A Genetic and Correlation Analysis of Liver Cholesterol Concentration in Rat Recombinant Inbred Strains Fed a High Cholesterol Diet

Identification of Mutated Srebf1 as a QTL Influencing Risk for Hepatic Steatosis in the Spontaneously Hypertensive Rat

“…We used a high-cholesterol diet in all experiments because this was the same diet used in the original QTL mapping studies. 5 In pilot studies, we also found that administration of the high-cholesterol diet was necessary to elicit significant differences in hepatic lipid levels between the SHR progenitor and the SHR-Chr.10 congenic strain. After 4 weeks of feeding the high-cholesterol diet, we euthanized the animals and obtained liver samples for measurement of hepatic cholesterol and for preparation of mRNA to quantify gene expression levels as described further below.…”

“…The assertion that Srebf1 is a QTL influencing hepatic cholesterol levels in the SHR-BN model is based on the stringent criteria proposed by Glazier et al for QTL identification in studies of complex traits 1 : 1st criterion, Linkage. In RI strains fed a high cholesterol diet, a QTL associated with effects on hepatic cholesterol levels maps to rat Chromosome 10 near the Srebf1 candidate gene 5,6 ; 2nd criterion, Fine mapping. In recombinant congenic sublines derived from the SHR and BN strains, the QTL regulating hepatic cholesterol levels and the Srebf1 positional candidate gene map to an interval of approximately 2.5 Mbp together with effects on mRNA and protein levels for SREBP-1; 3rd criterion, Sequence analysis.…”

“…Given our previous linkage results in RI strains, 5 we performed additional mapping studies in a congenic strain and congenic subline to test whether Srebf1 might represent the QTL on Chromosome 10 linked to hepatic cholesterol levels in the SHR. We derived the congenic strain, SHR.BN-D10Mgh3/Srebf1 (hereafter referred to as the SHR-Chr.10 congenic strain), by a backcross breeding protocol in which we replaced a 53.7 megabase pair (Mbp) segment of Chromosome 10 including Srebf1 in the SHR/Ola strain (hereafter referred to as the SHR progenitor strain) with the corresponding chromosome segment from the Brown Norway (BN/Crl) strain ( Figure 1).…”

“…This finding confirmed the original linkage results in the RI strains in which the putative Hcl1 QTL of SHR origin was associated with lower hepatic cholesterol levels than the QTL of BN origin. 5 Fine mapping of the Hcl1 QTL was achieved by comparison of the SHR-Chr.10 congenic strain to the SHR-Chr.10a congenic subline that is genetically identical except for approximately 2.5 Mbp of Chromosome 10 that includes the SHR variant of Srebf1 in the subline (Figures 1 and 2). This subline carrying the SHR Figure 2.…”

“…Recently, in recombinant inbred (RI) strains derived from the spontaneously hypertensive rat (SHR), we mapped a QTL on Chromosome 10 that was associated with effects on hepatic cholesterol levels in animals fed a high cholesterol diet (designated Hcl1 [Hepatic cholesterol level 1]). 5 We subsequently found that the SHR harbors a rare allele of the gene encoding sterol regulatory element binding protein 1 (SREBP-1) that maps to a region of rat Chromosome 10 overlapping the QTL regulating hepatic cholesterol levels. 6 The gene for SREBP-1 (Srebf1) has 2 alternate promoters and codes for 2 protein isoforms designated SREBP-1a and SREBP-1c, both of which are well known transcriptional regulators of hepatic cholesterol and fatty acid synthesis.…”