Genetic analysis of 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 of these diseases have yet to be identified systematically. In this study we availed of the Abcb4 (Mdr2) knockout (^-/-) mouse model, in which deficiency of the hepatobiliary phosphatidylcholine floppase leads to chronic cholestasis and fibrosis. As different mouse strains show variable fibrosis susceptibility, we applied a systematic approach to elucidate the genetic control of liver fibrosis in an experimental cross of ABCB4 deficient mice.

Methods: The Abcb4 ^-/- knockout was crossed from the fibrosis-resistant FVB-Abcb4 ^-/- mouse strain into 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 fibrosis 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, 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. In these susceptibility loci, we identified the genes Abcg5, Abcg8 and sterol carrier protein 2 (Scp2) that are functionally implicated in hepatobiliary cholesterol homeostasis and resemble creedal modifer genes.

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