Xavier Jeunemaitre

Functional Characterization of a Calcium-Sensing Receptor Mutation in Severe Autosomal Dominant Hypocalcemia with a Bartter-Like Syndrome

The extracellular Ca(2+)-sensing receptor (CaSR) plays an essential role in extracellular Ca(2+) homeostasis by regulating the rate of parathyroid hormone (PTH) secretion and the rate of calcium reabsorption by the kidney. Activation of the renal CaSR is thought to inhibit paracellular divalent cation reabsorption in the cortical ascending limb (cTAL) both directly and indirectly via a decrease in NaCl transport. However, in patients with autosomal dominant hypocalcemia (ADH), caused by CaSR gain-of-function mutations, a defect in tubular NaCl reabsorption with renal loss of NaCl has not been described so far. This article describes a patient with ADH due to a gain-of-function mutation in the CaSR, L125P, associated with a Bartter-like syndrome that is characterized by a decrease in distal tubular fractional chloride reabsorption rate and negative NaCl balance with secondary hyperaldosteronism and hypokalemia. The kinetics of activation of the L125P mutant receptor expressed in HEK-293 cells, assessed by measuring CaSR-stimulated changes in intracellular Ca(2+) and ERK activity, showed a dramatic reduction in the EC(50) for extracellular Ca(2+) compared with the wild-type and a loss-of-function mutant CaSR (I40F). This study describes the first case of ADH associated with a Bartter-like syndrome. It is herein proposed that the L125P mutation of the CaSR, which represents the most potent gain-of-function mutation reported so far, may reduce NaCl reabsorption in the cTAL sufficiently to result in renal loss of NaCl with secondary hyperaldosteronism and hypokalemia.

Genotype – phenotype analysis of angiotensinogen polymorphisms and essential hypertension: the importance of haplotypes

Objectives To better understand the relationship between angiotensinogen (AGT) genetic variation and essential hypertension, AGT genotypes and haplotypes were tested for association with hypertensive endophenotypes and essential hypertension. Methods Two hundred and fifty-six Hypertensive Pathotype (HyperPATH)/Specialized Center of Research (SCOR) cases and 126 controls were genotyped for 24 single-nucleotide polymorphisms (SNPs) in the AGT gene. SNPs and AGT haplotypes were tested for association with plasma AGT, renal plasma flow (RPF), and essential hypertension. Results New associations between essential hypertension, plasma AGT, and RPF are reported for alleles −1178G, 6066A, 6152A, 6233C, and 12822C. The maximum odds ratio for association of hypertension and AGT genetic variation was 2.3 [95% confidence interval (CI) 1.5–3.8; P < 0.0003] for allele 6233C. Previous associations for −1074T, −532T, −217A, −6A, and 4072C are confirmed (P < 0.05). Sodium depletion enhances associations between AGT SNPs and plasma AGT. Most individually associated SNPs, including −6A and 4072C, are found on a common complete AGT haplotype, H4 (frequency = 0.09). Individuals with haplotype H4 have significantly higher plasma AGT and reduced RPF (P < 0.003 and P < 0.0002, respectively). Other common haplotypes are not associated with increased plasma AGT levels in this data set despite the presence of the −6A and 4072C alleles, suggesting that AGT haplotype H4 is more predictive of elevated plasma AGT than is −6A or 4072C. Conclusion This study demonstrates the importance of analyzing haplotypes in addition to single genotypes in association studies. By demonstrating the dependence of AGT associations on sodium depletion status, it helps to explain previous conflicting association results.

Phenotype–genotype correlation in antenatal and neonatal variants of Bartter syndrome

BACKGROUNDnAnte/neonatal Bartter syndrome (BS) is a hereditary salt-losing tubulopathy due to mutations in genes encoding proteins involved in NaCl reabsorption in the thick ascending limb of Henles loop. Our aim was to study the frequency, clinical characteristics and outcome of each genetic subtype.nnnMETHODSnCharts of 42 children with mutations in KCNJ1 (n = 19), SLC12A1 (n = 13) CLCNKB (n = 6) or BSND (n = 4) were retrospectively analysed. The median follow-up was 8.3 [0.4-18.0] years.nnnRESULTSnWe describe 24 new mutations: 10 in KCNJ1, 11 in SLC12A1 and 3 in CLCNKB. The onset of polyhydramnios, birth term, height and weight were similar for all groups; three patients had no history of polyhydramnios or premature birth and had CLCNKB mutations according to a less severe renal sodium wasting. Contrasting with these data, patients with CLCNKB had the lowest potassium (P = 0.006 versus KCNJ1 and P = 0.034 versus SLC12A1) and chloride plasma concentrations (P = 0.039 versus KCNJ1 and P = 0.024 versus SLC12A1) and the highest bicarbonataemia (P = 0.026 versus KCNJ1 and P = 0.014 versus SLC12A1). Deafness at diagnosis was constant in patients with BSND mutations; transient neonatal hyperkalaemia was present in two-thirds of the children with KCNJ1 mutations. Nephrocalcinosis was constant in KCNJ1 and SLC12A1 but not in BSND and CLCNKB patients. In most cases, water/electrolyte supplementation + indomethacin led to catch-up growth. Three patients developed chronic renal failure: one with KCNJ1 mutations during the second decade of age and two with CLCNKB and BSND mutations and without nephrocalcinosis during the first year of life.nnnCONCLUSIONSnWe confirmed in a large cohort of ante/ neonatal BS that deafness, transient hyperkalaemia and severe hypokalaemic hypochloraemic alkalosis orientate molecular investigations to BSND, KCNJ1 and CLCNKB genes, respectively. Chronic renal failure is a rare event, associated in this cohort with three genotypes and not always associated with nephrocalcinosis.

Prevalence of primary hyperaldosteronism in mild to moderate hypertension without hypokalaemia

Screening for primary hyperaldosteronism (PHA) is often indicated in individuals with resistant hypertension or hypokalaemia. However, in the far larger subset of the hypertensive population who do not fit into these criteria, the evidence for screening is conflicting and dependent on the disease prevalence. The purpose of this study was to examine the prevalence of PHA in a large population with mild to moderate hypertension and without hypokalaemia using a carefully controlled study protocol including a normotensive control population. Hypertensive subjects underwent medication washout and both hypertensive and normotensive subjects placed on a high-sodium diet prior to biochemical and haemodynamic testing. Study specific cutoff values were based on results from the normotensive population studied under identical conditions. A screening test (serum aldosterone/PRA ratio [ARR]>25 with a serum aldosterone level >8u2009ng/dl) was followed by a confirmatory test (urine aldosterone excretion rate [AER] >17u2009μg/24u2009h) to demonstrate evidence of PHA. An elevated ARR with a concomitant elevated serum aldosterone was present in 26 (7.5%) individuals. Of these, 11 (3.2%) had an elevated AER, consistent with evidence of PHA. Individuals with PHA had higher blood pressure and lower serum potassium levels while on a high-sodium diet. Sodium restriction neutralized these differences between PHA and essential hypertensives. The prevalence of PHA in this mild to moderate hypertensive population without hypokalaemia is at most 3.2%, a rate that might lead to excessive false positives with random screening in comparable populations. Hyperaldosteronism, when present, is responsive to sodium restriction.

Mitral valve disease—morphology and mechanisms

Mitral valve disease is a frequent cause of heart failure and death. Emerging evidence indicates that the mitral valve is not a passive structure, but—even in adult life—remains dynamic and accessible for treatment. This concept motivates efforts to reduce the clinical progression of mitral valve disease through early detection and modification of underlying mechanisms. Discoveries of genetic mutations causing mitral valve elongation and prolapse have revealed that growth factor signalling and cell migration pathways are regulated by structural molecules in ways that can be modified to limit progression from developmental defects to valve degeneration with clinical complications. Mitral valve enlargement can determine left ventricular outflow tract obstruction in hypertrophic cardiomyopathy, and might be stimulated by potentially modifiable biological valvular–ventricular interactions. Mitral valve plasticity also allows adaptive growth in response to ventricular remodelling. However, adverse cellular and mechanobiological processes create relative leaflet deficiency in the ischaemic setting, leading to mitral regurgitation with increased heart failure and mortality. Our approach, which bridges clinicians and basic scientists, enables the correlation of observed disease with cellular and molecular mechanisms, leading to the discovery of new opportunities for improving the natural history of mitral valve disease.

Inheritance of arterial lesions in renal fibromuscular dysplasia

We have previously shown that patients with renal fibromuscular dysplasia (FMD) have asymptomatic carotid lesions and that familial forms may occur. The objective of this study was to test whether carotid lesions could be detected in relatives of familial cases. High-resolution echotracking of the carotid artery was performed in 47 relatives of 13 cases from six families. This non-invasive investigation led to a semiquantitative arterial score that was compared with that obtained for 47 controls matched for age and sex and that for 125 sporadic cases. Familial resemblance was tested by using a generalized estimating equation approach taking into account the clustering of scores in families. As expected, FMD cases had a significantly higher score than controls (4.02 vs 2.52, P<10−5). Familial cases were not significantly different from sporadic cases. Of interest, the 47 apparently healthy relatives of familial cases had also a high carotid score (4.17), very significantly higher than that of controls (2.52, P<10−5) even though lower than the corresponding index FMD cases (4.81, P=0.01). Segregation analysis showed that 52% of the descendants of subjects with a score >4 had a score >4, a proportion consistent with autosomal-dominant transmission of the trait. Altogether these results strengthen the hypothesis of renal FMD being a systemic arterial disease and argue for a familial resemblance that may be due to a major genetic effect. The carotid score obtained by high-resolution echotracking may provide a non-invasive surrogate marker for renal FMD of potential value for use in linkage strategies on large pedigrees.

WNK1-related Familial Hyperkalemic Hypertension results from an increased expression of L-WNK1 specifically in the distal nephron

Significance Hypertension, one of the most common morbidity factors worldwide, is caused mostly by a deregulation of salt transport by the kidney, but underlying mechanisms remain elusive. The serine-threonine kinase With No lysine (K) 1 (WNK1) has been identified as a regulator of ion transport in the distal nephron, because its mutations cause Familial Hyperkalemic Hypertension (FHHt), a rare form of human hypertension. We generated a mouse model of WNK1-FHHt that fully recapitulates the disease and studied changes in expression of the Na+ and K+ transporters and channels involved in the maintenance of a correct blood pressure. Our study provides insights into the mechanisms underlying the pathogenesis of WNK1-FHHt and further corroborates the importance of WNK1 in ion homeostasis and blood pressure. Large deletions in the first intron of the With No lysine (K) 1 (WNK1) gene are responsible for Familial Hyperkalemic Hypertension (FHHt), a rare form of human hypertension associated with hyperkalemia and hyperchloremic metabolic acidosis. We generated a mouse model of WNK1-associated FHHt to explore the consequences of this intronic deletion. WNK1+/FHHt mice display all clinical and biological signs of FHHt. This phenotype results from increased expression of long WNK1 (L-WNK1), the ubiquitous kinase isoform of WNK1, in the distal convoluted tubule, which in turn, stimulates the activity of the Na–Cl cotransporter. We also show that the activity of the epithelial sodium channel is not altered in FHHt mice, suggesting that other mechanisms are responsible for the hyperkalemia and acidosis in this model. Finally, we observe a decreased expression of the renal outer medullary potassium channel in the late distal convoluted tubule of WNK1+/FHHt mice, which could contribute to the hyperkalemia. In summary, our study provides insights into the in vivo mechanisms underlying the pathogenesis of WNK1-mediated FHHt and further corroborates the importance of WNK1 in ion homeostasis and blood pressure.

Association Between 2 Angiographic Subtypes of Renal Artery Fibromuscular Dysplasia and Clinical Characteristics

Background— Initially based on histology, the diagnosis of renal artery fibromuscular dysplasia (FMD) is now based mostly on angiographic appearance because arterial tissue samples are rarely available. This retrospective cross-sectional study aimed to assess the clinical relevance of a binary angiographic classification of FMD lesions (unifocal or multifocal) based on computed tomographic or magnetic resonance angiography. Methods and Results— Adult patients diagnosed with FMD in a single tertiary care center for hypertension management were identified by screening of electronic files. FMD lesions were reviewed and classified according to computed tomography or magnetic resonance angiography as multifocal if there were at least 2 stenoses in the same arterial segment; otherwise, they were classified as unifocal. Of 337 patients with established renal artery FMD, 276 (82%) were classified as multifocal. Patients with unifocal and multifocal lesions differed significantly in median age at diagnosis of FMD (30 and 49 years) and hypertension (26 and 40 years), sex distribution (female:male ratio, 2:1 and 5:1), initial blood pressure (157/97 and 146/88 mm Hg), current smoking (50% and 26%), prevalence of unilateral renal artery lesions (79% and 38%), presence of kidney asymmetry (33% and 10%), renal revascularization procedures (90% and 35%), and hypertension cure rates in patients who underwent revascularization (54% and 26%). Conclusions— A binary angiographic classification into unifocal or multifocal renal artery FMD is straightforward and discriminates 2 groups of patients with different clinical phenotypes.

Familial Aggregation of Low-Renin Hypertension

Low-renin hypertension, representing roughly one quarter of all essential hypertension, is widely recognized by distinct physiological features, including salt-sensitivity, diuretic- responsiveness, and a favorable natural history. Although certain demographic features including age, ethnicity, and diabetes mellitus predispose to low-renin hypertension, these factors account for only a minority of cases. We examined familial concordance for renin status in 119 families with 257 hypertensive members. Low-renin was defined rigorously by plasma renin activity ≤0.69 ng angiotensin I/L per second, drawn when subjects had achieved balance after 5 to 7 days on a 10 mmol sodium diet and had stood upright for at least 1 hour. Given the prevalence of low-renin hypertension in our general population, low-renin hypertension was significantly more concordant among siblings than expected by chance (P =0.01). There were twice as many low-renin families as expected (10.9% versus 5.5%), in sharp contrast to the normal-renin state, in which the observed and expected were similar (61.0% versus 58.6%). These results were independent of age, race, and gender. Variance in renin status attributable to family membership was 35%. Association studies were performed on 8 polymorphisms in 5 candidate genes, and significant association was confirmed with the G460W polymorphism of the adducin gene. Familial determinants, which are probably but not definitely genetic, contribute to the low-renin hypertension state.

PHACTR1 Is a Genetic Susceptibility Locus for Fibromuscular Dysplasia Supporting Its Complex Genetic Pattern of Inheritance

Fibromuscular dysplasia (FMD) is a nonatherosclerotic vascular disease leading to stenosis, dissection and aneurysm affecting mainly the renal and cerebrovascular arteries. FMD is often an underdiagnosed cause of hypertension and stroke, has higher prevalence in females (~80%) but its pathophysiology is unclear. We analyzed ~26K common variants (MAF>0.05) generated by exome-chip arrays in 249 FMD patients and 689 controls. We replicated 13 loci (P<10−4) in 402 cases and 2,537 controls and confirmed an association between FMD and a variant in the phosphatase and actin regulator 1 gene (PHACTR1). Three additional case control cohorts including 512 cases and 669 replicated this result and overall reached the genomic level of significance (OR = 1.39, P = 7.4×10−10, 1,154 cases and 3,895 controls). The top variant, rs9349379, is intronic to PHACTR1, a risk locus for coronary artery disease, migraine, and cervical artery dissection. The analyses of geometrical parameters of carotids from ~2,500 healthy volunteers indicate higher intima media thickness (P = 1.97×10−4) and wall to lumen ratio (P = 0.002) in rs9349379-A carriers, suggesting indices of carotid hypertrophy previously described in carotids of FMD patients. Immunohistochemistry detected PHACTR1 in endothelium and smooth muscle cells of FMD and normal human carotids. The expression of PHACTR1 by genotypes in primary human fibroblasts showed higher expression in rs9349379-A carriers (N = 86, P = 0.003). Phactr1 knockdown in zebrafish resulted in dilated vessels indicating subtle impaired vascular development. We report the first susceptibility locus for FMD and provide evidence for a complex genetic pattern of inheritance and indices of shared pathophysiology between FMD and other cardiovascular and neurovascular diseases.