Quantitative trait locus analysis of susceptibility to diet-induced atherosclerosis in recombinant inbred mice

Identification of Quantitative Trait Loci That Determine Plasma Total-Cholesterol and Triglyceride Concentrations in DDD/Sgn and C57BL/6J Inbred Mice

DDD/Sgn mice have significantly higher plasma lipid concentrations than C57BL/6J mice. In the present study, we performed quantitative trait loci (QTL) mapping for plasma total-cholesterol (CHO) and triglyceride (TG) concentrations in reciprocal F₂ male intercross populations between the two strains. By single-QTL scans, we identified four significant QTL on chromosomes (Chrs) 1, 5, 17, and 19 for CHO and two significant QTL on Chrs 1 and 12 for TG. By including cross direction as an interactive covariate, we identified separate significant QTL on Chr 17 for CHO but none for TG. When the large phenotypic effect of QTL on Chr 1 was controlled by composite interval mapping, we identified three additional significant QTL on Chrs 3, 4, and 9 for CHO but none for TG. QTL on Chr 19 was a novel QTL for CHO and the allelic effect of this QTL significantly differed between males and females. Whole-exome sequence analysis in DDD/Sgn mice suggested that Apoa2 and Acads were the plausible candidate genes underlying CHO QTL on Chrs 1 and 5, respectively. Thus, we identified a multifactorial basis for plasma lipid concentrations in male mice. These findings will provide insight into the genetic mechanisms of plasma lipid metabolism.

Responsiveness of cardiometabolic-related microbiota to diet is influenced by host genetics

Intestinal microbial community structure is driven by host genetics in addition to environmental factors such as diet. In comparison with environmental influences, the effect of host genetics on intestinal microbiota, and how host-driven differences alter host metabolism is unclear. Additionally, the interaction between host genetics and diet, and the impact on the intestinal microbiome and possible down-stream effect on host metabolism is not fully understood, but represents another aspects of inter-individual variation in disease risk. The objectives of this study were to investigate how diet and genetic background shape microbial communities, and how these diet- and genetic-driven microbial differences relate to cardiometabolic phenotypes. To determine these effects, we used the 8 progenitor strains of the collaborative cross/diversity outbred mapping panels (C57BL/6J, A/J, NOD/ShiLtJ, NZO/HILtJ, WSB/EiJ, CAST/EiJ, PWK/PhJ, and 129S1/SvImJ). 16s rRNA profiling of enteric microbial communities in addition to the assessment of phenotypes central to cardiometabolic health was conducted under baseline nutritional conditions and in response to diets varying in atherogenic nutrient (fat, cholesterol, cholic acid) composition. These studies revealed strain-driven differences in enteric microbial communities which were retained with dietary intervention. Diet–strain interactions were seen for a core group of cardiometabolic-related microbial taxa. In conclusion, these studies highlight diet and genetically regulated cardiometabolic-related microbial taxa. Furthermore, we demonstrate the progenitor model is useful for nutrigenomic-based studies and screens seeking to investigate the interaction between genetic background and the phenotypic and microbial response to diet.

High-resolution association mapping of atherosclerosis loci in mice

OBJECTIVE

The purpose of this study was to fine map previously identified quantitative trait loci affecting atherosclerosis in mice using association analysis.

METHODS AND RESULTS

We recently showed that high-resolution association analysis using common inbred strains of mice is feasible if corrected for population structure. To use this approach for atherosclerosis, which requires a sensitizing mutation, we bred human apolipoprotein B-100 transgenic mice with 22 different inbred strains to produce F1 heterozygotes. Mice carrying the dominant transgene were tested for association with high-density single nucleotide polymorphism maps. Here, we focus on high-resolution mapping of the previously described atherosclerosis 30 locus on chromosome 1. Compared with the previous linkage analysis, association improved the resolution of the atherosclerosis 30 locus by more than an order of magnitude. Using expression quantitative trait locus analysis, we identified one of the genes in the region, desmin, as a strong candidate.

CONCLUSIONS

Our high-resolution mapping approach accurately identifies and fine maps known atherosclerosis quantitative trait loci. These results suggest that high-resolution genome-wide association analysis for atherosclerosis is feasible in mice.

Pituitary control of cholesterol metabolism in normal and LDL receptor knock-out mice: Effects of hypophysectomy and growth hormone treatment

The pituitary is important in the control of lipid metabolism and studies of hypophysectomized (Hx) rats have shown strong effects of growth hormone (GH) on bile acid synthesis, hepatic LDL receptor (LDLR) expression and on the sensitivity to dietary cholesterol. It is unclear if mice may be used in such studies. The aim of the current study was to evaluate if Hx mice may be used to further explore how GH modulates cholesterol and bile acid metabolism, and to define the importance of the LDLR in this regulation by studying LDLR-deficient mice (LDLRko). Experiments on three mouse strains showed that, following Hx, HDL were reduced and LDL increased. Cholesterol/fat feeding of Hx mice increased serum cholesterol levels 2- to 3-fold. Serum triglycerides were reduced 50% in Hx mice; a further 30% reduction was seen after dietary cholesterol/fat. A serum marker for CYP7A1-mediated bile acid synthesis (C4) increased 2-fold in intact mice on cholesterol/fat diet. In Hx mice C4 levels were reduced by 50% as compared to intact controls, but were unexpectedly increased to levels seen in normal mice upon cholesterol/fat feeding. Hx of LDLRko mice moderately increased LDL-cholesterol and reduced triglycerides and GH treatment attenuated these effects; serum C4 levels were increased by GH treatment in all groups. In conclusion, mice can be used to explore the role of the pituitary in lipid metabolism. CYP7A1 is generally reduced in Hx mice but has a normal stimulatory response following dietary cholesterol suggesting that faulty regulation of CYP7A1 is not important for the reduced resistance to dietary cholesterol in Hx mice. Further, the LDLR is only to a minor part involved in the pituitary regulation of serum cholesterol in mice.

Understanding the human condition: experimental strategies in mammalian genetics

Mice have become the mammalian model of choice for the application of genetics in biomedical research due to the evolutionary conservation of physiological systems and their attendant pathologies among all mammals as well as the exceptional power of genetic research technologies in the species. Beginning from aberrant phenotypes, a large number of mouse mutants and natural polymorphisms have been cloned, providing much information about the molecular basis of physiological processes. Additionally, the variable expression of these mutations in different inbred strain backgrounds has demonstrated the importance of modifier genes, which are also susceptible to cloning. Research efforts are keeping pace with these developments. In the area of gene discovery, large, government-funded mutagenesis programs now exist, and as a matter of great practical importance, recent evidence suggests that the same genes may be involved in the natural polymorphisms affecting disease in mice and humans. In parallel, dramatic advances are also being made in our ability to measure physiological processes in mice, and the advent of expression profiling promises revolutionary advances in understanding phenotype at the molecular level. Gene-driven approaches have relied on engineering the mouse genome, including adding, subtracting, and replacing genes and, most recently, the ability to control gene activity reversibly. Together, these multiple advances in our technical abilities have created extraordinary opportunities for future discovery.

ENU large-scale mutagenesis and quantitative trait linkage (QTL) analysis in mice: novel technologies for searching polygenetic determinants of craniofacial abnormalities

Discrepancies in size and shape of the jaws are the underlying etiology in many orthodontic and orthognathic surgery patients. Genetic factors combined with environmental interactions have been postulated to play a causal or contributory role in these craniofacial abnormalities. Along with the soon-to-be-available complete human and mouse genomic sequence data, mouse mutants have become a valuable tool in the functional mapping of genes involved in the development of human maxillofacial dysmorphologies. We review two powerful methods in such efforts: N-ethyl-N-nitrosourea (ENU) large-scale mutagenesis and quantitative trait linkage (QTL) analysis. The former aims at producing a plethora of novel variants of particular trait(s), and ultimately mapping the point mutations responsible for the appearance of these new traits. In contrast, the latter applies intensive breeding and mapping techniques to identify multiple loci (and, subsequently, genes) contributing to the phenotypic difference between the tested strains. A prerequisite for either approach to studying variations in the traits of interest is the application of effective mouse cephalometric phenotype analysis and rapid DNA mapping techniques. These approaches will produce a wealth of new data on critical genes that influence the size and shape of the human face.

Quantitative trait locus mapping for atherosclerosis susceptibility

PURPOSE OF REVIEW

Atherosclerosis is a complex trait with both environmental and genetic aspects. Although some progress has been made in defining genes associated with atherosclerosis in humans, animal models have been useful in learning about pathways and genes involved in atherogenesis. This review describes an unbiased genetic mapping method called quantitative trait locus mapping and progress in using this method to identify genes that alter atherosclerosis susceptibility in mice.

RECENT FINDINGS

Approximately 10 well defined genetic loci have been described that are associated with lesion severity in diet-induced or gene knockout mouse models of atherosclerosis. Recently, two of these genetic loci were narrowed considerably by analysis of genetic recombinants within these loci. In addition, a computational method to discover quantitative trait loci has been applied to atherosclerosis. However, none of the genes responsible for these atherosclerosis quantitative trait loci has been definitively identified. The recent completion of the mouse draft genome should facilitate the task of identifying these genes.

SUMMARY

Quantitative trait locus mapping studies in mouse models of atherosclerosis have defined genetic regions that alter lesion severity. The identification of the responsible genes may lead to insights into the pathogenesis of atherosclerosis as well as to candidates for human genetic association studies.

In silico quantitative trait locus map for atherosclerosis susceptibility in apolipoprotein E-deficient mice

OBJECTIVE

Atherosclerosis susceptibility is a genetic trait that varies between mouse strains. The goal of this study was to use a public mouse single nucleotide polymorphism (SNP) database to define the genetic loci that are associated with this trait, without the need to perform strain intercrosses that are normally required to obtain these loci.

METHODS AND RESULTS

Apolipoprotein E (apoE)-deficient mice on 6 inbred genetic backgrounds were compared for atherosclerosis lesion size in the aortic root in 2 independent studies. After normalization to the C57BL/6 strain that was used in both studies, lesion areas were found in the following rank order: DBA/2J>C57BL/6>129/SV-ter>AKR/J approximately BALB/cByJ approximately C3H/HeJ. The log lesion difference in phenotypes between each of the 15 heterologous strain pairs was determined. A mouse SNP database was then used to calculate the genetic differences between the 15 strain pairs in partially overlapping 30-cM bins across the mouse genome. Correlation analyses were preformed to analyze the genetic and phenotypic differences among the strain pairs for each genetic region. The genetic regions with the highest correlations define the in silico quantitative trait loci (QTL) associated with the atherosclerosis phenotype. Five in silico atherosclerosis QTL were identified on chromosomes 1, 10, 14, 15, and 18. The loci on chromosomes 1, 10, 14, and 18 overlap with suggestive atherosclerosis QTL identified through analyses of an F(2) cohort derived from apoE-deficient mice on the C57BL/6 and FVB/N strains.

CONCLUSIONS

The 5 identified in silico QTL are candidates for further study to confirm the presence and identity of atherosclerosis susceptibility genes within these loci.

Macrophage scavenger receptor (SR-A I/II) deficiency reduced diet-induced atherosclerosis in C57BL/6J mice

The effects of SR-A I/II deficiency and a synthetic anti-oxidant BO-653 on a diet-induced atherosclerosis in C57BL/6J, an inbred strain known to be susceptible to diet-induced atherosclerotic lesion formation, were examined. Quantitative analysis of the extent of atherosclerotic lesions in the mice fed the high-fat diet revealed that the atherosclerotic lesion area in SR-A I/II mutants was significantly reduced by 70% compared to wild type mice. A similar level of lesion reduction (75%) was found in wild type mice fed the high-fat diet supplemented with 0.6% BO-653 compared to those without BO-653. Thus, for C57BL/6J in the setting of prolonged exposure to a high-fat diet, defect of SR-A I/II expression is significantly protective against the development of atherosclerosis, as is the synthetic anti-oxidant BO-653. These results indicate that SR-A I/II has a crucial role in atherosclerotic lesion formation with uptake of oxidized-LDL in this mouse model.

Genetic background determines the extent of atherosclerosis in ApoE-deficient mice

Two strains of ApoE-deficient mice were found to have markedly different plasma lipoprotein profiles and susceptibility to atherosclerosis when fed either a low-fat chow or a high-fat Western-type diet. FVB/NJ ApoE-deficient (FVB E0) mice had higher total cholesterol, HDL cholesterol, ApoA1, and ApoA2 levels when compared with C57BL/6J ApoE-deficient (C57 E0) mice. At 16 weeks of age, mean aortic root atherosclerotic lesion area was 7- to 9-fold higher in chow diet-fed C57 E0 mice and 3.5-fold higher in Western diet-fed C57 E0 mice compared with FVB E0 mice fed similar diets. Lesion area in chow diet-fed first-generation mice from a strain intercross was intermediate in size compared with parental values. The distribution of the lesion area in 150 chow diet-fed second-generation progeny spanned the range of the lesion area in both parental strains. There were no correlations between total cholesterol, non-HDL cholesterol, HDL cholesterol, ApoA1, ApoA2, ApoJ, or anti-cardiolipin antibodies and lesion area in the second-generation progeny. Thus, a genomic approach may succeed in identifying the genes responsible for the variation in atherosclerosis susceptibility in these 2 strains of ApoE-deficient mice, which could not be explained by measured plasma parameters.

Quantitative trait loci on chromosomes 1 and 4 affect lipid phenotypes in the rat

The spontaneously hypertensive rat (SHR/Mol) and the spontaneously diabetic BB/OK rat were crossed, and the F1 hybrids were backcrossed onto the BB/OK rat in order to search for quantitative trait loci (QTL) affecting serum total cholesterol and triglycerides on chromosomes 1, 3, 4, 10, 13, 18, and X. On chromosome 4 a QTL for triglyceride levels (lod score 3.3) was found within the region flanked by the D4Mit9 and Il-6 markers. Suggestive linkage (lod score 1.9) was found for total cholesterol on chromosome 4 at the Spr locus. Also, on chromosome 1 suggestive linkage for both investigated traits was found at marker D1Mit14 (lod score 1.9 for triglycerides, 2.1 for total cholesterol). The results of the study could contribute to the explanation of the genetic basis of lipid abnormalities, which are a common feature of pathological disorders such as coronary heart disease, hypertension, or non-insulin-dependent diabetes.

Mapping of mouse obesity genes: A generic approach to a complex trait

Identification of genes underlying any complex trait such as obesity is an important and difficult problem in genetics. Traditional candidate gene approaches cannot be relied on to identify all of the genes influencing a complex trait, and positional cloning is very laborious. With the advent of new tools and methods, however, comprehensive approaches to the identification of any genes underlying complex traits are now available. Quantitative trait locus (QTL) mapping is a general technique to map Mendelian factors influencing complex traits. The QTL approach involves the crossing of two strains that differ in the trait of interest to produce F2 or back-cross progeny, individually phenotyping and genotyping each progeny, and statistically associating the typed markers and the phenotype. QTL mapping has been used in the last 4 years to map genes for a wide variety of traits, including body weight and growth, obesity, atherosclerosis and susceptibility to cancer in the mouse, and hypertension, hyperactivity and arthritis in the rat. QTL mapping has also been used to map genes in pigs, poultry, cows, fish and plants. Once a trait has been located in a chromosomal subregion, identifying the underlying gene remains a significant problem. A monogenic model must be developed, isolating one gene influencing a trait from other genes affecting the same phenotype. Then the positional candidate strategy, which relies on a combination of mapping to a chromosomal subregion followed by a survey of the interval to see if attractive candidates reside there, becomes practical.

Novel quantitative trait loci for blood pressure and related traits on rat chromosomes 1, 10, and 18

Hypertension and diabetes mellitus are known to be frequently associated. The genetic dissection of diseases such as hypertension or diabetes mellitus is possible by using experimental crosses, which allow identification of loci influencing phenotypic traits (quantitative trait loci - QTLs). In this study the spontaneously hypertensive rat (SHR) and spontaneously diabetic, but normotensive rat (BB/OK) were crossed and the F2 population was analysed in order to search for QTLs on selected chromosomes (1, 10, 18) for blood pressure and some metabolic traits related to diabetes, renal function and hypertension. There were 3 regions found on chromosome 1 which showed linkage to blood pressure. The strongest evidence for linkage was observed between loci Igf2 and D1Mgh12. On chromosome 10 there was a QTL for blood pressure found between Ppy and Abp and on chromosome 18 there were three regions (Ttr-Grl, Tilp-Gja1, Olf-D18Mit9) with linkage to blood pressure. Since the 24 hr albumin and phosphate excretion correlated with blood pressure in F2 hybrids, the same regions were linked to both parameters. Region with linkage to serum concentrations of cholesterol (probably located beyond the terminal marker Ttr of the linkage group) were also found. The results of this study with a new F2(BB x SHR) population confirm the existence of previously described blood pressure loci (Sa and Bp2) and showed novel QTLs on chromosomes 1, 10 and 18.

Complex genetic control of HDL levels in mice in response to an atherogenic diet. Coordinate regulation of HDL levels and bile acid metabolism

Inbred strains of mice differ in susceptibility to atherogenesis when challenged with a high fat, high cholesterol diet containing 0.5% cholic acid. Studies of recombinant inbred (RI) strains derived from the susceptible strain C57BL/6J (B6) and the resistant strains C3H/HeJ (C3H) and BALB/cJ have revealed an association between fatty streak lesion size and a decrease in high density lipoprotein (HDL) levels on the diet. To better understand the genetic factors contributing to HDL metabolism and atherogenesis in response to the diet, we studied mice derived from an intercross between B6 and C3H using a complete linkage map approach. A total of 185 female progeny were typed for 134 genetic markers spanning the mouse genome, resulting in an average interval of about 10 cM between markers. A locus on distal chromosome 1 containing the apolipoprotein AII gene was linked to HDL-cholesterol levels on both the chow and the atherogenic diets, but this locus did not contribute to the decrease in HDL-cholesterol in response to the diet. At least three distinct genetic loci, on chromosomes 3, 5, and 11, exhibited evidence of linkage to a decrease in HDL-cholesterol after a dietary challenge. Since a bile acid (cholic acid) is required for the diet induced changes in HDL levels and for atherogenesis in these strains, we examined cholesterol-7-alpha hydroxylase (C7AH) expression. Whereas B6 mice exhibited a large decrease in C7AH mRNA levels in response to the diet, C3H showed an increase. Among the intercross mice, multiple loci contributed to the regulation of C7AH mRNA levels in response to the diet, the most notable of which coincided with the loci on chromosomes 3, 5, and 11 controlling HDL levels in response to the diet. None of these loci were linked to the C7AH structural gene which we mapped to proximal chromosome 4. These studies reveal coordinate regulation of C7AH expression and HDL levels, and they indicate that the genetic factors controlling HDL levels are more complex than previously suggested by studies of RI strains. Furthermore, we observed that two of the loci for C7AH expression contributed to differences in gallstone formation between these strains.