Gitelman-Like Syndrome Caused by Pathogenic Variants in mtDNA

Background: Gitelman syndrome is the most frequent hereditary salt-losing tubulopathy characterized by hypokalemic alkalosis and hypomagnesemia. Gitelman syndrome is caused by biallelic pathogenic variants in SLC12A3, encoding the Na+-Cl− cotransporter (NCC) expressed in the distal convoluted tubule. Pathogenic variants of CLCNKB, HNF1B, FXYD2, or KCNJ10 may result in the same renal phenotype of Gitelman syndrome, as they can lead to reduced NCC activity. For approximately 10 percent of patients with a Gitelman syndrome phenotype, the genotype is unknown. Methods: We identified mitochondrial DNA (mtDNA) variants in three families with Gitelman-like electrolyte abnormalities, then investigated 156 families for variants in MT-TI and MT-TF, which encode the transfer RNAs for phenylalanine and isoleucine. Mitochondrial respiratory chain function was assessed in patient fibroblasts. Mitochondrial dysfunction was induced in NCC-expressing HEK293 cells to assess the effect on thiazide-sensitive 22Na+ transport. Results: Genetic investigations revealed four mtDNA variants in 13 families: m.591C>T (n=7), m.616T>C (n=1), m.643A>G (n=1) (all in MT-TF), and m.4291T>C (n=4, in MT-TI). Variants were near homoplasmic in affected individuals. All variants were classified as pathogenic, except for m.643A>G, which was classified as a variant of uncertain significance. Importantly, affected members of six families with an MT-TF variant additionally suffered from progressive chronic kidney disease. Dysfunction of oxidative phosphorylation complex IV and reduced maximal mitochondrial respiratory capacity were found in patient fibroblasts. In vitro pharmacological inhibition of complex IV, mimicking the effect of the mtDNA variants, inhibited NCC phosphorylation and NCC-mediated sodium uptake. Conclusion: Pathogenic mtDNA variants in MT-TF and MT-TI can cause a Gitelman-like syndrome. Genetic investigation of mtDNA should be considered in patients with unexplained Gitelman syndrome-like tubulopathies

Les grandes avancées en néphro-génétique pédiatrique

L’essor de la génétique au cours des dernières décennies a permis des avancées majeures dans la compréhension des mécanismes conduisant aux maladies rénales héréditaires. Des premières études par clonage positionnel jusqu’à l’avènement du séquençage à haut débit (NGS), les techniques d’analyse du génome sont devenues de plus en plus performantes, avec un niveau de résolution extraordinaire. Les prix de séquençage se sont effondrés, passant d’un million de dollars (environ 940 millions d’euros) pour le séquençage du génome de James Watson en 2008, à quelques centaines d’euros pour le séquençage d’un génome aujourd’hui. Le diagnostic moléculaire tient ainsi une place centrale pour le diagnostic des patients et influe sur la prise en charge thérapeutique dans de nombreuses situations. Mais si le NGS est un outil performant pour l’identification de variants impliqués dans les maladies, il expose au risque de surinterprétation de certains variants, conduisant à des diagnostics erronés. Dans cette revue, nous proposons une brève rétrospective des étapes essentielles qui ont conduit aux connaissances actuelles et au développement du NGS pour l’étude des néphropathies héréditaires de l’enfant. Nous développerons ensuite les principales néphropathies héréditaires et les mécanismes moléculaires sous-jacents

Diversity of functional alterations of the ClC‐5 exchanger in the region of the proton glutamate in patients with Dent disease 1

International audienceMutations in the CLCN5 gene encoding the 2Cl- /1H+ exchanger ClC-5 are associated with Dent disease 1, an inherited renal disorder characterized by low molecular weight (LMW) proteinuria and hypercalciuria. In the kidney, ClC-5 is mostly localized in proximal tubule cells where it is thought to play a key role in the endocytosis of LMW proteins. Here, we investigated the consequences of eight previously reported pathogenic missense mutations of ClC-5 surrounding the "proton glutamate" that serves as a crucial H+ -binding site for the exchanger. A complete loss of function was observed for a group of mutants that were either retained in the endoplasmic reticulum of HEK293T cells or unstainable at plasma membrane due to proteasomal degradation. In contrast, the currents measured for a second group of mutations in X. laevis oocytes were reduced. Molecular Dynamics simulations performed on a ClC-5 homology model demonstrated that such mutations may alter ClC-5 protonation by interfering with the water pathway. Analysis of clinical data from patients harboring these mutations demonstrated no phenotype/genotype correlation. This study reveals that mutations clustered in a crucial region of ClC-5 have diverse molecular consequences in patients with Dent disease 1, ranging from altered expression to defects in transport. This article is protected by copyright. All rights reserved

When a maternal heterozygous mutation of the CYP24A1 gene leads to infantile hypercalcemia through a maternal uniparental disomy of chromosome 20

International audienceBackgroundInfantile hypercalcemia is an autosomal recessive disorder caused either by mutations in the CYP24A1 gene (20q13.2) or in the SLC34A1 gene (5q35.3). This disease is characterized by hypercalcemia, hypercalciuria and nephrocalcinosis in paediatric patients.Maternal uniparental disomy of chromosome 20 [UPD(20)mat], resulting in aberrant expression of imprinted transcripts at the GNAS locus, is a poorly characterized condition. UPD(20)mat patients manifest a phenotype similar to that of Silver-Russell syndrome and small for gestational age-short stature.Case presentationWe report here the genetic and clinical characterization of a male child with a phenotype of infantile hypercalcemia, postnatal growth retardation, and minor dysmorphic features. Genetic analysis using a next generation sequencing panel revealed a homozygous pathogenic variant of CYP24A1. The absence of the variant in the father led to microsatellite segregation analysis, suggestive of UPD. SNP-array revealed a large terminal copy neutral loss of heterozygosity leading to CYP24A1 homozygosity. SNP-array data of parent–child trio confirmed a UPD(20)mat responsible for both infantile hypercalcemia and Silver-Russell syndrome-like traits.ConclusionThis is the first report of uniparental disomy of chromosome 20 revealed by infantile hypercalcemia related to CYP24A1 biallelic homozygous variants, underlying the importance of controlling allelic segregation in cases of homozygosity

Prenatal hyperechogenic kidneys in three cases of infantile hypercalcemia associated with SLC34A1 mutations

International audiencePrenatal diagnosis of hyperechogenic kidneys is associated with a wide range of etiologies and prognoses. The recent advances in fetal ultrasound associated with the development of next-generation sequencing for molecular analysis have enlarged the spectrum of etiologies, making antenatal diagnosis a very challenging discipline. Of the various known causes of hyperechogenic fetal kidneys, calcium and phosphate metabolism disorders represent a rare cause. An accurate diagnosis is crucial for providing appropriate genetic counseling and medical follow-up after birth

High-throughput sequencing contributes to the diagnosis of tubulopathies and familial hypercalcemia hypocalciuria in adults

Hereditary tubulopathies are rare diseases with unknown prevalence in adults. Often diagnosed in childhood, hereditary tubulopathies can nevertheless be evoked in adults. Precise diagnosis can be difficult or delayed due to insidious development of symptoms, comorbidities and polypharmacy. Here we evaluated the diagnostic value of a specific panel of known genes implicated in tubulopathies in adult patients and compared to our data obtained in children. To do this we analyzed 1033 non-related adult patients of which 744 had a clinical diagnosis of tubulopathy and 289 had a diagnosis of familial hypercalcemia with hypocalciuria recruited by three European reference centers. Three-quarters of our tubulopathies cohort included individuals with clinical suspicion of Gitelman syndrome, kidney hypophosphatemia and kidney tubular acidosis. We detected pathogenic variants in 26 different genes confirming a genetic diagnosis of tubulopathy in 29% of cases. In 16 cases (2.1%) the genetic testing changed the clinical diagnosis. The diagnosis of familial hypercalcemia with hypocalciuria was confirmed in 12% of cases. Thus, our work demonstrates the genetic origin of tubulopathies in one out of three adult patients, half of the rate observed in children. Hence, establishing a precise diagnosis is crucial for patients, in order to guide care, to survey and prevent chronic complications, and for genetic counselling. At the same time, this work enhances our understanding of complex phenotypes and enriches the database with the causal variants described

Resistance to Insulin in Patients with Gitelman Syndrome and a Subtle Intermediate Phenotype in Heterozygous Carriers: A Cross-Sectional Study

BACKGROUND: Gitelman syndrome is a salt-losing tubulopathy caused by mutations in the SLC12A3 gene, which encodes the thiazide-sensitive sodium-chloride cotransporter. Previous studies suggested an intermediate phenotype for heterozygous carriers. METHODS: To evaluate the phenotype of heterozygous carriers of pathogenic SLC12A3 mutations, we performed a cross-sectional study of patients with Gitelman syndrome, heterozygous carriers, and healthy noncarriers. Participants measured their BP at home for three consecutive days before hospital admission for blood and urine sampling and an oral glucose tolerance test. RESULTS: We enrolled 242 participants, aged 18-75 years, including 81 heterozygous carriers, 82 healthy noncarriers, and 79 patients with Gitelman syndrome. The three groups had similar age, sex ratio, and body mass index. Compared with healthy noncarriers, heterozygous carriers showed significantly higher serum calcium concentration (P=0.01) and a trend for higher plasma aldosterone (P=0.06), but measures of home BP, plasma and urine electrolytes, renin, parathyroid hormone, vitamin D, and response to oral glucose tolerance testing were similar. Patients with Gitelman syndrome had lower systolic BP and higher heart rate than noncarriers and heterozygote carriers; they also had significantly higher fasting serum glucose concentration, higher levels of markers of insulin resistance, and a three-fold higher sensitivity to overweight. According to oral glucose tolerance testing, approximately 14% of patients with Gitelman syndrome were prediabetic, compared with 5% of heterozygous carriers and 4% of healthy noncarriers. CONCLUSIONS: Heterozygous carriers had a weak intermediate phenotype, between that of healthy noncarriers and patients with Gitelman syndrome. Moreover, the latter are at risk for development of type 2 diabetes, indicating the heightened importance of body weight control in these patients

Clinical characteristics of familial hypocalciuric hypercalcaemia type 1: A multicentre study of 77 adult patients

International audienceObjectiveFamilial hypocalciuric hypercalcaemia type 1 (FHH1), related to heterozygous loss‐of‐function mutations of the calcium‐sensing receptor gene, is the main differential diagnosis for primary hyperparathyroidism. The aim of our study was to describe clinical characteristics of adult patients living in France with a genetically confirmed FHH1.Design and patientsThis observational, retrospective, multicentre study included 77 adults, followed up in 32 clinical departments in France, with a genetic FHH1 diagnosis between 2001 and 2012.ResultsHypercalcaemia was diagnosed at a median age of 53 years [IQR: 38‐61]. The diagnosis was made after clinical manifestations, routine analysis or familial screening in 56, 34 and 10% of cases, respectively, (n = 58; data not available for 19 patients). Chondrocalcinosis was present in 11/51 patients (22%), bone fractures in 8/56 (14%) and renal colic in 6/55 (11%). The median serum calcium was 2.74 mmol/L [IQR: 2.63‐2.86 mmol/L], the median plasma parathyroid hormone level was 4.9 pmol/L [3.1‐7.1], and the median 24‐hour urinary calcium excretion was 2.8 mmol/24 hours [IQR: 1.9‐4.0]. Osteoporosis (dual X‐ray absorptiometry) or kidney stones (renal ultrasonography) were found in 6/38 patients (16%) and 9/32 patients (28%), respectively. Fourteen patients (18%) underwent parathyroid surgery; parathyroid adenoma was found in three patients (21%) and parathyroid hyperplasia in nine patients (64%). No correlation between genotype and phenotype was established.ConclusionThis large cohort study demonstrates that FHH1 clinical characteristics can be atypical in 33 patients (43%). Clinicians should be aware of this rare differential diagnosis in order to adopt an appropriate treatment strategy

Chromosomal microarray analysis in fetuses with an isolated congenital heart defect: A retrospective, nationwide, multicenter study in France

International audienceObjectives Congenital heart defects (CHDs) may be isolated or associated with other malformations. The use of chromosome microarray (CMA) can increase the genetic diagnostic yield for CHDs by between 4% and 10%. The objective of this study was to evaluate the value of CMA after the prenatal diagnosis of an isolated CHD. Methods In a retrospective, nationwide study performed in France, we collected data on all cases of isolated CHD that had been explored using CMAs in 2015. Results A total of 239 fetuses were included and 33 copy number variations (CNVs) were reported; 19 were considered to be pathogenic, six were variants of unknown significance, and eight were benign variants. The anomaly detection rate was 10.4% overall but ranged from 0% to 16.7% as a function of the isolated CHD in question. The known CNVs were 22q11.21 deletions (n = 10), 22q11.21 duplications (n = 2), 8p23 deletions (n = 2), an Alagille syndrome (n = 1), and a Kleefstra syndrome (n = 1). Conclusion The additional diagnostic yield was clinically significant (3.1%), even when anomalies in the 22q11.21 region were not taken into account. Hence, patients with a suspected isolated CHD and a normal karyotype must be screened for chromosome anomalies other than 22q11.21 duplications and deletions

Pseudoxanthoma Elasticum overlaps Hereditary Spastic Paraplegia Type 56

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