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

Burden of Mendelian disorders in a large Middle Eastern biobank

BACKGROUND

Genome sequencing of large biobanks from under-represented ancestries provides a valuable resource for the interrogation of Mendelian disease burden at world population level, complementing small-scale familial studies.

METHODS

Here, we interrogate 6045 whole genomes from Qatar-a Middle Eastern population with high consanguinity and understudied mutational burden-enrolled at the national Biobank and phenotyped for 58 clinically-relevant quantitative traits. We examine a curated set of 2648 Mendelian genes from 20 panels, annotating known and novel pathogenic variants and assessing their penetrance and impact on the measured traits.

RESULTS

We find that 62.5% of participants are carriers of at least 1 known pathogenic variant relating to recessive conditions, with homozygosity observed in 1 in 150 subjects (0.6%) for which Peninsular Arabs are particularly enriched versus other ancestries (5.8-fold). On average, 52.3 loss-of-function variants were found per genome, 6.5 of which affect a known Mendelian gene. Several variants annotated in ClinVar/HGMD as pathogenic appeared at intermediate frequencies in this cohort (1-3%), highlighting Arab founder effect, while others have exceedingly high frequencies (> 5%) prompting reconsideration as benign. Furthermore, cumulative gene burden analysis revealed 56 genes having gene carrier frequency > 1/50, including 5 ACMG Tier 3 panel genes which would be candidates for adding to newborn screening in the country. Additionally, leveraging 58 biobank traits, we systematically assess the impact of novel/rare variants on phenotypes and discover 39 candidate large-effect variants associating with extreme quantitative traits. Furthermore, through rare variant burden testing, we discover 13 genes with high mutational load, including 5 with impact on traits relevant to disease conditions, including metabolic disorder and type 2 diabetes, consistent with the high prevalence of these conditions in the region.

CONCLUSIONS

This study on the first phase of the growing Qatar Genome Program cohort provides a comprehensive resource from a Middle Eastern population to understand the global mutational burden in Mendelian genes and their impact on traits in seemingly healthy individuals in high consanguinity settings.

Multi-Omic Approaches to Identify Genetic Factors in Metabolic Syndrome

Metabolic syndrome (MetS) is a highly heritable disease and a major public health burden worldwide. MetS diagnosis criteria are met by the simultaneous presence of any three of the following: high triglycerides, low HDL/high LDL cholesterol, insulin resistance, hypertension, and central obesity. These diseases act synergistically in people suffering from MetS and dramatically increase risk of morbidity and mortality due to stroke and cardiovascular disease, as well as certain cancers. Each of these component features is itself a complex disease, as is MetS. As a genetically complex disease, genetic risk factors for MetS are numerous, but not very powerful individually, often requiring specific environmental stressors for the disease to manifest. When taken together, all sequence variants that contribute to MetS disease risk explain only a fraction of the heritable variance, suggesting additional, novel loci have yet to be discovered. In this article, we will give a brief overview on the genetic concepts needed to interpret genome-wide association studies (GWAS) and quantitative trait locus (QTL) data, summarize the state of the field of MetS physiological genomics, and to introduce tools and resources that can be used by the physiologist to integrate genomics into their own research on MetS and any of its component features. There is a wealth of phenotypic and molecular data in animal models and humans that can be leveraged as outlined in this article. Integrating these multi-omic QTL data for complex diseases such as MetS provides a means to unravel the pathways and mechanisms leading to complex disease and promise for novel treatments. © 2022 American Physiological Society. Compr Physiol 12:1-40, 2022.

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.

Integrative genomic analysis of blood pressure and related phenotypes in rats

Despite remarkable progress made in human genome-wide association studies, there remains a substantial gap between statistical evidence for genetic associations and functional comprehension of the underlying mechanisms governing these associations. As a means of bridging this gap, we performed genomic analysis of blood pressure (BP) and related phenotypes in spontaneously hypertensive rats (SHR) and their substrain, stroke-prone SHR (SHRSP), both of which are unique genetic models of severe hypertension and cardiovascular complications. By integrating whole-genome sequencing, transcriptome profiling, genome-wide linkage scans (maximum n=1415), fine congenic mapping (maximum n=8704), pharmacological intervention and comparative analysis with transcriptome-wide association study (TWAS) datasets, we searched causal genes and causal pathways for the tested traits. The overall results validated the polygenic architecture of elevated BP compared with a non-hypertensive control strain, Wistar Kyoto rats (WKY); e.g. inter-strain BP differences between SHRSP and WKY could be largely explained by an aggregate of BP changes in seven SHRSP-derived consomic strains. We identified 26 potential target genes, including rat homologs of human TWAS loci, for the tested traits. In this study, we re-discovered 18 genes that had previously been determined to contribute to hypertension or cardiovascular phenotypes. Notably, five of these genes belong to the kallikrein-kinin/renin-angiotensin systems (KKS/RAS), in which the most prominent differential expression between hypertensive and non-hypertensive alleles could be detected in rat Klk1 paralogs. In combination with a pharmacological intervention, we provide in vivo experimental evidence supporting the presence of key disease pathways, such as KKS/RAS, in a rat polygenic hypertension model.

Proteomic Changes in the Plasma of Broiler Chickens with Femoral Head Necrosis

Femoral head necrosis (FHN) is a skeletal problem in broiler chickens, where the proximal femoral head cartilage shows susceptibility to separation from its growth plate. The selected birds with FHN showed higher body weights and reduced plasma cholesterol. The proteomic differences in the plasma of healthy and FHN-affected chickens were explored using matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS) and liquid chromatography/electrospray ionization-tandem mass spectrometry (LC-MS/MS) to prospect for protein biomarkers. We isolated two differentially expressed low molecular weight proteins and identified them by MALDI peptide mass fingerprinting as fibrinogen- and fetuin-derived peptides, respectively. These peptides were reduced in birds susceptible to femoral head problems. Quantitation of LC-MS/MS spectra showed elevated levels of gallinacin-9, apolipoprotein A1, and hemoglobin and reduced levels of alpha-1-acid glycoprotein, albumin, and SPINK7 proteins in FHN. These results suggest that the bodyweight and the lipid profiles along with the above proteins can be useful as noninvasive biomarkers of FHN.

Involvement of microRNA214 and transcriptional regulation in reductions in mevalonate pyrophosphate decarboxylase mRNA levels in stroke-prone spontaneously hypertensive rat livers

Hypocholesterolemia has been epidemiologically identified as one of the causes of stroke (cerebral hemorrhage). We previously reported that lower protein levels of mevalonate pyrophosphate decarboxylase (MPD), which is responsible for reducing serum cholesterol levels in stroke-prone spontaneously hypertensive rats (SHRSP), in the liver were caused by a reduction in mRNA levels. However, the mechanism responsible for reducing MPD expression levels in the SHRSP liver remains unclear. Thus, we compared microRNA (miR)-214 combined with the 3'-untranslated region of MPD mRNA and heterogeneous nuclear RNA (hnRNA) between SHRSP and normotensive Wistar Kyoto rats (WKY). miR-214 levels in the liver were markedly higher in SHRSP than in WKY, whereas hnRNA levels were significantly lower. These results indicate that the upregulation of miR-214 and downregulation of MPD transcription in the liver both play a role in the development of hypocholesterolemia in SHRSP.

Unavailability of liver triacylglycerol increases serum cholesterol concentration induced by dietary cholesterol in exogenously hypercholesterolemic (ExHC) rats

BACKGROUND

Exogenously hypercholesterolemic (ExHC) rats develop hypercholesterolemia and low hepatic triacylglycerol (TAG) levels when dietary cholesterol is loaded. The responsible gene Smek2 was identified via linkage analysis using the original strain Sprague-Dawley (SD) rats. In this study, we compared SD and ExHC rats to investigate a relationship between hypercholesterolemia and the low hepatic TAG levels observed in ExHC rats.

METHODS

Male 4-weeks-old ExHC and SD rats were fed a 1% cholesterol diet for 1 week. Serum and liver parameters were analyzed. Gene expression and enzyme activities related to TAG metabolism were also assessed.

RESULTS

We reproducibly observed higher serum cholesterol and lower hepatic TAG levels in ExHC rats than in SD rats. Golgi apparatus in the livers of ExHC rats secreted β-very-low-density lipoprotein (β-VLDL) that had higher cholesterol ester (CE) and lower TAG content than those in the β-VLDL secreted by SD rats. Gene expression related to fatty acid and TAG synthesis in ExHC rats was lower than that in SD rats. Enzymatic activities for fatty acid synthesis were also relatively lower in ExHC rats. Moreover, the fatty acid composition of hepatic and serum CE in ExHC rats showed that these CEs were not modified after secretion from the liver despite the similar activities of serum lecithin-cholesterol acyltransferase (LCAT) in ExHC rats to those in SD rats.

CONCLUSIONS

Low production of liver TAG and secretion of CE-rich, TAG-poor β-VLDL without modification by LCAT in the circulation contributed to hypercholesterolemia induced by dietary cholesterol in ExHC rats.

Comprehensive QTL analysis of serum cholesterol levels before and after a high-cholesterol diet in SHRSP

The stroke-prone spontaneously hypertensive rat (SHRSP) showed an exaggerated response to a high-fat, high-cholesterol (HFC) diet, and the resulting reactive hypercholesterolemia was suggested to exacerbate the atherogenic process in this rat. We thus performed a quantitative trait locus (QTL) analysis on the serum cholesterol level of SHRSP before and after the HFC diet, with the final goal being the identification of the genetic mechanisms of its reactive hypercholesterolemia. Three hundred fifty-eight F2 rats between SHRSP and Wistar-Kyoto rat were employed in the study. The serum cholesterol and apoprotein E were measured before and after 2 wk of feeding with the HFC diet. Multiple QTLs for the basal cholesterol level were identified on chromosomes 1 and 5, whereas those for the postdietary cholesterol level were on chromosomes 7, 15, and 16. The cholesterol QTLs before and after HFC diet did not overlap with one another, implying that the involved metabolic processes were considerably different between the two conditions. Supporting this, VLDL and LDL cholesterol were the major components of the postdietary serum cholesterol, whereas the basal cholesterol level consisted mainly of HDL cholesterol. A substantial difference of the QTLs between males and females was observed, especially after the HFC diet. The QTL on chromosome 15 had an inverse effect on the cholesterol level, suggesting that the congenic substitution of the SHRSP fragment with that of Wistar-Kyoto rats could induce a greater cholesterol level in SHRSP. This observation is significant in establishing a new model for atherosclerosis with hypertension in 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.

Genetic determinants of metabolic syndrome components in the stroke-prone spontaneously hypertensive rat

OBJECTIVE

The metabolic syndrome is a complex multifactorial disease, which results from interactions between genes on multiple chromosomes and environmental factors. Animal models may facilitate genetic analysis of complex phenotypes by allowing complete control of environmental conditions and the ability to produce designer strains.

METHODS

Stroke-prone spontaneously hypertensive (SHRSP) and Wistar-Kyoto (WKY) rat strains were used to construct congenic (SP.WKYGla2a), consomic (SP.WKYGlaYw, WKY.SPGlaYs) and double-introgressed (SP.WKYGla2aYw) strains, which were characterized for metabolic syndrome phenotypes (systolic blood pressure, glucose tolerance and lipid profile) after feeding a 60% fructose diet for 14 days.

RESULTS

The Y consomic strain (SP.WKYGlaYw) demonstrated that the WKY Y chromosome significantly lowered triglyceride levels (3.77 +/- 0.60 versus 9.09 +/- 1.47 mmol/l; P < 0.001) and improved glucose tolerance [area under the curve (AUC): 26.93 +/- 0.81 versus 31.47 +/- 0.89; P < 0.05] compared with SHRSP. The chromosome 2 congenic strain (SP.WKYGla2a) exhibited significantly improved glucose tolerance (AUC: 28.19 +/- 1.17 versus 31.47 +/- 0.89; P < 0.05) and lower systolic blood pressure (161.2 +/- 6.2 versus 179.7 +/- 3.9 mmHg; P < 0.05) compared with SHRSP. 2 x 2 factorial ANOVA identified a significant interaction for glucose metabolism (P = 0.004) in the double-introgressed strain (SP.WKYGla2aYw) between chromosome 2 and Y.

CONCLUSIONS

These results identify novel interacting regions on chromosome 2 and the Y chromosome influencing a cluster of metabolic and cardiovascular phenotypes. Translation to clinical studies will facilitate genetic dissection of human metabolic syndrome.

Distinct QTLs are linked to cardiac left ventricular mass in a sex-specific manner in a normotensive inbred rat inter-cross

Genetic mapping of the progeny of an F(2) inter-cross between WKY and WKHA rats had previously allowed us to detect male-specific linkage between locus Cm 24 and left ventricular mass index (LVMI). By further expanding that analysis, we detected additional loci that were all linked to LVMI in a sex-specific manner despite their autosomal location. In males, we detected one additional locus (Lvm 8) on Chromosome 5 (LOD=3.4), the two loci Lvm 13 (LOD=4.5) and Lvm 9 (LOD=2.8) on Chromosome 17, and locus Lvm 10 (LOD=4.2) on Chromosome 12. The locus Lvm 13 had the same boundaries as locus Cm 26 previously reported by others using a different cross. None of these loci showed linkage to LVM in females. In contrast, we identified in females the novel locus Lvm 11 on Chromosome 15 (LOD=2.8) and locus Lvm 12 (LOD=2.7) that had the same boundaries on Chromosome 3 as locus Cm 25 detected previously by others using a cross of other normotensive strains. In prepubertal males, there were no differences in the width of cardiomyocytes from WKY and WKHA rats, but cardiomyocytes from WKHA became progressively wider than that of WKY as sexual maturation progressed. Altogether, these results provide evidence that distinct genes may influence LVMI of rats in a sex-dependent manner, maybe by involving sex-specific interactions of sex steroids with particular genes involved in the determination of LVMI and/or cardiomyocyte width.

Genetic analysis of diet-induced hypercholesterolemia in exogenously hypercholesterolemic rats

The exogenously hypercholesterolemic (ExHC) rat is an established strain that exhibits a polygenic syndrome of hypercholesterolemia after feeding on a cholesterol-containing diet, and the extent of this differs between male and female rats in the strain. The present study was performed to determine the genetic background of diet-induced hypercholesterolemia in ExHC rats. We used quantitative trait locus (QTL) analyses of the F2 progeny derived from ExHC and Brown-Norway rats. Rats were fed a diet containing 1% cholesterol, and a genome-wide scan was then performed. Significant QTLs for serum total cholesterol levels were revealed on chromosomes 5 and 14 in the vicinity of markers D5Rat95 and D14Rat43, having maximum logarithm of the odds scores of 6.0 and 5.8, respectively. A suggestive QTL for the trait was also detected on chromosome 3 at D3Rat140. In particular, the QTL on chromosome 5 was specific for female rats. These loci were novel QTLs for post-dietary serum total cholesterol levels. In addition, cross-mating analysis in F1 generations suggested that the responsiveness to dietary cholesterol in ExHC rats is partly attributable to X-linked inheritance. Identifying such genetic factors may be useful in predicting the risks associated with diet-induced hypercholesterolemia in humans.

Identification of quantitative trait loci for cardiac hypertrophy in two different strains of the spontaneously hypertensive rat

Cardiac hypertrophy and left ventricular hypertrophy are known to be substantially controlled by genetic factors. As an experimental model, we undertook genome-wide screens for cardiac mass in F2 populations bred from the stroke-prone spontaneously hypertensive rats (SHRSP) and normal spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) of a Japanese colony. Two F2 cohorts were independently produced: F2(SHRSP x WKY) (110 male and 110 female rats) and F2(SHR x WKY) (151 male rats). The ratio of heart weight to body weight (Hw/Bw) was evaluated at 12 months of age in F2(SHRSP x WKY) after salt-loading for 7 months, and at around 15 weeks of age in F2(SHR x WKY) who had been fed a normal rat chow diet. Subsequent to an initial screen with 251 markers in F2(SHRSP x WKY) male progeny, 170 and 161 markers were selected and characterized in F2(SHRSP x WKY) female progeny and F2(SHR x WKY) male progeny, respectively. Markers from four chromosomal regions showed suggestive or significant linkage to Hw/Bw. The strongest and the most consistent linkage was found in the vicinity of D3Mgh16 on rat chromosome (RNO) 3 (a maximal log of the odds score reached 4.0 to 6.6 across the F2 populations studied). In the other three regions on RNO6, RNO10 and RNO13, the degree of linkage was more prominent in either males or females. These data provide solid evidence for a "principal" RNO3 quantitative trait loci regulating Hw/Bw in SHRSP and SHR, and also suggest the possible presence of sexual dimorphism in regard to genetic susceptibility for cardiac hypertrophy.

Dynamic genetic architecture of metabolic syndrome attributes in the rat

The polydactylous rat strain (PD/Cub) is a highly inbred (F > 90) genetic model of metabolic syndrome. The aim of this study was to analyze the genetic architecture of the metabolic derangements found in the PD/Cub strain and to assess its dynamics in time and in response to diet and medication. We derived a PD/Cub × BN/Cub (Brown Norway) F2 intercross population of 149 male rats and performed metabolic profiling and genotyping and multiple levels of genetic linkage and statistical analyses at five different stages of ontogenesis and after high-sucrose diet feeding and dexamethasone administration challenges. The interval mapping analysis of 83 metabolic and morphometric traits revealed over 50 regions genomewide with significant or suggestive linkage to one or more of the traits in the segregating PD/Cub × BN/Cub population. The multiple interval mapping showed that, in addition to “single” quantitative train loci, there are more than 30 pairs of loci across the whole genome significantly influencing the variation of particular traits in an epistatic fashion. This study represents the first whole genome analysis of metabolic syndrome in the PD/Cub model and reveals several new loci previously not connected to the genetics of insulin resistance and dyslipidemia. In addition, it attempts to present the concept of “dynamic genetic architecture” of metabolic syndrome attributes, evidenced by shifts in the genetic determination of syndrome features during ontogenesis and during adaptation to the dietary and pharmacological influences.

Evaluation of insulin resistance linkage to rat chromosome 4 in SHR of a Japanese colony

The spontaneously hypertensive rat (SHR) is a model of human insulin resistance syndrome. Quantitative trait loci for cellular defects in glucose and fatty acid metabolism have been mapped to an overlapping region of rat chromosome (RNO) RNO4 in SHR of the National Institute of Health colony, where a deletion in the Cd36 gene has been implicated as the causative mutation of insulin resistance. The present study has examined the potential presence of RNO4 linkage to a series of metabolic phenotypes in F(2) progeny derived from SHR of a Japanese colony (SHR/Izm) without the Cd36 mutation. Our data demonstrate that 'major' insulin resistance gene(s) are unlikely to exist on RNO4 in SHR/Izm and in vitro phenotypes measured in isolated adipocytes do not cosegregate in the F(2) population studied. Thus, it seems to be difficult to explain the underlying genetic mechanisms of insulin resistance by a single major gene on RNO4.

Eae19, a new locus on rat chromosome 15 regulating experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) and its animal model, myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis (MOG-EAE), share a complex genetic predisposition with contributions from the major histocompatibility complex class II genes and many other genes. Linkage mapping in F(2) crosses between the susceptible DA rat strain and the resistant ACI or BN rat strains in various models of autoimmune neuroinflammation have repeatedly displayed suggestive linkage to a region on rat chromosome 15. A direct study of this region was undertaken in congenic strains by transferring resistant ACI alleles to the susceptible DA background. Phenotypic analysis demonstrated lower maximal and cumulative EAE scores in the DA.ACI-D15Rat6-D15Rat71 (C15), DA.ACI-D15Rat6-D15Rat48, D15Rat126-D15Rat71 (C15R3b), and DA.ACI-D15Rat23-D15rat71 (C15R4) strains compared to the parental DA rat strain. Linkage analysis was then performed in a (DA x PVG.AV1)F(7) advanced intercross line, resulting in a LOD score of 4.7 for the maximal EAE score phenotype at the peak marker D15Rat71 and a confidence interval of 13 Mb, overlapping with the congenic fragment defined by the C15R3b and the C15R4 strains. Thus, a new MOG-EAE locus with the designation Eae19 is identified on rat chromosome 15. There are 32 confirmed or predicted genes in the confidence interval, including immune-responsive gene 1 and neuronal ceroid lipofuscinose gene 5. Definition of loci such as Eae19 enables the characterization of genetically regulated, evolutionary conserved disease pathways in complex neuroinflammatory diseases.

Genome-wide searches for blood pressure quantitative trait loci in the stroke-prone spontaneously hypertensive rat of a Japanese colony

OBJECTIVES

Although several quantitative trait loci for blood pressure have been reported in stroke-prone spontaneously hypertensive rats (SHRSP), the results are not always concordant among different crosses. To evaluate potential confounding factors in linkage analysis, we performed genome-wide screens in F2 populations derived from SHRSP and Wistar-Kyoto rats of a Japanese colony.

METHODS

Two F cohorts were independently produced: F2-1 (110 male and 110 female rats), and F2-2 (174 male and 184 female rats). Blood pressure was measured longitudinally (from 2 to 5 months of age and 1 month after salt-loading) in F2-1, while it was measured at 13 weeks of age in F2-2. Subsequent to an initial screen with 251 markers in F2-1 male progeny, 170 markers were selected and characterized in the remaining populations.

RESULTS

When 578 rats were analyzed together, markers from five chromosomal regions showed significant linkage to blood pressure at 13 weeks of age. The strongest and the most consistent linkage was found on rat chromosome 1 (a maximal log of the odds score reached 8.3). In the other regions, the degree of linkage was more prominent in either of sexes. Some evidence of age-specific and sex-specific linkage was detected in five additional regions in the F2-1 cohort. In the Japanese colony, however, there was no significant linkage to several chromosomal regions previously reported in other SHRSP colonies.

CONCLUSIONS

Our data provide solid evidence of a chromosome-1 linkage and demonstrate the importance of aging, sex, and dietary manipulation in linkage analysis. Also, the combination of parental rat strains seems to be critical when searching for blood pressure quantitative trait loci.

A new essential hypertension susceptibility locus on chromosome 2p24-p25, detected by genomewide search

Essential hypertension (EH) is a complex disorder that results from the interaction of a number of susceptibility genes and environmental factors. We studied an isolated Sardinian village (Talana) in which the prevalence of hypertension is comparable to that in most Western populations. Talana exhibits features, such as slow demographic growth, high inbreeding, a low number of founders, stable lifestyle and culture, and accurate genealogical records, that make it suitable for the study of complex disorders. Clinical assessment of the entire adult population (N= approximately 1,000) identified approximately 100 hypertensive subjects. For our study, we selected the individuals with the most-severe EH (i.e., diastolic blood pressure >100 mm Hg), belonging to a single deep-rooted pedigree (12 generations), whose common ancestors lived in the 17th century. We performed a three-stage genomewide search using 36 affected individuals, by means of parametric linkage and allele-sharing approaches. LOD scores >1 were observed on chromosomes 1, 2, 13, 15, 17, and 19 (stage I). The most striking result was found in a 7.57-cM region on chromosome 2p24-p25. All five nonparametric linkage statistics estimated by the SimWalk2 program lie above the significance threshold of P<.008 for the whole region. Similar significance was obtained for 2p24-25 when parametric linkage (LOD score 1.99) and linkage disequilibrium mapping (P=.00006) were used, suggesting that a hypertension-susceptibility locus is located between D2S2278 and D2S168. This finding is strengthened by a recent report of linkage with marker D2S168 in a hypertensive sib-pair sample from China.

Evaluation of the atrial natriuretic peptide gene in stroke

The atrial natriuretic peptide (ANP) gene was, though inconclusive, implied to be etiologically related to stroke in rats and recently in humans. The present study tested the candidacy of ANP for stroke susceptibility by a combination of molecular genetic approaches. First, we undertook an association study using a reported ANP variant, G664A, in two case-control panels independently collected, which involved 970 Japanese subjects. Second, we compared the rat ANP gene sequences and neighboring marker alleles among stroke-prone SHR (SHRSP), normal SHR and WKY of an original inbred colony and we also compared brain ANP expression between SHRSP and normal SHR. In humans, we found no significant association between the 664A variant and stroke in the studied population. In rats, 21 polymorphic sites were identified by direct sequencing of 2170-bp ANP fragments, from which two distinct alleles, SHRSP- and WKY-types, were inferred. From a genealogical point of view, our data indicated that an SHRSP-type allele could not play a determinant role in stroke-proneness. Overall results did not support the disease relevance of ANP, disagreeing with previous reports. Thus, considerable caution should be taken when one attempts to transfer findings in the animal model to humans.

Quantitative trait loci for body weight, blood pressure, blood glucose, and serum lipids: linkage analysis with wild rats (Rattus norvegicus)

To study polygenetically inherited human diseases like hypertension, inbred rat strains are usually the preferred models. Because many inbred generations under optimized environmental conditions may have led to the survival of "silent" disease genes, we used a cross between one wild rat and genetically hypertensive SHR rats to analyze quantitative trait loci (QTLs) of blood pressure and related traits. The (Wild x SHR)F1 hybrids were transferred into a pathogen-free environment by wet-hysterectomy and were backcrossed onto SHR to generate first backcross hybrids (BC1). Progeny from one F1 female (n = 72) were phenotypically and genetically characterized to map QTLs. Significant, subsignificant, and suggestive evidence was found for more sex-specific than common linkage of blood pressure and most blood-pressure-related traits. Male- and female-specific regions were determined on different chromosomes for blood pressures (Chrs. 2 and 7 vs 5 and 11), body weight (Chrs. 10 vs 18), and blood glucose (Chr. 17 vs 20). A linkage in both males and females was shown for serum triglycerides on chromosomes 6 and 17, respectively, and blood glucose on chromosome 15. For serum total cholesterol, a significant linkage was found on chromosome 14 only in males. Our findings not only indicate the complex character of quantitative traits per se but also show impressively their dependence on sex, age, and strains in cosegregation analysis.

Sex-Specific and Sex-Independent Quantitative Trait Loci for Facets of the Metabolic Syndrome in WOKW Rats

WOKW rats develop a complete metabolic syndrome closely resembling human disease. Since genetic studies using male (WOKW x DA)F2 progeny showed that several independent genetic factors were involved, a polygenic basis for the syndrome in WOKW was assumed. However, because the metabolic syndrome in human clearly demonstrates sex differences, we have extended our study to include both male and female (WOKW x DA)F2 progeny in a genome-wide scan. Male- or female-specific quantitative trait loci (QTLs) were mapped for body weight, body mass index, adiposity index and serum insulin on chromosomes 1 and 5, serum triglycerides on chromosomes 4, 7, 11, and 16, serum total and high density lipoprotein cholesterol on chromosomes 3, 4, 5, 10, and 17, and serum leptin on chromosomes 8 and 16 as well as blood glucose and glucose tolerance (AUC) on chromosomes 3, 4 and 17. QTLs for both, males and females were only found for body weight on chromosome 1 and for serum total cholesterol on chromosome 3 and 10. These findings clearly demonstrate that there are sex-specific and sex-independent QTLs for facets of the metabolic syndrome in WOKW rats.

Molecular genetics of essential hypertension: recent results and emerging strategies

Efforts to identify hypertension-predisposition genetic loci have focused largely on candidate gene strategies, in which specific candidates have been tested for linkage and association with blood pressure or the diagnosis of hypertension. A variety of candidate genes have been investigated, including loci involving the renin-angiotensin-aldosterone system, sodium epithelial channel, catecholaminergic/adrenergic function, renal kallikrein system, alpha-adducin, and others involving lipoprotein metabolism, hormone receptors, and growth factors. These studies, and more recently, several genome-wide scans, have yielded highly promising results suggesting a number of potential candidate genes and genomic regions that may contribute to blood pressure variation. The results also point to the need for more robust phenotypes that are intermediate in the pathogenetic development of high blood pressure. Additional methods and strategies for improving genetic studies of human hypertension include comparative genomics, in which results from animal studies are used to target potential blood pressure loci, the use of newly developed quantitative tests of linkage and association, comprehensive single-nucleotide polymorphism discovery in candidate loci, and the use of single-nucleotide polymorphisms in cladistic/haplotype analyses and genome-wide searches.