Augmented rififylin is a risk factor linked to aberrant cardiomyocyte function, short-QT interval and hypertension

E3 ubiquitin ligase rififylin has yin and yang effects on rabbit cardiac transient outward potassium currents (Ito) and corresponding channel proteins

Genome-wide association studies have reported a correlation between a SNP of the RING finger E3 ubiquitin protein ligase rififylin (RFFL) and QT interval variability in humans (Newton-Cheh et al., 2009). Previously, we have shown that RFFL downregulates expression and function of the human-like ether-a-go-go-related gene potassium channel and corresponding rapidly activating delayed rectifier potassium current (IKr) in adult rabbit ventricular cardiomyocytes. Here, we report that RFFL also affects the transient outward current (Ito), but in a peculiar way. RFFL overexpression in adult rabbit ventricular cardiomyocytes significantly decreases the contribution of its fast component (Ito,f) from 35% to 21% and increases the contribution of its slow component (Ito,s) from 65% to 79%. Since Ito,f in rabbits is mainly conducted by Kv4.3, we investigated the effect of RFFL on Kv4.3 expressed in HEK293A cells. We found that RFFL overexpression reduced Kv4.3 expression and corresponding Ito,f in a RING domain-dependent manner in the presence or absence of its accessory subunit Kv channel-interacting protein 2. On the other hand, RFFL overexpression in Kv1.4-expressing HEK cells leads to an increase in both Kv1.4 expression level and Ito,s, similarly in a RING domain-dependent manner. Our physiologically detailed rabbit ventricular myocyte computational model shows that these yin and yang effects of RFFL overexpression on Ito,f, and Ito,s affect phase 1 of the action potential waveform and slightly decrease its duration in addition to suppressing IKr. Thus, RFFL modifies cardiac repolarization reserve via ubiquitination of multiple proteins that differently affect various potassium channels and cardiac action potential duration.

Quantitative proteomic analysis to identify differentially expressed proteins in the persistent atrial fibrillation using TMT coupled with nano-LC-MS/MS

Persistent atrial fibrillation (PeAF) is a progressive cardiovascular disease with a high risk for most patients after diagnosis. Poor molecular description of PeAF has led to unsatisfactory interpretation of the pathogenesis of it, resulting in the lack of effective treatments. The aim of the present study was to find several new potential biomarkers for early prevention, diagnosis and treatment of this disease and explore the underlying molecular mechanisms. An absolute quantitation Tandem Mass Tag (TMT)-liquid chromatography-tandem mass spectrometry (LC-MS/MS) approach was applied to identify differentially expressed proteins (DEPs) in left atrial appendage. Totally, 4682 proteins were identified and 4159 proteins were quantified. Compared with control subjects, 118 DEPs (85 upregulated proteins and 33 downregulated proteins) were identified in the atrial tissues of PeAF patients. Using String software, a regulatory network containing 87 nodes and 244 edges was built, and the functional enrichment showed that DEPs were predominantly involved in protein digestion and absorption, regulation of metabolism and focal adhesion. Four proteins, collagen 1 (COL-I), collagen 2 (COL-II), ras-related protein 1 (RAP1) and leucine-rich alpha-2-glycoprotein 1 (LRG1) were selected for validation using Western blot analysis to distinguish PeAF patients and control subjects. The present results provide a comprehensive understanding of the pathophysiological mechanisms of PeAF and the validated biomarkers for the diagnosis of PeAF, which facilitate the development of therapeutic targets.

Chromosomal Substitution Strategies to Localize Genomic Regions Related to Complex Traits

Chromosomal substitution strategies provide a powerful tool to anonymously reveal the relationship between DNA sequence variants and a normal or disease phenotype of interest. Even in this age of CRISPR-Cas9 genome engineering, the knockdown or overexpression of a gene provides relevant information to our understanding of complex disease only when a close association of an allelic variant with the phenotype has first been established. Limitations of genetic linkage approaches led to the development of more efficient breeding strategies to substitute chromosomal segments from one animal strain into the genetic background of a different strain, enabling a direct comparison of the phenotypes of the strains with variant(s) that differ only at a defined locus. This substitution can be a whole chromosome (consomic), a part of a chromosome (congenic), or as small as only a single or several alleles (subcongenics). In contrast to complete knockout of a specific candidate gene of interest, which simply studies the effects of complete elimination of the gene, the substitution of naturally occurring variants can provide special insights into the functional actions of wild-type alleles. Strategies for production of these inbred strains are reviewed, and a number of examples are used to illustrate the utility of these model systems. Consomic/congenic strains provide a number of experimental advantages in the study of functions of genes and their variants, which are emphasized in this article, such as replication of experimental studies; determination of temporal relationships throughout a life; rigorously controlled experiments in which relations between genotype and phenotype can be tested with the confounding effects of heterogeneous genetic backgrounds, both targeted and multilayered; and "omic" studies performed at many levels of functionality, from molecules to organelles, cells to organs, and organs to organismal behavior across the life span. The application of chromosomal substitution strategies and development of consomic/congenic rat and mouse strains have greatly expanded our knowledge of genomic variants and their phenotypic relationship to physiological functions and to complex diseases such as hypertension and cancer. © 2020 American Physiological Society. Compr Physiol 10:365-388, 2020.

Dissecting Epistatic QTL for Blood Pressure in Rats: Congenic Strains versus Heterogeneous Stocks, a Reality Check

Advances in molecular genetics have provided well-defined physical genetic maps and large numbers of genetic markers for both model organisms and humans. It is now possible to gain a fundamental understanding of the genetic architecture underlying quantitative traits, of which blood pressure (BP) is an important example. This review emphasizes analytical techniques and results obtained using the Dahl salt-sensitive (S) rat as a model of hypertension by presenting results in detail for three specific chromosomal regions harboring genetic elements of increasing complexity controlling BP. These results highlight the critical importance of genetic interactions (epistasis) on BP at all levels of structure, intragenic, intergenic, intrachromosomal, interchromosomal, and across whole genomes. In two of the three examples presented, specific DNA structural variations leading to biochemical, physiological, and pathological mechanisms are well defined. This proves the usefulness of the techniques involving interval mapping followed by substitution mapping using congenic strains. These classic techniques are compared to newer approaches using sophisticated statistical analysis on various segregating or outbred model-organism populations, which in some cases are uniquely useful in demonstrating the existence of higher-order interactions. It is speculated that hypertension as an outlier quantitative phenotype is dependent on higher-order genetic interactions. The obstacle to the identification of genetic elements and the biochemical/physiological mechanisms involved in higher-order interactions is not theoretical or technical but the lack of future resources to finish the job of identifying the individual genetic elements underlying the quantitative trait loci for BP and ascertaining their molecular functions. © 2019 American Physiological Society. Compr Physiol 9:1305-1337, 2019.

LITAF (Lipopolysaccharide-Induced Tumor Necrosis Factor) Regulates Cardiac L-Type Calcium Channels by Modulating NEDD (Neural Precursor Cell Expressed Developmentally Downregulated Protein) 4-1 Ubiquitin Ligase

BACKGROUND

The turnover of cardiac ion channels underlying action potential duration is regulated by ubiquitination. Genome-wide association studies of QT interval identified several single-nucleotide polymorphisms located in or near genes involved in protein ubiquitination. A genetic variant upstream of LITAF (lipopolysaccharide-induced tumor necrosis factor) gene prompted us to determine its role in modulating cardiac excitation.

METHODS

Optical mapping was performed in zebrafish hearts to determine Ca2+ transients. Live-cell confocal calcium imaging was performed on adult rabbit cardiomyocytes to determine intracellular Ca2+handling. L-type calcium channel (LTCC) current (ICa,L) was measured using whole-cell recording. To study the effect of LITAF on Cav1.2 (L-type voltage-gated calcium channel 1.2) channel expression, surface biotinylation, and Westerns were performed. LITAF interactions were studied using coimmunoprecipitation and in situ proximity ligation assay.

RESULTS

LITAF knockdown in zebrafish resulted in a robust increase in calcium transients. Overexpressed LITAF in 3-week-old rabbit cardiomyocytes resulted in a decrease in ICa,L and Cavα1c abundance, whereas LITAF knockdown increased ICa,L and Cavα1c protein. LITAF-overexpressing decreases calcium transients in adult rabbit cardiomyocytes, which was associated with lower Cavα1c levels. In tsA201 cells, overexpressed LITAF downregulated total and surface pools of Cavα1c via increased Cavα1c ubiquitination and its subsequent lysosomal degradation. We observed colocalization between LITAF and LTCC in tsA201 and cardiomyocytes. In tsA201, NEDD (neural precursor cell expressed developmentally downregulated protein) 4-1, but not its catalytically inactive form NEDD4-1-C867A, increased Cavα1c ubiquitination. Cavα1c ubiquitination was further increased by coexpressed LITAF and NEDD4-1 but not NEDD4-1-C867A. NEDD4-1 knockdown abolished the negative effect of LITAF on ICa,L and Cavα1c levels in 3-week-old rabbit cardiomyocytes. Computer simulations demonstrated that a decrease of ICa,L current associated with LITAF overexpression simultaneously shortened action potential duration and decreased calcium transients in rabbit cardiomyocytes.

CONCLUSIONS

LITAF acts as an adaptor protein promoting NEDD4-1-mediated ubiquitination and subsequent degradation of LTCC, thereby controlling LTCC membrane levels and function and thus cardiac excitation.

QTL mapping of rat blood pressure loci on RNO1 within a homologous region linked to human hypertension on HSA15

Fine-mapping of regions linked to the inheritance of hypertension is accomplished by genetic dissection of blood pressure quantitative trait loci (BP QTLs) in rats. The goal of the current study was to further fine-map two genomic regions on rat chromosome 1 with opposing blood pressure effects (BP QTL1b1 and BP QTL1b1a), the homologous region of which on human chromosome 15 harbors BP QTLs. Two new substrains were constructed and studied from the previously reported BP QTL1b1, one having significantly lower systolic BP by 17 mmHg than that of the salt-sensitive (S) rat (P = 0.007). The new limits of BP QTL1b1 were between 134.09 Mb and 135.40 Mb with a 43% improvement from the previous 2.31 Mb to the current 1.31 Mb interval containing 4 protein-coding genes (Rgma, Chd2, Fam174b, and St8sia2), 2 predicted miRNAs, and 4 lncRNAs. One new substrain was constructed and studied from the previously reported BPQTL1b1a having a significantly higher systolic BP by 22 mmHg (P = 0.006) than that of the S rat. The new limits of BPQTL1b1a were between 133.53 Mb and 134.52 Mb with a 32% improvement from the previous1.45 Mb to the current 990.21 Kb interval containing 1 protein-coding gene, Mctp2, and a lncRNA. The congenic segments of these two BP QTLs overlapped between 134.09 Mb and 134.52 Mb. No exonic variants were detected in any of the genes. These findings reiterate complexity of genetic regulation of BP within QTL regions, where elements beyond protein-coding sequences could be factors in controlling BP.

Trafficking of the human ether-a-go-go-related gene (hERG) potassium channel is regulated by the ubiquitin ligase rififylin (RFFL)

The QT interval is an important diagnostic feature on surface electrocardiograms because it reflects the duration of the ventricular action potential. A previous genome-wide association study has reported a significant linkage between a single-nucleotide polymorphism ∼11.7 kb downstream of the gene encoding the RING finger ubiquitin ligase rififylin (RFFL) and variability in the QT interval. This, along with results in animal studies, suggests that RFFL may have effects on cardiac repolarization. Here, we sought to determine the role of RFFL in cardiac electrophysiology. Adult rabbit cardiomyocytes with adenovirus-expressed RFFL exhibited reduced rapid delayed rectifier current (I _Kr). Neonatal rabbit cardiomyocytes transduced with RFFL-expressing adenovirus exhibited reduced total expression of the potassium channel ether-a-go-go-related gene (rbERG). Using transfections of 293A cells and Western blotting experiments, we observed that RFFL and the core-glycosylated form of the human ether-a-go-go-related gene (hERG) potassium channel interact. Furthermore, RFFL overexpression led to increased polyubiquitination and proteasomal degradation of hERG protein and to an almost complete disappearance of I _Kr, which depended on the intact RING domain of RFFL. Blocking the ER-associated degradation (ERAD) pathway with a dominant-negative form of the ERAD core component, valosin-containing protein (VCP), in 293A cells partially abolished RFFL-mediated hERG degradation. We further substantiated the link between RFFL and ERAD by showing an interaction between RFFL and VCP in vitro We conclude that RFFL is an important regulator of voltage-gated hERG potassium channel activity and therefore cardiac repolarization and that this ubiquitination-mediated regulation requires parts of the ERAD pathway.

Fine mapping of epistatic genetic determinants of blood pressure on rat chromosome 5

BACKGROUND

Previously, using linkage analysis and substitution mapping, two closely-linked interactive blood pressure quantitative trait loci (QTLs), BP QTL1 and BP QTL2, were located within a 13.96 Mb region from 117894038 to 131853815 bp (RGSC 3.4 version) on rat chromosome 5 (RNO5). This was done by using a series of congenic strains consisting of genomic segments of the Dahl salt-sensitive (S) rat substituted with that of the normotensive Lewis (LEW) rat. The interactive nature of the two loci was further confirmed by the construction and characterization of a panel of S.LEW bicongenic strains and corresponding S.LEW monocongenic strains, which provided definitive evidence of epistasis (genetic interaction) between BP QTL1 (7.77 Mb) and BP QTL2 (4.18 Mb). The purpose of this work was to further map these interacting QTLs.

METHOD

A new panel of seven new S.LEW bicongenic strains was constructed and characterized for BP.

RESULTS

The data obtained from these new strains further resolved BP QTL1 from 7.77 to 2.93 Mb. Further, BP QTL2 was traceable as not being a single QTL, but a composite of at least three QTLs, LEW alleles at two of which located within 2.26 Mb and 175 kb lowered BP but the third one located within 1.31 Mb increased BP.

CONCLUSION

Lack of coding variation within any of the regions further mapped within the previous QTL2 suggests noncoding variation as likely responsible for the observed epistasis.

Targeted disruption of regulated endocrine-specific protein ( Resp18) in Dahl SS/Mcw rats aggravates salt-induced hypertension and renal injury

Hypertension is a classic example of a complex polygenic trait, impacted by quantitative trait loci (QTL) containing candidate genes thought to be responsible for blood pressure (BP) control in mammals. One such mapped locus is on rat chromosome 9, wherein the proof for a positional candidate gene, regulated endocrine-specific protein-18 ( Resp18) is currently inadequate. To ascertain the status of Resp18 as a BP QTL, a custom targeted gene disruption model of Resp18 was developed on the Dahl salt-sensitive (SS) background. As a result of this zinc-finger nuclease (ZFN)-mediated disruption, a 7 bp deletion occurred within exon 3 of the Resp18 locus. Targeted disruption of Resp18 gene locus in SS rats decreases its gene expression in both heart and kidney tissues regardless of their dietary salt level. Under a high-salt dietary regimen, both systolic and diastolic BP of Resp18^mutant rats were significantly increased compared with SS rats. Resp18^mutant rats demonstrated increased renal damage, as evidenced by higher proteinuria and increased renal fibrosis compared with SS rats. Furthermore, under a high-salt diet regimen, the mean survival time of Resp18^mutant rats was significantly reduced compared with SS rats. These findings serve as evidence in support of Resp18 as a gene associated with the development of hypertension and renal disease.

A personalized, multiomics approach identifies genes involved in cardiac hypertrophy and heart failure

A traditional approach to investigate the genetic basis of complex diseases is to identify genes with a global change in expression between diseased and healthy individuals. However, population heterogeneity may undermine the effort to uncover genes with significant but individual contribution to the spectrum of disease phenotypes within a population. Here we investigate individual changes of gene expression when inducing hypertrophy and heart failure in 100 + strains of genetically distinct mice from the Hybrid Mouse Diversity Panel (HMDP). We find that genes whose expression fold-change correlates in a statistically significant way with the severity of the disease are either up or down-regulated across strains, and therefore missed by a traditional population-wide analysis of differential gene expression. Furthermore, those "fold-change" genes are enriched in human cardiac disease genes and form a dense co-regulated module strongly interacting with the cardiac hypertrophic signaling network in the human interactome. We validate our approach by showing that the knockdown of Hes1, predicted as a strong candidate, induces a dramatic reduction of hypertrophy by 80-90% in neonatal rat ventricular myocytes. Our results demonstrate that individualized approaches are crucial to identify genes underlying complex diseases as well as to develop personalized therapies.

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.

Positional cloning of quantitative trait nucleotides for blood pressure and cardiac QT-interval by targeted CRISPR/Cas9 editing of a novel long non-coding RNA

Multiple GWAS studies have reported strong association of cardiac QT-interval to a region on HSA17. Interestingly, a rat locus homologous to this region is also linked to QT-intervals. The high resolution positional mapping study located the rat QT-interval locus to a <42.5kb region on RNO10. This region contained no variants in protein-coding sequences, but a prominent contiguous 19bp indel polymorphism was noted within a novel predicted long non-coding RNA (lncRNA), which we named as Rffl-lnc1. To assess the candidacy of this novel lncRNA on QT-interval, targeted CRISPR/Cas9 based genome-engineering approaches were applied on the rat strains used to map this locus. Targeted disruption of the rat Rffl-lnc1 locus caused aberrant, short QT-intervals and elevated blood pressure. Further, to specifically examine the significance of the 19bp polymorphism within the Rffl-lnc1 locus, a CRISPR/Cas9 based targeted knock-in rescue model was constructed by inserting the 19bp into the strain which contained the deletion polymorphism. The knock-in alleles successfully rescued the aberrant QT-interval and blood pressure phenotypes. Further studies revealed that the 19bp polymorphism was necessary and sufficient to recapitulate the phenotypic effect of the previously mapped <42.5kb rat locus. To our knowledge, this study is the first demonstration of a combination of both CRISPR/Cas9 based targeted disruption as well as CRISPR/Cas9 based targeted knock-in rescue approaches applied for a mammalian positional cloning study, which defines the quantitative trait nucleotides (QTNs) within a rat long non-coding RNA as being important for the pleiotropic regulation of both cardiac QT-intervals and blood pressure.

Diseases of the cardiovascular system such as essential hypertension do not have a clear cause, but are known to run in families. The inheritance patterns of essential hypertension and other cardiac diseases suggest that they are not due to a single defective gene but instead are caused by multiple genetic defects that are inherited together in a patient. This complex inheritance makes it difficult to pinpoint the underlying defects. Here, we describe a panel of genetically-engineered rats, using which we have discovered a novel gene, which does not code for any protein, as a gene required for maintenance of normal blood pressure. Structural defects within this non-coding RNA cause hypertension and cardiac short-QT interval. Further, by performing genome surgery to correct the gene defect, we demonstrate the precise error in nucleotides that was inherited and caused hypertension and cardiac short-QT interval syndrome.

Postnatal Cardiovascular Consequences in the Offspring of Pregnant Rats Exposed to Smoking and Smoking Cessation Pharmacotherapies

Approximately 20% of pregnant women smoke despite intentions to quit. Smoking cessation drugs, such as nicotine replacement therapy (NRT) and bupropion, are recommended treatments. Adverse cardiovascular outcomes in offspring have raised concerns about NRT's safety during pregnancy. However, the effect of bupropion is unknown. Using a rat model, we determined whether NRT and bupropion interventions during pregnancy are safer than continued smoking on offspring's cardiovascular function. Male offspring of controls and dams exposed to cigarette smoke (1.6 packs/day, inhalation), nicotine (2 mg/kg/d subcutaneously), and bupropion (13 mg/kg twice daily orally) were assessed for fetoplacental weight, cardiac function, blood pressure, and vascular reactivity. Fetoplacental weights were decreased and spontaneous beating and intracellular calcium in neonatal cardiomyocytes were increased in smoking, nicotine, and bupropion offspring; however, these effects were more accentuated in smoking followed by nicotine and bupropion offspring. Increased heart rate and decreased cardiac output, stroke volume, and left ventricular percent posterior wall thickening were observed in smoking, nicotine, and bupropion offspring. The left ventricular mass was reduced in smoking and nicotine but not in bupropion offspring. Blood pressure was higher with decreased endothelium-dependent relaxation and exaggerated vascular contraction to angiotensin II in smoking and nicotine offspring, with more pronounced dysfunctions in smoking than nicotine offspring. Maternal bupropion did not impact offspring's blood pressure, endothelium-dependent relaxation, and vascular contraction. In conclusion, maternal nicotine intervention adversely affects offspring's cardiovascular outcomes, albeit less severely than continued smoking. However, bupropion causes cardiac derangement in offspring but does not adversely affect blood pressure and vascular function.

High-resolution mapping of a novel rat blood pressure locus on chromosome 9 to a region containing the Spp2 gene and colocalization of a QTL for bone mass

Through linkage analysis of the Dahl salt-sensitive (S) rat and the spontaneously hypertensive rat (SHR), a blood pressure (BP) quantitative trait locus (QTL) was previously located on rat chromosome 9. Subsequent substitution mapping studies of this QTL revealed multiple BP QTLs within the originally identified logarithm of odds plot by linkage analysis. The focus of this study was on a 14.39 Mb region, the distal portion of which remained unmapped in our previous studies. High-resolution substitution mapping for a BP QTL in the setting of a high-salt diet indicated that an SHR-derived congenic segment of 787.9 kb containing the gene secreted phosphoprotein-2 (Spp2) lowered BP and urinary protein excretion. A nonsynonymous G/T polymorphism in the Spp2 gene was detected between the S and S.SHR congenic rats. A survey of 45 strains showed that the T allele was rare, being detected only in some substrains of SHR and WKY. Protein modeling prediction through SWISSPROT indicated that the predicted protein product of this variant was significantly altered. Importantly, in addition to improved cardiovascular and renal function, high salt-fed congenic animals carrying the SHR T variant of Spp2 had significantly lower bone mass and altered bone microarchitecture. Total bone volume and volume of trabecular bone, cortical thickness, and degree of mineralization of cortical bone were all significantly reduced in congenic rats. Our study points to opposing effects of a congenic segment containing the prioritized candidate gene Spp2 on BP and bone mass.

Pleiotropic Effect of a High Resolution Mapped Blood Pressure QTL on Tumorigenesis

This study is focused on a translationally significant, genome-wide-association-study (GWAS) locus for cardiovascular disease (QT-interval) on human chromosome 17. We have previously validated and high resolution mapped the homologous genomic segment of this human locus to <42.5 kb on rat chromosome 10. This <42.5 kb segment in rats regulates both QT-interval and blood pressure and contains a single protein-coding gene, rififylin (Rffl). The expression of Rffl in the hearts and kidneys is differential between Dahl S and S.LEW congenic rats, which are the strains used for mapping this locus. Our previous study points to altered rate of endocytic recycling as the underlying mechanism, through which Rffl operates to control both QT-interval and blood pressure. Interestingly, Rffl also contributes to tumorigenesis by repressing caspases and tumor suppressor genes. Moreover, the expression of Methyl-CpG Binding Domain Protein 2 (Mbd2) in the hearts and kidneys is also higher in the S.LEW congenic strain than the background (control) Dahl S strain. Mbd2 can repress methylated tumor suppressor genes. These data suggest that the S.LEW congenic strain could be more susceptible to tumorigenesis. To test this hypothesis, the S and S.LEW strains were compared for susceptibility to azoxymethane-induced colon tumors. The number of colon tumors was significantly higher in the S.LEW congenic strain compared with the S rat. Transcriptomic analysis confirmed that the chemical carcinogenesis pathway was significantly up-regulated in the congenic strain. These studies provide evidence for a GWAS-validated genomic segment on rat chromosome 10 as being important for the regulation of cardiovascular function and tumorigenesis.

Dr Lewis Kitchener Dahl, the Dahl rats, and the "inconvenient truth" about the genetics of hypertension

Lewis K. Dahl is regarded as an iconic figure in the field of hypertension research. During the 1960s and 1970s he published several seminal articles in the field that shed light on the relationship between salt and hypertension. Further, the Dahl rat models of hypertension that he developed by a selective breeding strategy are among the most widely used models for hypertension research. To this day, genetic studies using this model are ongoing in our laboratory. While Dr. Dahl is known for his contributions to the field of hypertension, very little, if any, of his personal history is documented. This article details a short biography of Dr. Lewis Dahl, the history behind the development of the Dahl rats and presents an overview of the results obtained through the genetic analysis of the Dahl rat as an experimental model to study the inheritance of hypertension.

Mutation within the hinge region of the transcription factor Nr2f2 attenuates salt-sensitive hypertension

Genome-wide association studies (GWAS) have prioritized a transcription factor, nuclear receptor 2 family 2 (NR2F2), as being associated with essential hypertension in humans. Here we provide evidence that validates this association and indicates that Nr2f2 is a genetic determinant of blood pressure (BP). Using the zinc-finger nuclease technology, the generation of a targeted Nr2f2-edited rat model is reported. The resulting gene-edited rats have a 15 bp deletion in exon 2 leading to a five-amino-acid deletion in the hinge region of the mutant Nr2f2 protein. Both systolic and diastolic blood pressures of the Nr2f2(mutant) rats are significantly lower than controls. Because the hinge region of Nr2f2 is required for interaction with Friend of Gata2 (Fog2), protein-protein interaction is examined. Interaction of Nr2f2(mutant) protein with Fog2 is greater than that with the wild-type Nr2f2, indicating that the extent of interaction between these two transcription factors critically influences BP.

The role of TGFβ1 and LRG1 in cardiac remodelling and heart failure

Heart failure is a life-threatening condition that carries a considerable emotional and socio-economic burden. As a result of the global increase in the ageing population, sedentary life-style, increased prevalence of risk factors, and improved survival from cardiovascular events, the incidence of heart failure will continue to rise. Despite the advances in current cardiovascular therapies, many patients are not suitable for or may not benefit from conventional treatments. Thus, more effective therapies are required. Transforming growth factor (TGF) β family of cytokines is involved in heart development and dys-regulated TGFβ signalling is commonly associated with fibrosis, aberrant angiogenesis and accelerated progression into heart failure. Therefore, a potential therapeutic pathway is to modulate TGFβ signalling; however, broad blockage of TGFβ signalling may cause unwanted side effects due to its pivotal role in tissue homeostasis. We found that leucine-rich α-2 glycoprotein 1 (LRG1) promotes blood vessel formation via regulating the context-dependent endothelial TGFβ signalling. This review will focus on the interaction between LRG1 and TGFβ signalling, their involvement in the pathogenesis of heart failure, and the potential for LRG1 to function as a novel therapeutic target.

RING finger protein RNF207, a novel regulator of cardiac excitation

Two recent studies (Newton-Cheh, C. et al. (2009) Common variants at ten loci influence QT interval duration in the QTGEN Study. Nat. Genet. 41, 399-406 and Pfeufer, A. et al. (2009) Common variants at ten loci modulate the QT interval duration in the QTSCD Study. Nat. Genet. 41, 407-414) identified an association, with genome-wide significance, between a single nucleotide polymorphism within the gene encoding RING finger protein 207 (RNF207) and the QT interval. We sought to determine the role of RNF207 in cardiac electrophysiology. Morpholino knockdown of RNF207 in zebrafish embryos resulted in action potential duration prolongation, occasionally a 2:1 atrioventricular block, and slowing of conduction velocity. Conversely, neonatal rabbit cardiomyocytes infected with RNF207-expressing adenovirus exhibited shortened action potential duration. Using transfections of U-2 OS and HEK293 cells, Western blot analysis and immunocytochemistry data demonstrate that RNF207 and the human ether-a-go-go-related gene (HERG) potassium channel interact and colocalize. Furthermore, RNF207 overexpression significantly elevated total and membrane HERG protein and HERG-encoded current density by ∼30-50%, which was dependent on the intact N-terminal RING domain of RNF207. Finally, coexpression of RNF207 and HSP70 increased HERG expression compared with HSP70 alone. This effect was dependent on the C terminus of RNF207. Taken together, the evidence is strong that RNF207 is an important regulator of action potential duration, likely via effects on HERG trafficking and localization in a heat shock protein-dependent manner.

Multiple blood pressure loci with opposing blood pressure effects on rat chromosome 1 in a homologous region linked to hypertension on human chromosome 15

Genetic dissection of blood pressure (BP) quantitative trait loci (QTLs) in rats has facilitated the fine-mapping of regions linked to the inheritance of hypertension. The goal of the current study was to further fine-map one such genomic region on rat chromosome 1 (BPQTL1b1), the homologous region of which on human chromosome 15 harbors BP QTLs, as reported by four independent studies. Of the six substrains constructed and studied, the systolic BP of two of the congenic strains were significantly lower by 36 and 27 mm Hg than that of the salt-sensitive (S) rat (P < 0.0001, P = 0.0003, respectively). The congenic segments of these two strains overlapped between 135.12 and 138.78 Mb and contained eight genes and two predicted miRNAs. None of the annotations had variants within expressed sequences. These data taken together with the previous localization resolved QTL1b1 with a 70% improvement from the original 7.39 Mb to the current 2.247 Mb interval. Furthermore, the systolic BP of one of the congenic substrains was significantly higher by 20 mm Hg (P < 0.0001) than the BP of the S rat. The limits of this newly identified QTL with a BP increasing effect (QTL1b1a) were between 134.12 and 135.76 Mb, spanning 1.64 Mb, containing two protein-coding genes, Mctp2 and Rgma, and a predicted miRNA. There were four synonymous variants within Mctp2. These data provide evidence for two independent BP QTLs with opposing BP effects within the previously identified BP QTL1b1 region. Additionally, these findings illustrate the complexity underlying the genetic mechanisms of BP regulation, wherein inherited elements beyond protein-coding sequences or known regulatory regions could be operational.

Theoretical model for gene-gene, gene-environment, and gene-sex interactions based on congenic-strain analysis of blood pressure in Dahl salt-sensitive rats

There is a significant literature describing quantitative trait loci (QTL) controlling blood pressure (BP) in the Dahl salt-sensitive (S) rat. In studies to identify the genes underlying BP QTL it has been common practice to place chromosomal segments from low BP strains on the genetic background of the S rat and then reduce the congenic segments by substitution mapping. The present work suggests a model to simulate genetic interactions found using such congenic strains. The QTL are considered to be switches that can be either in series or in parallel represented by the logic operators AND or OR, respectively. The QTL switches can be on/off switches but are also allowed specific leak properties. The QTL switches are represented by a "universal" switch consisting of two molecules binding to form a complex. Genetic inputs enter the model as allelic products of one of the binding molecules and environmental variation (including dietary salt- and sex-related differences) enters as an influence on the concentration of the other binding molecule. The pairwise interactions of QTL are very well simulated and fall into recognizable patterns. There is, however, often more than one assumed model to predict a given pattern so that all patterns do not necessarily have a unique solution. Nevertheless, the models obtained provide a framework for placing the QTL in pathways relative to one another. Moreover, based on their leak properties pairs of QTL could be identified in which one QTL may alter the properties of the other QTL.

Isolation and high-throughput sequencing of two closely linked epistatic hypertension susceptibility loci with a panel of bicongenic strains

Interactions or epistasis between genetic factors may contribute to "missing heritability." While linkage analyses detect epistasis, defining the limits of the interacting segments poses a significant challenge especially when the interactions are between loci in close proximity. The goal of the present study was to isolate two such epistatic blood pressure (BP) loci on rat chromosome 5. A panel of S.LEW bicongenic strains along with the corresponding monocongenic strains was constructed. BP of each set comprising of one bicongenic and two corresponding monocongenic strains were determined along with the parental Salt-sensitive (S) strain. Epistasis was observed in one out of four sets of congenic strains, wherein systolic blood pressures (SBP) of the two monocongenic strains S.LEW(5)x6Bx9x5a and S.LEW(5)x6Bx9x5b were comparable to that of S, but the SBP of the bicongenic strain S.LEW(5)x6Bx9x5 (157 ± 4.3 mmHg) was significantly lower than that of S (196 ± 6.8 mmHg, P < 0.001). A two-way ANOVA indicated significant interactions between the LEW alleles at the two loci. The interacting loci were 2.02 Mb apart and located within genomic segments spanning 7.77 and 4.18 Mb containing 7,360 and 2,753 candidate variants, respectively. The current study demonstrates definitive evidence for epistasis and provides genetic tools for further dissection of the isolated epistatic BP loci.

Targeted disruption of Adamts16 gene in a rat genetic model of hypertension

A disintegrin-like metalloproteinase with thrombospondin motifs-16 (Adamts16) is an important candidate gene for hypertension. The goal of the present study was to further assess the candidacy of Adamts16 by targeted disruption of this gene in a rat genetic model of hypertension. A rat model was generated by manipulating the genome of the Dahl Salt-sensitive (S) rat using zinc-finger nucleases, wherein the mutant rat had a 17 bp deletion in the first exon of Adamts16, introducing a stop codon in the transcript. Systolic blood pressure (BP) of the homozygous Adamts16(mutant) rats was lower by 36 mmHg compared with the BP of the S rats. The Adamts16(mutant) rats exhibited significantly lower aortic pulse wave velocity and vascular media thickness compared with S rats. Scanning electron and fluorescence microscopic studies indicated that the mechanosensory cilia of vascular endothelial cells from the Adamts16(mutant) rats were longer than that of the S rats. Furthermore, Adamts16(mutant) rats showed splitting and thickening of glomerular capillaries and had a longer survival rate, compared with the S rats. Taken together, these physiological observations functionally link Adamts16 to BP regulation and suggest the vasculature as the potential site of action of Adamts16 to lower BP.

Construction of two novel reciprocal conplastic rat strains and characterization of cardiac mitochondria

Because of the lack of appropriate animal models, the potentially causal contributions of inherited mitochondrial genomic factors to complex traits are less well studied compared with inherited nuclear genomic factors. We previously detected variations between the mitochondrial DNA (mtDNA) of the Dahl salt-sensitive (S) rat and the spontaneously hypertensive rat (SHR). Specifically, multiple variations were detected in mitochondrial genes coding for subunits of proteins essential for electron transport, in mitochondrial reactive oxygen species production, and within the D-loop region. To evaluate the effects of these mtDNA variations in the absence of the corresponding nuclear genomic factors as confounding variables, novel reciprocal strains of S and SHR were constructed and characterized. When compared with that of the S rat, the heart tissue from the S.SHR(mt) conplastic strain wherein the mtDNA of the S rat was substituted with that of the SHR had a significant increase in mtDNA copy number and decrease in mitochondrial reactive oxygen species production. A corresponding increase in aerobic treadmill running capacity and a significant increase in survival that was not related to changes in blood pressure were observed in the S.SHR(mt) rats compared with the S rat. The reciprocal SHR.S(mt) rats did not differ from the SHR in any phenotype tested, suggesting lower penetrance of the S mtDNA on the nuclear genomic background of the SHR. These novel conplastic strains serve as invaluable tools to further dissect the relationship between heart function, aerobic fitness, cardiovascular disease progression, and mortality.

Increased Expression of Rififylin in A < 330 Kb Congenic Strain is Linked to Impaired Endosomal Recycling in Proximal Tubules

Cell surface proteins are internalized into the cell through endocytosis and either degraded within lysosomes or recycled back to the plasma membrane. While perturbations in endosomal internalization are known to modulate renal function, it is not known whether similar alterations in recycling affect renal function. Rififylin is a known regulator of endocytic recycling with E3 ubiquitin protein ligase activity. In this study, using two genetically similar strains, the Dahl Salt-sensitive rat and an S.LEW congenic strain, which had allelic variants within a < 330 kb segment containing rififylin, we tested the hypothesis that alterations in endosomal recycling affect renal function. The congenic strain had 1.59-fold higher renal expression of rififylin. Transcriptome analysis indicated that components of both endocytosis and recycling were upregulated in the congenic strain. Transcription of Atp1a1 and cell surface content of the protein product of Atp1a1, the alpha subunit of Na⁺K⁺ATPase were increased in the proximal tubules from the congenic strain. Because rififylin does not directly regulate endocytosis and it is also a differentially expressed gene within the congenic segment, we reasoned that the observed alterations in the transcriptome of the congenic strain constitute a feedback response to the primary functional alteration of recycling caused by rififylin. To test this, recycling of transferrin was studied in isolated proximal tubules. Recycling was significantly delayed within isolated proximal tubules of the congenic strain, which also had a higher level of polyubiquitinated proteins and proteinuria compared with S. These data provide evidence to suggest that delayed endosomal recycling caused by excess of rififylin indirectly affects endocytosis, enhances intracellular protein polyubiquitination and contributes to proteinuria.

Use of contiguous congenic strains in analyzing compound QTLs

Genetic analysis of polygenic traits in rats and mice has been very useful for finding the approximate chromosomal locations of the genes causing quantitative phenotypic variation, so-called quantitative trait loci (QTL). Further localization of the causative genes and their ultimate identification has, however, proven to be slow and frustrating. A major technique for gene identification in such models utilizes series of congenic strains with progressively smaller chromosomal segments introgressed from one inbred strain into another inbred strain. Under the assumption that a single causative locus underlies a QTL, nested series of congenic strains were earlier suggested as an appropriate configuration for the congenic strains. It is now known that most QTL are compound, that is, the QTL signal is caused by clusters of loci where alleles exert positive, negative, and interactive effects on the trait in a given strain comparison. It is argued that in this situation an initial series of nonoverlapping contiguous congenic strains over a relatively large chromosomal region will lead to a better appreciation of the underlying complexity of the QTL and therefore more rapid gene identification. Examples from the literature where this strategy would be helpful, as well as a case where it would be potentially counterproductive, are given.

Refined mapping of blood pressure quantitative trait loci using congenic strains developed from two genetically hypertensive rat models

Previously linkage and substitution mapping were conducted between the Dahl Salt-sensitive (S) rat and the Spontaneously Hypertensive Rat (SHR) to address the hypothesis that genetic contributions to blood pressure (BP) in two genetically hypertensive rat strains are different. Among the BP quantitative trait loci (QTLs) detected, two are located on chromosome 9 within large genomic segments. The goal of the current study was to develop new iterations of congenic substrains, to further resolve both of these BP QTLs on chromosome 9 as independent congenic segments. A total of 10 new congenic substrains were developed and characterized. The newly developed congenic substrains S.SHR(9)x8Ax11A and S.SHR(9)x10Ax1, with introgressed segments of 2.05 and 6.14 Mb, represented the shortest genomic segments. Both of these congenic substrains, S.SHR(9)x8Ax11A and S.SHR(9)x10Ax1 lowered BP of the S rat by 56 mm Hg (P<0.001) and 15 mm Hg (P<0.039), respectively. The BP measurements were corroborated by radiotelemetry. Urinary protein excretion was significantly lowered by SHR alleles within S.SHR(9)x10Ax1 but not by S.SHR(9)x8Ax11A. The shorter of the two congenic segments, 2.05 Mb was further characterized and found to contain a single differentially expressed protein-coding gene, Tomoregulin-2 (Tmeff2). The protein expression of Tmeff2 was higher in the S rat compared with S.SHR(9)x8Ax11A, which also had lower cardiac hypertrophy as measured by echocardiography. Tmeff2 is known to be upregulated in patients from multiple cohorts with cardiac hypertrophy. Taken together, Tmeff2 can be prioritized as a candidate gene for hypertension and associated cardiac hypertrophy in both rats and in humans.