Familial hypomagnesemia with hypercalciuria and nephrocalcinosis

Characterization of ROMK cellular heterogeneity along the mouse kidney thick ascending limb

The renal thick ascending limb (TAL) plays a key role in water and ion homeostasis. Apical potassium secretion via the renal outer medullary potassium channel (ROMK) is essential for transepithelial sodium reabsorption via the furosemide-sensitive Na-K-2Cl-cotransporter and creates the electrochemical gradient for paracellular ion transport through Claudin tight junction proteins. Interestingly, the TAL exhibits transcriptomic heterogeneity and variable apical ROMK abundance. Single-cell RNA sequencing suggests that the cortical TAL consists of at least three distinct cell types, but whether ROMK distribution aligns with these types remains unclear. We analyzed perfusion-fixed mouse kidneys using RNAscope in situ hybridization (ISH), iterative indirect immunofluorescence imaging (4i multiplexing), and machine learning. ROMK mRNA expression was seen in all TAL cells. In contrast, apical ROMK protein abundance was found on almost all macula densa (MD) cells but was heterogeneous along the rest of the TAL. In the remaining TAL, only about 60% of the TAL cells had strong apical ROMK staining, while 40% lacked apical ROMK but showed weak perinuclear signals. ISH revealed that apical ROMK-positive cells express Ptger3 mRNA, whereas apical ROMK-negative cells express Foxq1 mRNA. Multiplexing analysis showed that ROMK-positive cells form Claudin-10b-positive tight junctions, while ROMK-negative cells form Claudin-16/19-positive junctions and express basolateral Kir4.1. Despite universal ROMK mRNA expression, apical ROMK distribution aligns with molecularly distinct TAL cell types. This unique ROMK expression pattern suggests functional heterogeneity for ROMK along the TAL.

Assessment of bone status and bone turnover in pediatric patients with familiar hypomagnesemia with hypercalciuria and nephrocalcinosis

Familiar hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare monogenic tubulopathy. Although some of its features are potentially harmful for skeletal homeostasis, this problem has not been systematically evaluated so far. To evaluate bone mineral density (BMD) in correlation with selected mineral parameters and bone turnover markers (BTMs) to determine the risk of bone mass loss in pediatric patients with FHHNC. The study comprised 28 FHHNC patients aged 4-18 years and 33 healthy, sex - and age matched controls. 6 FHHNC patients showed normal kidney function whereas the remaining 22 presented with CKD grade II- III (median eGFR 73 ml/min/1.73m2). In both groups, serum levels of calcium (sCa), phosphate (sP), magnesium (sMg), 25(OH)D3, 1.25 (OH)2D3, parathormone (PTH) and selected BTMs [BAP, OC, PINP, CTX-I, OPG, SCL, FGF23 and soluble Klotho protein (sKL)] as well as 24-hour urinary calcium excretion (24 h-uCa) were assessed. In addition, BMD of the lumbar spine by DXA method was evaluated. 3 (10.7%) of FHHNC patients showed low BMD (Z-score < -2). Although median Z-score was lower in FHHNC group in comparison to controls, the difference was not significant. FHHNC patients had significantly higher median PTH, 1.25(OH)2D3 and 24 h-uCa values as well as lower sMg. Of the BTMs, they had significantly higher FGF23 and CTX-I levels. CTX-I correlated positively with PTH, FGF23 and SCL but negatively with sMg. Moreover, FGF23 and PTH correlated negatively with sKL. Negative correlation between PTH and sMg was noticed. No significant correlations between measured BTMs and eGFR, sCa, sP, 25(OH)D3, 1.25 (OH)2D3 as well as 24 h uCa were found. None of BTMs significantly correlated with BMD. The results show that pediatric FHHNC patients, regardless of CKD may be at risk for increased bone resorption. Although its pathomechanism is complex, the trigger seems to be Mg depletion, aggravating secondary hyperparathyroidism and leading to the activation of osteolytic processes. However, their clinical significance is unknown, since only minority of patients show osteopenia. Therefore, follow-up of BMD and bone- related laboratory parameters including CTX-I seem to be essential in patients' monitoring, especially in adults with FHHNC.

Understanding Rare Kidney Stone Diseases: A Review

Rare kidney stone diseases typically present with nephrolithiasis or nephrocalcinosis in childhood or adolescence. Affected individuals might face kidney injury and even kidney failure related to associated complications. Screening blood and urine tests recommended for patients with nephrolithiasis/nephrocalcinosis help guide further evaluation, and the increased availability and decreased costs of genetic testing can facilitate the diagnosis of hereditary stone conditions. Genetic testing should be considered when there are clinical clues of an increased likelihood of an inherited condition such as consanguinity, nephrolithiasis in young children, nephrolithiasis in multiple family members, repeated episodes of nephrolithiasis, or kidney failure with nephrolithiasis or nephrocalcinosis. Adult and pediatric nephrologists and urologists should have a basic understanding of monogenic rare kidney stone diseases and their associated diagnoses, treatments, and complications. In many disorders, early detection allows for the initiation of tailored therapies that may alter the natural history of the disease, preserve kidney function, and modify extra renal manifestations.

Identification of modifier gene variants overrepresented in familial hypomagnesemia with hypercalciuria and nephrocalcinosis patients with a more aggressive renal phenotype

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an ultra-rare autosomal recessive renal tubular disease with an incidence of <1/1.000.000 individuals, caused by loss-of-function mutations in CLDN16 and CLDN19. Our study includes a unique cohort representing all known FHHNC patients in Spain, with 90% harbouring mutations in CLDN19. Of these, 70% carry the p.G20D mutation in homozygosis. Despite this high genetic homogeneity, our FHHNC cohort display a high phenotypic variability, even among siblings harbouring identical mutations. Patients were stratified at the extremes of the renal phenotype according to their estimated glomerular filtration rate annual decline and subjected to whole exome sequencing (WES) aiming to find candidate phenotype-modifier genes. Initial statistical analysis by SKAT-O identified numerous variants, which were then filtered based on P-value <0.01 and kidney expression. A thorough prioritization strategy was then applied by an exhaustive disease knowledge-driven exploitation of data from public databases (Human Protein Atlas, GWAS catalog, GTEx) to further refine candidate genes. Odds ratios were also calculated to identify potential risk variants. This analysis pipeline suggested several gene variants associated with a higher risk of developing a more aggressive renal phenotype. While these findings hint at the existence of genetic modifiers in FHHNC, further research is needed to confirm their role and potential clinical significance. Clinical decisions should not be based on these preliminary findings, and additional cohorts should be studied to validate and expand upon our results. This exploratory study provides a foundation for future investigations into the genetic factors influencing FHHNC progression and may contribute to our understanding of the disease's variable expressivity potentially enabling the implementation of more tailored therapeutic strategies.

Atypical fundoscopic manifestation with good visual prognosis in familial hypomagnesemia with hypercalciuria and nephrocalcinosis

PURPOSE

Pathogenic variants in the CLDN19 gene are responsible for Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis (FHHNC) with ocular pathology (MIM *248190). Our objective was to delineate the ophthalmological and genetic manifestations of a patient with FHHNC and a pathogenic variant in CLDN19.

CASE REPORT

A 25-year-old woman presented with renal involvement and a best-corrected visual acuity of 20/25 in the right eye and finger-counting ability in the left eye. The patient exhibited high myopia, convergent strabismus, and chorioretinal atrophic plaques in the perifoveal and peripapillary areas. We conducted a comprehensive ophthalmological examination, including refraction, fundoscopy, color and autofluorescence retinography, optical coherence tomography, and electrophysiology tests. Additionally, next-generation sequencing was performed using Illumina NextSeq500. We identified a homozygous missense variant, c.59G>A p.Gly20Asp, in the CLDN19 gene as the cause of renal and ocular manifestations.

CONCLUSION

FHHNC is associated with various ocular alterations. The unique retinal disorders described in this article suggest a more favorable visual prognosis compared to those previously reported in the literature. Determining the phenotypic diversity of this disease may aid in the diagnosis and management of future cases.

Fanconi-Bickel syndrome complicated by nephrocalcinosis and GFR decline

Fanconi-Bickel syndrome (FBS) is a rare genetic disorder of carbohydrate metabolism due to pathogenic variants in SLC2A2, a gene encoding glucose transporter 2 (GLUT2), which leads to accumulation of glycogen in the kidney and liver. While consequential complex proximal tubular dysfunction is well acknowledged in the literature, long-term trajectories of kidney function in patients with FBS have not been well characterized, and kidney biopsy is performed infrequently. Here, we report on a patient with FBS followed from infancy through young adulthood who presented early on with hypercalciuria, phosphaturia, and hypophosphatemia, complicated by chronic kidney disease development during childhood. Kidney biopsy, in addition to a widespread glycogen accumulation in proximal tubular epithelial cells, demonstrated medullary nephrocalcinosis. Screening for nephrocalcinosis may be warranted in pediatric patients with FBS, along with close surveillance of their kidney function.

Identification of a Novel Homozygous Missense Mutation in the CLDN16 Gene to Decipher the Ambiguous Clinical Presentation Associated with Autosomal Dominant Hypocalcaemia and Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis in an Indian Family

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHNNC) is a rare autosomal recessive renal tubulopathy disorder characterized by excessive urinary loss of calcium and magnesium, polyuria, polydipsia, bilateral nephrocalcinosis, progressive chronic kidney disease, and renal failure. Also, sometimes amelogenesis imperfecta and severe ocular abnormalities are involved. The CLDN-16 and CLDN-19 genes encode the tight junction proteins claudin-16 and claudin-19, respectively, in the thick ascending loop of Henle in the kidney, epithelial cells of the retina, dental enamel, etc. Loss of function of the CLDN-16 and/or CLDN-19 genes leads to FHHNC. We present a case of FHHNC type 1, which was first confused with autosomal dominant hypocalcaemia (ADH) due to the presence of a very low serum parathyroid hormone (PTH) concentration and other similar clinical features before the genetic investigations. After the exome sequencing, FHHNC type 1 was confirmed by uncovering a novel homozygous missense mutation in the CLDN-16 gene (Exon 2, c.374 T > C) which causes, altered protein structure with F55S. Associated clinical, biochemical, and imaging findings also corroborate final diagnosis. Our findings expand the spectrum of the CLDN-16 mutation, which will further help in the genetic diagnosis and management of FHNNC.

The role of claudins in homeostasis

Sequential expression of claudins, a family of tight junction proteins, along the nephron mirrors the sequential expression of ion channels and transporters. Only by the interplay of transcellular and paracellular transport can the kidney efficiently maintain electrolyte and water homeostasis in an organism. Although channel and transporter defects have long been known to perturb homeostasis, the contribution of individual tight junction proteins has been less clear. Over the past two decades, the regulation and dysregulation of claudins have been intensively studied in the gastrointestinal tract. Claudin expression patterns have, for instance, been found to be affected in infection and inflammation, or in cancer. In the kidney, a deeper understanding of the causes as well as the effects of claudin expression alterations is only just emerging. Little is known about hormonal control of the paracellular pathway along the nephron, effects of cytokines on renal claudin expression or relevance of changes in paracellular permeability to the outcome in any of the major kidney diseases. By summarizing current findings on the role of specific claudins in maintaining electrolyte and water homeostasis, this Review aims to stimulate investigations on claudins as prognostic markers or as druggable targets in kidney disease.

Compounded Effervescent Magnesium for Familial Hypomagnesemia: A Case Report

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessive disorder affecting <1/1,000,000 people. It is caused by mutations in the CLDN16 (FHHNC Type 1) or CLDN19 (FHHNC Type 2) genes, which are located on Chromosomes 3q27 and 1p34.2, respectively. There are no drug therapies for this condition. Although magnesium salts represent an important class of compounds and exhibit various therapeutic actions as a supplement for magnesium deficiency in FHHNC, various formulations on the market have different bioavailability. We report the case of a patient with FHNNC first treated, in our Pediatric Institute, with high doses of magnesium pidolate and magnesium and potassium citrate. The patient began to neglect this therapy after experiencing frequent daily episodes of diarrhoea. Our pharmacy received a request for an alternative magnesium supplement that would better comply by ensuring a good magnesium intake which will result in adequate blood magnesium levels. In response, we developed a galenic compound in the form of effervescent magnesium. Here, we report on the promise of this formulation not only for better compliance than pidolate, but also for better bioavailability.

Furosemide rescues hypercalciuria in familial hypomagnesaemia with hypercalciuria and nephrocalcinosis model

AIM

Perturbed calcium homeostasis limits life expectancy in familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC). This rare disease occurs by loss-of-function mutations in CLDN16 or CLDN19 genes, causing impaired paracellular reabsorption of divalent cations along the cortical thick ascending limb (cTAL). Only partial compensation takes place in the ensuing late distal convoluted tubule, connecting tubule, and collecting duct, where the luminal transient receptor potential channel V5 (TRPV5), as well as basolateral plasma membrane calcium ATPase (PMCA) and sodium-potassium exchanger (NCX1) mediate transcellular Ca²⁺ reabsorption. The loop diuretic furosemide induces compensatory activation in these distal segments. Normally, furosemide enhances urinary calcium excretion via inhibition of the aforementioned cTAL. As Ca²⁺ reabsorption in the cTAL is already severely impaired in FHHNC patients, furosemide may alleviate hypercalciuria in this disease by activation of the distal transcellular Ca²⁺ transport proteins.

METHODS

Cldn16-deficient mice (Cldn16^-/- ) served as a FHHNC model. Wild-type (WT) and Cldn16^-/- mice were treated with furosemide (7 days of 40 mg/kg bw) or vehicle. We assessed renal electrolyte handling (metabolic cages) and key divalent transport proteins.

RESULTS

Cldn16^-/- mice show higher Ca²⁺ excretion than WT and compensatory stimulation of Cldn2, TRPV5, and NCX1 at baseline. Furosemide reduced hypercalciuria in Cldn16^-/- mice and enhanced TRPV5 and PMCA levels in Cldn16^-/- but not in WT mice.

CONCLUSIONS

Furosemide significantly reduces hypercalciuria, likely via upregulation of luminal and basolateral Ca²⁺ transport systems in the distal nephron and collecting duct in this model for FHHNC.

A clinical approach to tubulopathies in children and young adults

Kidney tubules are responsible for the preservation of fluid, electrolyte and acid-base homeostasis via passive and active mechanisms. These physiological processes can be disrupted by inherited or acquired aetiologies. The net result is a tubulopathy. It is important to make a prompt and accurate diagnosis of tubulopathies in children and young adults. This allows timely and appropriate management, including disease-specific therapies, and avoids complications such as growth failure. Tubulopathies can present with a variety of non-specific clinical features which can be diagnostically challenging. In this review, we build from this common anatomical and physiological understanding to present a tangible appreciation of tubulopathies as they are likely to be clinically encountered among affected children and young adults.

The membrane-associated protein 17 (MAP17) is up-regulated in response to empagliflozin on top of RAS blockade in experimental diabetic nephropathy

Sodium-glucose cotransporter 2 inhibitors (SGLT2i) have proven to delay diabetic kidney disease (DKD) progression on top of the standard of care with the renin-angiotensin system (RAS) blockade. The molecular mechanisms underlying the synergistic effect of SGLT2i and RAS blockers is poorly understood. We gave a SGLT2i (empagliflozin), an angiotensin-converting enzyme inhibitor (ramipril), or a combination of both drugs for 8 weeks to diabetic (db/db) mice. Vehicle-treated db/db and db/m mice were used as controls. At the end of the experiment, mice were killed, and the kidneys were saved to perform a differential high-throughput proteomic analysis by mass spectrometry using isobaric tandem mass tags (TMT labeling) that allow relative quantification of the identified proteins. The differential proteomic analysis revealed 203 proteins differentially expressed in one or more experimental groups (false discovery rate < 0.05 and Log2 fold change ≥ ±1). Fourteen were differentially expressed in the kidneys from the db/db mice treated with empagliflozin with ramipril. Among them, MAP17 was up-regulated. These findings were subsequently validated by Western blot. The combined therapy of empagliflozin and ramipril up-regulated MAP17 in the kidney of a diabetic mice model. MAP17 is a major scaffolding protein of the proximal tubular cells that places transporters together, namely SGLT2 and NHE3. Our results suggest that SGLT2i on top of RAS blockade may protect the kidney by boosting the inactivation of NHE3 via the up-regulation of key scaffolder proteins such as MAP17.