Identification of interacting modifier loci of liver fibrosis in Abcb4/Mdr2 deficient mice

Abstract

Functional variants in the ATP-binding cassette transporter gene ABCB4, have been identified in various cholestatic liver diseases, including progressive intrahepatic familial cholestasis. Phenotypic presentations of patients harbouring ABCB4 mutations differ markedly, and to date no modifier loci have been identified in human cohorts. We recently reported a single modifier locus on chromosome (chr) 4 (Grünhage et al. AASLD 2008) in a backcross of Abcb4 deficient mice. We now analysed these mice with regards to locus interactions influencing liver fibrosis.Fibrosis was monitored in an F2 cross of fibrosis-susceptible BALB-Abcb4-/- and resistant FVB-Abcb4-/- mice. As a quantitative trait, we determined hepatic collagen contents at 16 weeks of age. The F2 mice were genotyped for 270 single nucleotide polymorphisms, and mapped for multiple quantitative trait loci (QTL) that determine liver fibrosis, using R/qtl. We generated 183 F2 mice that displayed marked variation of biliary fibrosis and hepatic collagen accumulation (mean 300.9µg/g, range 72.9–778.5µg/g). Consistent with our previous results, we detected a potential single locus on chr 4 (LOD 2.1). Pairwise QTL scans identified two interactions between loci on chr 2 and 9 as well as chr 11 and 12. Composite interval mapping showed that the maker on chr 12 (rs13481500) was significantly associated with liver fibrosis dependent on the interacting locus at chr 11 (rs6346368; LOD 5.06). Permutation tests confirmed this marker as a significant modifier locus (pgw<0.05). Homozygous carriers of the risk alleles showed significantly increased collagen contents (489.4 vs.300.2µg/g, p<0.05). In addition to our previously reported single QTL on chr 4, we identified two interacting loci on chr 11 and 12, influencing fibrosis progression in Abcb4-/- mice. Fine mapping experiments to identify candidate genes at these loci may contribute to a better understanding of phenotypic heterogeneity in humans with ABCB4 deficiency.