Hypomagnesaemia with varying degrees of extrarenal symptoms as a consequence of heterozygous CNNM2 variants.

Variants in the CNNM2 gene are causative for hypomagnesaemia, seizures and intellectual disability, although the phenotypes can be variable. This study aims to understand the genotype-phenotype relationship in affected individuals with CNNM2 variants by phenotypic, functional and structural analysis of new as well as previously reported variants. This results in the identification of seven variants that significantly affect CNNM2-mediated Mg2+ transport. Pathogenicity of these variants is further supported by structural modelling, which predicts CNNM2 structure to be affected by all of them. Strikingly, seizures and intellectual disability are absent in 4 out of 7 cases, indicating these phenotypes are caused either by specific CNNM2 variant only or by additional risk factors. Moreover, in line with sporadic observations from previous reports, CNNM2 variants might be associated with disturbances in parathyroid hormone and Ca2+ homeostasis.

Autosomal dominant distal renal tubular acidosis in two pediatric patients with mutations in the SLC4A1 gene. Can the maximum urinary pCO2 test be normal?

Primary distal renal tubular acidosis (dRTA) is a rare tubulopathy characterised by the presence of hyperchloremic metabolic acidosis. It is caused by the existence of a defect in the function of the H+ -ATPase located on the luminal side of the α-intercalated cells or the Cl - HCO3- (AE1) anion exchanger located on the basolateral side. Patients do not acidify the urine after acid overload (NH4Cl) or after stimulating H+ secretion by obtaining a high intratubular concentration of an anion such as chlorine (pH is measured) or HCO3- (urinary pCO2 is measured). We present a family with autosomal dominant dRTA produced by a heterozygous mutation in the SLC4A1 gene in which the two paediatric members showed a test of normal maximum urinary pCO2. Our hypothesis is that since the H + -ATPase is intact, at least initially, the stimulation induced by intratubular electronegativity to secrete H + could be effective, which would allow the maximum urinary pCO2 to be paradoxically normal, which could explain the onset, moderate presentation of symptoms and late diagnosis in patients with this mutation. This is the first documented case of a dominant dRTA in Mexico.

Acidosis tubular renal distal autosómica dominante en dos pacientes pediátricos con mutaciones en el gen SLC4A1. ¿La prueba de la pCO2 urinaria máxima puede ser normal? / Autosomal dominant distal renal tubular acidosis in two pediatric patients with mutations in the SLC4A1 gene. Can the maximum urinary pCO2 test be normal?

La acidosis tubular renal distal (ATRd) primaria es una tubulopatía poco frecuente caracterizada por la presencia de acidosis metabólica hiperclorémica. Está generada por la existencia de un defecto en la función de la H+-ATPasa situada en el lado luminal de las células α-intercaladas o del intercambiador de aniones Cl−-HCO3− (AE1) ubicado en el lado basolateral. Los pacientes no acidifican la orina tras una sobrecarga ácida (NH4Cl) o tras estimular la secreción de H+ mediante la obtención de una elevada concentración intratubular de un anión como cloro (se mide el pH) o HCO3− (se mide la pCO2 urinaria). Se presenta una familia con ATRd autosómica dominante producida por una mutación heterocigota en el gen SLC4A1 en la que los 2 miembros en edad pediátrica mostraron una prueba de la pCO2 urinaria máxima normal. Nuestra hipótesis es que al estar intacta, al menos inicialmente, la H+-ATPasa, podría ser efectivo el estímulo inducido por la electronegatividad intratubular para secretar H+ lo que permitiría que la pCO2 urinaria máxima fuera paradójicamente normal, lo que pudiera explicar el inicio tardío, la presentación moderada de los síntomas y el diagnóstico en edades más avanzadas, en los pacientes con dicha mutación. Este es el primer caso documentado de una ATRd dominante en México. (AU)

Primary distal renal tubular acidosis (dRTA) is a rare tubulopathy characterized by the presence of hyperchloremic metabolic acidosis. It is caused by the existence of a defect in the function of the H+-ATPase located on the luminal side of the α-intercalated cells or the Cl− HCO3− (AE1) anion exchanger located on the basolateral side. Patients do not acidify the urine after acid overload (NH4Cl) or after stimulating H+ secretion by obtaining a high intratubular concentration of an anion such as chlorine (pH is measured) or HCO3− (urinary pCO2 is measured). We present a family with autosomal dominant dRTA produced by a heterozygous mutation in the SLC4A1 gene in which the two pediatric members showed a test of normal maximum urinary pCO2. Our hypothesis is that since the H+-ATPase is intact, at least initially, the stimulation induced by intratubular electronegativity to secrete H+ could be effective, which would allow the maximum urinary pCO2 to be paradoxically normal, which could explain the onset, moderate presentation of symptoms and late diagnosis in patients with this mutation. This is the first documented case of a dominant dRTA in Mexico. (AU)

Clinical Findings and Genetic Analysis of Nine Mexican Families with Bartter Syndrome.

BACKGROUND: Bartter's syndrome (BS) is a group of salt-wasting tubulopathies characterized by hypokalemia, metabolic alkalosis, hypercalciuria, secondary hyperaldosteronism, and low or normal blood pressure. Loss-of-function variants in genes encoding for five proteins expressed in the thick ascending limb of Henle in the nephron, produced different genetic types of BS. AIM: Clinical and genetic analysis of families with Antenatal Bartter syndrome (ABS) and with Classic Bartter syndrome (CBS). METHODS: Nine patients from unrelated non-consanguineous Mexican families were studied. Massive parallel sequencing of a gene panel or whole-exome sequencing was used to identify the causative gene. RESULTS: Proband 1 was homozygous for the pathogenic variant p.Arg302Gln in the SLC12A1 gene encoding for the sodium-potassium-chloride NKCC2 cotransporter. Proband 3 was homozygous for the nonsense variant p.Cys308* in the KCNJ1 gene encoding for the ROMK potassium channel. Probands 7, 8, and 9 showed variants in the CLCKNB gene encoding the chloride channel ClC-Kb: proband 7 was compound heterozygous for the deletion of the entire gene and the missense change p.Arg438Cys; proband 8 presented a homozygous deletion of the whole gene and proband 9 was homozygous for the nonsense mutation p.Arg595*. A heterozygous variant of unknown significance was detected in the SLC12A1 gene in proband 2, and no variants were found in SLC12A1, KCNJ1, BSND, CLCNKA, CLCNKB, and MAGED2 genes in probands 4, 5, and 6. CONCLUSIONS: Genetic analysis identified loss-of-function variants in the SLC12A1, KCNJ1, and CLCNKB genes in four patients with ABS and in the CLCNKB gene in two patients with CBS.

[Major advances in pediatric nephro-genetics]. / Les grandes avancées en néphro-génétique pédiatrique.

The rise of genetics in the last decades has allowed major advances in the understanding of the mechanisms leading to inherited kidney diseases. From the first positional cloning studies to the advent of high-throughput sequencing (NGS), genome analysis technologies have become increasingly efficient, with an extraordinary level of resolution. Moreover, sequencing prices have decreased from one million dollars for the sequencing of James Watson's genome in 2008, to a few hundred dollars for the sequencing of a genome today. Thus, molecular diagnosis has a central place in the diagnosis of these patients and influences the therapeutic management in many situations. However, although NGS is a powerful tool for the identification of variants involved in diseases, it also exposes to the risk of over-interpretation of certain variants, leading to erroneous diagnoses, requiring the use of specialists. In this review, we first propose a brief retrospective of the essential steps that led to the current knowledge and the development of NGS for the study of hereditary nephropathies in children. This review is then an opportunity to present the main hereditary nephropathies and the underlying molecular mechanisms. Among them, we emphasize ciliopathies, congenital anomalies of the kidney and urinary tract, podocytopathies and tubulopathies.

Title: Les grandes avancées en néphro-génétique pédiatrique. Abstract: 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.

Relationship between clinical phenotype and in vitro analysis of 13 NPT2c/SCL34A3 mutants.

Biallelic pathogenic variants in the SLC34A3 gene, encoding for the NPT2c cotransporter, cause Hereditary Hypophosphatemic Rickets with Hypercalciuria (HHRH). However, the associated phenotype is highly variable. In addition, mice deleted for Slc34a3 exhibit a different phenotype compared to humans, without urinary phosphate leakage. The mechanisms by which SLC34A3 variants disrupt phosphate/calcium metabolism are un-completely understood. In this study we explored these mechanisms in vitro using SLC34A3 variants identified in patients with urinary phosphate leakage. We analyzed the consequences of these variants on NPT2c function and the link with the phenotype of the patients. We studied 20 patients with recurrent nephrolithiasis and low serum phosphate concentration harboring variants in the SLC34A3 gene. Half of the patients carried homozygous or composite heterozygous variants. Three patients had in addition variants in SLC34A1 and SLC9A3R1 genes. All these patients benefited from a precise analysis of their phenotype. We generated 13 of these mutants by site-directed mutagenesis. Then we carried out transient transfections of these mutants in HEK cells and measured their phosphate uptake capacity under different conditions. Among the 20 patients included, 3 had not only mutations in NPT2c but also in NPT2a or NHERF1 genes. Phosphate uptake was decreased in 8 NPT2c mutants studied and normal for 5. Four variants were initially categorized as variants of uncertain significance. Expression of the corresponding mutants showed that one did not modify phosphate transport, two reduced it moderately and one abolished it. Co-transfection of the NPT2c mutants with the wild-type plasmid of NPT2c or NPT2a did not reveal dominant negative effect of the mutants on NPT2c-mediated phosphate transport. A detailed analysis of patient phenotypes did not find a link between the severity of the disorder and the level of phosphate transport impairment. NPT2c mutations classified as ACMG3 identified in patients with renal phosphate leak should be characterized by in vitro study to check if they alter NPT2c-mediated phosphate transport since phosphate uptake capacity may not be affected. In addition, research for mutations in NHERF1 and NPT2a genes should always be associated to NPT2c sequencing.

Long-Read Sequencing Identifies Novel Pathogenic Intronic Variants in Gitelman Syndrome.

BACKGROUND: Gitelman syndrome is a salt-losing tubulopathy characterized by hypokalemic alkalosis and hypomagnesemia. It is caused by homozygous recessive or compound heterozygous pathogenic variants in SLC12A3 , which encodes the Na + -Cl - cotransporter (NCC). In up to 10% of patients with Gitelman syndrome, current genetic techniques detect only one specific pathogenic variant. This study aimed to identify a second pathogenic variant in introns, splice sites, or promoters to increase the diagnostic yield. METHODS: Long-read sequencing of SLC12A3 was performed in 67 DNA samples from individuals with suspected Gitelman syndrome in whom a single likely pathogenic or pathogenic variant was previously detected. In addition, we sequenced DNA samples from 28 individuals with one variant of uncertain significance or no candidate variant. Midigene splice assays assessed the pathogenicity of novel intronic variants. RESULTS: A second likely pathogenic/pathogenic variant was identified in 45 (67%) patients. Those with two likely pathogenic/pathogenic variants had a more severe electrolyte phenotype than other patients. Of the 45 patients, 16 had intronic variants outside of canonic splice sites (nine variants, mostly deep intronic, six novel), whereas 29 patients had an exonic variant or canonic splice site variant. Midigene splice assays of the previously known c.1670-191C>T variant and intronic candidate variants demonstrated aberrant splicing patterns. CONCLUSION: Intronic pathogenic variants explain an important part of the missing heritability in Gitelman syndrome. Long-read sequencing should be considered in diagnostic workflows for Gitelman syndrome.

Genetic basis of nephrogenic diabetes insipidus.

Nephrogenic diabetes insipidus is defined as an inability to concentrate urine due to a complete or partial alteration of the renal tubular response to arginine vasopressin hormone, resulting in excessive diluted urine excretion. Hereditary forms are caused by molecular defects in the genes encoding either of the two main renal effectors of the arginine vasopressin pathway: the AVPR2 gene, which encodes for the type 2 vasopressin receptor, or the AQP2 gene, which encodes for the water channel aquaporin-2. About 90% of cases of nephrogenic diabetes insipidus result from loss-of-function variants in the AVPR2 gene, which are inherited in a X-linked recessive manner. The remaining 10% of cases result from loss-of-function variants in the AQP2 gene, which can be inherited in either a recessive or a dominant manner. The main symptoms of the disease are polyuria, chronic dehydration and hypernatremia. These symptoms usually occur in the first year of life, although some patients present later. Diagnosis is based on abnormal response in urinary osmolality after water restriction and/or administration of exogenous vasopressin. Treatment involves ensuring adequate water intake on demand, possibly combined with thiazide diuretics, non-steroidal anti-inflammatory drugs, and a low-salt and protein diet. In this review, we provide an update on current understanding of the molecular basis of inherited nephrogenic insipidus diabetes.

Possible role for rare TRPM7 variants in patients with hypomagnesaemia with secondary hypocalcaemia.

BACKGROUND: Hypomagnesaemia with secondary hypocal-caemia (HSH) is a rare autosomal recessive disorder caused by pathogenic variants in TRPM6, encoding the channel-kinase transient receptor potential melastatin type 6. Patients have very low serum magnesium (Mg2+) levels and suffer from muscle cramps and seizures. Despite genetic testing, a subgroup of HSH patients remains without a diagnosis. METHODS: In this study, two families with an HSH phenotype but negative for TRPM6 pathogenic variants were subjected to whole exome sequencing. Using a complementary combination of biochemical and functional analyses in overexpression systems and patient-derived fibroblasts, the effect of the TRPM7-identified variants on Mg2+ transport was examined. RESULTS: For the first time, variants in TRPM7 were identified in two families as a potential cause for hereditary HSH. Patients suffer from seizures and muscle cramps due to magnesium deficiency and episodes of hypocalcaemia. In the first family, a splice site variant caused the incorporation of intron 1 sequences into the TRPM7 messenger RNA and generated a premature stop codon. As a consequence, patient-derived fibroblasts exhibit decreased cell growth. In the second family, a heterozygous missense variant in the pore domain resulted in decreased TRPM7 channel activity. CONCLUSIONS: We establish TRPM7 as a prime candidate gene for autosomal dominant hypomagnesaemia and secondary hypocalcaemia. Screening of unresolved patients with hypocalcaemia and secondary hypocalcaemia may further establish TRPM7 pathogenic variants as a novel Mendelian disorder.

Prenatal bone abnormalities in three cases of familial hypocalciuric hypercalcemia.

INTRODUCTION: Prenatal diagnosis of bone and mineralization anomalies is associated with a wide range of etiologies and prognoses. The improvement of antenatal ultrasound combined with the development of molecular diagnosis in genetics has transformed antenatal medicine into a challenging discipline. Of the various known causes of bone abnormalities and hypomineralization, calcium and phosphate metabolism disorders are exceptional. An accurate diagnosis is crucial for providing appropriate genetic counseling and medical follow-up after birth. CASE: We report on three siblings with severe bone abnormalities diagnosed during the second trimester ultrasound of pregnancy. Postnatal follow-up showed transitory hyperparathyroidism, with hypercalcemia and hypocalciuria. METHODS: Sanger sequencing performed after birth in the three newborns revealed a monoallelic pathogenic variant in the CASR gene, encoding the calcium sensing receptor, confirming the diagnosis of familial hypocalciuric hypercalcemia, paternally inherited. Postnatal evolution was favorable after treatment with a calcimimetic agent. CONCLUSIONS: Previously, prenatal bone abnormalities caused by familial hypocalciuric hypercalcemia had only been described in one patient. This entity should be considered as differential diagnosis of bones abnormalities. Knowing about this unusual etiology is important to guide the diagnosis, the prenatal counseling and to improve medical management.

How the diagnosis and the management of genetic renal phosphate leak impact the life of kidney stone formers?

Genetic renal phosphate leak is one of the rare disorders in recurrent stone formers with absorptive hypercalciuria. Diagnosis and appropriate management may change the life of patients. To provide answers on how and when to make the diagnosis of genetic renal phosphate leak and how medical management prevents the recurrences and changes patients' life, we conducted a retrospective study including nine patients with recurrent nephrolithiasis and a confirmed genetic mutation of a phosphate transporter between 2008 and 2019 in our multidisciplinary center at the Pitié Salpetriere Hospital, Paris, France. We compared the number and the annual rate of urological intervention before and after the diagnosis and management using the Wilcoxon test. A qualitative survey was done to evaluate the quality of life of patients. A total of 9 patients were included in this study. Patient baseline characteristics and elements supporting the diagnosis are described. We showed an effective decrease in urological intervention number (p = 0.0078) and annual rate (p = 0.0117) after the diagnosis and the appropriate management, and an improvement in the patients' quality of life. The diagnosis and the appropriate management of genetic renal phosphate leak disorder improve the quality of life by preventing stone recurrence and decreasing the number of surgical intervention.

Parathyroid hormone and phosphate homeostasis in patients with Bartter and Gitelman syndrome: an international cross-sectional study.

BACKGROUND: Small cohort studies have reported high parathyroid hormone (PTH) levels in patients with Bartter syndrome and lower serum phosphate levels have anecdotally been reported in patients with Gitelman syndrome. In this cross-sectional study, we assessed PTH and phosphate homeostasis in a large cohort of patients with salt-losing tubulopathies. METHODS: Clinical and laboratory data of 589 patients with Bartter and Gitelman syndrome were provided by members of the European Rare Kidney Diseases Reference Network (ERKNet) and the European Society for Paediatric Nephrology (ESPN). RESULTS: A total of 285 patients with Bartter syndrome and 304 patients with Gitelman syndrome were included for analysis. Patients with Bartter syndrome type I and II had the highest median PTH level (7.5 pmol/L) and 56% had hyperparathyroidism (PTH >7.0 pmol/L). Serum calcium was slightly lower in Bartter syndrome type I and II patients with hyperparathyroidism (2.42 versus 2.49 mmol/L; P = .038) compared to those with normal PTH levels and correlated inversely with PTH (rs -0.253; P = .009). Serum phosphate and urinary phosphate excretion did not correlate with PTH. Overall, 22% of patients had low serum phosphate levels (phosphate-standard deviation score < -2), with the highest prevalence in patients with Bartter syndrome type III (32%). Serum phosphate correlated with tubular maximum reabsorption of phosphate/glomerular filtration rate (TmP/GFR) (rs 0.699; P < .001), suggesting renal phosphate wasting. CONCLUSIONS: Hyperparathyroidism is frequent in patients with Bartter syndrome type I and II. Low serum phosphate is observed in a significant number of patients with Bartter and Gitelman syndrome and appears associated with renal phosphate wasting.

Calcium pyrophosphate crystal deposition in a cohort of 57 patients with Gitelman syndrome.

OBJECTIVE: Gitelman syndrome (GS) is the most frequent salt-wasting genetic tubulopathy and a source of hypokalaemia and hypomagnesemia. Chondrocalcinosis (CC) is a frequent feature of GS. The aim of our study was to determine the prevalence, distribution patterns, clinical phenotypes and risk factors for CC in GS. METHODS: This prospective study of a cohort of 57 patients with GS included a systematic screening for CC by peripheral joint radiography, cervical spine CT and joint US. The prevalence of cervical C1-C2 CC by CT was compared between 33 GS patients and sex- and age-matched controls. Clinical and biochemical features were analysed to identify factors associated with CC. RESULTS: Mean (s.d.) age of patients was 46.5 (12.4) years, 66.7% were women and 93.0% carried SLC12A3 mutations. Mean serum magnesium level was 0.60 (0.30) mmol/l. CC was observed in 79% of patients, with the highest prevalence at the cervical spine (81.8%) followed by the knee (52.6%), wrist (50.9%), ankle (38.6%), TM joint (36.4%), shoulder (33.3%), hip (22.8%), elbow (14.0%) and sclerochoroid (12.1%). Prevalence of CC at the C1-C2 level was higher in the GS cohort than control group (72.7% vs 9.1%) (adjusted odds ratio 21.0, 95% CI 2.8, 156.1, P = 0.003). Independent factors associated with CC were low serum magnesium level and age. CONCLUSION: GS was associated with widespread CC, favoured by aging and hypomagnesemia. The C1-C2 level was the most affected site. Follow-up of this unique cohort will help understanding the clinical consequences of CC, especially the precise characterization of pyrophosphate arthropathy.

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.

Genetic testing in the diagnosis of chronic kidney disease: recommendations for clinical practice.

The overall diagnostic yield of massively parallel sequencing-based tests in patients with chronic kidney disease (CKD) is 30% for paediatric cases and 6-30% for adult cases. These figures should encourage nephrologists to frequently use genetic testing as a diagnostic means for their patients. However, in reality, several barriers appear to hinder the implementation of massively parallel sequencing-based diagnostics in routine clinical practice. In this article we aim to support the nephrologist to overcome these barriers. After a detailed discussion of the general items that are important to genetic testing in nephrology, namely genetic testing modalities and their indications, clinical information needed for high-quality interpretation of genetic tests, the clinical benefit of genetic testing and genetic counselling, we describe each of these items more specifically for the different groups of genetic kidney diseases and for CKD of unknown origin.

Pathophysiological aspects of the thick ascending limb and novel genetic defects: HELIX syndrome and transient antenatal Bartter syndrome.

The thick ascending limb plays a central role in human kidney physiology, participating in sodium reabsorption, urine concentrating mechanisms, calcium and magnesium homeostasis, bicarbonate and ammonium homeostasis, and uromodulin synthesis. This review aims to illustrate the importance of these roles from a pathophysiological point of view by describing the interactions of the key proteins of this segment and by discussing how recently identified and long-known hereditary diseases affect this segment. The descriptions of two recently described salt-losing tubulopathies, transient antenatal Bartter syndrome and HELIX syndrome, which are caused by mutations in MAGED2 and CLDN10 genes, respectively, highlight the role of new players in the modulation of sodium reabsorption the thick ascending limb.

Primary Hyperparathyroidism in Homozygous Sickle Cell Patients: A Hemolysis-Mediated Hypocalciuric Hypercalcemia Phenotype?

Primary hyperparathyroidism (pHPT) has been reported to have a higher prevalence in sickle cell disease (SCD) patients, including a high rate of recurrence following surgery. However, most patients are asymptomatic at the time of diagnosis, with surprisingly infrequent hypercalciuria, raising the issue of renal calcium handling in SCD patients. We conducted a retrospective study including (1) 64 hypercalcemic pHPT non-SCD patients; (2) 177 SCD patients, divided into two groups of 12 hypercalcemic pHPT and 165 non-pHPT; (3) eight patients with a diagnosis of familial hypocalciuric hypercalcemia (FHH). Demographic and biological parameters at the time of diagnosis were collected and compared between the different groups. Determinants of fasting fractional excretion of calcium (FeCa2+) were also analyzed in non-pHPT SCD patients. Compared to non-SCD pHPT patients, our data show a similar ionized calcium and PTH concentration, with a lower plasmatic calcitriol concentration and a lower daily urinary calcium excretion in pHPT SCD patients (p < 0.0001 in both cases). Fasting FeCa2+ is also surprisingly low in pHPT SCD patients, and thus inadequate to be considered hypercalcemia, recalling the FHH phenotype. FeCa2+ is also low in the non-pHPT SCD control group, and negatively associated with PTH and hemolytic biomarkers such as LDH and low hemoglobin. Our data suggest that the pHPT biochemical phenotype in SCD patients resembles the FHH phenotype, and the fasting FeCa2+ association with chronic hemolysis biomarkers strengthens the view of a potential pharmacological link between hemolytic by-products and calcium reabsorption, potentially through a decreased calcium-sensing receptor (CaSR) activity.

The variety of genetic defects explains the phenotypic heterogeneity of Familial Hyperkalemic Hypertension.

INTRODUCTION: Familial hyperkalemic hypertension is a rare inherited form of arterial hypertension. Four genes are responsible for this disease, the variants of these genes cause disruption in the regulation of ion transport in the distal renal tubule. Whether the genotype explains the large phenotypic heterogeneity has not been fully explored. METHODS: We retrospectively analyzed clinical and genetic data of 153 cases (84 probands, 69 relatives) with familial hyperkalemic hypertension. RESULTS: Pathogenic variants (25 novel variants) were identified as follows: KLHL3 (n = 50), CUL3 (n = 16), WNK1 acidic motif (n = 11), WNK4 acidic motif (n = 4) and WNK1 intron 1 deletions (n = 3). De novo cases were mainly observed in the CUL3-related cases (9 of 12) and recessive cases were only observed in KLHL3-related cases (14 of 50). More severe forms were observed in recessive KLHL3 and CUL3 cases that were also associated with growth retardation. Patients with WNK1 acidic motif variants had a typical biological phenotype and lower frequency of hypertension conversely to WNK4 variants affecting the same motif. Patients with heterozygous KLHL3 and WNK1 deletions had milder forms. Familial screening in 178 relatives allowed detection and care for 69 positive cases. Blood pressure and hyperkalemia were improved by hydrochlorothiazide in all groups. CONCLUSIONS: This study confirms the phenotypic variability ranging from the severe and early forms associated with CUL3 and recessive KLHL3 genotypes through intermediate forms associated with KLHL3 dominant, WNK4 and WNK1 deletion to mild form associated with WNK1 acidic motif genotype and reinforces the interest of genetic screening to better orientate medical care and genetic counseling.

mTOR-Activating Mutations in RRAGD Are Causative for Kidney Tubulopathy and Cardiomyopathy.

BACKGROUND: Over the last decade, advances in genetic techniques have resulted in the identification of rare hereditary disorders of renal magnesium and salt handling. Nevertheless, approximately 20% of all patients with tubulopathy lack a genetic diagnosis. METHODS: We performed whole-exome and -genome sequencing of a patient cohort with a novel, inherited, salt-losing tubulopathy; hypomagnesemia; and dilated cardiomyopathy. We also conducted subsequent in vitro functional analyses of identified variants of RRAGD, a gene that encodes a small Rag guanosine triphosphatase (GTPase). RESULTS: In eight children from unrelated families with a tubulopathy characterized by hypomagnesemia, hypokalemia, salt wasting, and nephrocalcinosis, we identified heterozygous missense variants in RRAGD that mostly occurred de novo. Six of these patients also had dilated cardiomyopathy and three underwent heart transplantation. We identified a heterozygous variant in RRAGD that segregated with the phenotype in eight members of a large family with similar kidney manifestations. The GTPase RagD, encoded by RRAGD, plays a role in mediating amino acid signaling to the mechanistic target of rapamycin complex 1 (mTORC1). RagD expression along the mammalian nephron included the thick ascending limb and the distal convoluted tubule. The identified RRAGD variants were shown to induce a constitutive activation of mTOR signaling in vitro. CONCLUSIONS: Our findings establish a novel disease, which we call autosomal dominant kidney hypomagnesemia (ADKH-RRAGD), that combines an electrolyte-losing tubulopathy and dilated cardiomyopathy. The condition is caused by variants in the RRAGD gene, which encodes Rag GTPase D; these variants lead to an activation of mTOR signaling, suggesting a critical role of Rag GTPase D for renal electrolyte handling and cardiac function.