Genetic analysis of spontaneous (non-toxic) liver fibrosis in a congenic mouse intercross

Background: Mutations in the ABCB4 (ATP-binding casette, subfamily B, member 4) gene cause cholestatic liver diseases including progressive intrahepatic familial cholestasis (PFIC). Modifier genes have yet to be identified systematically. In this study we used the Abcb4 (Mdr2) knockout (-/-) mouse model. The deficiency of the hepatobiliary phosphatidylcholine floppase leads to chronic cholestasis, liver injury and fibrosis. As different mouse strains show varying fibrosis susceptibility, we applied a systematic approach to elucidate the genetic control of liver fibrosis in an experimental cross of ABCB4 deficient congenic strains.

Methods: The Abcb4 knockout was transferred from the fibrosis-resistant FVB-Abcb4-/- strain to the susceptible BALB/cJ strain by repeated backcrossing. To identify genetic modifiers that contribute to the fibrosis susceptibility linked to ABCB4 deficiency, we crossed these two congenic strains to generate an F2 intercross population. By quantitative trait locus (QTL) analysis differences in disease progression were mapped to polymorphic genetic regions across the whole genome. Single and two-dimensional QTL scans were applied to identify modifiers and pairwise gene interactions.

Results: Compared to FVB-Abcb4-/- mice, the BALB-Abcb4-/- mice progress to higher fibrosis stages. The heterogenic F2 population shows marked phenotypic variation. Whereas single modifiers demonstrate minor effects, gene-gene interaction scans identified a significant interaction of two QTLs on chromosomes 4 and 17. Underlying these loci we identified the genes Abcg5, Abcg8 (ATP binding cassette subfamily G members 5 and 8) and sterol carrier protein 2 (Scp2) that are functionally related with hepatobiliary cholesterol homeostasis and resemble creedal modifer genes.

Conclusions: The congenic Balb-Abcb4 knockout mouse allows the genomic exploration of a spontaneous, non-toxic disease model of a human gene defect. The experimental cross of the two genetic backgrounds with distinct fibrosis susceptibility enables the identification of Abcb4-dependent modifiers of cholestatic liver diseases.