transport was impaired in T296I patient-derived organoids, but could be rescued via lipid nanoparticle-mediated FXR WT mRNA delivery, indicating the variant is responsible for the identified reduced BSEP expression. Using molecular dynamics simulations, we observed a reduced transitioning from the inactive to the active state for the T296I variant, indicating a molecular mechanism underlying the reduced activity. To our knowledge, this is the first study to describe the conformational change from an inactive to an active state of the FXR LBD. This might be useful for new therapeutic approaches targeting the activation of FXR.