Antenatal Bartter Syndrome Research Articles

X-linked transient antenatal Bartter syndrome related to MAGED2 gene: Enriching the phenotypic description and pathophysiologic investigation

PurposeTransient Bartter syndrome related to pathogenic variants of MAGED2 is the most recently described antenatal Bartter syndrome. Despite its transient nature, it is the most severe form of Bartter syndrome in the perinatal period. Our aim was to describe 14 new cases and to try to explain the incomplete penetrance in women. MethodsWe report on 14 new cases, including 3 females, and review the 40 cases described to date. We tested the hypothesis that MAGED2 is transcriptionally regulated by differential methylation of its CpG-rich promotor by pyrosequencing of DNA samples extracted from fetal and adult leukocytes and kidney samples. ResultsAnalysis of the data from 54 symptomatic patients showed spontaneous resolution of symptoms in 27% of cases, persistent complications in 41% of cases, and fatality in 32% of cases. Clinical anomalies were reported in 76% of patients, mostly renal anomalies (52%), cardiovascular anomalies (29%), and dysmorphic features (13%). A developmental delay was reported in 24% of patients. Variants were found in all regions of the gene. Methylation analysis of the MAGED2 CpG-rich promotor showed a correlation with gender, independent of age, tissue or presence of symptoms, excluding a role for this mechanism in the incomplete penetrance in women. ConclusionThis work enriches the phenotypic and genetic description of this recently described disease and deepens our understanding of the pathophysiological role and regulation of MAGED2. Finally, by describing the wide range of outcomes in patients, this work opens the discussion on genetic counseling offered to families.

Protein Quality Control of NKCC2 in Bartter Syndrome and Blood Pressure Regulation.

Mutations in NKCC2 generate antenatal Bartter syndrome type 1 (type 1 BS), a life-threatening salt-losing nephropathy characterized by arterial hypotension, as well as electrolyte abnormalities. In contrast to the genetic inactivation of NKCC2, inappropriate increased NKCC2 activity has been associated with salt-sensitive hypertension. Given the importance of NKCC2 in salt-sensitive hypertension and the pathophysiology of prenatal BS, studying the molecular regulation of this Na-K-2Cl cotransporter has attracted great interest. Therefore, several studies have addressed various aspects of NKCC2 regulation, such as phosphorylation and post-Golgi trafficking. However, the regulation of this cotransporter at the pre-Golgi level remained unknown for years. Similar to several transmembrane proteins, export from the ER appears to be the rate-limiting step in the cotransporter's maturation and trafficking to the plasma membrane. The most compelling evidence comes from patients with type 5 BS, the most severe form of prenatal BS, in whom NKCC2 is not detectable in the apical membrane of thick ascending limb (TAL) cells due to ER retention and ER-associated degradation (ERAD) mechanisms. In addition, type 1 BS is one of the diseases linked to ERAD pathways. In recent years, several molecular determinants of NKCC2 export from the ER and protein quality control have been identified. The aim of this review is therefore to summarize recent data regarding the protein quality control of NKCC2 and to discuss their potential implications in BS and blood pressure regulation.

AUP1 Regulates the Endoplasmic Reticulum-Associated Degradation and Polyubiquitination of NKCC2.

Inactivating mutations of kidney Na-K-2Cl cotransporter NKCC2 lead to antenatal Bartter syndrome (BS) type 1, a life-threatening salt-losing tubulopathy. We previously reported that this serious inherited renal disease is linked to the endoplasmic reticulum-associated degradation (ERAD) pathway. The purpose of this work is to characterize further the ERAD machinery of NKCC2. Here, we report the identification of ancient ubiquitous protein 1 (AUP1) as a novel interactor of NKCC2 ER-resident form in renal cells. AUP1 is also an interactor of the ER lectin OS9, a key player in the ERAD of NKCC2. Similar to OS9, AUP1 co-expression decreased the amount of total NKCC2 protein by enhancing the ER retention and associated protein degradation of the cotransporter. Blocking the ERAD pathway with the proteasome inhibitor MG132 or the α-mannosidase inhibitor kifunensine fully abolished the AUP1 effect on NKCC2. Importantly, AUP1 knock-down or inhibition by overexpressing its dominant negative form strikingly decreased NKCC2 polyubiquitination and increased the protein level of the cotransporter. Interestingly, AUP1 co-expression produced a more profound impact on NKCC2 folding mutants. Moreover, AUP1 also interacted with the related kidney cotransporter NCC and downregulated its expression, strongly indicating that AUP1 is a common regulator of sodium-dependent chloride cotransporters. In conclusion, our data reveal the presence of an AUP1-mediated pathway enhancing the polyubiquitination and ERAD of NKCC2. The characterization and selective regulation of specific ERAD constituents of NKCC2 and its pathogenic mutants could open new avenues in the therapeutic strategies for type 1 BS treatment.

Identification of a novel intronic mutation of MAGED2 gene in a Chinese family with antenatal Bartter syndrome

BackgroundAntenatal Bartter syndrome is a life-threatening disease caused by a mutation in the MAGED2 gene located on chromosome Xp11. It is characterized by severe polyhydramnios and extreme prematurity. While most reported mutations are located in the exon region, variations in the intron region are rarely reported.MethodsIn our study, we employed whole exome sequencing and Sanger sequencing to genotype members of this family. Additionally, a minigene assay was conducted to evaluate the impact of genetic variants on splicing.ResultsOur findings reveal a novel intronic variant (NM_177433.3:c.1271 + 4_1271 + 7delAGTA) in intron 10 of the MAGED2 gene. Further analysis using the minigene assay demonstrated that this variant activated an intronic cryptic splice site, resulting in a 96 bp insertion in mature mRNA.ConclusionsOur results indicate that the novel intronic variant (c.1271 + 4_1271 + 7delAGTA) in intron 10 of the MAGED2 gene is pathogenic. This expands the mutation spectrum of MAGED2 and highlights the significance of intronic sequence analysis.

Successful antenatal treatment of MAGED2-related Bartter syndrome and review of treatment options and efficacy.

A new form of transient antenatal Bartter syndrome (aBS) was recently identified that is associated with the X-linked MAGED2 variant. Case reports demonstrate that this variant leads to severe polyhydramnios that may result in preterm birth or pregnancy loss. There is limited but promising evidence that amnioreductions may improve fetal outcomes in this rare condition. We report a woman with two affected pregnancies. In the first pregnancy, the patient was diagnosed with mild-to-moderate polyhydramnios in the second trimester that ultimately resulted in preterm labor and delivery at 25weeks with fetal demise. Whole exome sequencing of the amniotic fluid sample resulted after the pregnancy loss and revealed a c.1337G>A MAGED2 variant that was considered diagnostically. The subsequent pregnancy was confirmed by chorionic villi sampling to also be affected by this variant. The pregnancy was managed with frequent ultrasounds and three amnioreductions that resulted in spontaneous vaginal delivery at 37weeks and 6days of a viable newborn with no evidence of overt electrolyte abnormalities suggesting complete resolution. A detailed review of the published cases of MAGED2-related transient aBS is provided. Our review focuses on individuals who received antenatal treatment. A total of 31 unique cases of MAGED2-related transient aBS were compiled. Amnioreduction was performed in 23 cases and in 18 cases no amnioreduction was performed. The average gestational age at delivery was significantly lower in cases without serial amnioreduction (28.7 vs. 30.71weeks, p=0.03). Neonatal mortality was seen in 5/18 cases without serial amnioreduction, and no mortality was observed in the cases with serial amnioreduction. In cases of second trimester severe polyhydramnios without identifiable cause, whole exome sequencing should be considered. Intensive ultrasound surveillance and serial amnioreduction is recommended for the management of MAGED2-related transient aBS.

BARTTER SYNDROME TYPE I. CLINICAL CASE OF A PREGNANT WOMAN WITH POLYHYDRAMNIOS

Bartter syndrome is an inherited renal tubular disorder characterized by salt wasting, hypokalemia and metabolic acidosis due to impaired salt reabsorption in the ascending limb of the loop of Henle in the renal nephrons. Currently, five types of the disease are known. While the clinical signs of type 3 (classical type) appear in childhood and later ages, types 1, 2, 4, 5 (antenatal type) are characterized by severe polyhydramnios in the antenatal period and antenatal fetal death. This article is about a pregnant woman carrying a fetus with antenatal Bartter syndrome type I.

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

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. Clinical and genetic analysis of families with Antenatal Bartter syndrome (ABS) and with Classic Bartter syndrome (CBS). 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. 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. 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.

An Unusual Case of BSND Gene–Related (Type IV) Bartter Syndrome Presenting as Antenatal Bartter Syndrome: A Case Report and Review of Literature

Abstract Bartter syndrome is a group of autosomal recessive renal tubular disorders; it has two types of presentation: antenatal and classic. The antenatal type presents as severe unexplained polyhydramnios in the second trimester. This is due to fetal urinary losses of sodium, chloride, and potassium, leading to fetal polyuria. The classic type presents in the late neonatal or infancy stage, with dehydration, dyselectrolytemia, failure to thrive, and nephrocalcinosis. Antenatal scans are normal in such cases. Type I and II Bartter syndrome presents in the antenatal period, whereas type IV has a classic presentation. We describe an unusual case of type IVa Bartter syndrome presenting in the antenatal period, with severe polyhydramnios. The initial diagnosis was made based on amniotic fluid chloride levels and later confirmed by performing a genetic test. Genetic testing is important for confirming diagnosis and prognostication regarding the condition.

Bartter Syndrome-Related Variants Distribution: Brazilian Data and Its Comparison with Worldwide Cohorts

Background: Genetic testing is recommended for accurate diagnosis of Bartter syndrome (BS) and serves as a basis for implementing specific target therapies. However, populations other than Europeans and North Americans are underrepresented in most databases and there are uncertainties in the genotype-phenotype correlation. We studied Brazilian BS patients, an admixed population with diverse ancestry. Methods: We evaluated the clinical and mutational profile of this cohort and performed a systematic review of BS mutations from worldwide cohorts. Results: Twenty-two patients were included; Gitelman syndrome was diagnosed in 2 siblings with antenatal BS and congenital chloride diarrhea in 1 girl. BS was confirmed in 19 patients: BS type 1 in 1 boy (antenatal BS); BS type 4a in 1 girl and BS type 4b in 1 girl, both of them with antenatal BS and neurosensorial deafness; BS type 3 (CLCNKB mutations): 16 cases. The deletion of the entire CLCNKB (1–20 del) was the most frequent variant. Patients carrying the 1–20 del presented earlier manifestations than those with other CLCNKB-mutations and the presence of homozygous 1–20 del was correlated with progressive chronic kidney disease. The prevalence of the 1–20 del in this BS Brazilian cohort was similar to that of Chinese cohorts and individuals of African and Middle Eastern descent from other cohorts. Conclusion: This study expands the genetic spectrum of BS patients with different ethnics, reveals some genotype/phenotype correlations, compares the findings with other cohorts, and provides a systematic review of the literature on the distribution of BS-related variants worldwide.

A case with prenatal molecular diagnosis of X-linked transient antenatal Bartter syndrome

Early-onset polyhydramnios during pregnancy can be caused by X-linked transient antenatal Bartter syndrome. Most of the reported cases were molecularly diagnosed after birth, whereas few cases were diagnosed in the fetus period. We received a pregnant woman who had polyhydramnios detected by ultrasound imaging at 25 weeks of gestation, and treated with magnesium sulfate, indomethacin and an amnioreduction at 30 weeks of gestation, whereas amniotic fluid decreased spontaneously since 32 weeks of gestation. Prenatal molecular testing showed the fetus carried MAGED2 hemizygous variant c.967C>T [p. (Asp323*)] inherited from the mother. The preterm boy did not present with polyuria and electrolytes and acid-base imbalance in the early neonatal period, and had good development without polyuria at the age of 20 months. We presented the phenotypes of a Chinese case with a prenatal diagnosis of X-linked transient antenatal Bartter syndrome and his response to prenatal indomethacin treatment. Early identification of the condition helps to provide appropriate prenatal genetic counseling and postnatal management.

Reciprocal Regulation of MAGED2 and HIF-1α Augments Their Expression under Hypoxia: Role of cAMP and PKA Type II.

Hypoxia stabilizes the transcription factor HIF-1α, which promotes the transcription of many genes essential to adapt to reduced oxygen levels. Besides proline hydroxylation, expression of HIF-1α is also regulated by a range of other posttranslational modifications including phosphorylation by cAMP-dependent protein kinase A (PKA), which stabilizes HIF-1α. We recently demonstrated that MAGED2 is required for cAMP generation under hypoxia and proposed that this regulation may explain the transient nature of antenatal Bartter syndrome (aBS) due to MAGED2 mutations. Consequently, we sought to determine whether hypoxic induction of HIF-1α requires also MAGED2. In HEK293 and HeLa cells, MAGED2 knock-down impaired maximal induction of HIF-1α under physical hypoxia as evidenced by time-course experiments, which showed a signification reduction of HIF-1α upon MAGED2 depletion. Similarly, using cobalt chloride to induce HIF-1α, MAGED2 depletion impaired its appropriate induction. Given the known effect of the cAMP/PKA pathway on the hypoxic induction of HIF-1α, we sought to rescue impaired HIF-1α induction with isoproterenol and forskolin acting upstream and downstream of Gαs, respectively. Importantly, while forskolin induced HIF-1α above control levels in MAGED2-depleted cells, isoproterenol had no effect. To further delineate which PKA subtype is involved, we analyzed the effect of two PKA inhibitors and identified that PKA type II regulates HIF-1α. Interestingly, MAGED2 mRNA and protein were also increased under hypoxia by a cAMP mimetic. Moreover, MAGED2 protein expression also required HIF-1α. Thus, our data provide evidence for reciprocal regulation of MAGED2 and HIF-1α under hypoxia, revealing therefore a new regulatory mechanism that may further explain the transient nature of aBS caused by MAGED2 mutations.

Phenotypic and genotypic characteristics of children with Bartter syndrome.

Bartter syndrome (BS) is a group of autosomal-recessive tubular disorders and it is classified into five genetic subtypes. BS can also be classified by phenotype (antenatal, classic). Patients with mutations in the same gene can present different phenotypes. In the present study, target gene sequencing was performed to evaluate the genotype-phenotype relationship. Biochemical, clinical and renal ultrasonography results were collected at presentation and the last clinic visit. Genetic analyses were performed. The findings of patients with classical BS (cBS) and antenatal BS (aBS) at presentation and the last visit were compared. Our study included 21 patients (12 female, 57.1%) from 20 families with BS. The median age at diagnosis was 8 months and the median follow-up period was 39 months. The most frequent complaint was growth failure. We have found 18 different types of mutations in four genes, including nine in the CLCNKB gene, seven in the SLCA12A1 gene, one in the KCNJ1 gene and one in the BSND gene. In ten patients, nine different types of CLCNKB gene mutations were detected, five of them were novel. Seven different mutations in the SLC12A1 gene were detected in eight patients, five of them were novel. Compared to patients with aBS and cBS, prematurity was significantly higher in the group with aBS. Nephrocalcinosis was present in only one patient with cBS, all the ten hypercalciuric patients with aBS had nephrocalcinosis at the time of diagnosis and the last visit. The mean height standard deviation score (SDS) of patients with aBS were significantly lower than the cBS group at the time of presentation. The mean weight SDS at the time of presentation was worse in patients with aBS than in patients with cBS. The mean plasma potassium and chloride concentrations were significantly lower in the patients with cBS at the time of diagnosis. This investigation revealed the mutation characteristics and phenotype-genotype relationship of our patients and provided valuable data for genetic counseling.

MAGED2 controls vasopressin-induced aquaporin-2 expression in collecting duct cells

Mutations in the Melanoma-Associated Antigen D2 (MAGED2) cause antenatal Bartter syndrome type 5 (BARTS5). This rare disease is characterized by perinatal loss of urinary concentration capability and large urine volumes. The underlying molecular mechanisms of this disease are largely unclear. Here, we study the effect of MAGED2 knockdown on kidney cell cultures using proteomic and phosphoproteomic analyses. In HEK293T cells, MAGED2 knockdown induces prominent changes in protein phosphorylation rather than changes in protein abundance. MAGED2 is expressed in mouse embryonic kidneys and its expression declines during development. MAGED2 interacts with G-protein alpha subunit (GNAS), suggesting a role in G-protein coupled receptors (GPCR) signalling. In kidney collecting duct cell lines, Maged2 knockdown subtly modulated vasopressin type 2 receptor (V2R)-induced cAMP-generation kinetics, rewired phosphorylation-dependent signalling, and phosphorylation of CREB. Maged2 knockdown resulted in a large increase in aquaporin-2 abundance during long-term V2R activation. The increase in aquaporin-2 protein was mediated transcriptionally. Taken together, we link MAGED2 function to cellular signalling as a desensitizer of V2R-induced aquaporin-2 expression. SignificanceIn most forms of Bartter Syndrome, the underlying cause of the disease is well understood. In contrast, the role of MAGED2 mutations in a newly discovered form of Bartter Syndrome (BARTS5) is unknown. In our manuscript we could show that MAGED2 modulates vasopressin-induced protein and phosphorylation patterns in kidney cells, providing a broad basis for further studies of MAGED2 function in development and disease.

Prenatal diagnosis of MAGED2 gene mutation causing transient antenatal Bartter syndrome

Transient antenatal Bartter syndrome due to melanoma-associated antigen D2 gene mutation is a newly reported type of Bartter syndrome. Its characteristics include an X-linked inheritance pattern, early-onset hydramnios, and spontaneous disappearance of symptoms after childbirth. To date, there have been no reports of prenatally diagnosed cases. We herein present the case of a preterm male born to a mother with early-onset hydramnios and a family history of X-linked idiopathic hydramnios. We suspected melanoma-associated antigen D2 gene mutation and performed direct sequencing. As a result, we were able to prenatally establish a diagnosis of transient Bartter syndrome due to a melanoma-associated antigen D2 gene mutation.

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.

Differential diagnosis of perinatal Bartter, Bartter and Gitelman syndromes.

The common finding of hypokalemic alkalosis in several unrelated disorders may confound the early diagnosis of salt-losing tubulopathy (SLT). Antenatal Bartter syndrome (BS) must be considered in idiopathic early-onset polyhydramnios. Fetal megabladder in BS may allow its distinction from third-trimester polyhydramnios that occurs in congenital chloride diarrhea (CCD). Fetal megacolon occurs in CCD while fecal chloride >90 mEq/L in infants is diagnostic. Failure-to-thrive, polydipsia and polyuria in early childhood are the hallmarks of classic BS. Unlike BS, there is low urinary chloride in hypokalemic alkalosis of intractable emesis and cystic fibrosis. Rarely, renal salt wasting may result from cystinosis, Dent disease, disorders of paracellular claudin-10b and Kir4.1 potassium-channel deficiency. Acquired BS may result from calcimimetic up-regulation of a calcium-sensing receptor or autoantibody inactivation of sodium chloride co-transporters in Sjögren syndrome. A relatively common event of heterozygous gene mutations for Gitelman syndrome increases the likelihood of its random occurrence in certain diseases of adult onset. Finally, diuretic abuse is the most common differential diagnosis of SLT. Unlike the persistent elevation in BS, urinary chloride concentration losses waxes and wanes on day-to-day assessment in patients with diuretic misuse.

Syndromes de Bartter–Gitelman

Bartter–Gitelman syndromes are rare inherited autosomal recessive salt-losing tubulopathies characterized by severe and chronic hypokalemia associated with metabolic alkalosis and secondary hyperaldosteronism. Bartter syndrome results from a furosemide-like defect in sodium reabsorption in the Henle's loop leading to hypercalciuria and defect in urinary concentration capacity. The antenatal Bartter syndrome is defined by polyhydramnios and an infantile polyuria with severe dehydration whereas classic Bartter syndrome appears during childhood or adulthood. Gitelman syndrome is a thiazide-like salt-losing tubulopathy. It is associated with hypomagnesemia, hypocalciuria without defect in urinary concentration capacity. The diagnosis is most often made in adolescents or adults. Clinical symptoms include tetany, delay in the height-weight growth curves, chronic tiredness, muscle weakness, myalgia and vertigo. Nephrocalcinosis in Bartter syndrome could lead to chronic kidney disease. Antenatal Bartter syndrome requires hospitalization in intensive care unit to manage the severe newborn dehydration. Chondrocalcinosis is the major complication in the Gitelman syndrome. The corner stones of treatment is the fluid and electrolyte management Bartter and Gitelman syndromes need lifelong oral supplementations of potassium, salt (Bartter) and magnesium (Gitelman). Indomethacin is efficient to reduce water and electrolyte loss in Bartter. In Gitelman, potassium-sparing diuretics may be helping for severe hypokaliemia but they will reinforce hypovolemia.

Clinical Case of Pregnancy and Follow-Up of Bartter Syndrome (Type II) with a Novel Mutation

Background: Bartter syndrome is a rare autosomal recessive inherited salt wasting tubulopathy, it`s incidence proportion is 1.2 cases per 1.000.000 live births. The present case - report discusses a clinical case of an antenatal Bartter syndrome (type II) with a novel mutation and it`s course from antenatal presentation to 6 months postpartum. Case Presentation: The case-report discusses a clinical case of an antenatal Bartter syndrome (type II) with a novel homozygous missense variant mutation in KCNJ1 gene: c.554C>T (p. Pro185Leu). Symptoms presented from 24 weeks of pregnancy as premature labour threats, maternal dyspnoea and severe polyhydramnios (amniotic fluid index 36 cm). Therapeutic interventions included use of indomethacin, dexamethasone, micronized progesterone and three consequent amnioreductions. Pregnancy was prolonged until 32 weeks and induced due to severe reoccurring polyhydramnios, progressing maternal dyspnoea and inability to perform next amnioreduction. Labour was complicated by severe placental abruption and new born – boy was referred to neonatal intensive care unit. Neonatal period was complicated by electrolyte abnormalities: hyponatremia, hypochloremic metabolic alkalosis, transient hyperkalaemia that gradually developed into hypokalaemia, hypercalcemia and elevated rennin and aldosterone levels characteristic to type II Bartter syndrome. At 6 months (corrected age 4 months) he is gaining weight within normal ranges and his psychomotor development is ahead of his corrected age, without any need for daily medications. Conclusion: The present case report describes the clinical course of a Bartter syndrome is of high importance, due to the reason that it shows clinical course of patient with novel mutation and offers one of the ways how to manage the disease. The described novel mutation may have favourable prognosis for neonate. The pregnancy should be managed as high-risk pregnancy with expertise in perinatal diagnostics and interventions. Early recognition, and interventions, are and essential to prolong a pregnancy and lessen prematurity complications.

MON-LB68 Dual Pathologic Mutations in SLC12A1 and GALNT3 Lead to Impaired Mineral Homeostasis

BackgroundLoss-of-function mutations in SLC12A and GALNT3 individually have effects on calcium and phosphorus metabolism but the combined effect has not been described.Case: A 1-week-old female infant presented with hyponatremia (129 mmol/L), hypokalemia (3.1 mmol/L) and metabolic alkalosis (29 mmol/L). Pregnancy was complicated by polyhydramnios requiring several amnioreduction procedures and parental consanguinity. Urine studies showed sodium, potassium, and chloride loss with hyperreninemia (490 ng/ml/h) and hyperaldosteronism (210 ng/dL). Genetic testing revealed a homozygous mutation in SLC12A1 leading to a diagnosis of severe antenatal Bartter syndrome. The patient was started on sodium and potassium supplements with improvement in electrolytes. A few weeks later, she was found to have mild hypercalcemia (11.1 mg/dL) with elevated PTH (102 pg/mL) and high 1,25(OH)2D levels (224 pg/mL) suggestive of primary hyperparathyroidism. While hypercalcemia and hyperparathyroidism have been described in Bartter syndrome, the infant was also noted to have hyperphosphatemia (8.5 mg/dL), which was not expected. Further review of her chromosomal microarray showed a homozygous mutation in GALNT3, an enzyme that normally protects fibroblast growth factor 23 (FGF23) from proteolytic cleavage through glycosylation. Low FGF23 would subsequently result in hyperphosphatemia as well as elevated 1,25(OH)2D, and could contribute to elevated PTH. She was started on Calcilo formula and sevelamer with improvements in her hypercalcemia and hyperphosphatemia. Conclusions: To our knowledge, this is the first patient known to have mutations in both SLC12A1 and GALNT3 leading to derangements in mineral homeostasis. Patients with Bartter syndrome typically have hypercalciuria and increased risk of nephrocalcinosis, variable serum calcium levels, and possibly decreased serum phosphate with decreased urine phosphate reabsorption. In contrast, GALNT3 mutations result in hyperphosphatemia and reduced phosphaturia. By approximately 1 month of age, our patient developed hypercalcemia, hyperphosphatemia and elevated PTH. While hypercalcemic, hypercalciuric hyperparathyroidism has been described in Bartter syndrome and could be the driver of increased PTH in our patient, the GALNT3 mutation leading to low FGF23 must also be considered. Thus, patients with dual mutations in SLC12A1 and GALNT3 require close monitoring as symptomatic electrolyte imbalances may present at variable times.

A novel SLC12A1 mutation in Bedouin kindred with antenatal Bartter syndrome type I.

Four affected individuals of consanguineous kindred presented at infancy with an apparently autosomal recessive syndrome of polyuria and hypokalemic metabolic alkalosis, following maternal polyhydramnios and premature delivery, culminating in severe failure to thrive. Hypercalciuria, nephrocalcinosis, and hyperaldosteronism were further apparent as well as an unusual finding of intermittent hypernatremia. Additionally, all patients demonstrated variable micrognathia with upper respiratory airway abnormalities. As neither postnatal hyperkalemia nor permanent hearing deficits were shown, clinical assessment was consistent with antenatal Bartter syndrome (ABS) type I, which was never described before in the Israeli Bedouin population. Through genome-wide linkage analysis, we identified a single ∼3.3Mbp disease-associated locus on chromosome 15q21.1, segregating within the pedigree. Whole-exome sequencing revealed a single novel homozygous missense mutation within this locus, in SLC12A1, encoding the Na-K-Cl cotransporter, NKCC2, in accordance with the clinical diagnosis. In this concise study, we report a novel missense mutation within the SLC12A1 gene, causing a severe form of ABS type I, the first to be described in Israeli Bedouins, with unusual clinical features of hypernatremia caused by nephrogenic diabetes insipidus and putatively related micrognathia with upper airway abnormalities .