Genome-wide scan for modifier genes of liver fibrosis in Abcb4/Mdr2 deficient mice

Aims: In a various cholestatic liver diseases including progressive intrahepatic familial cholestasis mutations in the ATP-binding cassette transporter gene ABCB4 encoding the hepatic phospholipid flippase have been identified. Of note, the phenotypic presentation of patients harboring ABCB4 mutations differs markedly, and no genotype-phenotype correlations could be established. To identify potential modifier genes, Abcb4/Mdr2 knock-out mice were backcrossed on two different genetic backgrounds (BALB/cJ, FVB/NJ) that strongly differ in their propensity to develop liver fibrosis.

Material and methods: Spontaneous (biliary) fibrosis was monitored in progeny from an F2 cross of Abcb4 deficient mice on the fibrosis-susceptible BALB/cJ and the resistant FVB/NJ background. As quantitative trait, we determined hepatic collagen contents at 16 weeks of age. The F2 progeny were genotyped for 278 single nucleotide polymorphisms, and quantitative trait loci (QTL) that determine hepatic fibrosis were identified by genome-wide regression analysis and interval mapping, using R/qtl.

Results: We generated 210 F2 mice that displayed marked variation of biliary fibrosis and hepatic collagen accumulation (mean 300.9 μg/g, range 72.9–778.5 μg/g). QTL analysis identifies a modifier locus on chromosome 4 that significantly affects hepatic collagen contents with a LOD score of 3.4. Genotype-phenotype correlations showed evidence for a dominant model of inheritance with a significant increase of hepatic collagen levels in progeny carrying the at-risk genotype (330.8±137.1 vs. 261.5±108.1 μg/g; p<0.001).

Conclusions: This is the first study to identify a potential modifier locus for the progression of fibrosis in the Abcb4/Mdr2 knock-out mouse model of biliary fibrosis. Fine mapping experiments to identify candidate genes at this locus may contribute to a better understanding of phenotypic heterogeneity in humans with ABCB4 deficiency.