Annie Ménard

Epistasis, Not Numbers, Regulates Functions of Clustered Dahl Rat Quantitative Trait Loci Applicable to Human Hypertension

Quantitative trait loci (QTLs) for blood pressure (BP) were found on chromosome 10 of Dahl salt-sensitive rats and are potentially important to human essential hypertension. But their identities and how they influence BP together were not known. Presently, we first fine mapped existing QTLs, C10QTL1, C10QTL2, and C10QTL3, by constructing congenic strains. In the process, a new QTL, C10QTL4, was identified. Because the intervals harboring C10QTL1 and C10QTL4 contain a maximum of 16 and 10 possible genes, respectively, a limited number of specific gene targets has been identified to be QTLs residing in human homologous regions on chromosome 17. Moreover, because none of these candidates encodes a gene known to influence BP, the 2 QTLs will represent novel genes for BP regulations. Second, we used congenic strains with QTL combinations to analyze the interactions between the QTLs. Consequently, a double combination of C10QTL4 and C10QTL1 possessed the same BP as each of the 2 QTLs alone. BP of a triple combination of C10QTL4, C10QTL1, and C10QTL3 was not different from BP of the C10QTL4 and C10QTL1 double combination. These results demonstrate that C10QTL4, C10QTL1, and C10QTL3 are epistatic to one another in their BP effects. In contrast, when adding C10QTL2 into the triple formation of the 3 QTLs above to create a quadruple QTL combination, BP increased proportionately, indicating that C10QTL2 acts independently of C10QTL4, C10QTL1, and C10QTL3. The epistatic and additive interactions uncovered in the animal model will help elucidate similar interactions playing a role in human essential hypertension.

Submegabase resolution of epistatically interacting quantitative trait loci for blood pressure applicable for essential hypertension

Objective Although genetic mapping of quantitative trait loci for blood pressure to large chromosome segments is readily achievable, their final identification confronts formidable hurdles. Restriction of the genes lodging in one quantitative trait locus interval to experimental limitation can facilitate their positional cloning. We previously delineated several quantitative trait loci for blood pressure on chromosome 10 of Dahl salt-sensitive rats, but their chromosome delimitations were either large or not definitive. Methods In this study, we systematically and comprehensively constructed congenic strains with submegabase (Mb) genome resolution and analyzed their blood pressure by telemetry. Results Three quantitative trait loci have been conclusively delimited by three congenic strains, each independently lowering the blood pressure. Their intervals are demarcated by genomic regions between 350 and 910 kilobases (kb) in size. Two of the three quantitative trait loci share an epistatic relationship and are separated from one another by less than 170 kb. Two additional quantitative trait loci for blood pressure were also tentatively delineated and their intervals range from 520 kb to 1.75 Mb. Possible genes dwelling in each quantitative trait locus-interval number between 11 and 17. None of these genes is known to exert a functional impact on blood pressure. Work is underway to find candidate genes with mutations that could be responsible for the blood pressure effect. Conclusion Novel diagnostic, prognostic, preventive and/or therapeutic targets for essential hypertension and hypertension-associated diseases are likely to emerge from the identification of these quantitative trait loci. Potential applications of these quantitative trait loci to humans are suggested from the positive results from several association studies, demonstrating the existence of quantitative trait loci in the broad homologous regions.

Distinct quantitative trait loci for kidney, cardiac, and aortic mass dissociated from and associated with blood pressure in Dahl congenic rats

Blood pressure (BP) is largely determined by quantitative trait loci (QTLs) in Dahl salt-sensitive (DSS) rats. Little is known about QTLs controlling kidney (K), cardiac (C), and aortic (A) mass (i.e. Km, Cm, and Am, respectively) of DSS rats independent of BP. Their identification can facilitate our understanding of end organ damage. In this work, 36 congenic strains were employed to define QTLs for Km, Cm, and Am either independent of or associated with BP. Five new QTLs, i.e., KmQTLs, that influence Km independent of Cm, Am, and BP were defined. Four new CakmQTLs were defined for Cm, Am, and Km independent of BP. Among them, the CakmC10QTL1 interval contained 13 genes and undefined loci, and none was known to influence the phenotypes in question, paving the way for a novel gene discovery. Among 17 individual QTLs for BP, 14 also affected Cm, Km, and Am, i.e., they are BpcakmQTLs. In contrast, one BpQTL had no effect on Cm, Am, and Kam. Therefore, BP and Cm, Am, and Km have distinct and shared genetic determinants. The discovery of individual Km and Cakm QTLs will likely facilitate the identification of mechanisms underlying renal, cardiac, and/or aortic hypertrophy independent of hypertension.

α-Kinase 2 is a novel candidate gene for inherited hypertension in Dahl rats.

Objectives The interval harboring a quantitative trait locus for blood pressure (BP), C18QTL3, contains β-2 adrenergic receptor (Adrb2) and neural precursor cell expressed, developmentally downregulated 4-like (Nedd4l) genes. None of the other genes in the C18QTL3-residing interval is known to affect BP. The identification of C18QTL3 might uncover a brand new gene that could prosper into a novel diagnostic and/or therapeutic target for essential hypertension, if neither Adrb2 nor Nedd4l could be upheld as candidate genes. Methods Congenic fine resolution was combined with gene analyses. Results The gene encoding α-kinase 2 (Alpk2) contains a three base-pair deletion and multiple nonconserved mutations in its coding region in Dahl salt-sensitive (DSS) rats. In contrast, the gastrin-releasing peptide gene (Grp) possesses two nonconserved mutations, designated as single nucleotide polymorphisms 1 and 2 (i.e. SNP1 and SNP2), but could not be supported as a candidate gene because the C18S.L14 congenic strain displayed a homozygous DSS genotype at both SNP1 and SNP2. Furthermore, Adrb2 and Nedd4l could not account for the BP-diminishing effect of Lewis alleles in C18S.L14, as their DSS alleles bear functionally identical domains as those of Lewis, and no evidence of differential expression and splicing was evident. No significant nucleotide variations were found in 13 other genes closely linked to Alpk2. Conclusion Alpk2 emerged as a strong candidate gene for C18QTL3. The present study is the first to implicate Alpk2 in the genetics of polygenic hypertension and paves the way for novel gene discovery.

Novel genes as primary triggers for polygenic hypertension.

Objectives The discovery of causative genes leading to hypertension in animal models can reveal new mechanistic insights into blood pressure (BP) regulations. Previously, we isolated segments that harbor BP quantitative trait loci (QTLs) on rat chromosome 10 as defined by congenic strains made from crosses of inbred hypertensive Dahl salt-sensitive (DSS) and normotensive Lewis rats. The aim of the current study was to identify hypertension-causing genes for each QTL. Methods Molecular analysis was performed. Results A systematic and comprehensive molecular analysis divulged particular genes that carry nonconserved mutations. Specifically, the proline rich 11 gene is likely responsible for C10QTL5. C10QTL1 is one of five genes, namely Benzodiazepine receptor associated protein 1, Loc689764, myotubularin related protein 4, protein phosphatase 1E, PP2C domain containing and ring finger protein 43. Loc100363423 with no known function is a candidate for C10QTL3. The ATP-binding cassette, subfamily A (ABC1), member 8a gene is probably responsible for C10QTL2. Conclusions Primary genes initiating polygenic hypertension are those not known to be involved in BP modulation. Novel pathways towards BP homeostasis appear to underlie the functionality of C10QTL5, C10QTL1 and C10QTL3 and C10QTL2. Moreover, these genes may become innovative targets for the diagnosis and therapeutics of essential hypertension.

Cardiac pathways distinguish two epistatic modules enacting BP quantitative trait loci and candidate gene analysis

Animal models emulating essential hypertension are an informative means by which to elucidate the physiological mechanisms and gene–gene interactions underlying blood pressure (BP) regulation. We have localized earlier quantitative trait loci (QTLs) for BP on Chromosome (Chr) 2 of Dahl salt-sensitive (DSS) rats, but their chromosome delineations were too large for gene identification. To advance toward positional cloning of these QTLs, we constructed congenic strains that systematically dissect a Chr 2 segment with no overlaps. BP and cardiac functions were measured by telemetry and echocardiography. Six QTLs were delimited, each independently influencing BP. The intervals lodging two of them harbor 10–15 genes and undefined loci. These six QTLs can be grouped into two epistatic modules distinguishable by cardiac pathways/cascades. None of the genes known to exert physiological effects on BP in the segments harboring the six QTLs are leading candidates, as their protein products are the same in DSS rats and similar to those in their Milan normotensive counterparts. Specifically, the lack of an amino-acid alteration, coupled with a lack of difference in the α1-Na-K-ATPase activity, excluded ATPase, Na+/K+-transporting, α-1 polypeptide as a candidate gene for C2QTL6. The identification of the six QTLs will likely develop into a novel diagnostic and/or therapeutic target for essential hypertension and hypertension-associated diseases.

Normotension in Lewis and Dahl salt-resistant rats is governed by different genes

Objectives Inbred rodent models simulating essential hypertension and normotension are useful tools in discovering genes controlling blood pressure (BP) homeostasis. An analysis of a F2 population made from crosses of hypertensive Dahl salt-sensitive (DSS) and normotensive Lewis rats did not detect a BP quantitative trait locus (QTL) on chromosome 7 (Chr 7). However, false negativity could not be excluded. If a BP QTL could be proven to exist, what gene(s) may be responsible for this QTL. Methods We first constructed reciprocal congenic strains for a Chr 7 segment and determined functional domains of prominent candidate genes. Results A congenic strain made in the DSS rat background exhibited a BP effect, indicating that a BP QTL, C7QTL, inhabits Chr 7. Contrarily, a congenic strain constructed in the Lewis rat background did not change BP, demonstrating a dependence of C7QTL on the DSS rats environment. Among the candidate genes, tachykinin 2 (Tac2), neurexophilin 4 (Nxph4) and retinol dehydrogenase 2 (Rdh2) bear nonsynonymous changes comparing DSS and Lewis rats, but are the same comparing DSS and Dahl salt-resistant (DSR) rats. In contrast, the Lewis alleles of 11-beta-hydroxylase (Cyp11b1), aldosterone synthase (Cyp11b2) and Cytochrome P-450 11B3 (Cyp11b3) are identical to those of DSS rats, but different from those of DSR rats. Conclusion Thus, the failure to detect a linkage between a Chr 7 segment and BP in F2(DSS × Lewis) can be attributed to false negativity. Tac2, Nxph4 and Rdh2 are priority candidate genes for C7QTL. Lewis and DSR rats are both normotensive, but their underlying genetic determinants are different.

Distinct genomic replacements from Lewis correct diastolic dysfunction, attenuate hypertension, and reduce left ventricular hypertrophy in Dahl salt-sensitive rats.

Background Hypertension and diastolic heart failure are two common cardiovascular diseases that inflict heavy morbidity and mortality, yet relatively little is understood about their pathophysiology. The identification of quantitative trait loci for blood pressure is important in unveiling the causes of polygenic hypertension. Although Dahl salt-sensitive strain is also an excellent model for the study of diastolic heart failure, virtually nothing is known about the quantitative trait loci determining diastolic heart failure. Diastolic dysfunction often represents the onset of diastolic heart failure. Methods We first characterized the cardiac phenotype of Dahl salt-sensitive strain and normotensive Lewis control rats by echocardiography to ascertain diastolic function. We then analyzed corresponding features of four newly developed and two existing congenic strains, each of which carrys a specific chromosome substitution of Dahl salt-sensitive strain by its Lewis homologue and each lowering blood pressure. Results Dahl salt-sensitive strain displayed diastolic dysfunction that was rectified in two of six congenic strains, designated as positive congenic strains, which represent the first rodent models exhibiting functional normalization of diastolic dysfunction caused by naturally occurring genetic variants. The two positive congenic strains also showed a reduction in left ventricular mass. In contrast, four of six congenic strains did not change diastolic function despite their blood pressure-lowering effects. Conclusion Genes present in the replaced chromosome segments of the two positive congenic strains are not commonly known to affect blood pressure, diastolic function or left ventricular mass. Consequently, novel prognostic, diagnostic and therapeutic strategies for hypertensive diastolic heart failure likely emerge from this work.

Sexual Dimorphism on Hypertension of Quantitative Trait Loci Entrapped in Dahl Congenic Rats

Although it is well-known that quantitative trait loci (QTLs) influence blood pressure (BP) in male Dahl salt-sensitive rats (DSS), few studies have been carried out to ascertain the BP effect of these QTLs in females. In the current work, we analyzed BP of seven selected congenic strains constructed in the DSS background. One QTL, C8QTL2, exhibited similar effects on systolic (SAP), diastolic (DAP), and mean arterial (MAP) pressures in females as previously shown in males. In contrast, six QTLs that previously demonstrated influences on SAP, DAP, and MAP in males did not have effects in females. These male-specific QTLs are likely regulated differently in males than in females and emphasize the necessity of identifying female-specific QTLs for diagnosing and treating hypertension in women. Current findings may have implications in genetic research of essential hypertension, and association and linkage analyses should be performed in separate genders. Men and women may possess distinctive as well as shared genetic determinants for SAP, DAP, and MAP. The data on a single gene or marker might be pooled from both genders only when evidence favors the sex-independence in a study.

Hypertension Suppression, Not a Cumulative Thrust of Quantitative Trait Loci, Predisposes Blood Pressure Homeostasis to Normotension.

Background—Genetics of high blood pressure (BP) has revealed causes of hypertension. The cause of normotension, however, is poorly understood. Inbred Lewis rats sustain normotension despite a genetic push in altering BP. It was unknown whether this rigid resistance to BP changes is because of an insufficient hypertensive impact from limited alleles of quantitative trait loci (QTLs) or because of an existence of a master control superseding the combined strength of hypertensive QTL alleles. Methods and Results—Currently, BP-elevating QTL alleles from hypertensive Dahl salt-sensitive rats (DSS) replaced those of Lewis on chromosomes 7, 8, 10, and 17 on the Lewis background. These hypertensive QTL alleles were then merged to systematically achieve multiple combinations. Results showed that there was no quantitative correlation between BP variations and the number of hypertensive QTL alleles, and that BP was only slightly elevated from a combined force of normotensive alleles from 7 QTLs. Thus, a genetic factor aside from the known QTLs seemed to be at play in preserving normotension and act as a hypertension suppressor. A follow-up study using consecutive backcrosses from Dahl salt-sensitive rats and Lewis identified a chromosome segment where a hypertension suppressor might reside. Conclusions—Our results provide the first evidence that normotension is not enacted via a numeric advantage of BP-lowering QTL alleles, and instead can be achieved by a particular genetic component actively suppressing hypertensive QTL alleles. The identification of this hypertension suppressor could result in formulating unique diagnostic and therapeutic targets, and above all, preventive measures against essential hypertension.