Dissection of genetic determinants of fibrogenesis in liver and heart

Background: Recently we observed that long-term administration of carbon tetrachloride (CCl₄) to mice induces fibrosis in both liver and heart. To dissect common and organ-specific mechanisms of fibrogenesis, we employed BXD recombinant inbred lines as a genetic reference population (GRP). Our aim was to identify potential candidate genes with effects on hepatic and cardiac fibrogenesis.

Methods: Thirty BXD lines were used for genome-wide quantitative trait loci (QTL)-analyses. Fibrosis was induced by CCl₄ for six weeks (0.7 mg/kg, 12 i.p. injections). The hepatic and cardiac QTLs linked to collagen accumulation were screened for potential candidates by expression QTL (eQTL)-analyses, availing of transcriptomic data of CCl₄-treated BXDs (Affy 1.0 ST arrays). For candidate-gene verification, we compared CCl₄-challenged Raf-kinase inhibitor 1 (Rkip1) -knockout (Rkip ^-/-) and wild-type mice. Collagen accumulation was quantified in histological liver and heart sections after Sirius red staining.

Results: Significant differences of hepatic and cardiac fibrogenesis were mapped and identified coinciding QTLs on chromosomes 5 and 18. eQTL analyses revealed Rkip1 as major cis-QTG (LRS = 64.6), and evidence exists affirming a functional link to fibrogenesis. After fibrosis induction, Rkip ^-/- mice showed significantly (p < 0.05) less cardiac collagen accumulation, whereas no differences were observed in liver.

Conclusions: This study demonstrates that CCl₄-induced fibrosis is a systemic model that allows comparative analysis of fibrogenic mechanisms in liver and heart in identical individuals. QTL analyses identified common and organ-specific QTLs of fibrogenesis. Rkip1 deficiency has an anti-fibrotic effect in heart but not in liver, pointing to organ-specific mechanisms of fibrosis progression and resolution in this model.