Generation and characterization of a mouse model for bile salt export pump deficiency with the p.E297G mutation

Bile Salt Export Pump (BSEP) deficiency is a rare genetic cholestatic liver disease, often necessitating liver transplantation. The p.E297G missense mutation is associated with residual BSEP function in vitro and delayed need for transplantation in patients. We aimed to generate a p.E297G BSEP knock-in (BSEP^E297G) mouse model to evaluate interventions to improve residual BSEP function.

We generated BSEP^E297G mice by CRISPR-Cas9 technology. BSEP^E297G mice and wild type (WT) littermates were characterized for BSEP expression and liver pathology at 14 weeks of age. Maximal BSEP transport capacity without and after 4-phenylbutyrate (4-PB) treatment were determined in vivo by quantification of biliary bile acid secretion during intravenous infusion of increasing dosages of tauroursodeoxycholic acid (TUDCA) in WT, BSEP^E297G and BSEP^-/- mice.

Western blot analysis showed immature BSEP protein in BSEP^E297G livers. Median plasma AST was three-fold higher in BSEP^E297G mice (Males: 198 vs. 60 U/L; Females: 188 vs. 50 U/L; each p < 0.001) while plasma bile acid levels were higher in female BSEP^E297G mice compared to WT (Females: 36 vs. 6 μM, p < 0.001; Males: 11 vs. 3 μM, p = 0.07). Histological analysis revealed features of cholestatic liver pathology in BSEP^E297G mice. TUDCA infusion strongly increased biliary bile acid secretion in WT but not in BSEP^E297G and BSEP^-/- mice. 4-PB treatment did not enhance bile acid transport capacity in BSEP^E297G mice.

BSEP^E297G mice display a BSEP deficiency phenotype with a strongly reduced hepatobiliary bile acid transport capacity. The expression of immature BSEP protein suggests the potential to assess correctors of the BSEP functionality in vivo.