370 SYSTEMIC AND ORGAN-SPECIFIC GENETIC DETERMINANTS OF FIBROGENESIS IN LIVER AND HEART

Abstract

Background: Various chronic human diseases result in organ fibrosis, i.e. excessive accumulation of extracellular matrix. Fibrosis is a multifactorial trait determined by both genetic and exogenous factors such as xenobiotics. Our aim is to compare fibrogenesis in liver and heart in a systemic fibrosis model to dissect common and organ-specific mechanisms of fibrosis. Furthermore, we employed a systems genetics approach to identify genetic modifiers of hepatic and cardiac fibrosis. Method: We tested the parental strains (C57BL/6J [B] and DBA/2J [D], 6/group) for the following fibrosis models: (i) N-nitro-Larginine-methylester (L-NAME), a NO synthase inhibitor, provided in drinking water (5 weeks, ad libitum); (ii) CCl4, administered i.p. (0.7mg/kg, 12 injections for 6 weeks). Collagen accumulation was quantified in histological liver and heart sections after Sirius red staining. The proliferation marker Ki-67 and fibroblast marker fibronectin were determined by immunohistochemistry. Thirty BxD recombinant inbred lines, genotyped for >13,000 markers, served as genetic reference population for the identification of genetic risk factors by genome-wide quantitative trait locus analysis (QTL). Results: Whereas L-NAME treatment did not induce hepatic fibrosis, CCl4 induced fibrosis in liver and heart, a hitherto unknown phenomenon. We observed marked differences for both hepatic and cardiac fibrosis between C57BL/6J and DBA/2J strains. Cardiac fibroblast proliferation and collagen accumulation was enhanced in DBA/2J mice (collagen area [%] in liver B =2.5%, D =7.2%, P = 0.0002; in heart: B = 0.6%, D =1.2%, P = 0.006). CCl4-challenged BxD lines also differed in fibrosis susceptibility. Collagen levels in livers and hearts of BxDs were correlated significantly (R = 0.39, P < 0.002). QTL analyses identified several tissue-specific loci as well as an overlap of fibrosis associated peaks on chromosome 18 in liver and heart. Conclusions: CCl4 induces systemic fibrosis in liver and heart and therefore meets the requirements as a fibrotic model to dissect common and organ specific modifiers of fibrogenesis. In the genetic reference population, hepatic and cardiac fibrosis susceptibility were correlated significantly, suggesting common genetic determinants of fibrogenesis. The overlap of tissue-specific QTL peaks on chromosome 18 supports the assumption of systemic profibrogenic core pathways.