Quantitative trait loci influencing cholesterol and phospholipid phenotypes map to chromosomes that contain genes regulating blood pressure in the spontaneously hypertensive rat

Research on Experimental Hypertension in Prague (1966-2009)

The study of ontogenetic aspects of water and electrolyte metabolism performed in the Institute of Physiology (Czechoslovak Academy of Sciences) led to the research on the increased susceptibility of immature rats to salt-dependent forms of hypertension since 1966. Hemodynamic studies in developing rats paved the way to the evaluation of hemodynamic mechanisms during the development of genetic hypertension in SHR. A particular attention was focused on altered renal function and kidney damage in both salt and genetic hypertension with a special respect to renin-angiotensin system. Renal damage associated with hypertension progression was in the center of interest of several research groups in Prague. The alterations in ion transport, cell calcium handling and membrane structure as well as their relationship to abnormal lipid metabolism were studied in a close cooperation with laboratories in Munich, Glasgow, Montreal and Paris. The role of NO and oxidative stress in various forms of hypertension was a subject of a joint research with our Slovak colleagues focused mainly on NO-deficient hypertension elicited by chronic L-NAME administration. Finally, we adopted a method enabling us to evaluate the balance of vasoconstrictor and vasodilator mechanisms in BP maintenance. Using this method we demonstrated sympathetic hyperactivity and relative NO deficiency in rats with either salt-dependent or genetic hypertension. At the end of the first decennium of this century we were ready to modify our traditional approach towards modern trends in the research of experimental hypertension. Keywords: Salt-dependent hypertension o Genetic hypertension o Body fluids o Hemodynamics o Ion transport o Cell membrane structure and function o Renal function o Renin-angiotensin systems.

Nutrigenetic Interaction of Spontaneously Hypertensive Rat Chromosome 20 Segment and High-Sucrose Diet Sensitizes to Metabolic Syndrome

Several corresponding regions of human and mammalian genomes have been shown to affect sensitivity to the manifestation of metabolic syndrome via nutrigenetic interactions. In this study, we assessed the effect of sucrose administration in a newly established congenic strain BN.SHR20, in which a limited segment of rat chromosome 20 from a metabolic syndrome model, spontaneously hypertensive rat (SHR), was introgressed into Brown Norway (BN) genomic background. We mapped the extent of the differential segment and compared the genomic sequences of BN vs. SHR within the segment in silico. The differential segment of SHR origin in BN.SHR20 spans about 9 Mb of the telomeric portion of the short arm of chromosome 20. We identified non-synonymous mutations e.g., in ApoM, Notch4, Slc39a7, Smim29 genes and other variations in or near genes associated with metabolic syndrome in human genome-wide association studies. Male rats of BN and BN.SHR20 strains were fed a standard diet for 18 weeks (control groups) or 16 weeks of standard diet followed by 14 days of high-sucrose diet (HSD). We assessed the morphometric and metabolic profiles of all groups. Adiposity significantly increased only in BN.SHR20 after HSD. Fasting glycemia and the glucose levels during the oral glucose tolerance test were higher in BN.SHR20 than in BN groups, while insulin levels were comparable. The fasting levels of triacylglycerols were the highest in sucrose-fed BN.SHR20, both compared to the sucrose-fed BN and the control BN.SHR20. The non-esterified fatty acids and total cholesterol concentrations were higher in BN.SHR20 compared to their respective BN groups, and the HSD elicited an increase in non-esterified fatty acids only in BN.SHR20. In a new genetically defined model, we have isolated a limited genomic region involved in nutrigenetic sensitization to sucrose-induced metabolic disturbances.

An evolutionarily-conserved promoter allele governs HMG-CoA reductase expression in spontaneously hypertensive rat

3-Hydroxy-3-methyl glutaryl-coenzyme A reductase (Hmgcr) encodes the rate-limiting enzyme in the cholesterol biosynthesis pathway. The regulation of Hmgcr in rat models of genetic hypertension (viz. Spontaneously Hypertensive Rat [SHR] and its normotensive control Wistar/Kyoto [WKY] strain) is unclear. Interestingly, Hmgcr transcript and protein levels are diminished in liver tissues of SHR as compared to WKY. This observation is consistent with the diminished plasma cholesterol level in SHR animals. However, the molecular basis of these apparently counter-intuitive findings remains completely unknown. Sequencing of the Hmgcr promoter in SHR and WKY strains reveals three variations: A-405G, C-62T and a 11 bp insertion (-398_-388insTGCGGTCCTCC) in SHR. Among these variations, A-405G occurs at an evolutionarily-conserved site among many mammals. Moreover, SHR-Hmgcr promoter displays lower activity than WKY-Hmgcr promoter in various cell lines. Transient transfections of Hmgcr-promoter mutants and in silico analysis suggest altered binding of Runx3 and Srebf1 across A-405G site. On the other hand, C-62T and -398_-388insTGCGGTCCTCC variations do not appear to contribute to the reduced Hmgcr promoter activity in SHR as compared to WKY. Indeed, chromatin immunoprecipitation assays confirm differential binding of Runx3 and Srebf1 to Hmgcr promoter leading to reduced expression of Hmgcr in SHR as compared to WKY under basal as well as cholesterol-modulated conditions. Taken together, this study provides, for the first time, molecular basis for diminished Hmgcr expression in SHR animals, which may account for the reduced circulating cholesterol level in this widely-studied model for cardiovascular diseases.

Complement Factor B Is a Determinant of Both Metabolic and Cardiovascular Features of Metabolic Syndrome

CFB (complement factor B) is elevated in adipose tissue and serum from patients with type 2 diabetes mellitus and cardiovascular disease, but the causal relationship to disease pathogenesis is unclear. Cfb is also elevated in adipose tissue and serum of the spontaneously hypertensive rat, a well-characterized model of metabolic syndrome. To establish the role of CFB in metabolic syndrome, we knocked out the Cfb gene in the spontaneously hypertensive rat. Cfb-/- rats showed improved glucose tolerance and insulin sensitivity, redistribution of visceral to subcutaneous fat, increased adipocyte mitochondrial respiration, and marked changes in gene expression. Cfb-/- rats also had lower blood pressure, increased ejection fraction and fractional shortening, and reduced left ventricular mass. These changes in metabolism and gene expression, in adipose tissue and left ventricle, suggest new adipose tissue-intrinsic and blood pressure-independent mechanisms for insulin resistance and cardiac hypertrophy in the spontaneously hypertensive rat. In silico analysis of the human CFB locus revealed 2 cis-regulated expression quantitative trait loci for CFB expression significantly associated with visceral fat, circulating triglycerides and hypertension in genome-wide association studies. Together, these data demonstrate a key role for CFB in the development of spontaneously hypertensive rat metabolic syndrome phenotypes and of related traits in humans and indicate the potential for CFB as a novel target for treatment of cardiometabolic disease.

Serum Triglyceride Lowering Effect of Cilnidipine in Patients With Essential Hypertension

Background

Many epidemiological studies have established the relationship between hypertension and dyslipidemia. Calcium channel blockers (CCBs) are one of the first-line drugs for newly diagnosed patients with essential hypertension. Cilnidipine as a newer CCB acting by blocking both L- and N-type calcium channels possesses additional beneficial effects apart from lowering blood pressure (BP). The aim of this study was to evaluate the effectiveness of cilnidipine in patients with essential hypertension with borderline dyslipidemia and its effects on lipid profile.

Methods

Out of 45 enrolled patients, who fulfilled the inclusion criteria, only 37 completed the study. Cilnidipine was started at 10 mg/day, and then adjusted to 5 - 20 mg/day to achieve the target blood pressure.

Results

After 12 weeks of study, patients showed significant reduction in systolic blood pressure, diastolic blood pressure, mean BP, heart rate and serum triglyceride level from baseline values (P < 0.00).

Conclusion

In clinical setting where both hypertension and hypertriglyceridemia exist, cilnidipine can be a promising drug of choice.

Atherogenic Risk Assessment among Persons Living in Rural Uganda

Background. Hypertension and dyslipidemia are independent risk factors for coronary heart disease and commonly coexist. Cardiovascular risk can be reliably predicted using lipid ratios such as the atherogenic index, a useful prognostic parameter for guiding timely interventions. Objective. We assessed the cardiovascular risk profile based on the atherogenic index of residents within a rural Ugandan cohort. Methods. In 2011, a population based survey was conducted among 7507 participants. Sociodemographic characteristics, physical measurements (blood pressure, weight, height, and waist and hip circumference), and blood sampling for nonfasting lipid profile were collected for each participant. Atherogenic risk profile, defined as logarithm base ten of (triglyceride divided by high density lipoprotein cholesterol), was categorised as low risk (<0.1), intermediate risk (0.1-0.24), and high risk (>0.24). Results. Fifty-five percent of participants were female and the mean age was 49.9 years (SD ± 20.2). Forty-two percent of participants had high and intermediate atherogenic risk. Persons with hypertension, untreated HIV infection, abnormal glycaemia, and obesity and living in less urbanised villages were more at risk. Conclusion. A significant proportion of persons in this rural population are at risk of atherosclerosis. Key identified populations at risk should be considered for future intervention against cardiovascular related morbidity and mortality. The study however used parameters from unfasted samples that may have a bearing on observed results.

Atherogenic Risk Assessment among Persons Living in Rural Uganda

Background. Hypertension and dyslipidemia are independent risk factors for coronary heart disease and commonly coexist. Cardiovascular risk can be reliably predicted using lipid ratios such as the atherogenic index, a useful prognostic parameter for guiding timely interventions.Objective. We assessed the cardiovascular risk profile based on the atherogenic index of residents within a rural Ugandan cohort.Methods. In 2011, a population based survey was conducted among 7507 participants. Sociodemographic characteristics, physical measurements (blood pressure, weight, height, and waist and hip circumference), and blood sampling for nonfasting lipid profile were collected for each participant. Atherogenic risk profile, defined as logarithm base ten of (triglyceride divided by high density lipoprotein cholesterol), was categorised as low risk (<0.1), intermediate risk (0.1–0.24), and high risk (>0.24).Results. Fifty-five percent of participants were female and the mean age was 49.9 years (SD±20.2). Forty-two percent of participants had high and intermediate atherogenic risk. Persons with hypertension, untreated HIV infection, abnormal glycaemia, and obesity and living in less urbanised villages were more at risk.Conclusion. A significant proportion of persons in this rural population are at risk of atherosclerosis. Key identified populations at risk should be considered for future intervention against cardiovascular related morbidity and mortality. The study however used parameters from unfasted samples that may have a bearing on observed results.

Chromosomal assignment of quantitative trait loci influencing baseline circulating total cholesterol level in male laboratory mice: report of a consomic strain survey and comparison with published results

Background

An important risk for atherosclerosis is a low level of HDL cholesterol. Baseline HDL cholesterol is under complex genetic and environmental control. Here we report on results of male mice from a consomic strain survey and the parental inbred strains for baseline circulating total cholesterol concentration, which is almost the same as HDL cholesterol in chow fed mice. The consomic strains have been derived from C57BL/6J (host strain) and A/J (donor strain) inbred lines. The work contributes to the value of the mouse as an animal model for studying the genetic background of differences in baseline circulating total and HDL cholesterol levels.

Results

The consomic strain survey suggested that mouse chromosomes 1, 7, 9, 14, 16, 17, 19, X, and Y contained at least one quantitative trait locus that is involved in baseline circulating total cholesterol concentration. All consomic lines, for which there is evidence that the substituted chromosome contains a quantitative trait locus, increase compared to the host strain baseline circulating total cholesterol concentration. Since there is evidence that ‘body weight’, ‘age at blood sampling’, ‘time of the day blood was collected’, and ‘season’ influence this phenotype, additional statistical analyses (with these variables as covariates) were performed. Now there is only evidence for quantitative trait loci on chromosomes 1, 8, 12, and Y. Taken the present results together with previous consomic strain surveys there is evidence that all mouse chromosomes carry quantitative trait loci that control baseline circulating total cholesterol levels. There was however little agreement between the present consomic strain results and previous sets of data. This might be explained by seasonal effects and differences in methodological variables such as age of the mice, fasting versus non-fasting, percentage of dietary fat, unanesthetized versus anesthetized mice, and the daily light–dark cycle.

Conclusions

The present findings, when compared with previous consomic strain surveys, clearly illustrate the complexity of the genetic-environmental architecture for the regulation of baseline circulating total cholesterol levels in mice. Different data can be obtained from different labs and it underscores that animal geneticists should present as accurate a picture as possible of the laboratory mouse’s environment.

Genomic determinants of triglyceride and cholesterol distribution into lipoprotein fractions in the rat

The plasma profile of major lipoprotein classes and its subdivision into particular fractions plays a crucial role in the pathogenesis of atherosclerosis and is a major predictor of coronary artery disease. Our aim was to identify genomic determinants of triglyceride and cholesterol distribution into lipoprotein fractions and lipoprotein particle sizes in the recombinant inbred rat set PXO, in which alleles of two rat models of the metabolic syndrome (SHR and PD inbred strains) segregate together with those from Brown Norway rat strain. Adult male rats of 15 PXO strains (n = 8–13/strain) and two progenitor strains SHR-Lx (n = 13) and BXH2/Cub (n = 18) were subjected to one-week of high-sucrose diet feeding. We performed association analyses of triglyceride (TG) and cholesterol (C) concentrations in 20 lipoprotein fractions and the size of major classes of lipoprotein particles utilizing 704 polymorphic microsatellite markers, the genome-wide significance was validated by 2,000 permutations per trait. Subsequent in silico focusing of the identified quantitative trait loci was completed using a map of over 20,000 single nucleotide polymorphisms. In most of the phenotypes we identified substantial gradient among the strains (e.g. VLDL-TG from 5.6 to 66.7 mg/dl). We have identified 14 loci (encompassing 1 to 65 genes) on rat chromosomes 3, 4, 7, 8, 11 and 12 showing suggestive or significant association to one or more of the studied traits. PXO strains carrying the SHR allele displayed significantly higher values of the linked traits except for LDL-TG and adiposity index. Cholesterol concentrations in large, medium and very small LDL particles were significantly associated to a haplotype block spanning part of a single gene, low density lipoprotein receptor-related protein 1B (Lrp1b). Using genome-wide association we have identified new genetic determinants of triglyceride and cholesterol distribution into lipoprotein fractions in the recombinant inbred panel of rat model strains.

LIPITENSION: Interplay between dyslipidemia and hypertension

The burden of cardiovascular disease (CVD) is increasing worldwide. The increase in the burden is a major concern in developing countries like India. It is well-established that hypertension and dyslipidemia are the two major contributing risk factors for CVD. Various epidemiological studies have shown the prevalence of the co-existence of hypertension and dyslipidemia, in the range of 15 to 31%. The co-existence of the two risk factors has more than an additive adverse impact on the vascular endothelium, which results in enhanced atherosclerosis, leading to CVD. This review emphasizes on the 'co-existence and interplay of dyslipidemia and hypertension'. The authors have termed the co-existence as, 'LIPITENSION'. The term LIPITENSION may help clinicians in easy identification and aggressive management of the two conditions together, ultimately preventing future cardiovascular events.

Integrated genomic approaches to identification of candidate genes underlying metabolic and cardiovascular phenotypes in the spontaneously hypertensive rat

The spontaneously hypertensive rat (SHR) is a widely used rodent model of hypertension and metabolic syndrome. Previously we identified thousands of cis-regulated expression quantitative trait loci (eQTLs) across multiple tissues using a panel of rat recombinant inbred (RI) strains derived from Brown Norway and SHR progenitors. These cis-eQTLs represent potential susceptibility loci underlying physiological and pathophysiological traits manifested in SHR. We have prioritized 60 cis-eQTLs and confirmed differential expression between the parental strains by quantitative PCR in 43 (72%) of the eQTL transcripts. Quantitative trait transcript (QTT) analysis in the RI strains showed highly significant correlation between cis-eQTL transcript abundance and clinically relevant traits such as systolic blood pressure and blood glucose, with the physical location of a subset of the cis-eQTLs colocalizing with “physiological” QTLs (pQTLs) for these same traits. These colocalizing correlated cis-eQTLs (c3-eQTLs) are highly attractive as primary susceptibility loci for the colocalizing pQTLs. Furthermore, sequence analysis of the c3-eQTL genes identified single nucleotide polymorphisms (SNPs) that are predicted to affect transcription factor binding affinity, splicing and protein function. These SNPs, which potentially alter transcript abundance and stability, represent strong candidate factors underlying not just eQTL expression phenotypes, but also the correlated metabolic and physiological traits. In conclusion, by integration of genomic sequence, eQTL and QTT datasets we have identified several genes that are strong positional candidates for pathophysiological traits observed in the SHR strain. These findings provide a basis for the functional testing and ultimate elucidation of the molecular basis of these metabolic and cardiovascular phenotypes.

Refined mapping of the renal failure RF-3 quantitative trait locus

Rf-3, a quantitative trait locus (QTL) on rat chromosome 3, affects the development of CKD in Fawn-Hooded Hypertensive (FHH) rats. This QTL spans 110 Mb and approximately 1400 genes; therefore, narrowing the position of this locus is necessary to elucidate potential candidate genes. Here, we used congenic models and comparative genomics to refine the Rf-3 candidate region. We generated congenic lines carrying smaller intervals (subcongenics) of the Rf-3 region and used these lines to reduce the Rf-3 candidate region by 94% (to 7.1 Mb). We used comparative genomics to identify QTL for both nephropathy and albuminuria in the syntenic region of this interval for both human and mouse. We also used the overlapping homologous regions to reduce the number of likely positional candidate genes to 13 known or predicted genes. By combining congenic models and cross-species studies, we narrowed the list of candidate genes to a level that we could sequence the whole interval to further identify the causative gene in future studies.

Use of rat genomics for investigating the metabolic syndrome

The spontaneously hypertensive rat (SHR) is the most widely used animal model of essential hypertension and accompanying metabolic disturbances. In this model, the use of whole genome sequencing and gene expression profiling techniques, linkage and correlation analyses in recombinant inbred strains, and in vitro and in vivo functional studies in congenic and transgenic lines has recently enabled molecular identification of quantitative trait loci (QTLs) relevant to the metabolic syndrome: (1) a deletion variant in Cd36 (fatty acid translocase) responsible for QTLs on chromosome 4 associated with dyslipidemia, insulin resistance and hypertension, (2) mutated Srebf1 (sterol regulatory element binding factor 1) as a QTL on chromosome 10 influencing dietary-induced changes in hepatic cholesterol levels, and (3) Ogn (osteoglycin) as a QTL on chromosome 17 associated with left ventricular hypertrophy. In addition, selective replacement of the mitochondrial genome of the SHR with the mitochondrial genome of the Brown Norway rat influenced several major metabolic risk factors for type 2 diabetes and provided evidence that spontaneous variation in the mitochondrial genome per se can promote systemic metabolic disturbances relevant to the pathogenesis of metabolic syndrome. Owing to recent progress in the development of rat genomic resources, the pace of QTL identification and discovery of new disease mechanisms can be expected to accelerate in the near future.

Molecular Genetics of Experimental Hypertension and the Metabolic Syndrome

Genetic studies of human and experimental hypertension provide a means to identify key pathways that predispose individuals to increased blood pressure and associated risk factors for cardiovascular and metabolic diseases. The pathways so identified can then serve as targets for therapeutic intervention. This article discusses genetic studies in animal models of hypertension in which specific genes have been identified that regulate blood pressure and biochemical features of the metabolic syndrome. Consistent with studies in humans with monogenic disorders of blood pressure regulation, studies in rat models have demonstrated that naturally occurring genetic variation in pathways regulating sodium chloride transport can contribute to inherited variation in blood pressure. Such studies have also indicated that naturally occurring variation in genes, such as Cd36, that regulate fatty acid metabolism and ectopic accumulation of fat and fat metabolites can influence both biochemical and hemodynamic features of the metabolic syndrome and mediate the antidiabetic effects of drugs that activate the peroxisome proliferator-activated receptor-gamma. Angiotensin II receptor blockers with the ability to selectively modulate activity of peroxisome proliferator-activated receptor-gamma and expression of genes in these fat metabolism pathways may represent useful prototypes for a new class of transcription modulating drugs aimed at treating patients with hypertension and the metabolic syndrome.

Endotoxin-induced acute lung injury is enhanced in rats with spontaneous hypertension

1. Acute lung injury (ALI), or acute respiratory distress syndrome, is a major cause of mortality in endotoxaemia. The present study tested whether the endotoxaemia-induced changes and associated ALI were enhanced in rats with established hypertension and to examine the possible mechanisms involved. 2. Fifty spontaneously hypertensive rats (SHR) and the same number of normotensive Wistar Kyoto (WKY) rats, aged 12-15 weeks, were used. The experiments were performed in conscious, unanaesthetized rats. Endotoxaemia was produced by intravenous lipopolysaccharide (LPS; 10 mg/kg). N(G)-Nitro-L-arginine methyl ester (L-NAME; 10 mg/kg, i.v.), L-N(6)-(1-iminoethyl)-lysine (L-Nil; 5 mg/kg, i.v.) and 3-morpholinosydnonimine (SIN-1; 5 mg/kg, i.v.) were given 5 min before LPS to observe the effects of nitric oxide synthase (NOS) inhibition and nitric oxide (NO) donation. 3. We monitored arterial pressure and heart rate and evaluated ALI by determining the lung weight/bodyweight ratio, lung weight gain, leakage of Evans blue dye, the protein concentration in bronchoalveolar lavage and histopathological examination. Plasma nitrate/nitrite, methyl guanidine, pro-inflammatory cytokines, including tumour necrosis factor-alpha and interleukin-1beta, and lung tissue cGMP were determined. Expression of mRNA for inducible and endothelial NOS was examined using reverse transcription-polymerase chain reaction. 4. Lipopolysaccharide caused systemic hypotension, ALI and increases in plasma nitrate/nitrite, methyl guanidine, pro-inflammatory cytokines and lung cGMP content. The LPS-induced changes were greater in SHR than in WKY rats. Pretreatment with L-NAME or L-Nil attenuated, whereas the NO donor SIN-1 aggravated, the endotoxin-induced changes. 5. In conclusion, rats with genetic hypertension are more susceptible to endotoxaemia and this results in a greater extent of ALI compared with normotensive WKY rats.

Genetic control of plasma lipid levels in a cross derived from normoglycaemic Brown Norway and spontaneously diabetic Goto-Kakizaki rats

AIMS/HYPOTHESIS

Dyslipidaemia is a main component of the insulin resistance syndrome. The inbred Goto-Kakizaki (GK) rat is a model of spontaneous type 2 diabetes and insulin resistance, which has been used to identify diabetes-related susceptibility loci in genetic crosses. The objective of our study was to test the genetic control of lipid metabolism in the GK rat and investigate a possible relationship with known genetic loci regulating glucose homeostasis in this strain.

MATERIALS AND METHODS

Plasma concentration of triglycerides, phospholipids, total cholesterol, HDL, LDL and VLDL cholesterol were determined in a cohort of 151 hybrids of an F2 cross derived from GK and non-diabetic Brown Norway (BN) rats. Data from the genome-wide scan of the F2 hybrids were used to test for evidence of genetic linkage to the lipid quantitative traits.

RESULTS

We identified statistically significant quantitative trait loci (QTLs) that control the level of plasma phospholipids and triglycerides (chromosome 1), LDL cholesterol (chromosome 3) and total and HDL cholesterol (chromosomes 1 and 5). These QTLs do not coincide with previously identified diabetes susceptibility loci in a similar cross. The significance of lipid QTLs mapped to chromosomes 1 and 5 is strongly influenced by sex.

CONCLUSION/INTERPRETATION

We established that several genetic loci control the quantitative variations of plasma lipid variables in a GKxBN cross. They appear to be distinct from known GK diabetes QTLs, indicating that lipid metabolism and traits directly relevant to glucose and insulin regulation are controlled by different gene variants in this strain combination.

Statins: another class of antihypertensive agents?

The assessment of global cardiovascular risk is an essential step in the management of atherosclerotic disease prevention. Among the risk factors to be addressed are hypertension and hyperlipidaemia; these commonly coexist. A neutral or lipid-friendly antihypertensive agent is probably useful in the presence of lipid abnormalities. Similarly, statins have been shown to decrease cardiovascular risk in hypertensive patients. There is also experimental and clinical evidence that statins have blood pressure (BP)-lowering effects. In this review, we discuss the beneficial effects of statins on BP, and provide an overview of the underlying pathophysiology. We also consider the evidence justifying the use of statins in the management of hypertensive patients.

Genetic analysis of complex cardiovascular traits in the spontaneously hypertensive rat

Identification of the genetic determinants of common diseases is a major challenge for current biomedical research. Combining linkage analyses of essentially monogenic cis-regulated expression phenotypes with oligogenic intermediate physiological phenotypes represents a promising approach for identification of quantitative trait loci at the molecular level. In the present review, a genetic analysis of cardiovascular phenotypes studied at several levels of complexity in rat recombinant inbred strains is described.

The therapeutic potential of targeting drugs at transcription factors

The pivotal role of gene transcription in a plethora of biological processes indicates that transcription represents a suitable target for potential therapeutic intervention. Ultimately, the pathophysiology of numerous human disease processes must be understood in terms of changes in gene expression within relevant body cell types. There is mounting evidence that genetic variation in transcription factors and/or their binding-site sequences, as well as environmentally induced malfunctioning of these proteins, contribute to common multifactorial disorders including cancer, diabetes, ischaemic heart disease and neural disorders. Even in 'non-inheritable' infectious diseases, alterations of host-cell transcriptional regulation play an important role in pathogenesis. The enormous progress in understanding the mechanisms of transcriptional control offers hope for the development of a new generation of drugs. Such compounds could be specifically designed to modulate either the synthesis of transcription factors, the regulation of their activity by small-molecule bioligands or phosphorylation events, their interactions with activator/repressor proteins or their binding to DNA. Given the remarkable specificity of this approach, it is anticipated that these agents will provide superior tools for the prevention and treatment of a diverse panel of clinical disorders in the not too distant future.

Integrated transcriptional profiling and linkage analysis for identification of genes underlying disease

Integration of genome-wide expression profiling with linkage analysis is a new approach to identifying genes underlying complex traits. We applied this approach to the regulation of gene expression in the BXH/HXB panel of rat recombinant inbred strains, one of the largest available rodent recombinant inbred panels and a leading resource for genetic analysis of the highly prevalent metabolic syndrome. In two tissues important to the pathogenesis of the metabolic syndrome, we mapped cis- and trans-regulatory control elements for expression of thousands of genes across the genome. Many of the most highly linked expression quantitative trait loci are regulated in cis, are inherited essentially as monogenic traits and are good candidate genes for previously mapped physiological quantitative trait loci in the rat. By comparative mapping we generated a data set of 73 candidate genes for hypertension that merit testing in human populations. Mining of this publicly available data set is expected to lead to new insights into the genes and regulatory pathways underlying the extensive range of metabolic and cardiovascular disease phenotypes that segregate in these recombinant inbred strains.

Use of a dense single nucleotide polymorphism map for in silico mapping in the mouse

Rapid expansion of available data, both phenotypic and genotypic, for multiple strains of mice has enabled the development of new methods to interrogate the mouse genome for functional genetic perturbations. In silico mapping provides an expedient way to associate the natural diversity of phenotypic traits with ancestrally inherited polymorphisms for the purpose of dissecting genetic traits. In mouse, the current single nucleotide polymorphism (SNP) data have lacked the density across the genome and coverage of enough strains to properly achieve this goal. To remedy this, 470,407 allele calls were produced for 10,990 evenly spaced SNP loci across 48 inbred mouse strains. Use of the SNP set with statistical models that considered unique patterns within blocks of three SNPs as an inferred haplotype could successfully map known single gene traits and a cloned quantitative trait gene. Application of this method to high-density lipoprotein and gallstone phenotypes reproduced previously characterized quantitative trait loci (QTL). The inferred haplotype data also facilitates the refinement of QTL regions such that candidate genes can be more easily identified and characterized as shown for adenylate cyclase 7.

Enhanced insulin secretion and cholesterol metabolism in congenic strains of the spontaneously diabetic (Type 2) Goto Kakizaki rat are controlled by independent genetic loci in rat chromosome 8

AIMS/HYPOTHESIS

Genetic investigations in the spontaneously diabetic (Type 2) Goto Kakizaki (GK) rat have identified quantitative trait loci (QTL) for diabetes-related phenotypes. The aims of this study were to refine the chromosomal mapping of a QTL ( Nidd/gk5) identified in chromosome 8 of the GK rat and to define a pathophysiological profile of GK gene variants underlying the QTL effects in congenics.

METHODS

Genetic linkage analysis was carried out with chromosome 8 markers genotyped in a GKxBN F2 intercross previously used to map diabetes QTL. Two congenic strains were designed to contain GK haplotypes in the region of Nidd/gk5 transferred onto a Brown Norway (BN) genetic background, and a broad spectrum of diabetes phenotypes were characterised in the animals.

RESULTS

Results from QTL mapping suggest that variations in glucose-stimulated insulin secretion in vivo, and in body weight are controlled by different chromosome 8 loci (LOD3.53; p=0.0004 and LOD4.19; p=0.00007, respectively). Extensive physiological screening in male and female congenics at 12 and 24 weeks revealed the existence of GK variants at the locus Nidd/gk5, independently responsible for significantly enhanced insulin secretion and increased levels of plasma triglycerides, phospholipids and HDL, LDL and total cholesterol. Sequence polymorphisms detected between the BN and GK strains in genes encoding ApoAI, AIV, CIII and Lipc do not account for these effects.

CONCLUSIONS/INTERPRETATION

We refined the localisation of the QTL Nidd/gk5 and its pathophysiological characteristics in congenic strains derived for the locus. These congenic strains provide novel models for testing the contribution of a subset of GK alleles on diabetes phenotypes and for identifying diabetes susceptibility genes.

Influence of sex and diet on quantitative trait loci for HDL cholesterol levels in an SM/J by NZB/BlNJ intercross population

To investigate the dependence of HDL quantitative trait loci (QTL) on sex and diet, we generated a large intercross population of mice from parental strains SM/J and NZB/BlNJ. We measured HDL levels in progeny fed a chow diet and measured them again after 6, 12, and 16 weeks of feeding a high-fat, high-cholesterol diet. QTL analysis was performed on the 260 female and 253 male F(2) progeny. A total of 13 significant QTL were found. Four QTL were specific to female mice: Hdlq23 (Chr 6, 26 cM), Hdlq26 (Chr 10, 70 cM), Hdlq27 (Chr 15, 48 cM), and Hdlq32 (Chr 19, 40 cM). One significant QTL was specific to male mice: Hdlq29 (Chr 17, 36 cM). In addition, several QTL were found to have effects that were dependent on diet. Sex- and diet-dependent effects were characterized using a linear model-based genome scan method that avoids the potential pitfalls of subdivided data analysis. The dependence of QTL effects on sex suggests an important role for the sex hormones in HDL regulation. We recommend that sex should be explicitly accounted for in future studies in the genetics of HDL regulation in both mice and humans.

Genome-Wide Scan Identifies Novel QTLs for Cholesterol and LDL Levels in F2[Dahl R×S]-Intercross Rats

Hypercholesterolemia is a significant risk factor for coronary artery disease development. Genes influencing nonmonogenic hypercholesterolemia susceptibility in humans remain to be identified. Animal models are key investigative systems because major confounding variables such as diet, activity, and genetic background can be controlled. We performed a 121-marker, total genome-analysis of an F2[Dahl RxS]-intercross selected for contrasting parental strain susceptibilities for hyperlipidemia on regular rat diets at 6 months of age. Quantitative traits studied were plasma total cholesterol, triglyceride, HDL, and LDL levels adjusted for obesity. Genome-wide analysis of 200 F2-intercross male rats detects two QTLs with highly significant linkage for total cholesterol (TC) on chromosome (chr) 5-133.3 Mbp (LOD 5.8), and chr5-54.2 Mbp (LOD 4.8), and two QTLs with significant linkage for TC: on chromosome 8, chr8-60.4 Mbp (LOD 3.8), and chromosome 2, chr2-243.5 Mbp (LOD 3.4). A QTL for LDL with significant linkage is detected on chromosome 5, chr5-104 Mbp (LOD 3.7). These QTLs contribute from 7% to 12% of total trait variance, respectively, with Dahl-S allele effects resulting in increased TC and LDL levels consistent with hyperlipidemia susceptibility in the parental Dahl-S rat strain. Predicted QTL-peaks do not coincide with previous genome scans. Human homologues of two TC-QTLs span genes listed in a LocusLink profile for cholesterol. Only suggestive loci were detected for HDL and total triglyceride levels. Altogether, the data demonstrates the contribution of multiple QTLs to hypercholesterolemia making a multipathway pathogenic framework imperative. QTL-peak candidate genes delineated are syntenic between rat and human genomes, increasing clinical relevance and mandating further study.

Cytokine mRNA expression in unilateral ischemic-reperfused rat lung with salt solution supplemented with low-endotoxin or standard bovine serum albumin

Our aim was to determine whether cytokine mRNA expression is induced by experimental manipulation including artificial perfusate or ischemia-reperfusion (I/R) in an isolated, perfused rat lung model. Constant pulmonary flow [Krebs-Henseleit solution supplemented with lowendotoxin (LE) or standard (ST) bovine serum albumin 4%, 0.04 ml/g body wt] and ventilation were maintained throughout. Right and left pulmonary arteries were isolated, and the left pulmonary artery was occluded for 60 min and then reperfused for 30 min. Analysis of tumor necrosis factor-α, IL-1β, IL-6, IL-10, and IFN-γ mRNA expression by RT-PCR and evaluation of vascular permeability by bronchoalveolar lavage (BAL) fluid albumin content were conducted separately in right and left lung. Both LE and ST groups (each 12 rats) showed increases in vascular permeability by I/R (BAL fluid albumin content: 5.53 ± 1.55 vs. 15.63 ± 8.87 and 4.76 ± 2.71 vs. 16.72 ± 4.85 mg·ml BAL fluid-1·g lung dry wt-1, mean ± SD; right vs. left lung in LE and ST groups, P < 0.05 between right and left). Cytokine mRNA expression was significantly higher in the I/R lung than in the control lung in the LE group, whereas it was higher in the control lung in the ST group ( P < 0.05). mRNAs of not only proinflammatory but also anti-inflammatory cytokines were expressed in I/R lung, which are expected to aggravate I/R injury. The reversed pattern of cytokine mRNA expression in the ST group was possibly due to the longer perfusion of control lung with perfusate containing endotoxin, which caused no lung damage without I/R.

Genetic Models in Applied Physiology. HXB/BXH rat recombinant inbred strain platform: a newly enhanced tool for cardiovascular, behavioral, and developmental genetics and genomics

This review deals with the largest set of rat recombinant inbred (RI) strains and summarizes past and recent accomplishments with this platform for genetic mapping and analyses of divergent and complex traits. This strain, derived by crossing the spontaneously hypertensive rat, SHR/Ola, with a Brown Norway congenic, BN-Lx, carrying polydactyly-luxate syndrome, is referred to as HXB/BXH. The RI strain set has been used for linkage and association studies to identify quantitative trait loci for numerous cardiovascular phenotypes, including arterial pressure, stress-elicited heart rate, and pressor response, and metabolic traits, including insulin resistance, dyslipidemia and glucose handling, and left ventricular hypertrophy. The strain's utility has been enhanced with development of a new framework marker-based map and strain distribution patterns of polymorphic markers. Quantitative trait loci for behavioral traits mapped include loci for startle motor response and habituation, anxiety and locomotion traits associated with elevated plus maze, and conditioned taste aversion. The polydactyly-luxate syndrome Lx mutation has allowed the study of alleles important to limb development and malformation phenotypes as well as teratogens. The RI strains have guided development of numerous congenic strains to test locus assignments and to study the effect of genetic background. Although these strains were originally developed to aid in studies of rat genetic hypertension and morphogenetic abnormalities, this rodent platform has been shown to be equally powerful for a wide spectrum of traits and endophenotypes. These strains provide a ready and available vehicle for many physiological and pharmacological studies.

Segment of rat chromosome 20 regulates diet-induced augmentations in adiposity, glucose intolerance, and blood pressure

Previous linkage and association studies have suggested that a region of human chromosome 6 containing the tumor necrosis factor (TNF)-alpha gene is involved in the pathogenesis of obesity and obesity-associated hypertension. The aim of the present investigation was to establish whether a segment of rat chromosome 20 (RNO20), which also contains the TNF-alpha gene, determines diet-induced changes in adiposity and blood pressure (BP). The results showed that a transfer of the RNO20 segment from the normotensive Brown Norway (BN) rat onto the background of the spontaneously hypertensive rat (SHR) is associated with a significantly greater increase in adiposity, glucose intolerance, circulating leptin levels, and BP during 12-week, high-fat-diet feeding. In contrast, the transfer is not associated with significant changes in these variables during 12-week, normal-diet feeding. In addition, sequencing of the TNF-alpha gene revealed differences between SHR and BN in the 5'- and 3'-regulatory regions of the gene. Subsequent analyses of TNF-alpha gene expression in fat, muscle, and liver, however, did not provide support for the functional involvement of these differences. In summary, the investigated RNO20 segment contains 1 or more gene variants that affect adiposity, glucose tolerance, serum leptin levels, and BP, but only when the animals are exposed to a particular environment, ie, high-fat-diet feeding. Further studies are needed to identify genes mediating these effects. Considering current changes in our lifestyle involving an increased calorie and fat intake, we believe that gene-environment interactions, such as those described here, play an important role in the current epidemic of obesity and obesity-associated hypertension.

Liver copper content of rats hypo- or hyperresponsive to dietary cholesterol

The question addressed is whether cholesterol intake reduces the hepatic copper content in rats. For this purpose we have compared the hepatic copper content of two selected rat inbred strains after feeding the animals a control or a high fat, high cholesterol diet. One strain was dietary cholesterol resistant (SHR/OlaIpcv), whereas the other strain was susceptible to dietary cholesterol (BN-Lx/Cub). Dietary cholesterol-susceptible rats have a lower baseline hepatic copper content when compared with their resistant counterparts. The consumption of a hypercholesterolemic diet decreased the liver copper concentration (expressed in microg/g dry weight) to about the same extent in both strains. However, dietary cholesterol did not reduce the absolute (expressed as microg/whole liver) and relative (expressed as microg/whole liver/100 g body weight) copper store of rats. The decrease of liver copper concentration after the high fat, high cholesterol diet is probably not caused by a decrease in whole hepatic copper content, but rather due to dietary-induced hepatomegaly.

Quantitative trait loci influencing blood and liver cholesterol concentration in rats

OBJECTIVE

The LEW/OlaHsd and BC/CpbU rat inbred strains differ markedly in blood and hepatic cholesterol levels before and after a cholesterol-rich diet. To define loci controlling these traits and related phenotypes, an F2 population derived from these strains was genetically analyzed.

METHODS AND RESULTS

For each of the 192 F2 animals, phenotypes were determined, and genomic DNA was screened for polymorphic microsatellite markers. Significant quantitative trait loci (QTLs) were detected for basal serum cholesterol level on chromosome 1 (D1Rat335-D1Rat27: total population, lod score 9.6; females, lod score 10.3) and chromosome 7 (D7Rat69: males, lod score 4.1), for postdietary serum cholesterol level on chromosome 2 (D2Rat69: total population, lod score 4.4) and chromosome 16 (D16Rat6-D16Rat44: total population, lod score 3.3), for postdietary serum phospholipid level on chromosome 11 (D11Rat10: total population, lod score 4.1; females, lod score 3.6), and for postdietary serum aldosterone level on chromosome 1 (D1Rat14: females, lod score 3.7) and chromosome 18 (D18Rat55-D18Rat8: females, lod score 2.9). In addition, QTLs with borderline significance were found on chromosomes 3, 5 to 11, 15, and 18.

CONCLUSIONS

QTLs involved in blood and/or hepatic cholesterol concentrations (or related phenotypes) in the rat were identified. This contributes to the value of the rat as an animal model in studies researching the role of cholesterol in the pathogenesis of atherosclerosis and other cholesterol-related diseases.

A genome scan for Loci associated with aerobic running capacity in rats

Aerobic capacity is a complex trait that defines the efficiency to use atmospheric oxygen as an electron acceptor in energy transfer. Copenhagen (COP) and DA inbred rat strains show a wide difference in a test for aerobic treadmill running and serve as contrasting genetic models for aerobic capacity. A genome scan was carried out on an F(2)(COP x DA) segregating population (n=224) to detect quantitative trait loci (QTLs) associated with aerobic running capacity. Linkage analysis revealed a significant QTL on chromosome 16 (lod score, 4.0). A suggestive linkage was found near the p-terminus of chromosome 3 (lod score, 2.2) with evidence of an interaction with another QTL on chromosome 16 (lod score, 2.9). All three QTLs showed a dominant mode of inheritance in which the presence of at least one DA allele was associated with a greater distance run. These results represent the first aerobic capacity QTLs identified in genetic models.

Heart rate and blood pressure quantitative trait loci for the airpuff startle reaction

The airpuff startle reaction is a probe of sensori-autonomic processing and is useful for studies of genetic control of stress-induced cardiovascular activity. Using a Wistar-Kyoto-Spontaneously Hypertensive Rat F2 cross, we reported an airpuff-elicited strain-dependent and trial-dependent bradycardia, the absence of which cosegregated with hypertension. Here, we use the mapping power of the HXB-BXH recombinant inbred rat strains (n=23) to locate quantitative trait loci (QTL) for this and associated cardiovascular phenotypes. Rats (12 weeks old), with indwelling femoral arterial catheters, were subjected to repeated airpuff startle stimuli (100 ms, 12.5 psi, 28 trials). Basal mean arterial pressure (MAP), delta MAP, and delta heart rate response to airpuff stimuli were analyzed as the average over 28 trials. There was a significant strain effect on the cardiovascular phenotypes measured. One QTL for the bradycardia elicited by the first airpuff stimulus was identified on chromosome 2 (D2rat 62/63; logarithm of odds [LOD] 2.9) mapping near a reported blood pressure locus. Further QTL were identified for basal MAP (RN08), stimulus-elicited tachycardia on trials 2 to 5 (RNO1 and RNO10), and delta MAP (RNO6). Our results indicate that chromosomes 1, 2, and 10 are involved in heart rate responses to airpuff startle stimulus, and chromosomes 6 and 8 are involved in pressor responses. This study is the first to identify stress-related heart rate loci and provides additional support for our prior cosegregation results. Furthermore, we have established the utility of this experimental paradigm to identify loci responsible for cardiovascular regulation during stress in genetic hypertensive models.

Sequencing and chromosomal assignment of the rat endothelial-derived lipase gene (Lipg)

Part of the nucleotide sequence of the Lipg gene in the rat was established using primers based on the mRNA sequence described in the mouse. The rat intron sequence served as a template for designing primers for the specific amplification of rat Lipg. A rat-hamster radiation hybrid (RH) panel was used for chromosomal assignment of the rat Lipg gene. The Lipg gene was found to be located on rat chromosome 18 in the vicinity of the marker D18Mit11; a region reported to be homologous with both human and mouse chromosome 18.

Y-chromosome transfer induces changes in blood pressure and blood lipids in SHR

Previous studies with chromosome-Y consomic strains of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats suggest that a quantitative trait locus for blood pressure regulation exists on chromosome Y. To test this hypothesis in the SHR-Brown Norway (BN) model and to study the effects of chromosome Y on lipid and carbohydrate metabolism, we produced a new consomic strain of SHR carrying the Y chromosome transferred from the BN rat. We found that replacing the SHR Y chromosome with the BN Y chromosome resulted in significant decreases in systolic and diastolic blood pressures in the SHR.BN-Y consomic strain (P<0.05). To elicit possible dietary-induced variation in lipid and glucose metabolism between the SHR progenitor and chromosome-Y consomic strains, we fed rats a high-fructose diet for 15 days in addition to the normal diet. On the high-fructose diet, the SHR.BN-Y consomic rats exhibited significantly increased levels of serum triglycerides and decreased levels of serum HDL cholesterol versus the SHR progenitor rats. Glucose tolerance and insulin/glucose ratios, however, were similar in both strains on both normal and high-fructose diets. These findings provide direct evidence that a gene or genes on chromosome Y contribute to the pathogenesis of spontaneous hypertension in the SHR-BN model. These results also indicate that transfer of the Y chromosome from the BN rat onto the SHR background exacerbates dietary-induced dyslipidemia in SHR. Thus, genetic variation in genes on the Y chromosome may contribute to variation in blood pressure and lipid levels and may influence the risk for cardiovascular disease.

Metabolic features in disease-resistant as well as in spontaneously hypertensive rats and newly established obese Wistar Ottawa Karlsburg inbred rats

OBJECTIVE

Studies, comparing several disease-prone and disease-resistant rat strains to elucidate the extent and severity of syndromes resembling human diseases are lacking. Therefore we studied the inbred rat strains BB/OK, BN/Crl, LEW/K and WKY/Crl in comparison with SHR/Mol and WOKW/K rats as models of metabolic syndrome.

DESIGN

Body weight and body mass index (BMI) were measured in 12 males of each strain at 14 weeks. In addition blood glucose, serum triglycerides, cholesterol, insulin and leptin were determined at 12, 13 and 14 weeks of age.

RESULTS

In contrast to SHR animals, WOKW rats develop a severe metabolic syndrome including obesity, hyperleptinemia, hyperinsulinemia and dyslipidemia.

CONCLUSION

We conclude that; (i) the choice of disease-resistant inbred rat strains as 'healthy controls' for a disease-prone strain has to be carefully evaluated; (ii) in comparison with SHR, WOKW rats develop most if not all facets of the metabolic syndrome described in human and (iii) as with the human disease the syndrome in rats is polygenic.

Chromosomal mapping of a major quantitative trait locus regulating compensatory renal growth in the rat

Despite extensive research conducted over the past century, the mechanisms of compensatory renal growth (CRG) remain a mystery. Insight into the mechanisms that regulate CRG might be gained by identifying genetic factors that influence this complex phenotype. In a large set of recombinant inbred strains derived from the spontaneously hypertensive rat and the Brown Norway rat, a genome scan for quantitative trait loci (QTL) that regulate CRG was performed. The CRG score was expressed as a ratio of the weight of the remnant right kidney at 8 wk of age to the weight of the left kidney at 5 wk of age, both adjusted for body weight. QTL mapping was performed using Map Manager QT and the strain distribution patterns of more than 600 genetic markers. It was found that CRG after unilateral nephrectomy is a multifactorially determined trait with a substantial genetic component. The heritability of CRG approached 40%. Genome wide scan analysis revealed significant evidence of linkage to a region of rat chromosome 4 designated Crg 1 that accounted for more than 50% of the additive genetic variance of CRG in the recombinant inbred strains. The detection of a major QTL influencing CRG in the rat should provide new opportunities for identifying mechanisms that regulate this historically enigmatic phenomenon and may also have implications for research on the pathogenesis of end-stage kidney disease.

Genetic dissection of the syndrome X in the rat

In 1988, Reaven used the term syndrome X to describe a relation between several disorders including hypertension, dyslipidemia, impaired glucose tolerance, obesity, and coronary heart disease. Despite a number of studies dealing with syndrome X, its genetic basis remains poorly understood. Regarding the complexity of this syndrome, it is important to use animal models developing the traits of the disease. Here we show a genetic dissection of syndrome X in the WOKW rat, an animal model of genetically determined syndrome X. We found a major quantitative trait locus (QTL) for glucose metabolism on chromosome 3 and further QTLs influencing obesity and body weight on chromosomes 1 and 5. Genetic determinants of dyslipidemia were mapped to chromosomes 4 and 17. In addition, suggestive linkage for serum insulin was found on chromosome 1 to the region previously shown to be associated with type-1 diabetes mellitus. This is the first study demonstrating independent genetic factors influencing traits of the syndrome X in the rat as well as a possible genetic relationships between syndrome X and diabetes mellitus. Moreover, regarding the close similarities between WOKW rat and human syndrome X, the study could help in a search of genetic factors involved in this complex metabolic disorder in human.

Identification of quantitative trait loci for serum cholesterol levels in stroke-prone spontaneously hypertensive rats

The stroke-prone spontaneously hypertensive rat (SHRSP) has been reported to show significantly lower levels of serum total cholesterol than the normotensive control strain Wistar-Kyoto rat (WKY). Because selective inbreeding was conducted for stroke proneness, this concomitantly inherited characteristic of SHRSP may play some pathophysiological role in stroke. We evaluated the genetic determinants of the cholesterol trait by estimating heritability and subsequently by undertaking a genome-wide screen with 161 genetic markers in F(2) progeny involving SHRSP and WKY (104 male and 106 female rats). Three quantitative trait loci (QTLs) were detected on rat chromosomes 5, 7, and 15. Markers from the linked region on chromosome 15 indicated significant evidence of linkage with a maximal log of the odds (LOD) score of 7.7, whereas those on chromosomes 5 and 7 cosegregated with the trait in a sex-specific manner (the QTL close to genetic marker D5 Mit5 reached an LOD score of 7.3 in males, and that close to D7 Mit10 reached an LOD score of 3.2 in females). The male-specific QTL on chromosome 5 appeared to overlap with previously reported QTLs for stroke-associated phenotypes, but an identical gene (or genes) appeared unlikely to control these and the cholesterol traits simultaneously. In the present study, serum cholesterol levels were shown to be highly genetically determined in SHRSP (the heritability estimates are 76% in males and 83% in females), and 3 QTLs with substantial effects were identified. Further work, however, is required to clarify whether the cholesterol trait is related to the etiology of stroke or has been retained by chance through the inbreeding process in SHRSP.

The promise of comparative genomics in mammals

Dense genetic maps of human, mouse, and rat genomes that are based on coding genes and on microsatellite and single-nucleotide polymorphism markers have been complemented by precise gene homolog alignment with moderate-resolution maps of livestock, companion animals, and additional mammal species. Comparative genetic assessment expands the utility of these maps in gene discovery, in functional genomics, and in tracking the evolutionary forces that sculpted the genome organization of modern mammalian species.

Genetics of Cd36 and the clustering of multiple cardiovascular risk factors in spontaneous hypertension

Disorders of carbohydrate and lipid metabolism have been reported to cluster in patients with essential hypertension and in spontaneously hypertensive rats (SHRs). A deletion in the Cd36 gene on chromosome 4 has recently been implicated in defective carbohydrate and lipid metabolism in isolated adipocytes from SHRs. However, the role of Cd36 and chromosome 4 in the control of blood pressure and systemic cardiovascular risk factors in SHRs is unknown. In the SHR. BN-Il6/Npy congenic strain, we have found that transfer of a segment of chromosome 4 (including Cd36) from the Brown Norway (BN) rat onto the SHR background induces reductions in blood pressure and ameliorates dietary-induced glucose intolerance, hyperinsulinemia, and hypertriglyceridemia. These results demonstrate that a single chromosome region can influence a broad spectrum of cardiovascular risk factors involved in the hypertension metabolic syndrome. However, analysis of Cd36 genotypes in the SHR and stroke-prone SHR strains indicates that the deletion variant of Cd36 was not critical to the initial selection for hypertension in the SHR model. Thus, the ability of chromosome 4 to influence multiple cardiovascular risk factors, including hypertension, may depend on linkage of Cd36 to other genes trapped within the differential segment of the SHR. BN-Il6/Npy strain.

A radiation hybrid map of the rat genome containing 5,255 markers

A whole-genome radiation hybrid (RH) panel was used to construct a high-resolution map of the rat genome based on microsatellite and gene markers. These include 3,019 new microsatellite markers described here for the first time and 1,714 microsatellite markers with known genetic locations, allowing comparison and integration of maps from different sources. A robust RH framework map containing 1,030 positions ordered with odds of at least 1,000:1 has been defined as a tool for mapping these markers, and for future RH mapping in the rat. More than 500 genes which have been mapped in mouse and/or human were localized with respect to the rat RH framework, allowing the construction of detailed rat-mouse and rat-human comparative maps and illustrating the power of the RH approach for comparative mapping.

Identification of Cd36 (Fat) as an insulin-resistance gene causing defective fatty acid and glucose metabolism in hypertensive rats

The human insulin-resistance syndromes, type 2 diabetes, obesity, combined hyperlipidaemia and essential hypertension, are complex disorders whose genetic basis is unknown. The spontaneously hypertensive rat (SHR) is insulin resistant and a model of these human syndromes. Quantitative trait loci (QTLs) for SHR defects in glucose and fatty acid metabolism, hypertriglyceridaemia and hypertension map to a single locus on rat chromosome 4. Here we combine use of cDNA microarrays, congenic mapping and radiation hybrid (RH) mapping to identify a defective SHR gene, Cd36 (also known as Fat, as it encodes fatty acid translocase), at the peak of linkage to these QTLs. SHR Cd36 cDNA contains multiple sequence variants, caused by unequal genomic recombination of a duplicated ancestral gene. The encoded protein product is undetectable in SHR adipocyte plasma membrane. Transgenic mice overexpressing Cd36 have reduced blood lipids. We conclude that Cd36 deficiency underlies insulin resistance, defective fatty acid metabolism and hypertriglyceridaemia in SHR and may be important in the pathogenesis of human insulin-resistance syndromes.

Effects of quantitative trait loci for lipid phenotypes in the rat are influenced by age

1. Previous study on the backcross hybrids derived from a cross of the spontaneously hypertensive rat (SHR/Mol) and the spontaneously diabetic BB/OK rat demonstrated the existence of quantitative trait loci (QTL) affecting lipid phenotypes on chromosome 4 and suggestive linkage of lipid phenotypes with markers on chromosome 1. Because the previous study was performed with backcross hybrids at 12 weeks of age and it is known that lipid phenotypes can show age-related differences, in the present study, the effect of QTL (chromosome 1 and 4) on serum triglycerides and cholesterol was longitudinally analysed between 20 and 32 weeks of age in backcross hybrids. 2. The results of the present study show that the effect of QTL on chromosome 4 (between It-6 and D4Mit9) for serum triglycerides was maximal at 20 weeks of age, but disappeared at 32 weeks of age. In contrast, the effect of QTL on serum total cholesterol on chromosome 4 (Npy-Spr) was maximal at 32 weeks of age. In contrast with the first study (12 weeks), the longitudinal investigation showed significant linkage of DIMit14 marker with lipid phenotypes on chromosome 1. 3. The results of the present study indicate the necessity of considering the role of age in QTL analysis of lipid phenotypes.

Transcription-modulating drugs: a new frontier in the treatment of essential hypertension

While the promises of gene therapy may be years away from realization, the therapeutic use of drugs that act by modifying gene transcription is a well-established practice in clinical medicine. Although transcription-modulating drugs are frequently used in many different specialties, the deliberate development and use of these agents in cardiovascular medicine has been comparatively limited. However, research advances in the area of gene transcription and in the molecular genetic regulation of blood pressure, insulin resistance, lipid metabolism, and cell growth are providing new opportunities for controlling the expression of genes that are relevant to the pathogenesis of cardiovascular disease and essential hypertension. These research advances are beginning to converge in the development of transcription-modulating drugs with the potential to attack genetically determined risk factors that often cluster in patients with essential hypertension. Ligand-activated transcription factors that serve as receptors for small lipophilic compounds such as the thiazolidinediones and retinoids represent examples of potential therapeutic targets with direct effects on the expression of genes relevant to the pathogenesis of essential hypertension and its complications. Mounting evidence suggesting that the superior cardiorenal protective properties of converting enzyme inhibitors are related in part to their ability to indirectly modify the expression of genes in the heart and vasculature provides provisional support for the clinical value of this therapeutic approach. Given the success of transcription-modulating drugs in the treatment of type II diabetes and many other clinical disorders, it is anticipated that these agents will be developed as tools for the prevention and treatment of hypertension and cardiovascular disease in the not too distant future.

Resistance to endotoxin shock in spontaneously hypertensive rats

Septic shock involves systemic vasodilation mediated by proinflammatory cytokines. In essential hypertension, vascular and immune dysfunctions are closely associated. The response of hypertensive animals compared with normotensive controls to endotoxin (lipopolysaccharide; LPS) challenge is not known. Age-matched (12 weeks) normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were exposed to intravenous injection of 10 mg/kg LPS. Survival rate at 24 hours was markedly higher in SHR than in WKY (12 of 15 and 3 of 15, respectively; P<0.01). Survival of LPS-injected SHR was not related to their hypertension because hydralazine-treated SHR with normalized pressure had similar survival rates, and WKY made hypertensive by clipping of one renal artery showed fatality similar to that of normotensive WKY. Continuous arterial pressure and sequential plasma levels of interleukin-6 (IL-6) and tumor necrosis factor (TNF) were measured in LPS-treated SHR and WKY. Both the duration of the delayed hypotensive phase and the systemic release of IL-6 were much lower in SHR than WKY, whereas both acute hypotension and plasma TNF peak were equivalent. We further explored in vitro the inflammatory response and showed that LPS-activated whole blood from SHR produced less TNF and IL-6 than WKY LPS-activated whole blood. Our results indicate that SHR have a greater ability to resist endotoxic shock than WKY. This is not related to their hypertension but is associated with an attenuated inflammatory response to LPS.