Frans Leenen

Carte électronique

Frans Leenen
Professeur auxiliaire

Pièce : Institut de cardiologie de l’Université d’Ottawa
phone : 613-696-7360
Télécopieur : 613-696-7125
Courriel professionnel : fhhleenen@gmail.com

Dr Frans Leenen

Biographie

Frans H.H. Leenen received his PhD and MD from the University of Utrecht, The Netherlands.  He completed his residencies in internal medicine and cardiology at the University of Utrecht Medical School and teaching hospitals.  He obtained postdoctoral research training at the University of Utrecht and the University of Pittsburgh.  Dr. Leenen is currently Professor of Medicine and Pharmacology at the University of Ottawa School of Medicine, and Director of the Hypertension Unit at the University of Ottawa Heart Institute.
Dr. Leenens current areas of research are 1) brain mechanisms determining sympathetic hyperactivity in salt-sensitive hypertension and heart failure with as primary focus the brain RAAS; and 2) genetic basis of salt-sensitive hypertension in rats and humans.  He is a fellow in the cardiovascular section of the American Physiological Society, a fellow of the Council for High Blood Pressure of the American Heart Association, a fellow of the International Academy of Cardiovascular Sciences, and honorary member of the High Blood Pressure Research Council of Australia. For many years he was a Career Investigator of the Heart and Stroke Foundation of Ontario, and since 2004 the first recipient of the Pfizer Research Chair in Hypertension, an endowed chair supported by Pfizer Canada, the Ottawa Heart Institute Foundation, and Canadian Institutes of Health Research.  He is also the recipient of several prestigious awards, including the Dedicated Service Award from the Heart and Stroke Foundation of Canada.  He was the RD Wright Lecturer at the 2009 Annual Scientific meeting of the High BP Research Council of Australia.  He has published more than 280 peer-reviewed papers in respected journals, such as the American Journal of Physiology, Hypertension, Circulation, and Circulation Research.  He is/was a member of editorial boards for Journal of Hypertension, Blood Pressure, Hypertension, Canadian Journal of Cardiology, American Journal of Cardiovascular Drugs, Vascular Health & Risk Management, Experimental & Clinical Cardiology, Current Opinion in Cardiology, Therapeutic Advances in Cardiovascular Disease, American Journal of Hypertension, The Open Hypertension Journal, Cardiology Research and Practice, World Journal of Cardiology.

Recent Publications

  • Huang BS, Leenen FHH. The brain renin-angiotensin-aldosterone system: a major mechanism for sympathetic hyperactivity and LV remodeling and dysfunction post MI. Current Heart Failure Reports 6:81-88, 2009.
  • Leenen FHH.  The central role of the brain aldosterone-“ouabain” pathway in salt sensitive hypertension.  Biochimica et Biophysica Acta, Special Issue “Molecular Basis of Disease Arterial Hypertension”  1082:1132-1139, 2010.
  • Leenen FHH, McInnis MH, Fodor G.  Obesity and the Prevalence and Management of Hypertension in Ontario.  American Journal of Hypertension 23:1000-1006, 2010.
  • Leenen FHH, Schiffrin EL.  Control rates of Hypertension in North America.  Editorial Commentary, Hypertension 56:571-572, 2010.
  • Huang BS, Ahmadi S, Ahmad M, White RA, Leenen FHH.  Central neuronal activation and pressor responses induced by circulating ANG II:  Role of brain aldosterone-“ouabain” pathway.  American Journal of Physiology  299:H422-H430, 2010
  • McAlister FA, Wilkins K, Joffres M, Leenen FHH, Fodor G, Gee M, Tremblay MS, Walker R. Johansen H, Campbell N.  Changes in hypertension awareness, treatment, and control rates in Canada over the past two decades.  Canadian Medical Association Journal, 183:1007-1013, 2011.

Present Research Group

Research Associates

  • Monir Ahmad, MD
  • Hong-Wei Wang, MD PhD
  • Bing Huang, MD, PhD

Students

  • Alex Gabor, Graduate Student (PhD)
  • Shahrier Amin, Graduate Student (PhD)
  • Missale Tiruneh, Graduate Student (MSc)
  • Katherine Westcott, Graduate Student (MSc)
  • Sara Ahmadi, Graduate Student (MSc)
  • Anastasia Drobysheva, Graduate Student (MSc)

Support Staff

  • Chelsea Kingsbury, Senor Research Coordinator
  • Roselyn White, Laboratory Manager
  • Danielle Oja, Manager, Hypertension Unit

Research Projects:

Title : Brain mechanisms and sympathetic hyperactivity in genetic models of salt-sensitive hypertension

Canadian Institutes of Health Research: 2010 2015

PI : Frans Leenen
Co-PI : Frederique Tesson

Balwant Tuana

Overview:

Two distinct mechanisms contribute to the CNS activation by high salt diet in genetically salt-sensitive rat strains such as Dahl S and SHR: 1) enhanced Na+-transport across the choroid plexus (CP) into the CSF, and 2) enhanced sympatho-excitatory and pressor-responses to [Na+]. Our studies provided strong evidence for a novel concept: an increase in CSF[Na+] activates aldosterone and ouabain production and release in the hypothalamus. Activation of this neuromodulatory pathway by [Na+] is enhanced in S vs R rats and appears to increase activity in angiotensinergic sympatho-excitatory pathways. Blockade of this neuromodulatory pathway or of AT1-receptors in the CNS prevents the salt-induced hypertension in Dahl S. Regarding mechanisms contributing to Na+-transport across the CP, by microarray we identified differential expression of 4 genes in the CP of Dahl S rats on both regular and high salt diet and of 39 genes only on high salt diet (table 1,2). We hypothesize that these differentially expressed genes will lead us to (new) mechanisms contributing to enhanced Na+-transport across the CP of Dahl S rats. The following lines of research will be pursued in Dahl S and 2 salt-resistant control strains, Dahl R (70% identical) and the consomic strain SS.BN13 (98% identical with Dahl S):

  • Microarray follow-up studies
  • Regulation of CSF and brain tissue [Na+]

As second hypothesis we propose that a chronic increase in CSF[Na+] activates angiotensinergic neurons in the SFO leading to an increase in production and release of aldosterone and ouabain by neuro-secretory neurons in the SON and PVN. The following lines of research will be pursued mainly in Dahl S.

1.1. Effects of high salt intake on aldosterone and ouabain content in brain nuclei involved in cardiovascular regulation.
1.2. Forebrain areas showing an increase in aldosterone or ouabain will be evaluated for changes in gene expression of enzymes contributing to aldosterone or ouabain production.
1.3. CNS functional pathways activated by [Na+], either by high salt diet or icv infusion of Na+-rich aCSF.

Staff:

  • Hong-Wei Wang, MD PhD, Research Associate
  • Bing Huang, MD, PhD, Research Associate

Title :Brain mechanisms determining sympathetic hyperactivity and Cardiac Dysfunction in rats post MI.

Canadian Institutes of Health Research: 2009 2014

PI : Frans Leenen

Overview:
Central Mechanisms and pathways post MI:  The CNS acts as the conductor integrating inputs from a variety of sources in the body, which leads to activation of CNS descending pathways controlling activity of important regulators of cardiovascular homeostasis post MI (Leenen, Circ Res 2007). In the CNS a cascade of neuromodulators/neurotransmitters is involved. Currently, activation of local aldosterone production and release appears as the primary step. What stimulus increases aldosterone production and where in the CNS is still unknown. The rapid increase in circulating angiotensin II post MI may activate neurons in forebrain circumventricular organs, particularly the SFO, and for the present grant we propose as novel hypothesis that angiotensinergic neurons projecting from the SFO activate aldosterone production and release which activates adjacent neurosecretory magnocellular neurons in the SON and PVN.  The subsequent enhanced ouabain release lowers the membrane potential particularly of parvocellular neurons in the PVN, thereby sensitizing these neurons to e.g. AT1-receptor stimulation leading to persistent activation of descending pathways. To address this concept the following approaches will be taken:

  • Assessment of gene expression for putative steroid biosynthetic pathways for aldosterone and ouabain in forebrain nuclei using both a candidate gene approach as well as microarray analysis for global, unbiased gene expression without and with blockade of specific mechanisms.
  •  Assessment of functional connections
  • Chemical lesioning of specific nuclei to assess the chronic impact of this neuron population.
  • Micro injection of specific blockers to identify specific mechanisms in specific nuclei.

Sympathetic Hyperactivity and Cardiac Remodeling and Dysfunction post MI: Regarding the peripheral mechanisms mediating the effects of the CNS on the heart post MI, it is likely that cardiac effects of central blockades depend on prevention of the increase in cardiac sympathetic activity and associated cardiac effects, both direct ones and indirect ones via stimulation of the cardiac RAAS. However, in rats post MI, treatment with non-selective ²-blockers has only minimal effects on LV remodeling and dysfunction whereas ²1-blockers show some benefits. Differences in loss of cardiomyocytes as a result of apoptosis may play a role. The RAAS (via AT1-receptors and MR) may induce cardiomyocyte apoptosis. ²1-receptor stimulation also induces apoptosis, whereas ²2- and ±1-receptors stimulation are inhibitory. Gradual loss of myocytes post MI due to apoptosis is well established to occur in the non-infarcted myocardium leading to hypertrophy of remaining myocytes, and progressive decrease in LV systolic function. We hypothesize that central blockades represent the most effective strategy to prevent inflammation and apoptosis and thereby maintain LV systolic function post MI and are significantly better than systemic ²1-blockade.
To address the above concepts the following approaches will be taken:

  • Assess effects of central blockades post MI
  • Compare effects of central blockades with those of ²1-receptor blockade post MI

This dual approach is unique for our group and critical for a better understanding of the pathophysiology and to translate new concepts into new/better mechanism-based approaches for interventions post MI to prevent myocyte loss and maintain LV function.

Staff:

  • Roselyn White, Laboratory Manager
  • Li Bi, Research Technician
  • Monir Ahmad, MD, Research Associate

Title: Genetic basis of salt sensitive hypertension in humans

Canadian Institutes of Health Research: 2008 2011

PI : Frederique Tesson, Frans Leenen
Co-PI : Alex Stewart

 Overview:
In the general population, high salt intake increases the blood pressure (BP) in the short-term and enhances the increase in BP with age. The BP response to salt is normally distributed with 2 extreme phenotypes salt sensitive (SS) and salt resistant (SR), and an intermediate phenotype. As many as 50% of hypertensive patients are SS in short term studies, compared to only ~ 10% in the normotensive population. In spite of numerous linkage and association studies of candidate genes for BP per se or BP response to salt, the genetic network responsible for BP variation remains elusive. Genes that are responsible for salt resistance have so far been largely ignored. A major limitation of genome-wide scan studies has been the limited assessment of both phenotype and genotype. In nearly all studies only office BPs were used. These are notoriously variable and can readily over-diagnose white-coat hypertension, or under-diagnose masked hypertension. Genome-wide scans so far have used widely spaced markers missing variations in intermediate chromosomal segments. To overcome previous limitations, we propose to use the Affymetrix 1.8 million genetic markers array, a population of Caucasian subjects with family history of early onset hypertension, carefully phenotyped for BP response to salt, and the high throughput genotyping made possible by the Centre for Cardiovascular Genetics at the Ottawa Heart Institute. We hypothesize that genes predisposing to salt-induced increases in BP can be identified by high density genome scan in a large population of men and women with early onset hypertension and a positive family history for hypertension, stratified based on the salt-response of BP on 24-hr ambulatory BP monitoring. To test the above hypothesis, the study has 6 specific aims:

Phase I population

  • To phenotype a prospective male and female hypertensive Caucasian population of 510 subjects for BP response to salt using 24-hour ambulatory BP monitoring.
  • To perform the first genome widE association study for both SNPs and CNVs (copy number variation) using high-density arrays to identify novel risk chromosomal loci for high vs minimal BP response to high salt intake.

Phase II population

  • To collect and phenotype a prospective large independent population of 1,500 individuals with elevated BP confirmed by 24-hour ambulatory BP monitoring and a control population of 1,500 normotensive individuals matched for sex and age.
  • Chromosomal loci showing an association with salt sensitive hypertension in aim 2 will be genotyped in an independent population of 1500 cases and 1500 controls.
  • Loci confirmed to be associated with hypertension in Aim 4 will undergo further genotyping utilizing additional adjacent customized SNPs to identify gene candidates and evaluate the influence of combinations of alleles.
  • To characterize the potential role of the associated variants.
  •  To assess salt-sensitivity of BP in the phase I population, subjects receive instructions to lower salt intake over a 2 week run-in period to ~100 mmol/day. On this restricted sodium diet for 6 weeks, they are randomized to salt-capsules (200 mmol/day) for either the first or second 3-week period. 24-hr urine sodium excretion and 24-hr ambulatory BP (ABP) are assessed at the end of each period. In the phase II population, all subjects are only studied once to assess 24-hr urinary sodium excretion and 24-hr ABP (off therapy for 2 weeks).


From a public health perspective, lowering the amount of salt added to foods is an important public health strategy. From an individual perspective, the impact of high salt intake on his/her cardiovascular system can vary from minimal to substantial and appears to a large extent genetically determined. The extent of this impact is clinically difficult to ascertain. Genetic diagnosis would be an ideal method of choice to advice life-style interventions for a particular individual. To achieve this goal, studies performing careful assessment of phenotype and genotypes are essential.

Staff:

  • Chelsea Kingsbury, Research Coordinator I
  • Natalie McInnis, Project Manager
  • Roselyn White, Laboratory Manager
  • Tracey Jackson, Lab Technician
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