Interacting QTLs for hepatic fibrosis in BALB/c and FVB/N strains of mice

Aims: Recently we identified genetic loci that determine susceptibility to liver fibrosis in experimental crosses between fibrosis-susceptible and resistant inbred strains of mice (Hillebrandt et al. Nat Genet 2005;37:835–43). Despite the potential significance of gene-gene interactions (epistatsis), their role in fibrosis susceptibility is unknown, since previous QTL analyses for liver fibrosis modeled the effects of single QTLs only. Our aim now was to identify interacting QTLs for liver fibrosis.

Methods: We intercrossed F₁ hybrids of the fibrosis-susceptible strain BALB/cJ and the resistant strain FVB/NJ to obtain 350 F₂ progeny. To induce liver fibrosis, we challenged the mice with CCl₄ (2ml/kg/wk i.p. for 6 wks). For phenotypic characterisation, we determined the histological stage of liver fibrosis and measured hepatic collagen contents. We performed a genome scan with microsatellite markers (average distance 14 cM) in 100 mice with the largest deviations from the mean to the upper and lower ends of the distribution of each trait, followed by simultaneous genome scans of all pairwise interactions, as implemented in R/qtl. LOD thresholds were estimated by permutation tests.

Results: In addition to the previously described fibrogenic QTLs (Hfib1 and Hfib2), we detected a new QTL on mouse chromosome 1. According to standard nomenclature, this major QTL with significant efects on the progression of liver fibrosis (LOD 3.9) is named Hfib3. Furthermore, the two-dimensional scans identified a locus on chromosome 18 that affects fibrosis by interacting with loci on chromosome 1 and 6. In addition, we found interacting gene pairs between chromosomes 6 and 7, 9 and 13 as well as 3 and 16. Of note, the interacting loci display no significance on their own; however, their interactions made notable effects on both the stage of fibrosis and hepatic collagen concentrations.

Conclusions: Our findings demonstrate the importance of both single susceptibility loci and gene-gene interactions in liver fibrogenesis and provide the basic framework to investigate how systems of genes work together affecting this complex trait during chronic liver injury.