Mapping of genetic loci predisposing to hypertriglyceridaemia in the hereditary hypertriglyceridaemic rat: analysis of genetic association with related traits of the insulin resistance syndrome

Towards Precision Medicine for Hypertension: A Review of Genomic, Epigenomic, and Microbiomic Effects on Blood Pressure in Experimental Rat Models and Humans

Compelling evidence for the inherited nature of essential hypertension has led to extensive research in rats and humans. Rats have served as the primary model for research on the genetics of hypertension resulting in identification of genomic regions that are causally associated with hypertension. In more recent times, genome-wide studies in humans have also begun to improve our understanding of the inheritance of polygenic forms of hypertension. Based on the chronological progression of research into the genetics of hypertension as the "structural backbone," this review catalogs and discusses the rat and human genetic elements mapped and implicated in blood pressure regulation. Furthermore, the knowledge gained from these genetic studies that provide evidence to suggest that much of the genetic influence on hypertension residing within noncoding elements of our DNA and operating through pervasive epistasis or gene-gene interactions is highlighted. Lastly, perspectives on current thinking that the more complex "triad" of the genome, epigenome, and the microbiome operating to influence the inheritance of hypertension, is documented. Overall, the collective knowledge gained from rats and humans is disappointing in the sense that major hypertension-causing genes as targets for clinical management of essential hypertension may not be a clinical reality. On the other hand, the realization that the polygenic nature of hypertension prevents any single locus from being a relevant clinical target for all humans directs future studies on the genetics of hypertension towards an individualized genomic approach.

Novel genes on rat chromosome 10 are linked to body fat mass, preadipocyte number and adipocyte size

BACKGROUND

The genetic architecture of obesity is multifactorial. We have previously identified a quantitative trait locus (QTL) on rat chromosome 10 in a F2 cross of Wistar Ottawa Karlsburg (WOKW) and Dark Agouti (DA) rats responsible for obesity-related traits. The QTL was confirmed in congenic DA.WOKW10 rats. To pinpoint the region carrying causal genes, we established two new subcongenic lines, L1 and L2, with smaller refined segments of chromosome 10 to identify novel candidate genes.

METHODS

All lines were extensively characterized under different diet conditions. We employed transcriptome analysis in visceral adipose tissue (VAT) by RNA-Seq technology to identify potential underlying genes in the segregating regions. Three candidate genes were measured in human paired samples of VAT and subcutaneous (SC) AT (SAT) (N=304) individuals with a wide range of body weight and glucose homeostasis parameters.

RESULTS

DA.WOKW and L1 subcongenic lines were protected against body fat gain under high-fat diet (HFD), whereas L2 and DA had significantly more body fat after high-fat feeding. Interestingly, adipocyte size distribution in SAT and epigonadal AT of L1 subcongenic rats did not undergo typical ballooning under HFD and the number of preadipocytes in AT was significantly elevated in L2 compared with L1 and parental rats. Transcriptome analysis identified three candidate genes in VAT on rat chromosome 10. In humans, these candidate genes were differentially expressed between SAT and VAT. Moreover, HID1 mRNA significantly correlates with parameters of obesity and glucose metabolism.

CONCLUSIONS

Our data suggest novel candidate genes for obesity that map on rat chromosome 10 in an interval 102.2-104.7 Mb and are strongly associated with body fat mass regulation, preadipocyte number and adipocyte size in rats. Among those genes, AT head involution defective (HID1) mRNA expression may be relevant for human fat distribution and glucose homeostasis.

Glucuronic acid epimerase is associated with plasma triglyceride and high-density lipoprotein cholesterol levels in Turks

We narrowed chromosome 15q21-23 linkage to plasma high-density lipoprotein cholesterol (HDL-C) levels in Turkish families by fine mapping, then focused on glucuronic acid epimerase (GLCE), a heparan sulfate proteoglycan (HSPG) biosynthesis enzyme. HSPGs participate in lipid metabolism along with apolipoprotein (apo) E. Of 31 SNPs in the GLCE locus, nine analyzed by haplotype were associated with HDL-C and triglyceride levels (permuted p = 0.006 and 0.013, respectively) in families. Of five tagging GLCE SNPs in two cohorts of unrelated subjects, three (rs16952868, rs11631403, and rs3865014) were associated with triglyceride and HDL-C levels in males (nonpermuted p < 0.05). The association was stronger in APOE 2/3 subjects (apoE2 has reduced binding to HSPGs) and reached multiple-testing significance (p < 0.05) in both males and females (n= 2612). Similar results were obtained in the second cohort (n= 1164). Interestingly, at the GLCE locus, bounded by recombination hotspots, Turks had a minor allele frequency of SNPs resembling Chinese more than European ancestry; adjoining regions resembled the European pattern. Studies of glce(+/-) apoe(-/-) mice fed a chow or high-fat diet supported a role for GLCE in lipid metabolism. Thus, SNPs in GLCE are associated with triglyceride and HDL-C levels in Turks, and mouse studies support a role for glce in lipid metabolism.

Investigating the effect of genetic background on proteinuria and renal injury using two hypertensive strains

An earlier linkage analysis conducted on a population derived from the Dahl salt-sensitive hypertensive (S) and the spontaneously hypertensive rat (SHR) identified 10 genomic regions linked to several renal and/or cardiovascular traits. In particular, loci on rat chromosomes (RNO) 8 and 13 were linked to proteinuria, albuminuria, and renal damage. At both loci, the S allele was associated with increased proteinuria and renal damage. The current study aimed to confirm the linkage analysis and to evaluate the effect of genetic background on the ability of each locus (either RNO8 or RNO13) to exert a phenotypic difference when placed on a genetic background either susceptible (S rat) or resistant (SHR) to the development of renal disease. Congenic strains developed to transfer genomic segments from either RNO8 or RNO13 from the SHR onto the S genetic background [S.SHR(8) or S.SHR(13)] demonstrated significantly reduced proteinuria and improved renal function. Both congenic strains demonstrated significantly reduced glomerular and tubular injury, with renal interstitial fibrosis as the predominant pathological difference compared with the S. In contrast, transfer of RNO8 or RNO13 genomic regions from the S onto the resistant SHR genetic background [SHR.S(8) or SHR.S(13)] yielded no significant difference in proteinuria or glomerular, tubular, or interstitial injury compared with SHR. These findings demonstrate that genetic context plays a significant and important role in the phenotypic expression of genes influencing proteinuria on RNO8 and RNO13.

A meta-analysis of QTL for diabetes-related traits in rodents

Crossbreeding studies in rodents have identified numerous quantitative trait loci (QTL) that are linked to diabetes-related component traits. To identify genetic consensus regions implicated in insulin action and glucose homeostasis, we have performed a meta-analysis of genomewide linkage scans for diabetes-related traits. From a total of 43 published genomewide scans we assembled a nonredundant collection of 153 QTL for glucose levels, insulin levels, and glucose tolerance. Collectively, these studies include data from 48 different parental strains and >11,000 individual animals. The results of the studies were analyzed by the truncated product method (TPM). The analysis revealed significant evidence for linkage of glucose levels, insulin levels, and glucose tolerance to 27 different segments of the mouse genome. The most prominent consensus regions [localized to chromosomes 2, 4, 7, 9, 11, 13, and 19; logarithm of odds (LOD) scores 10.5-17.4] cover approximately 11% of the mouse genome and collectively contain the peak markers for 47 QTL. Approximately half of these genomic segments also show significant linkage to body weight and adiposity, indicating the presence of multiple obesity-dependent and -independent consensus regions for diabetes-related traits. At least 84 human genetic markers from genomewide scans and >80 candidate genes from human and rodent studies map into the mouse consensus regions for diabetes-related traits, indicating a substantial overlap between the species. Our results provide guidance for the identification of novel candidate genes and demonstrate the presence of numerous distinct consensus QTL regions with highly significant LOD scores that control glucose homeostasis. An interactive physical map of the QTL is available online at http://www.diabesitygenes.org.

Triplet repeat in the Repin1 3'-untranslated region on rat chromosome 4 correlates with facets of the metabolic syndrome

BACKGROUND

Congenic and subcongenic rat strains confirmed the quantitative trait loci (QTLs) for facets of the metabolic syndrome between 60.53 and 77.11 Mb on chromosome 4. The analysis of candidate genes in this region favoured the replication initiator 1 (Repin1) characterized by a SNP in the coding region and a triplet repeat (TTT) in the 3'-untranslated region (3'UTR).

METHODS

We analysed nine rat strains (BB/OK, SHR, F344, BN, DA, LEW, hHTg, WOKW, and their founders WOK-F) and four wild rats on DNA (sequencing) and RNA level (gene expression in blood, liver, subcutaneous, and epididymal adipocytes). In addition, the rats were phenotypically characterized in order to link the rat phenotype to genotype differences in the QTL on chromosome 4.

RESULTS

Wild rats were heterozygous for the SNP (C/T), whereas all the inbred strains were homozygous. The shortest triplet repeat was found in SHR (5) and the highest was found in hHTg and WOKW (11), which developed metabolic disorders. The repeat number correlated with most phenotypic traits studied. Using linear multiple regression analysis with repeat size as the dependent variable and considering all the data of this study, it was clearly demonstrated that not only VLDL cholesterol and serum insulin but also the expression of Repin1 in the liver is significantly associated with the repeat size of the 3'UTR.

CONCLUSIONS

It is concluded that the triplet repeat expansion in 3'UTR is involved in metabolic alterations as found in hHTg and WOKW rats and that the functional unknown gene, Repin1, could be a novel candidate gene for the development of facets of the metabolic syndrome.

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.

Novel double-congenic strain reveals effects of spontaneously hypertensive rat chromosome 2 on specific lipoprotein subfractions and adiposity

We have developed a new, double-congenic rat strain BN- Lx.SHR2, which carries two distinct segments of chromosome 2 introgressed from the spontaneously hypertensive rat strain (SHR) into the genetic background of congenic strain BN- Lx, which was previously shown to express variety of metabolic syndrome features. In 16-wk-old male rats of BN- Lx and BN- Lx.SHR2 strains, we compared their glucose tolerance and triacylglycerol and cholesterol concentrations in 20 lipoprotein subfractions and the lipoprotein particle sizes under conditions of feeding standard and high-sucrose diets. Introgression of two distinct SHR-derived chromosome 2 segments resulted in decreased adiposity together with aggravation of glucose intolerance in the double-congenic strain. The BN- Lx.SHR2 rats were more sensitive to sucrose-induced rise in triacylglycerolemia. Although the total cholesterol concentrations of the two strains were comparable after the standard diet and even lower in BN- Lx.SHR2 after sucrose feeding, detailed analysis revealed that under both dietary conditions, the double-congenic strain had significantly higher cholesterol concentrations in low-density lipoprotein fractions and lower high-density lipoprotein fractions. We established a new inbred model showing dyslipidemia and mild glucose intolerance without obesity, attributable to specific genomic regions. For the first time, the chromosome 2 segments of SHR origin are shown to influence other than blood pressure-related features of metabolic syndrome or to be involved in relevant nutrigenomic interactions.

Protein remodeling of the heart ventricles in hereditary hypertriglyceridemic rat: effect of ACE-inhibition

The aim of this study was to determine whether protein remodeling of the heart ventricles and remodeling of the aorta were present in hereditary hypertriglyceridemic (hHTG) rats and whether treatment with the angiotensin-converting enzyme inhibitor, captopril could prevent these alterations. Three groups of rats were investigated in a four week experiment control Wistar /C/rats, hHTg rats, hHTg rats given captopril (100 mg/kg/day) (hHTg + CAP). In the hHTg group, the increased systolic blood pressure (SBP) was associated with hypertrophy of the LV and RV. Protein profile analysis revealed an enhancement of metabolic protein concentration in both ventricles. The concentration of total collagenous proteins was not changed in either ventricles. However, alterations in composition of cardiac collagen were detected, characterized by higher concentration of hydroxyproline in pepsin-insoluble fraction and lower concentration of hydroxyproline in pepsin soluble faction in the LV. Hypertrophy of aorta, associated with the reduction of nitric oxide dependent relaxation, was also present in hHTG rats. Captopril normalized SBP, reduced left ventricular hypertrophy (LVH), diminished metabolic protein concentration in both ventricles, and improved NO-dependent relaxation of the aorta. Furthermore, captopril partially reversed alterations in hydroxyproline concentration in soluble and insoluble collagenous fractions of the LV. We conclude that hypertrophy of both ventricles and the aorta are present in hHTG rats, along with protein remodeling of both ventricles. Captopril partially prevented left ventricular hypertrophy development and protein remodeling of the myocardium.

Cloning and characterization of Munc18c(L), a novel murine Munc18c gene paralog

We have identified and characterized a new mouse gene sequence, Munc18c(L), that appears closely related to the syntaxin-binding protein, Munc18c. The novel Munc18c(L) gene is comprised of 2 exons separated by a 600bp intron sequence with non-consensus donor and acceptor sites. Exons 1 and 2 of Munc18c(L) overlap with exons 1 through half of 9 of the Munc18c gene. The deduced amino acid sequence of Munc18c(L) is 271 amino acids long with homology to Munc18c protein ending at position 250. RT-PCR of murine tissues showed expression of Munc18c(L) in various tissues. RT-PCR carried out with a primer spanning the ATG codon and another one specific for the exon 2 of Munc18c(L) revealed two different transcripts of 0.8 and 1.4kbp in length. Using 5'-RACE, the start of Munc18c(L) exon 1 matches the one predicted for Munc18c, but the proximal promoter differ. This first identification of Munc18c(L) is vital in differentiating between Munc18c(L) and Munc18c and their potential roles in insulin-mediated glucose uptake.

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.

Mapping of genetic determinants of the sympathoneural response to stress

Activation of the sympathoadrenal system (SAS, comprising the sympathetic nervous system and the adrenal medulla) in response to stressful stimuli is an important defense mechanism as well as a contributor to several cardiovascular diseases. There is variability in the SAS response to stress, although the extent to which this is genetically regulated is unclear. Some rodent models, including the hereditary hypertriglyceridemic (hHTg) rat, are hyperresponsive to stress. We investigated whether quantitative trait loci (QTLs) that affect sympathoadrenal response to stress could be identified. Second filial generation rats ( n = 189) derived from a cross of the hHTg rat and the Brown Norway rat had plasma norepinephrine (NE) and epinephrine (Epi) levels, indices of activation of the sympathoneural and adrenal medulla components, respectively, measured in the resting state and in response to an immobilization stress. Responses were assessed early (20 min) and late (120 min) after the application of the stress. A genome scan was conducted using 153 microsatellite markers. Two QTLs (maximum peak LOD scores of 4.17 and 3.52, respectively) influencing both the early and late plasma NE response to stress were found on chromosome 10. Together, the QTLs accounted for ∼20% of the total variation in both the early and late NE responses in the F2rats. Interestingly, the QTLs had no effect on plasma Epi response to stress. These findings provide evidence for a genetic determination of the response of a specific component of the SAS response to stress. Genetically determined variation in sympathetic nervous system response to stress may contribute to cardiovascular diseases.

Phenotypic and genetic analyses of subcongenic BB.SHR rat lines shorten the region on chromosome 4 bearing gene(s) for underlying facets of metabolic syndrome

Congenic BB.SHR ( D4Got41-Npy-Tacr1; BB.4S) rats develop an incomplete metabolic syndrome with obesity, hyperleptinemia, and dyslipidemia compared with their progenitor strain, the diabetes-prone BB/OK rat. To narrow down the underlying gene(s), two subcongenic BB.SHR rat lines, briefly termed BB.4Sa and BB.4Sb, were generated. Male BB.4S ( n = 20), BB.4Sa ( n = 24), and BB.4Sb ( n = 26) were longitudinally characterized for facets of the metabolic syndrome and analyzed for expression of genes located in the region of interest in liver and blood. Body weight gain was comparable, serum triglycerides and leptin were significantly increased, and total cholesterol and HDL-cholesterol ratio were decreased in BB.4S compared with both subcongenics. Serum insulin was significantly higher in BB.4S and BB.4Sa than in BB.4Sb. The adiposity index showed a graduated decrease from BB.6S to BB.4Sb. Obvious differences in relative expression were found in 6 of 10 genes in liver and in 2 of 9 genes in blood. Only one gene, the eukaryotic translation initiation factor 2α kinase 3 ( Eif2ak3 also called Perk or Pek), was significantly less expressed in liver and in blood of both subcongenic BB.4Sa and BB.4Sb compared with their “parental” BB.4S rats. Based on the phenotype and genotype in BB.4S and its subcongenic derivatives, the most important region on chromosome 4 can be said to lie between D4Got72 and Tacr1. Eif2ak3 is mapped in this region. Considering the function of Eif2ak3, it may be a candidate gene for the development of glucose intolerance found in both subcongenics but not in BB.4S. Allelic variants between BB/OK and SHR could influence Eif2ak3 function, possibly leading not only to glucose intolerance but also to the disturbances in hepatic and renal function found in human Wolcott-Rallison syndrome.