Identification of NASH Quantitative Trait Loci (QTLs) using in silico mapping analysis in inbred mice

Abstract

Aims: Non-alcoholic steatohepatitis (NASH) is a polygenic disease associated with the metabolic syndrome, however, the pathogenesis is poorly understood. The methionine-choline deficient (MCD) diet is an extensively utilized animal model for progressive steatohepatitis. Recently in silico QTL analysis has been developed as a novel method for chromosomal mapping of complex disease-related traits (1). The aims of this study are to define strain-specific differences of the MCD model of NASH and to utilize in silico mapping analysis to identify gene loci associated with steatohepatitis. Methods: A/J, AKR/J, BALB/c/J, C57BL/6J and DBA/2J inbred strains of mice were fed an MCD diet for up to 25 weeks. ALT, weight loss (WL), liver/body weight ratio (LW) and hepatic triglycerides (HT) were measured. The QTLs responsible for steatohepatitis in mice were mapped in silico, employing the Roche Mouse SNP Database and the Chesler algorithm. Using the SNP distributions across the inbred strains, the number of allelic differences between 2 strains was calculated at 30cM chromosomal intervals. Phenotypic and genotypic distance arrays (1 per 30cM window) were composed, and linear regression was used to calculate the correlation coefficients between the phenotypic and each of the genotypic arrays. Results: Strain specific phenotypic differences of ALT, WL, LW, and HT levels are identified in the 5 strains. By haplotype mapping with 109 SNPs, strong positive correlations (standardized correlation >1.5) are detected between HT concentrations and gene loci (QTLs) on chromosomes (Chr) 2, 5, and 14. QTLs that determine ALT activities map to Chr 1, 4, 7, 12, and 14, three of which co-localize with gene loci affecting WL. Gene loci associated with LW map to Chr 6, 7, 11, 14, and 19. Of note, a region on Chr 14 harbors QTLs for ALT, WL, LW, and HT. Conclusions: These data indicate that multiple gene loci (Nash genes) affect the development of steatohepatitis in the MCD model of NASH. Four distinct phenotypic QTLs co-localize on Chr 14, and several loci affecting ALT activities co-localize with loci affecting WL, LW or HT accumulation. We speculate that many distinct but interacting Nash loci modulate the pathophysiological mechanisms responsible for NASH.