Genetic dissection of the fatty liver QTL Fl1sa by using congenic mice and identification of candidate genes in the liver and epididymal fat.

BACKGROUND: Nonalcoholic fatty liver disease (NAFLD) is a multifactorial disease caused by interactions between environmental and genetic factors. The SMXA-5 mouse is a high-fat diet-induced fatty liver model established from SM/J and A/J strains. We have previously identified Fl1sa, a quantitative trait locus (QTL) for fatty liver on chromosome 12 (centromere-53.06 Mb) of SMXA-5 mice. However, the chromosomal region containing Fl1sa was too broad. The aim of this study was to narrow the Fl1sa region by genetic dissection using novel congenic mice and to identify candidate genes within the narrowed Fl1sa region. RESULTS: We established two congenic strains, R2 and R3, from parental A/J-12SM and A/J strains. R2 and R3 strains have genomic intervals of centromere-29.20 Mb and 29.20-46.75 Mb of chromosome 12 derived from SM/J, respectively. Liver triglyceride content in R2 and R3 mice was significantly lower than that in A/J mice fed with a high-fat diet for 7 weeks. This result suggests that at least one of the genes responsible for fatty liver exists within the two chromosomal regions centromere-29.20 Mb (R2) and 29.20-46.75 Mb (R3). We found that liver triglyceride accumulation is inversely correlated with epididymal fat weight among the parental and congenic strains. Therefore, the ectopic fat accumulation in the liver may be due to organ-organ interactions between the liver and epididymal fat. To identify candidate genes in Fl1sa, we performed a DNA microarray analysis using the liver and epididymal fat in A/J and A/J-12SM mice fed with a high-fat diet for 7 weeks. In epididymal fat, mRNA levels of Zfp125 (in R2) and Nrcam (in R3) were significantly different in A/J-12SM mice from those in A/J mice. In the liver, mRNA levels of Iah1 (in R2) and Rrm2 (in R2) were significantly different in A/J-12SM mice from those in A/J mice. CONCLUSIONS: In this study, using congenic mice analysis, we narrowed the chromosomal region containing Fl1sa to two regions of mouse chromosome 12. We then identified 4 candidate genes in Fl1sa: Iah1 and Rrm2 from the liver and Zfp125 and Nrcam from epididymal fat.

Detection of differentially expressed candidate genes for a fatty liver QTL on mouse chromosome 12.

BACKGROUND: The SMXA-5 mouse is an animal model of high-fat diet-induced fatty liver. The major QTL for fatty liver, Fl1sa on chromosome 12, was identified in a SM/J × SMXA-5 intercross. The SMXA-5 genome consists of the SM/J and A/J genomes, and the A/J allele of Fl1sa is a fatty liver-susceptibility allele. The existence of the responsible genes for fatty liver within Fl1sa was confirmed in A/J-12(SM) consomic mice. The aim of this study was to identify candidate genes for Fl1sa, and to investigate whether the identified genes affect the lipid metabolism. RESULTS: A/J-12(SM) mice showed a significantly lower liver triglyceride content compared to A/J mice when fed the high-fat diet for 7 weeks. We detected differences in the accumulation of liver lipids in response to the high-fat diet between A/J and A/J-12(SM) consomic mice. To identify candidate genes for Fl1sa, we performed DNA microarray analysis using the livers of A/J-12(SM) and A/J mice fed the high-fat diet. The mRNA levels of three genes (Iah1, Rrm2, Prkd1) in the chromosomal region of Fl1sa were significantly different between the strains. Iah1 mRNA levels in the liver, kidney, and lung were significantly higher in A/J-12(SM) mice than in A/J mice. The hepatic Iah1 mRNA level in A/J-12(SM) mice was 3.2-fold higher than that in A/J mice. To examine the effect of Iah1 on hepatic lipid metabolism, we constructed a stable cell line expressing the mouse Iah1 protein in mouse hepatoma Hepa1-6 cells. Overexpression of Iah1 in Hepa1-6 cells suppressed the mRNA levels of Cd36 and Dgat2, which play important roles in triglyceride synthesis and lipid metabolism. CONCLUSIONS: These results demonstrated that Fl1sa on the proximal region of chromosome 12 affected fatty liver in mice on a high-fat diet. Iah1 (isoamyl acetate-hydrolyzing esterase 1 homolog) was identified as one of the candidate genes for Fl1sa. This study revealed that the mouse Iah1 gene regulated the expression of genes related to lipid metabolism in the liver.