Identifying novel genes for atherosclerosis through mouse-human comparative genetics

Novel strategy using F1-congenic mice for validation of QTLs: studies at the proximal chromosome 10 atherosclerosis susceptibility locus

OBJECTIVE

We have previously identified a quantitative trait locus (QTL) for atherosclerosis susceptibility on proximal chromosome 10 (Chr10) (Ath11) in independent crosses of FVB and C57BL/6 (B6) mice on the apolipoprotein E (ApoE-/-) and LDL receptor (LDLR-/-) deficient backgrounds. The aims of the current study were to (1) test a novel strategy for validating QTLs using interval-specific congenic strains that were heterozygous (F1) across the genome, (2) validate the Chr10 QTL, and (3) to assess whether the phenotype is transferable by bone marrow transplantation.

METHODS AND RESULTS

We generated Chr10 (0 to 21 cM) interval-specific mice on the F1.ApoE-/- background by crossing congenic FVB.ApoE-/-Chr10(B6/FVB) with B6.ApoE-/-, and B6.ApoE-/-Chr10(B6/FVB) with FVB.ApoE-/- mice. Lesion size was significantly larger in the resultant F1.ApoE-/-Chr10(FVB/FVB) mice compared to F1.ApoE-/-Chr10(B6/FVB) and F1.ApoE-/-Chr10(B6/B6) mice, validating the Chr10 QTL. The effect of the congenic interval was more robust on the F1.ApoE-/- than on the FVB.ApoE-/- and B6.ApoE-/- backgrounds. Bone marrow transplantation in congenic mice showed that the effect of the proximal Chr10 interval was not transferable by bone marrow-derived cells.

CONCLUSIONS

A novel strategy of congenic strains on an F1 background proved useful to validate an atherosclerosis susceptibility QTL on mouse proximal Chr10.

Congenic strains provide evidence that four mapped loci in chromosomes 2, 4, and 16 influence hypertension in the SHR

To dissect the genetic architecture controlling blood pressure (BP) regulation in the spontaneously hypertensive rat (SHR) we derived congenic rat strains for four previously mapped BP quantitative trait loci (QTLs) in chromosomes 2, 4, and 16. Target chromosomal regions from the Brown Norway rat (BN) averaging 13-29 cM were introgressed by marker-assisted breeding onto the SHR genome in 12 or 13 generations. Under normal salt intake, QTLs on chromosomes 2a, 2c, and 4 were associated with significant changes in systolic BP (13, 20, and 15 mmHg, respectively), whereas the QTL on chromosome 16 had no measurable effect. On high salt intake (1% NaCl in drinking water for 2 wk), the chromosome 16 QTL had a marked impact on SBP, as did the QTLs on chromosome 2a and 2c (18, 17, and 19 mmHg, respectively), but not the QTL on chromosome 4. Thus these four QTLs affected BP phenotypes differently: 1) in the presence of high salt intake (chromosome 16), 2) only associated with normal salt intake (chromosome 4), and 3) regardless of salt intake (chromosome 2c and 2a). Moreover, salt sensitivity was abrogated in congenics SHR.BN2a and SHR.BN16. Finally, we provide evidence for the influence of genetic background on the expression of the mapped QTLs individually or as a group. Collectively, these data reveal previously unsuspected nuances of the physiological roles of each of the four mapped BP QTLs in the SHR under basal and/or salt loading conditions unforeseen by the analysis of the F2 cross.

Blood pressure and heart rate QTL in mice of the B6/D2 lineage: sex differences and environmental influences

A quantitative trait locus (QTL) approach was used to define the genetic architecture underlying variation in systolic blood pressure (SBP) and heart rate (HR), measured indirectly on seven occasions by the tail cuff procedure. The tests were conducted in 395 F(2) adult mice (197 males, 198 females) derived from a cross of the C57BL/6J (B6) and DBA/2J (D2) strains and in 22 BXD recombinant-inbred (RI) strains. Interval mapping of F(2) data for the first 5 days of measurement nominated one statistically significant and one suggestive QTL for SBP on chromosomes (Chr) 4 and 14, respectively, and two statistically significant QTL for HR on Chr 1 (which was specific to female mice) and Chr 5. New suggestive QTL emerged for SBP on Chr 3 (female-specific) and 8 and for HR on Chr 11 for measurements recorded several weeks after mice had undergone stressful blood sampling procedures. The two statistically significant HR QTL were confirmed by analyses of BXD RI strain means. Male and female F(2) mice did not differ in SBP or HR but RI strain analyses showed pronounced strain-by-sex interactions and a negative genetic correlation between the two measures in both sexes. Evidence for a role for mitochondrial DNA was found for both HR and SBP. QTL for HR and SBP may differ in males and females and may be sensitive to different environmental contexts.

Mapping of quantitative trait loci for cholesterol, LDL, HDL, and triglyceride serum concentrations in pigs

The fine mapping of polymorphisms influencing cholesterol (CT), triglyceride (TG), and lipoprotein serum levels in human and mouse has provided a wealth of knowledge about the complex genetic architecture of these traits. The extension of these genetic analyses to pigs would be of utmost importance since they constitute a valuable biological and clinical model for the study of coronary artery disease and myocardial infarction. In the present work, we performed a whole genome scan for serum lipid traits in a half-sib Duroc pig population of 350 individuals. Phenotypic registers included total CT, TG, and low (LDL)- and high (HDL)-density lipoprotein serum concentrations at 45 and 190 days of age. This approach allowed us to identify two genomewide significant quantitative trait loci (QTL) for HDL-to-LDL ratio at 45 days (SSC6, 84 cM) and for TG at 190 days (SSC4, 23 cM) as well as a number of chromosomewide significant QTL. The comparison of QTL locations at 45 and 190 days revealed a notable lack of concordance at these two time points, suggesting that the effects of these QTL are age specific. Moreover, we have observed a considerable level of correspondence among the locations of the most significant porcine lipid QTL and those identified in humans. This finding might suggest that, in mammals, diverse polymorphisms located in a common set of genes are involved in the genetic variation of serum lipid levels.

Adiponectin gene and cardiovascular risk in type 2 diabetic patients: a review of evidences

Diabetic patients have a 3-fold higher risk of developing atherosclerosis and its clinical complications as compared to non-diabetic individuals. Part of the cardiovascular risk associated with diabetes is probably due to genetic determinants influencing both glucose homeostasis and the development of atherosclerosis. However, type 2 diabetes frequently coexists with other cardiovascular risk factors like arterial hypertension, central obesity and dyslipidemia. Genetic variability affecting many areas such as lipid and energy metabolisms, hypertension and haemodynamic mechanisms, blood clotting homeostasis, inflammation, and matrix turnover in the vascular wall will have an impact on the development of macrovascular complications in diabetic patients. Adiponectin is abundantly secreted by adipocytes. It plays important roles in lipid and glucose metabolisms and has direct anti-inflammatory and anti-atherogenic effects. In this review, we summarize recent data from the literature suggesting an implication of allelic variations of the adiponectin gene (ADIPOQ) in the genetic determinants of cardiovascular disease in diabetic subjects.

Human-mouse quantitative trait locus concordance and the dissection of a human neuroticism locus

BACKGROUND

Exploiting synteny between mouse and human disease loci has been proposed as a cost-effective method for the identification of human susceptibility genes. Here we explore its utility in an analysis of a human personality trait, neuroticism, which can be modeled in mice by tests of emotionality. We investigated a mouse emotionality locus on chromosome 1 that contains no annotated genes but abuts four regulators of G protein signaling, one of which (rgs2) has been previously identified as a quantitative trait gene for emotionality. This locus is syntenic with a human region that has been consistently implicated in the genetic aetiology of neuroticism.

METHODS

The functional candidacy of 29 murine sequence variants was tested by a combination of gel shift and transient transfection assays. Murine sequences that contained functional variants and exhibited significant cross-species conservation were prioritized for investigation in humans. Genetic association with neuroticism was tested in 1869 high and 2032 low unrelated individuals scored for neuroticism, selected from the extremes of 88,141 people from southwest England.

RESULTS

Fifteen sequence variants contributed to variation in the expression of rgs18, the gene lying at the edge of the quantitative trait loci (QTL) interval. There was no evidence of association between neuroticism and single nucleotide polymorphisms (SNPs) lying in the human regions homologous to those of mouse functional variants. One SNP, rs6428058, in a region of sequence conservation 644 kb upstream of RGS18, showed significant association (p = .000631).

CONCLUSIONS

It is unlikely that a single variant is responsible for the mouse emotionality locus on chromosome 1. This level of underlying genetic complexity means that although cross-species QTL concordance may be invaluable for the identification of human disease loci, it is unlikely to be as informative in the identification of human disease-causing variants.

Identifying the susceptibility genes for coronary artery disease: from hyperbole through doubt to cautious optimism

The genetic basis of coronary artery disease (CAD) is complex, and the fact that an alarmingly high proportion of reported associations between genetic variants and CAD are not replicated has generated uncertainty as to whether molecular genetics is ever going to deliver on the promises delivered in the late 1990s. However, during 2007, the first generation of large-scale genome-wide association studies using high-density, single nucleotide polymorphism genotyping arrays have revealed genetic variants that are robustly associated with CAD and CAD-related traits such as type 2 diabetes and obesity. In particular, a robust susceptibility locus for CAD has been identified on chromosome 9p21. Also, evidence has been obtained that multiple rare alleles with fairly strong phenotypic effects may contribute to the genetic heritability of CAD, in addition to common variants with a modest impact on risk. Furthermore, new mechanistic connections have been discovered between different common complex diseases including CAD. This review focuses on the challenges and recent advances of molecular genetics in dissecting the molecular pathophysiology of atherothrombosis and defining novel targets for treatment.

Increased ADAM17 mRNA expression and activity is associated with atherosclerosis resistance in LDL-receptor deficient mice

BACKGROUND

We have previously identified an atherosclerosis quantitative trait locus (QTL) on mouse chromosome (Chr) 12 in an F2-intercross of atherosclerosis-resistant FVB and atherosclerosis-susceptible C57BL/6 (B6) mice on the LDL-receptor deficient (LDL-/-) background. The aim of the present study was to identify potentially causative genes at this locus.

METHODS AND RESULTS

Expression QTL (eQTL) analysis of candidate genes in livers of F2-mice revealed that a disintegrin and metalloproteinase 17 (ADAM17) mRNA expression mapped to the physical position of ADAM17 on proximal Chr12 (21.6 Mb, LOD 3.3) and colocalized with the atherosclerosis QTL. The FVB allele was associated with significantly higher ADAM17 mRNA expression (39%) than the B6 allele. Likewise, ADAM17 mRNA levels in the parental strains were significantly elevated in FVB.LDLR-/- compared to B6.LDLR-/- mice in liver, macrophages, and aorta (68%, 58%, and 32%, respectively). Reporter gene assays revealed a genetic variant that might explain these expression differences. Moreover, FVB.LDLR-/- macrophages showed 5-fold increased PMA-induced shedding of tumor necrosis factor (TNF)-alpha and 32% increased release of TNF-receptor I compared to B6.LDLR-/-. The atherosclerosis locus and expression differences were confirmed in Chr12 interval-specific congenic mice.

CONCLUSIONS

Our data provide functional evidence for ADAM17 as a candidate gene of atherosclerosis susceptibility at the murine Chr12 QTL.

Quantitative trait locus analysis of carotid atherosclerosis in an intercross between C57BL/6 and C3H apolipoprotein E-deficient mice

BACKGROUND AND PURPOSE

Inbred mouse strains C57BL/6J (B6) and C3H/HeJ (C3H) exhibit marked differences in atherosclerotic lesion formation in the carotid arteries on the apolipoprotein E-deficient (apoE(-/-)) background when fed a Western diet. Quantitative trait locus analysis was performed on an intercross between B6.apoE(-/-) and C3H.apoE(-/-) mice to determine genetic factors contributing to variation in the phenotype.

METHODS

Female B6.apoE(-/-) mice were crossed with male C3H.apoE(-/-) mice to generate F(1) hybrids, which were intercrossed to generate 241 female F(2) progeny. At 6 weeks of age, F(2) mice were started on a Western diet. After being fed the diet for 12 weeks, F(2) mice were analyzed for phenotypes such as lesion size in the left carotid arteries and plasma lipid levels and typed for 154 genetic markers spanning the mouse genome.

RESULTS

One significant quantitative trait locus, named CAth1 (25 cM, log of the odds score: 4.5), on chromosome 12 and 4 suggestive quantitative trait loci, on chromosomes 1, 5, 6, and 11, respectively, were identified to influence carotid lesion size. One significant quantitative trait locus on distal chromosome 1 accounted for major variations in plasma low-density lipoprotein/very-low-density lipoprotein, high-density lipoprotein cholesterol, and triglyceride levels. Carotid lesion size was not significantly correlated with plasma low-density lipoprotein/very-low-density lipoprotein or high-density lipoprotein cholesterol levels.

CONCLUSIONS

These data indicate that the loci for carotid lesions do not overlap with those for aortic lesions as identified in a previous cross derived from the same parental strains, and carotid atherosclerosis and plasma lipids are controlled by separate genetic factors in the B6 and C3H mouse model.

Genetic control of lipids in the mouse cross DU6i x DBA/2

An F(2) pedigree based on the mouse lines DU6i and DBA/2 with extremely different growth and obesity characteristics was generated to search for QTLs affecting serum concentrations of triglycerides (TG), total cholesterol (CHOL), HDL cholesterol (HDL-C), and LDL cholesterol (LDL-C). Compared with many other studies, we searched for spontaneous genetic variants contributing to high lipid levels under a standard breeding diet. Significant QTLs for CHOL were identified on chromosomes 4 and 6, and a female-specific locus on chromosome 3. QTLs for HDL-C were detected on chromosome 11 for both sexes, and on chromosome 1 for females. These QTLs are located in syntenic human regions that have QTLs that have not been previously confirmed in animal studies. LDL-C QTLs have been mapped for both sexes to chromosome 8 and in males on chromosome 13. Epistatic interactions that significantly accounted for the phenotypic variance of HDL-C, CHOL, and LDL-C serum concentrations were also detected with one interaction between chromosomes 8 and 15, accounting for 22% of the observed variance in LDL-C levels. The identified loci coincide in part with regions controlling growth and obesity. Thus, multiple genes or pleiotropic effects may be assumed. The identified QTLs for cholesterol and its transport proteins as subcomponents of risk for coronary heart disease will further improve our understanding of the genetic net controlling plasma lipid concentrations.

Genetic analysis of albuminuria in a cross between C57BL/6J and DBA/2J mice

Chronic kidney disease (CKD) is a growing medical problem and a significant risk factor for the development of end-stage renal disease, cardiovascular disease, and cardiovascular mortality. The genetic basis of CKD is recognized, but knowledge of the specific genes that contribute to the onset and progression of kidney disease is limited, mainly because of the difficulty and expense of identifying genes underlying CKD in humans. Results from genetic studies of CKD in rodents often correspond to findings in humans; therefore, we used quantitative trait locus (QTL) analysis to detect genomic regions affecting albuminuria in a cross between C57BL/6J and DBA/2J mice, strains resistant and susceptible to CKD, respectively. We identified several independent and interacting loci affecting albuminuria, including one QTL on mouse chromosome (Chr) 2 that is concordant with QTL influencing urinary albumin excretion on rat Chr 3 and diabetic nephropathy on human Chr 20p. Because this QTL was identified in multiple mouse crosses, as well as in rats and in humans, we used comparative genomics, haplotype analysis, and expression profiling to narrow the initial QTL interval from 386 genes to 10 genes with known coding sequence polymorphisms or expression differences between the strains. These results support the continued use of multiple cross-mapping and cross-species comparisons to further our understanding of the genetic basis of kidney disease.

Genetic and genomic insights into the molecular basis of atherosclerosis

Atherosclerosis is a complex disease involving genetic and environmental risk factors, acting on their own or in synergy. Within the general population, polymorphisms within genes in lipid metabolism, inflammation, and thrombogenesis are probably responsible for the wide range of susceptibility to myocardial infarction, a fatal consequence of atherosclerosis. Genetic linkage studies have been carried out in both humans and mouse models to identify these polymorphisms. Approximately 40 quantitative trait loci for atherosclerotic disease have been found in humans, and approximately 30 in mice. Recently, genome-wide association studies have been used to identify atherosclerosis-susceptibility polymorphisms. Although discovering new atherosclerosis genes through these approaches remains challenging, the pace at which these polymorphisms are being found is accelerating due to rapidly improving bioinformatics resources and biotechnologies. The outcome of these efforts will not only unveil the molecular basis of atherosclerosis but also facilitate the discovery of drug targets and individualized medication against the disease.

Signal transducer of inflammation gp130 modulates atherosclerosis in mice and man

Liver-derived acute phase proteins (APPs) emerged as powerful predictors of cardiovascular disease and cardiovascular events, but their functional role in atherosclerosis remains enigmatic. We report that the gp130 receptor, which is a key component of the inflammatory signaling pathway within hepatocytes, influences the risk of atherosclerosis in a hepatocyte-specific gp130 knockout. Mice on an atherosclerosis-prone genetic background exhibit less aortic atherosclerosis (P < 0.05) with decreased plaque macrophages (P < 0.01). Translating these findings into humans, we show that genetic variation within the human gp130 homologue, interleukin 6 signal transducer (IL6ST), is significantly associated with coronary artery disease (CAD; P < 0.05). We further show a significant association of atherosclerotic disease at the ostium of the coronary arteries (P < 0.005) as a clinically important and heritable subphenotype in a large sample of families with myocardial infarction (MI) and a second independent population–based cohort. Our results reveal a central role of a hepatocyte-specific, gp130-dependent acute phase reaction for plaque development in a murine model of atherosclerosis, and further implicate IL6ST as a genetic susceptibility factor for CAD and MI in humans. Thus, the acute phase reaction should be considered an important target for future drug development in the management of CAD.

Identification of pathways for atherosclerosis in mice: integration of quantitative trait locus analysis and global gene expression data

We report a combined genetic and genomic analysis of atherosclerosis in a cross between the strains C3H/HeJ and C57BL/6J on a hyperlipidemic apolipoprotein E-null background. We incorporated sex and sex-by-genotype interactions into our model selection procedure to identify 10 quantitative trait loci for lesion size, revealing a level of complexity greater than previously thought. Of the known risk factors for atherosclerosis, plasma triglyceride levels and plasma glucose to insulin ratios were particularly strongly, but negatively, associated with lesion size. We performed expression array analysis for 23,574 transcripts of the livers and adipose tissues of all 334 F2 mice and identified more than 10,000 expression quantitative trait loci that either mapped to the gene encoding the transcript, implying cis regulation, or to a separate locus, implying trans-regulation. The gene expression data allowed us to identify candidate genes that mapped to the atherosclerosis quantitative trait loci and for which the expression was regulated in cis. Genes highly correlated with lesions were enriched in certain known pathways involved in lesion development, including cholesterol metabolism, mitochondrial oxidative phosphorylation, and inflammation. Thus, global gene expression in peripheral tissues can reflect the systemic perturbations that contribute to atherosclerosis.

Advances in QTL mapping in pigs

Over the past 15 years advances in the porcine genetic linkage map and discovery of useful candidate genes have led to valuable gene and trait information being discovered. Early use of exotic breed crosses and now commercial breed crosses for quantitative trait loci (QTL) scans and candidate gene analyses have led to 110 publications which have identified 1,675 QTL. Additionally, these studies continue to identify genes associated with economically important traits such as growth rate, leanness, feed intake, meat quality, litter size, and disease resistance. A well developed QTL database called PigQTLdb is now as a valuable tool for summarizing and pinpointing in silico regions of interest to researchers. The commercial pig industry is actively incorporating these markers in marker-assisted selection along with traditional performance information to improve traits of economic performance. The long awaited sequencing efforts are also now beginning to provide sequence available for both comparative genomics and large scale single nucleotide polymorphism (SNP) association studies. While these advances are all positive, development of useful new trait families and measurement of new or underlying traits still limits future discoveries. A review of these developments is presented.

The mouse as a model for human biology: a resource guide for complex trait analysis

The mouse has been a powerful force in elucidating the genetic basis of human physiology and pathophysiology. From its beginnings as the model organism for cancer research and transplantation biology to the present, when dissection of the genetic basis of complex disease is at the forefront of genomics research, an enormous and remarkable mouse resource infrastructure has accumulated. This review summarizes those resources and provides practical guidelines for their use, particularly in the analysis of quantitative traits.

Population and systems genetics analyses of cortisol in pigs divergently selected for stress

This study presents a systems genetic analysis on the physiology of cortisol in mice and pigs with an aim to show the potential of a comprehensive computational approach to quickly identify candidate genes and avoid a costly whole-genome quantitative trait locus (QTL) mapping. Population genetics analyses were performed on measurements of cortisol from a pig selection experiment. Expression QTL were mapped and gene networks were built using gene expressions for Crhr1 (corticotrophin-releasing hormone receptor) gene and single nucleotide polymorphisms from public mouse data. Results from mouse data were used to infer potential candidate regulatory genes involved in pig cortisol regulation, using a comparative or translational systems genetics approach. The pig data used were from a 10-yr divergent genetic selection experiment, providing data on 417 individuals. Population genetics analysis showed that cortisol is highly genetically determined with heritabilities of 0.40-0.70. Furthermore, a major gene with an additive effect of 86 ng/ml is segregating. Genetical-genomics investigations revealed two trans-acting eQTL for Crhr1 gene expression on chromosomes 2 and 13. Candidate gene search under trans-eQTL peaks yielded 63 genes for Crhr1 expression phenotypes. Functional links for Crhr1 genes with other genes/proteins in the gene network using mouse data were shown for the first 10 statistically significant genes involved. Results show translational or comparative systems genetics approaches reduce costs and time in large-scale genetics and "-omics" investigations. This is the first study to report a strong genetic basis for cortisol physiology using a systems approach.

Identification of atherosclerosis-modifying genes: pathogenic insights and therapeutic potential

Atherosclerosis is a common, complex trait, with genetic variation in many genes and the environment contributing to risk for this condition, which has multiple and highly variable phenotypic manifestations. Progress has been made in the identification of atherosclerosis-modifying genes in both human studies and through the use of animal models. In the future, it may be possible to administer a genetic test for variation in a handful of the most common atherosclerosis-modifier genes, and thus predict if a patient is likely to develop atherosclerosis and ischemic heart disease. These patients could then be treated aggressively to lower their low-density lipoprotein cholesterol levels and other risk factors, including counseling to make adjustments in their lifestyle. This review will summarize the methods and results thus far in the identification of atherosclerosis-modifier genes.

Genetics and heritability of coronary artery disease and myocardial infarction

A positive family history is frequently reported by patients with coronary artery disease (CAD) or myocardial infarction. For risk stratification, it is crucial to distinguish between accidental reoccurrence of sporadic cases and cases with a true heritable component of the conditions. A familial predisposition is assumed when a myocardial infarction is diagnosed by a male first degree relative before the 55th year of life or a female first degree relative before the 65th year of life. The current manuscript reviews major studies from which a familial risk of CAD or myocardial infarction can be inferred. Moreover, a brief overview summarizes the current results of molecular genetic research on chromosomal loci and genes relevant for CAD and myocardial infarction.

Identifying the genetic determinants of emotionality in humans; insights from rodents

The identification of human quantitative trait genes underlying variation in emotionality has proved to be both difficult and expensive. Whilst several QTL have been consistently identified and independently replicated across both phenotypes and populations, little progression has been made towards the identification of underlying genes or even variants. In this review we discuss the potential for using cross-species QTL concordance as a tool for QTL dissection in behaviour genetics, using an affect-related locus mapped to human chromosome 1 as an example.

Impact of chromosome 2 obesity loci on cardiovascular complications of insulin resistance in LDL receptor-deficient C57BL/6 mice

Previous characterization of mouse chromosome 2 identified genomic intervals that influence obesity, insulin resistance, and dyslipidemia. For this, resistant CAST/Ei (CAST) alleles were introgressed onto a susceptible C57BL/6J background to generate congenic strains with CAST alleles encompassing 67-162 Mb (multigenic obesity 6 [MOB6]) and 84-180 Mb (MOB5) from mouse chromosome 2. To examine the effects of each congenic locus on atherosclerosis and glucose disposal, we bred each strain onto a sensitizing LDL receptor-null (LDLR(-/-)) C57BL/6J background to predispose them to hypercholesterolemia and insulin resistance. LDLR(-/-) congenics and controls were characterized for measures of atherogenesis, insulin sensitivity, and obesity. We identified a genomic interval unique to the MOB6 congenic (72-84 Mb) that dramatically decreased atherosclerosis by approximately threefold and decreased insulin resistance. This region also reduced adiposity twofold. Conversely, the congenic region unique to MOB5 (162-180 Mb) increased insulin resistance but had little effect on atherosclerosis and adiposity. The MOB congenic intervals are concordant to human and rat quantitative trait loci influencing diabetes and atherosclerosis traits. Thus, our results define a strategy for studying the poorly understood interactions between diabetes and atherosclerosis and for identifying genes underlying the cardiovascular complications of insulin resistance.

QTL mapping for genetic determinants of lipoprotein cholesterol levels in combined crosses of inbred mouse strains

To identify additional loci that influence lipoprotein cholesterol levels, we performed quantitative trait locus (QTL) mapping in offspring of PERA/EiJxI/LnJ and PERA/EiJxDBA/2J intercrosses and in a combined data set from both crosses after 8 weeks of consumption of a high fat-diet. Most QTLs identified were concordant with homologous chromosomal regions that were associated with lipoprotein levels in human studies. We detected significant new loci for HDL cholesterol levels on chromosome (Chr) 5 (Hdlq34) and for non-HDL cholesterol levels on Chrs 15 (Nhdlq9) and 16 (Nhdlq10). In addition, the analysis of combined data sets identified a QTL for HDL cholesterol on Chr 17 that was shared between both crosses; lower HDL cholesterol levels were conferred by strain PERA. This QTL colocalized with a shared QTL for cholesterol gallstone formation detected in the same crosses. Haplotype analysis narrowed this QTL, and sequencing of the candidate genes Abcg5 and Abcg8 confirmed shared alleles in strains I/LnJ and DBA/2J that differed from the alleles in strain PERA/EiJ. In conclusion, our analysis furthers the knowledge of genetic determinants of lipoprotein cholesterol levels in inbred mice and substantiates the hypothesis that polymorphisms of Abcg5/Abcg8 contribute to individual variation in both plasma HDL cholesterol levels and susceptibility to cholesterol gallstone formation.

Pathogenesis of atherosclerosis

Atherosclerosis is a multifocal, smoldering, immunoinflammatory disease of medium-sized and large arteries fuelled by lipids. Endothelial cells, leukocytes, and intimal smooth muscle cells are the major players in the development of this disease. The most devastating consequences of atherosclerosis, such as heart attack and stroke, are caused by superimposed thrombosis. Therefore, the vital question is not why atherosclerosis develops but rather why atherosclerosis, after years of indolent growth, suddenly becomes complicated with luminal thrombosis. If thrombosis-prone plaques could be detected and thrombosis averted, atherosclerosis would be a much more benign disease. Approximately 76% of all fatal coronary thrombi are precipitated by plaque rupture. Plaque rupture is a more frequent cause of coronary thrombosis in men (approximately 80%) than in women (approximately 60%). Ruptured plaques are characterized by a large lipid-rich core, a thin fibrous cap that contains few smooth muscle cells and many macrophages, angiogenesis, adventitial inflammation, and outward remodeling. Plaque rupture is the most common cause of coronary thrombosis. Ruptured plaques and, by inference, rupture-prone plaques have characteristic pathoanatomical features that might be useful for their detection in vivo by imaging. This article describes the pathogenesis of atherosclerosis, how it begets thrombosis, and the possibility to detect thrombosis-prone plaques and prevent heart attack.

Nonconventional genetic risk factors for cardiovascular disease

Despite numerous advances made in identifying the genes for rare mendelian forms of cardiovascular disease (CVD), relatively little is known about the common, complex forms at the genetic level. Moreover, most genes that have been associated with CVD, whether they are single gene forms or more common forms of the disease, have primarily been involved in biochemical pathways related to what are considered "conventional" risk factors. However, recent genetic studies have begun to identify genes and pathways associated with CVD that would not be considered to underlie conventional risk factors. In this review, we discuss the evidence for this latter notion based on recent linkage and association studies in humans. As an example, we also illustrate how a combination of mouse and human genetics led to identification of the 5-lipoxygenase pathway for CVD, with potentially important implications for its treatment and diagnosis. We conclude with a discussion of the prospects for identifying CVD genes in the future and for potentially developing more effective therapeutic strategies.

Complex inheritance of the 5-lipoxygenase locus influencing atherosclerosis in mice

We previously mapped a locus on chromosome 6 with a large effect (LOD > 6) on aortic lesion size in a (C57BL/6J x CAST/Ei) F(2) cross and identified arachidonate 5-lipoxygenase (5LO) as a candidate gene in this region. Subsequent studies with the 5LO knockout model showed effects on atherosclerosis and aortic aneurysms. We now report detailed genetic analysis of the chromosome 6 locus. We created a panel of overlapping and reciprocal subcongenic lines from the B6.CAST Ldlr(-/-) chromosome 6 congenic strain (CON6.Ldlr(-/-)) and analyzed aortic lesion size in different subcongenic lines. Our results revealed that there are at least two subregions, designated as Ath37 and Ath38 that affect the size of aortic lesions independently of 5LO. We also showed that homozygote 5LO null mice develop smaller atherosclerotic lesions. We conclude that the relation between the mouse chromosome 6 locus and atherosclerosis is complex and is due to at least two genes with large effects within this region. This complexity should be considered when interpreting results of knockout studies.

Genetics of macrovascular complications in diabetes

Diabetic patients have a threefold higher risk of developing atherosclerosis and its clinical complications compared with nondiabetic individuals. Part of the cardiovascular risk associated with diabetes is probably due to genetic determinants influencing both glucose homeostasis and the development of atherosclerosis. However, type 2 diabetes frequently coexists with other cardiovascular risk factors such as arterial hypertension, central obesity, and dyslipidemia. Genetic variability affects many areas, such as lipid and energy metabolisms, hypertension and hemodynamic mechanisms, blood clotting homeostasis, inflammation, and matrix turnover in the vascular wall, and will have an impact on the development of macrovascular complications in diabetic patients. These individual susceptibility factors each contribute only a small increased risk interacting with environmental determinants.

Transcriptome profile of macrophages from atherosclerosis-sensitive and atherosclerosis-resistant mice

We performed a strain intercross between two apoE-deficient mouse strains with a large difference in lesion susceptibility and measured aortic root lesion area in 98 female F(2) progeny. Total RNA was prepared from bone marrow-derived macrophages, and RNA from the five mice with the smallest and largest lesions were used for microarray gene expression profiling. Remarkably, approximately 5% of the 12,288 expressed transcripts were differentially expressed in the atherosclerosis-susceptible and atherosclerosis-resistant bone marrow-derived macrophages (unadjusted p < 0.05), thus defining the transcriptome of macrophages associated with atherosclerosis susceptibility. Using more stringent criteria of twofold or greater change and p < 0.01, 116 and 70 transcripts were overexpressed in lesion-prone and lesion-resistant bone marrow-derived macrophages, respectively. Transcription factor binding site analysis identified two promoter elements that were found more often in the genes overexpressed in the large-lesion group, and one promoter element that was found more often in the small-lesion group. The combination of this expression profiling data with the genetic method of quantitative trait locus mapping should give powerful insights into the genes that affect atherosclerosis susceptibility in mice.

Psychiatric endophenotypes and the development of valid animal models

Endophenotypes are quantifiable components in the genes-to-behaviors pathways, distinct from psychiatric symptoms, which make genetic and biological studies of etiologies for disease categories more manageable. The endophenotype concept has emerged as a strategic tool in neuropsychiatric research. This emergence is due to many factors, including the modest reproducibility of results from studies directed toward etiologies and appreciation for the complex relationships between genes and behavior. Disease heterogeneity is often guaranteed, rather than simplified, through the current diagnostic system; inherent benefits of endophenotypes include more specific disease concepts and process definitions. Endophenotypes can be neurophysiological, biochemical, endocrine, neuroanatomical, cognitive or neuropsychological. Heritability and stability (state independence) represent key components of any useful endophenotype. Importantly, they characterize an approach that reduces the complexity of symptoms and multifaceted behaviors, resulting in units of analysis that are more amenable to being modeled in animals. We discuss the benefits of more direct interpretation of clinical endophenotypes by basic behavioral scientists. With the advent of important findings regarding the genes that predispose to psychiatric illness, we are at an important crossroads where, without anthropomorphizing, animal models may provide homologous components of psychiatric illness, rather than simply equating to similar (loosely analogized) behaviors, validators of the efficacy of current medications or models of symptoms. We conclude that there exists a need for increased collaboration between clinicians and basic scientists, the result of which should be to improve diagnosis, classification and treatment on one end and to increase the construct relevance of model organisms on the other.

Genomics of the future: identification of quantitative trait loci in the mouse

Positional cloning of quantitative trait loci in rodents is a common approach to identify genes involved in complex phenotypes, including genes important to human disease. However, cloning the causative genes has proved to be more difficult than determining their positions. New tools such as genomic sequence, clone libraries, and new genomic-based methods offer new approaches to identify these genes. Here we review how these new tools and approaches will improve our ability to discover the genes important in complex traits.

Meeting report for the 4th annual Complex Trait Consortium meeting: from QTLs to systems genetics

The fourth annual meeting of the Complex Trait Consortium (CTC) was held in Groningen, the Netherlands on June 26-29, 2005. This meeting, which set a new attendance record, followed three previous and highly successful meetings. The focus at this meeting was on continued resource development and the exiting new field of systems genetics, the integration and anchoring of multi-dimensional data-types to underlying genetic variation. A new aspect at the meeting was the three-minute, 'come see my poster' presentations that generated significant interaction at the poster sessions. If the 2005 meeting is an indicator of things to come, future meetings promise to offer even more exciting research efforts to integrate high-throughput biological measurements with genetic variation to unravel the mechanisms responsible for inter-individual variation.

Atherosclerosis susceptibility loci identified from a strain intercross of apolipoprotein E-deficient mice via a high-density genome scan

OBJECTIVE

Apolipoprotein (apo) E-deficient mice are hypercholesterolemic and develop atherosclerosis on low-fat chow diets; however, the genetic background strain has a large effect on atherosclerosis susceptibility. This study aimed to determine the genetic regions associated with strain effects on lesion area.

METHODS AND RESULTS

We performed a strain intercross between atherosclerosis sensitive DBA/2 and atherosclerosis resistant AKR apoE-deficient mice. Aortic root lesion area, total cholesterol, body weights, and complete blood counts were ascertained for 114 male and 95 female F2 progeny. A high-density genome scan was performed using a mouse single nucleotide polymorphism chip yielding 1967 informative polymorphic markers. Quantitative trait locus (QTL) statistical analyses were performed. Novel loci associated with lesion or log lesion area were identified for the female and male F2 cohorts. The atherosclerosis QTLs in female mice reside on chromosomes 15, 5, 3, and 13, and in male mice on chromosomes 17, 18, and 2. QTL were also identified for body weight, total cholesterol, and blood count parameters.

CONCLUSIONS

Loci were identified for atherosclerosis susceptibility in a strain intercross study. The identity of the responsible genes at these loci remains to be determined.

Diet and murine atherosclerosis

Lipid-enriched diets are often used to induce or accelerate the rate of atherosclerotic lesion development in murine models of atherosclerosis. It appears that the induction of persistent hypercholesterolemia to levels > or approximately to 300 mg/dL is required for the development of experimental atherosclerosis in the mouse. A variety of different diets have been used that vary in the level of cholesterol, the level and type of fatty acid, and the absence or presence of cholate. Each of these components as well as the protein source has been shown to influence lipoprotein level and/or atherosclerosis, with dietary cholesterol being the major proatherogenic component. In some instances the effects of these components on the expression of hepatic genes relevant to lipid homeostasis has been observed. An appreciation of the effect of the differences in diet composition on these processes is important to compare results from different atherosclerosis studies, so the composition of the diets used should always be reported or referenced. Cholate should not be used unless its effects are being specifically investigated.

Bioinformatics toolbox for narrowing rodent quantitative trait loci

Quantitative trait locus (QTL) analysis is a powerful method for localizing disease genes, but identifying the causal gene remains difficult. Rodent models of disease facilitate QTL gene identification, and causal genes underlying rodent QTL are often associated with the corresponding human diseases. Recently developed bioinformatics methods, including comparative genomics, combined cross analysis, interval-specific and genome-wide haplotype analysis, followed by sequence and expression analysis, each facilitated by public databases, provide new tools for narrowing rodent QTLs. Here we discuss each tool, illustrate its application and generate a bioinformatics strategy for narrowing QTLs. Combining these bioinformatics tools with classical experimental methods should accelerate QTL gene identification.