Familial hypomagnesemia with hypercalciuria and nephrocalcinosis: phenotype-genotype correlation and outcome in 32 patients with CLDN16 or CLDN19 mutations

Prevalence of kidney failure in adults diagnosed with hereditary tubulopathies

BACKGROUND

Inherited tubulopathies are rare kidney diseases with few data available in the literature regarding their long-term renal prognosis. This study aimed to evaluate the prevalence of kidney failure in adults with confirmed genetic tubulopathy and to describe the corresponding clinical and genetic findings.

METHODS

In this observational cohort study, we focused on genetic tubulopathies assumed to impact kidney function. In all adult patients genetically diagnosed in our laboratory between 2001 and 2019, we estimated Glomerular Filtration Rate (eGFR) at diagnosis using the Modification of diet in renal disease (MDRD) formula. Kidney failure was defined as an eGFR < 60 ml/min/1.73 m2.

RESULTS

A total of 2145 patients underwent genetic testing, confirming a genetic tubulopathy in 1031 cases (48%). We identified 116 patients out of 885 with available data with kidney failure, mostly diagnosed with Dent disease and distal renal tubular acidosis (respectively, 31% and 20%), followed by familial hypomagnesemia with hypercalciuria and nephrocalcinosis and renal hypophosphatemia/infantile hypercalcemia. Renal prognosis appeared particularly impacted in familial hypomagnesemia with hypercalciuria and nephrocalcinosis and Dent disease, while preserved in Gitelman syndrome.

CONCLUSION

In this cohort, 13% of adults with genetic tubulopathy had kidney failure at diagnosis, with this rate varying greatly according to tubulopathies and suggesting a significant impact on renal prognosis. Even in adults, genetic analyses yield a good diagnostic rate in selected patients, and should be performed as soon as possible, in order to improve the renal management of patients and their relatives.

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.

Association of urinary calcium excretion with chronic kidney disease in patients with type 2 diabetes

PURPOSE

Herein, we investigated the correlation between urinary calcium excretion (UCaE) and chronic kidney disease (CKD) in patients with type 2 diabetes mellitus (T2DM).

METHODS

From August 2018 to January 2023, a total of 2031 T2DM patients providing 24-h urine samples were included in the final analyses. Patients were separated into four cohorts, based on the UCaE quartiles. We then analyzed renal functional indicators like estimated glomerular filtration rate (eGFR) and urinary albumin excretion (UAE) among the four groups. Lastly, we utilized multivariable logistic regression models to investigate the correlation between UCaE and CKD.

RESULTS

After adjusting for confounding factors, we observed a decreasing trend in CKD prevalence (36.3%, 13.0%, 7.5%, and 6.6%, respectively, P < 0.001) across the UCaE quartiles. Albuminuria (55.5% vs. 40.0%, 36.5%, 37.4%) and macroalbuminuria prevalence (20.0% vs. 9.3%, 5.2%, 5.7%) in the lowest quartile were markedly elevated, compared to the remaining three quartiles (P < 0.001). Meanwhile, the eGFR level (P < 0.001) showed a clearly increasing trend across the UCaE quartiles, and patients with moderate-to-severe decreases in eGFR levels (with cutoff limits at 30-59, 15-30, and < 15 mL/min/1.73m2) were mostly found in the lowest quartile (P < 0.001). Logistic regression analysis revealed that patients in the lowest quartile experienced an enhanced prevalence of CKD, relative to those in the highest quartile (odds ratio: 5.90, 95% confidence interval: 3.60-9.67, P < 0.001).

CONCLUSION

Decreased UCaE was independently associated with the CKD prevalence in T2DM patients.

Insight into prevalence, etiology, and modalities of pediatric chronic dialysis: a comprehensive nationwide analysis

BACKGROUND

This study aimed to determine the prevalence and etiology of kidney failure (KF) among children below 15 years of age receiving chronic dialysis in Saudi Arabia and describe their dialysis modalities.

METHODS

This cross-sectional descriptive study was conducted on 8 August 2022, encompassing all 23 pediatric dialysis centers in Saudi Arabia. Data gathered comprised patient demographics, causes of KF, and the dialysis methods employed. Collected data underwent analysis to determine prevalence of children undergoing chronic dialysis, discern underlying causes of KF, and evaluate distribution of patients across different dialysis modalities.

RESULTS

The prevalence of children on chronic dialysis is 77.6 per million children living in Saudi Arabia, equating to 419 children. The predominant underlying cause of KF was congenital anomalies of the kidneys and urinary tract (CAKUT), representing a substantial 41% of cases. Following this, others or unknown etiologies accounted for a noteworthy 25% of cases, with focal segmental glomerulosclerosis (FSGS) comprising 13%, glomerulonephritis at 11%, and congenital nephrotic syndrome contributing 10% to etiological distribution. Regarding dialysis modalities employed, 67% of patients were on peritoneal dialysis (PD), while the remaining 33% were on hemodialysis (HD).

CONCLUSIONS

This first nationwide study of pediatric chronic dialysis in Saudi Arabia sheds light on the prevalence of children undergoing chronic dialysis and underlying causes of their KF, thereby contributing to our understanding of clinical management considerations. This research serves as a stepping stone for the development of national registries.

Monogenic features of urolithiasis: A comprehensive review

Objective

Urolithiasis formation has been attributed to environmental and dietary factors. However, evidence is accumulating that genetic background can contribute to urolithiasis formation. Advancements in the identification of monogenic causes using high-throughput sequencing technologies have shown that urolithiasis has a strong heritable component.

Methods

This review describes monogenic factors implicated in a genetic predisposition to urolithiasis. Peer-reviewed journals were evaluated by a PubMed search until July 2023 to summarize disorders associated with monogenic traits, and discuss clinical implications of identification of patients genetically susceptible to urolithiasis formation.

Results

Given that more than 80% of urolithiases cases are associated with calcium accumulation, studies have focused mainly on monogenetic contributors to hypercalciuric urolithiases, leading to the identification of receptors, channels, and transporters involved in the regulation of calcium renal tubular reabsorption. Nevertheless, available candidate genes and linkage methods have a low resolution for evaluation of the effects of genetic components versus those of environmental, dietary, and hormonal factors, and genotypes remain undetermined in the majority of urolithiasis formers.

Conclusion

The pathophysiology underlying urolithiasis formation is complex and multifactorial, but evidence strongly suggests the existence of numerous monogenic causes of urolithiasis in humans.

Review of childhood genetic nephrolithiasis and nephrocalcinosis

Nephrolithiasis (NL) is a common condition worldwide. The incidence of NL and nephrocalcinosis (NC) has been increasing, along with their associated morbidity and economic burden. The etiology of NL and NC is multifactorial and includes both environmental components and genetic components, with multiple studies showing high heritability. Causative gene variants have been detected in up to 32% of children with NL and NC. Children with NL and NC are genotypically heterogenous, but often phenotypically relatively homogenous, and there are subsequently little data on the predictors of genetic childhood NL and NC. Most genetic diseases associated with NL and NC are secondary to hypercalciuria, including those secondary to hypercalcemia, renal phosphate wasting, renal magnesium wasting, distal renal tubular acidosis (RTA), proximal tubulopathies, mixed or variable tubulopathies, Bartter syndrome, hyperaldosteronism and pseudohyperaldosteronism, and hyperparathyroidism and hypoparathyroidism. The remaining minority of genetic diseases associated with NL and NC are secondary to hyperoxaluria, cystinuria, hyperuricosuria, xanthinuria, other metabolic disorders, and multifactorial etiologies. Genome-wide association studies (GWAS) in adults have identified multiple polygenic traits associated with NL and NC, often involving genes that are involved in calcium, phosphorus, magnesium, and vitamin D homeostasis. Compared to adults, there is a relative paucity of studies in children with NL and NC. This review aims to focus on the genetic component of NL and NC in children.

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.

Ocular manifestations of the genetic renal tubulopathies

BACKGROUND

The genetic tubulopathies are rare and heterogenous disorders that are often difficult to identify. This study examined the tubulopathy-causing genes for ocular associations that suggested their genetic basis and, in some cases, the affected gene.

METHODS

Sixty-seven genes from the Genomics England renal tubulopathy panel were reviewed for ocular features, and for retinal expression in the Human Protein Atlas and an ocular phenotype in mouse models in the Mouse Genome Informatics database. The genes resulted in disease affecting the proximal tubules (n = 24); the thick ascending limb of the loop of Henle (n = 10); the distal convoluted tubule (n = 15); or the collecting duct (n = 18).

RESULTS

Twenty-five of the tubulopathy-associated genes (37%) had ocular features reported in human disease, 49 (73%) were expressed in the retina, although often at low levels, and 16 (24%) of the corresponding mouse models had an ocular phenotype. Ocular abnormalities were more common in genes affected in the proximal tubulopathies (17/24, 71%) than elsewhere (7/43, 16%). They included structural features (coloboma, microphthalmia); refractive errors (myopia, astigmatism); crystal deposition (in oxalosis, cystinosis) and sclerochoroidal calcification (in Bartter, Gitelman syndromes). Retinal atrophy was common in the mitochondrial-associated tubulopathies. Structural abnormalities and crystal deposition were present from childhood, but sclerochoroidal calcification typically occurred after middle age.

CONCLUSIONS

Ocular abnormalities are uncommon in the genetic tubulopathies but may be helpful in recognizing the underlying genetic disease. The retinal expression and mouse phenotype data suggest that further ocular associations may become apparent with additional reports. Early identification may be necessary to monitor and treat visual complications.

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.

Impact of urinary calcium excretion on kidney, bone, and cardiovascular systems in patients with bone biopsy proven osteoporosis: a longitudinal long-term follow-up study

The impact of urine calcium on kidney, bone, and cardiovascular systems in osteoporosis is not well-known. In this 7-year-follow-up study, high urine calcium did not affect kidney function but increased risk of kidney stones, while low urine calcium increased cardiovascular diseases. Maintaining normal urine calcium is beneficial for bone health.

PURPOSE

Hypercalciuria is common in patients with osteoporosis. However, the long-term effect of urinary calcium excretion (UCaE) on patients' health is not well-examined. The current study aims to assess the impact of UCaE on kidney, bone, and cardiovascular outcomes in patients with bone biopsy proven osteoporosis.

METHODS

Longitudinal study of all patients with osteoporosis who underwent bone biopsy and 24-h urine collection between 2008 and 2015 in the University of Kentucky. DXA scans, serum markers, kidney function, and cardiovascular events were recorded until last clinic visit in 2021. Exclusion criteria were secondary osteoporosis or conditions that might substantially impact UCaE. The significant results in univariate analysis were confirmed in multi-variable regression models involving clinically important covariates that might impact patients' outcomes.

RESULTS

Study included 230 patients with mean follow-up of 7.2 ± 2.9 years. The mean age was 61 years, and the mean eGFR at baseline was 85 ± 19 ml/min/1.73 m². Low bone turnover (LBT) was present in 57% and high bone turnover (HBT) in 43% of patients. Hypercalciuria was found in one-third of patients with no difference between LTB and HTB. UCaE correlated positively with eGFR but did not affect the rate of eGFR decline over time. Higher UCaE predicted kidney stones development. We observed U-shaped effect of UCaE on bone health. Hypercalciuria predicted loss of BMD at all sites, but also hypocalciuria was associated with higher loss in total hip BMD. Upper limb fractures were the most observed fractures, and their incidence was higher in patients with hyper- or hypo-calciuria. Lower UCaE independently predicted development of major adverse cardiac events (MACE) and cardiovascular disease (CVD).

CONCLUSION

UCaE correlated with eGFR but it did not affect the change of eGFR over time. Patients with normal UCaE had lower incidence of upper limb fractures and less reduction in BMD. Low UCaE predicted MACE and CVD.

The importance of kidney calcium handling in the homeostasis of extracellular fluid calcium

Extracellular fluid calcium concentration must be maintained within a narrow range in order to sustain many biological functions, encompassing muscle contraction, blood coagulation, and bone and tooth mineralization. Blood calcium value is critically dependent on the ability of the renal tubule to reabsorb the adequate amount of filtered calcium. Tubular calcium reabsorption is carried out by various and complex mechanisms in 3 distinct segments: the proximal tubule, the cortical thick ascending limb of the loop of Henle, and the late distal convoluted/connecting tubule. In addition, calcium reabsorption is tightly controlled by many endocrine, paracrine, and autocrine factors, as well as by non-hormonal factors, in order to adapt the tubular handling of calcium to the metabolic requirements. The present review summarizes the current knowledge of the mechanisms and factors involved in calcium handling by the kidney and, ultimately, in extracellular calcium homeostasis. The review also highlights some of our gaps in understanding that need to be addressed in the future.

Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis Due to CLDN16 Gene Mutations: Novel Findings in Two Cases with Diverse Clinical Features

Biallelic loss of function mutations in the CLDN16 gene cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), and chronic kidney disease. Here we report two cases of FHHNC with diverse clinical presentations and hypercalcemia in one as a novel finding. Pt#1 initially presented with urinary tract infection and failure to thrive at 5.5 months of age to another center. Bilateral nephrocalcinosis, hypercalcemia (Ca: 12.2 mg/dl), elevated parathyroid hormone (PTH) level, and hypercalciuria were detected. Persistently elevated PTH with high/normal Ca levels led to subtotal-parathyroidectomy at the age of 2.5. However, PTH levels remained elevated with progressive deterioration in renal function. At 9-year-old, she was referred to us for evaluation of hyperparathyroidism and, hypomagnesemia together with hypercalciuria, elevated PTH with normal Ca levels, and medullary nephrocalcinosis were detected. Compound heterozygosity of CLDN16 variants (c.715G>A, p.G239R; and novel c.360C>A, p.C120*) confirmed the diagnosis. Pt#2 was a 10-month-old boy, admitted with irritability and urinary crystals. Hypocalcemia, hypophosphatemia, elevated PTH and ALP, low 25(OH)D levels, and radiographic findings of rickets were detected. However, additional findings of hypercalciuria and bilateral nephrocalcinosis were inconsistent with the nutritional rickets. Low/normal serum Mg levels suggested the diagnosis of FHHNC which was confirmed genetically as a homozygous missense (c.602G > A; p.G201E) variant in CLDN16. Yet, hypocalcemia and hypomagnesemia persisted in spite of treatment. In conclusion, FHHNC may present with diverse clinical features with mild hypomagnesemia leading to secondary hyperparathyroidism with changing Ca levels from low to high. Early and accurate clinical and molecular genetic diagnosis is important for proper management.

The genetics of kidney stone disease and nephrocalcinosis

Kidney stones (also known as urinary stones or nephrolithiasis) are highly prevalent, affecting approximately 10% of adults worldwide, and the incidence of stone disease is increasing. Kidney stone formation results from an imbalance of inhibitors and promoters of crystallization, and calcium-containing calculi account for over 80% of stones. In most patients, the underlying aetiology is thought to be multifactorial, with environmental, dietary, hormonal and genetic components. The advent of high-throughput sequencing techniques has enabled a monogenic cause of kidney stones to be identified in up to 30% of children and 10% of adults who form stones, with ~35 different genes implicated. In addition, genome-wide association studies have implicated a series of genes involved in renal tubular handling of lithogenic substrates and of inhibitors of crystallization in stone disease in the general population. Such findings will likely lead to the identification of additional treatment targets involving underlying enzymatic or protein defects, including but not limited to those that alter urinary biochemistry.

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.

Determinants and Outcomes Associated With Urinary Calcium Excretion in Chronic Kidney Disease

CONTEXT

Abnormalities in calcium metabolism are common in chronic kidney disease (CKD). Diminished urinary calcium excretion may promote vascular calcification and increased urinary calcium excretion may lead to nephrolithiasis and nephrocalcinosis, conditions associated with CKD.

OBJECTIVE

To study predictors of urinary calcium excretion and its association with adverse clinical outcomes in CKD.

DESIGN, SETTING AND PATIENTS

This study assessed 3768 nondialysis participants in the Chronic Renal Insufficiency Cohort study from April 2003 to September 2008. Participants were followed up to October 2018.

EXPOSURE

Clinically plausible predictors of urinary calcium excretion and 24-h urinary calcium excretion at baseline.

MAIN OUTCOME MEASURES

Urinary calcium excretion; incident end stage kidney disease (ESKD), CKD progression [50% estimated glomerular filtration rate (eGFR) decline or incident ESKD], all-cause mortality, and atherosclerotic cardiovascular disease events.

RESULTS

eGFR was positive correlated with 24-h urinary calcium excretion. The variables most strongly associated with 24-h urinary calcium excretion in males and females were 24-h urinary sodium (β = 0.19 and 0.28, respectively), serum parathyroid hormone (β = -0.22 and -0.20, respectively), loop diuretics (β = 0.36 and 0.26, respectively), thiazide diuretics (β = -0.49 and -0.53, respectively), and self-identified black race (β = -0.23 and -0.27, respectively). Lower urinary calcium excretion was associated with greater risks of adverse outcomes, but these associations were greatly attenuated or nullified after adjustment for baseline eGFR.

CONCLUSION

Urinary calcium excretion is markedly lower in individuals with CKD compared to the general population. Determinants of urinary calcium excretion differed between sexes and levels of CKD. Associations between urinary calcium excretion and adverse clinical events were substantially confounded by eGFR.

Hereditary kidney diseases associated with hypomagnesemia

In the kidney, a set of proteins expressed in the epithelial cells of the thick ascending loop of Henle and the distal convoluted tubule directly or indirectly play important roles in the regulation of serum magnesium levels. Magnesium reabsorption in the thick ascending loop of Henle occurs through a passive paracellular pathway, while in the distal convoluted tubule, the final magnesium concentration is established through an active transcellular pathway. The players involved in magnesium reabsorption include proteins with diverse functions including tight junction proteins, cation and anion channels, sodium chloride cotransporter, calcium-sensing receptor, epidermal growth factor, cyclin M2, sodium potassium adenosine triphosphatase subunits, transcription factors, a serine protease, and proteins involved in mitochondrial function. Mutations in the genes that encode these proteins impair their function and cause different rare diseases associated with hypomagnesemia, which may lead to muscle cramps, fatigue, epileptic seizures, intellectual disability, cardiac arrhythmias, and chronic kidney disease. The purpose of this review is to describe the clinical and genetic characteristics of these hereditary kidney diseases and the current research findings on the pathophysiological basis of these diseases.

Clinical and Genetic Approach to Renal Hypomagnesemia

Magnesium (Mg²⁺) is an important intracellular cation and essential to maintain cell function including cell proliferation, immunity, cellular energy metabolism, protein and nucleic acid synthesis, and regulation of ion channels. Consequences of hypomagnesemia affecting multiple organs can be in overt or subtle presentations. Besides detailed history and complete physical examination, the assessment of urinary Mg²⁺ excretion is help to differentiate renal from extra-renal (gastrointestinal, tissue sequestration, and shifting) causes of hypomagnesemia. Renal hypomagnesemia can be caused by an increased glomerular filtration and impaired reabsorption in proximal tubular cells, thick ascending limb of the loop of Henle or distal convoluted tubules. A combination of renal Mg²⁺ wasting, familial history, age of onset, associated features, and exclusion of acquired etiologies point to inherited forms of renal hypomagnesemia. Based on clinical phenotypes, its definite genetic diagnosis can be simply grouped into specific, uncertain, and unknown gene mutations with a priority of genetic approach methods. An unequivocal molecular diagnosis could allow for prediction of clinical outcome, providing genetic counseling, avoiding unnecessary studies or interventions, and possibly uncovering the pathogenic mechanism. Given numerous identified genes responsible for Mg²⁺ transport in renal hypomagnesemia over the past two decades, several potential and specific molecular and cellular therapeutic strategies to correct hypomagnesemia are promising.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC; OMIM 248250) is a rare autosomal recessive kidney disease caused by mutations in the CLDN16 or CLDN19 genes encoding the proteins claudin-16 and claudin-19, respectively. These are involved in paracellular magnesium and calcium transport in the thick ascending limb of Henle's loop and account for most of the magnesium reabsorption in the tubules. FHHNC is characterized by hypomagnesaemia, hypercalciuria, and nephrocalcinosis, and progresses to kidney failure, requiring dialysis and kidney transplantation mainly during the second to third decades of life. Patients carrying CLDN19 mutations frequently exhibit associated congenital ocular defects leading to variable visual impairment. Despite this severe clinical course, phenotype variability even among siblings has been described in this disease, suggesting unidentified epigenetic mechanisms or other genetic or environmental modifiers. Currently, there is no specific therapy for FHHNC. Supportive treatment with high fluid intake and dietary restrictions, as well as magnesium salts, thiazides, and citrate, are commonly used in an attempt to retard the progression of kidney failure. A kidney transplant remains the only curative option for kidney failure in these patients. In this review, we summarize the current knowledge about FHHNC and discuss the remaining open questions about this disorder.

Congenital macular scars in siblings from CLDN19 mutations

We report 2 consecutive siblings who presented with unilateral and bilateral macular scars, respectively (initially presumed in the older sibling to be from congenital toxoplasmosis), who also developed chronic kidney disease. Both underwent genetic testing and were positive for a mutation in CLDN19, confirming the diagnosis of familial hypomagnesemia with hypercalciuria and nephrocalcinosis with severe ocular involvement. One of our patients had the unique finding of mild foveal hypoplasia, which is not typically associated with CLDN19 mutations.

Differential localization patterns of claudin 10, 16, and 19 in human, mouse, and rat renal tubular epithelia

Functional properties of the paracellular pathway depend critically on the set of claudins (CLDN) expressed at the tight junction. Two syndromes are causally linked to loss-of-function mutations of claudins: hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia (HELIX) syndrome caused by genetic variations in the CLDN10 gene and familial hypomagnesemia with hypercalciuria and nephrocalcinosis caused by genetic variations in the CLDN16 or CLDN19 genes. All three genes are expressed in the kidney, particularly in the thick ascending limb (TAL). However, localization of these claudins in humans and rodents remains to be delineated in detail. We studied the segmental and subcellular expression of CLDN10, CLDN16, and CLDN19 in both paraffin-embedded and frozen kidney sections from the adult human, mouse, and rat using immunohistochemistry and immunofluorescence, respectively. Here, CLDN10 was present in a subset of medullary and cortical TAL cells, localizing to basolateral domains and tight junctions in human and rodent kidneys. Weak expression was detected at the tight junction of proximal tubular cells. CLDN16 was primarily expressed in a subset of TAL cells in the cortex and outer stripe of outer medulla, restricted to basolateral domains and tight junctional structures in both human and rodent kidneys. CLDN19 predominantly colocalized with CLDN16 in tight junctions and basolateral domains of the TAL but was also found in basolateral and junctional domains in more distal sites. CLDN10 expression at tight junctions almost never overlapped with that of CLND16 and CLDN19, consistent with distinct junctional pathways with different permeation profiles in both human and rodent kidneys.NEW & NOTEWORTHY This study used immunohistochemistry and immunofluorescence to investigate the distribution of claudin 10, 16, and 19 in the human, mouse, and rat kidney. The findings showed distinct junctional pathways in both human and rodent kidneys, supporting the existence of different permeation profiles in all species investigated.

Heterogeneity is a common ground in familial hypomagnesemia with hypercalciuria and nephrocalcinosis caused by CLDN19 gene mutations

BACKGROUND

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare tubulopathy caused by mutations in the CLDN16 or CLDN19 genes. Patients usually develop hypomagnesemia, hypercalciuria, nephrocalcinosis and renal failure early in life. Patients with CLDN19 mutations may also have ocular abnormalities. Despite clinical variability, factors associated with kidney function impairment, especially in patients with CLDN19 mutations, have not been addressed.

METHODS

Retrospective multicenter study of 30 genetically confirmed FHHNC Spanish patients. We analyzed kidney function impairment considering as outcomes chronic kidney disease (CKD) stage 3 and annual estimated glomerular filtration rate (eGFR) decline, to identify factors associated with the different phenotypes.

RESULTS

Of thirty patients, 27 had mutations in the CLDN19 gene (20 homozygous for the p.G20D mutation) and 3 in the CLDN16. Age at diagnosis was 1.71 (0.67-6.04) years and follow-up time was 8.34 ± 4.30 years. No differences in CKD stage 3-free survival based on CLDN19 mutation (p.G20D homozygous vs. other mutations) or gender were found, although females seemed to progress faster than males. Patients with more pronounced eGFR decline had higher PTH levels at diagnosis than those with stable kidney function, despite similar initial eGFR. Approximately 60% of CLDN19 patients presented ocular abnormalities. Furthermore, we confirmed high phenotypic intrafamilial variability.

CONCLUSIONS

In a contemporary cohort of FHHNC patients with CLDN19 mutations, females seemed to progress to CKD-stage 3 faster than males. Increased PTH levels at baseline may indicate a more severe renal course. There was high phenotype variability among patients with CLDN19 mutations and kidney function impairment  differed even between siblings.

[Nephrocalcinosis in children]

Nephrocalcinosis is defined by calcium phosphate or calcium oxalate deposits in the kidney parenchyma, particularly in tubular epithelial cells and interstitial tissue. It should be differentiated from urolithiasis where calcium salts deposits are located in the kidney and urinary tract. The epidemiology of nephrocalcinosis in children is unknown but the condition is not so rare, with an increased incidence in preterm infants. Often detected as an incidental finding, nephrocalcinosis may be classified according to the radiological type: medullary, cortical or diffuse. Nephrocalcinosis in children can be caused by a variety of etiology. The most common causes concern medullary nephrocalcinosis and include hereditary tubular disorders, in particular distal renal tubular acidosis and Dent disease, metabolic disorders such as idiopathic hypercalciuria and hyperoxaluria, and iatrogenic causes such as vitamin D intoxication. In the newborn, the main cause is hypercalciuria of the premature baby, whose multifactorial origin is largely iatrogenic. Primary hyperoxaluria which can lead to early onset nephrocalcinosis and usually to chronic kidney disease should always be considered and further investigated. In order to provide a specific diagnosis, it is essential to take into account the family history, the clinical context and complete laboratory data. Early initiation of an appropriate etiological treatment is recommended and may prevent or delay the progression to chronic kidney disease in some cases.

Hypomagnesemia with Hypercalciuria Leading to Nephrocalcinosis, Amelogenesis Imperfecta, and Short Stature in a Child Carrying a Homozygous Deletion in the CLDN16 Gene

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessive disease caused by mutations in the CLDN16 or CLDN19 gene; however, few cases develop classical amelogenesis imperfecta. Herein, we report the case of a boy with early clinical renal manifestations that started at 1 year of age and presenting with dental hypoplasia and growth delay. The patient presented with vomiting, polyuria, and polydipsia. Apart from recurrent sterile leukocyturia, erroneously treated as infectious, he was normal, except for short stature and amelogenesis imperfecta with gradually discolored teeth. Laboratory tests revealed hyperparathyroidism, hypomagnesemia, severe hypercalciuria, and hypermagnesuria on 24-h urine testing. Helical computed tomography confirmed nephrocalcinosis. We performed whole-exome sequencing (WES) to test the hypothesis of FHHNC and oligogenic inheritance of amelogenesis. Analysis of the WES binary sequence alignment/map file revealed the presence of exon 1 of the CLDN16 and absence of the other exons [c.325_c918*? (E2_E5del)]. We confirmed a CLDN16 E2_E5 homozygous deletion by multiplex ligation-dependent probe amplification and polymerase chain reaction assays. Although most mutations causing FHHNC are missense and nonsense mutations in the CLDN16 or CLDN19 gene, large deletions occur and may be misled by WES, which is generally used for genetic screening of oligogenic disorders. The patient received cholecalciferol, magnesium oxide and potassium citrate. Later, the combination with hydrochlorothiazide plus amiloride was prescribed, with a good response during follow-up. Our report broadens the phenotype of FHHNC, including severe early-onset amelogenesis and short stature, and reinforces the phenotype-genotype correlation of the large deletion found in CLDN16.

Performance of ion chromatography to measure picomole amounts of magnesium in nanolitre samples

KEY POINTS

An UHPLC method to measure picomole amounts of magnesium has been developed. The method is sensitive, specific, accurate and reproducible. The method is suitable for quantifying magnesium transport across intact epithelia.

ABSTRACT

Magnesium is involved in many biological processes. Extracellular magnesium homeostasis mainly depends on the renal handling of magnesium, the study of which requires measurement of low concentrations of magnesium in renal tubular fluid. We developed an ultra-high-performance liquid chromatography method to measure millimolar concentrations of magnesium in nanolitre samples. Within-assay and between-assay coefficients of variation were lower than 5% and 6.6%, respectively. Measurement of magnesium concentration was linear (r²  = 0.9998) over the range 0-4 mmol/l. Absolute bias ranged from -0.03 to 0.05 mmol/l. The lower limit of quantification was 0.2 mmol/l. Recovery was 97.5-100.3%. No significant interference with calcium, another divalent cation present in the same samples, was detected. The method was successfully applied to quantify transepithelial magnesium transport by medullary and cortical thick ascending limbs during ex vivo microperfusion experiments. In conclusion, ultra-high-performance liquid chromatography is suitable for measurement of picomole amounts of magnesium in renal tubular fluid. The method allows detailed studies of transepithelial magnesium transport across native epithelium.

Novel compound heterozygous mutations of CLDN16 in a patient with familial hypomagnesemia with hypercalciuria and nephrocalcinosis

Background

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive tubulopathy characterized by excessive urinary wasting of magnesium and calcium, bilateral nephrocalcinosis, and progressive chronic renal failure in childhood or adolescence. FHHNC is caused by mutations in CLDN16 and CLDN19, which encode the tight‐junction proteins claudin‐16 and claudin‐19, respectively. Most of these mutations are missense mutations and large deletions are rare.

Methods

We examined the clinical and biochemical features of a Spanish boy with early onset of FHHNC symptoms. Exons and flanking intronic segments of CLDN16 and CLDN19 were analyzed by direct sequencing. We developed a new assay based on Quantitative Multiplex PCR of Short Fluorescent Fragments (QMPSF) to investigate large CLDN16 deletions.

Results

Genetic analysis revealed two novel compound heterozygous mutations of CLDN16, comprising a missense mutation, c.277G>A; p.(Ala93Thr), in one allele, and a gross deletion that lacked exons 4 and 5,c.(840+25_?)del, in the other allele. The patient inherited these variants from his mother and father, respectively.

Conclusions

Using direct sequencing and our QMPSF assay, we identified the genetic cause of FHHNC in our patient. This QMPSF assay should facilitate the genetic diagnosis of FHHNC. Our study provided additional data on the genotypic spectrum of the CLDN16 gene.

Channel functions of claudins in the organization of biological systems

Claudins are tight junction proteins mostly appreciated in their function of paracellular barrier-formation. Compared to a virtual absence of any tight junctions, their paracellular sealing role certainly stands out. Yet, it was recognized immediately after the discovery of the first claudins, that some members of the claudin protein family were able to convey size and charge selectivity to the paracellular pathway. Thus, paracellular permeability can be fine-tuned according to the physiological needs of a tissue by inserting these channel-forming claudins into tight junction strands. Precise permeability adjustment is further suggested by the presence of numerous isoforms of channel-forming claudins (claudin-10b-, -15-, -16-like isoforms) in various vertebrate taxa. Moreover, their expression and localization are controlled by multiple transcriptional and posttranslational mechanisms. Consequently, mutation or dysregulation of channel-forming claudins can cause severe diseases. The present review therefore aims at providing an up-to-date report of the current research on these aspects of channel-forming claudins and their possible implications on future developments.

Laboratory changes during adrenocorticotropic hormone therapy associated with renal calcified lesions

BACKGROUND

Renal calcified lesions are known as one of the complications during adrenocorticotropic hormone (ACTH) therapy for intractable epilepsy. However, laboratory changes during the therapy or laboratory features of high-risk cases with renal calcified lesions are yet to be clarified.

METHODS

In this study, 43 patients with West syndrome aged ≤2 years were included. We retrospectively reviewed age and body mass index at the beginning of ACTH therapy, as well as the amount of fluid intake, daily urinary volume, and laboratory data during therapy. In addition, we studied the urinary sediment of the cases with renal calcified lesions diagnosed by computed tomography.

RESULTS

After initiating ACTH treatment, urinary calcium (Ca)/creatinine ratio and urinary pH increased within 2 weeks. Urinary crystals and renal tubular epithelial cells (RTECs) in urinary sediment were frequently found in most cases. Urinary Ca levels, proteinuria or frequency of urinary crystals, and number of RTECs in the urinary sediment were significantly higher in patients with epithelial casts (ECs) or hematuria than in patients without these findings. Among the seven patients who underwent abdominal CT, ECs or hematuria were found only in those with renal calcified lesions. These findings suggested that patients with ECs or hematuria were more likely to have calcified lesions.

CONCLUSIONS

The risk of renal calcified lesions increased after 2 weeks of ACTH treatment. Abnormal findings in urinary sediments might be an early sign of renal calcification during ACTH therapy.

Claudins in Renal Physiology and Pathology

Claudins are integral proteins expressed at the tight junctions of epithelial and endothelial cells. In the mammalian kidney, every tubular segment express a specific set of claudins that give to that segment unique properties regarding permeability and selectivity of the paracellular pathway. So far, 3 claudins (10b, 16 and 19) have been causally traced to rare human syndromes: variants of CLDN10b cause HELIX syndrome and variants of CLDN16 or CLDN19 cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis. The review summarizes our current knowledge on the physiology of mammalian tight junctions and paracellular ion transport, as well as on the role of the 3 above-mentioned claudins in health and disease. Claudin 14, although not having been causally linked to any rare renal disease, is also considered, because available evidence suggests that it may interact with claudin 16. Some single-nucleotide polymorphisms of CLDN14 are associated with urinary calcium excretion and/or kidney stones. For each claudin considered, the pattern of expression, the function and the human syndrome caused by pathogenic variants are described.

Atypical presentation of familial hypomagnesemia with hypercalciuria and nephrocalcinosis in a patient with a new claudin-16 gene mutation

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive tubular disorder caused by mutations in genes that encode renal tight junction proteins claudin-16 or claudin-19, which are responsible for magnesium and calcium paracellular reabsorption in the thick ascending limb of Henle’s loop. Progressive renal failure is frequently present, and most of the patients require renal replacement therapy still during adolescence. In this case report, we describe a new homozygous missense mutation on CLDN16 gene (c.592G>C, Gly198Arg) in a 24-year-old male patient diagnosed with FHHNC after clinical investigation due to incidental detection of altered routine laboratorial tests, who was firstly misdiagnosed with primary hyperparathyroidism. In addition, it illustrates an atypical presentation of this disease, with late onset of chronic kidney disease, improving the phenotype-genotype knowledge of patients with FHHNC.

Exonic CLDN16 mutations associated with familial hypomagnesemia with hypercalciuria and nephrocalcinosis can induce deleterious mRNA alterations

BACKGROUND

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis type 1 is an autosomal recessive disease characterized by excessive renal magnesium and calcium excretion, bilateral nephrocalcinosis, and progressive chronic renal failure. This rare disease is caused by mutations in CLDN16 that encodes claudin-16, a tight-junction protein involved in paracellular reabsorption of magnesium and calcium in the renal tubule. Most of these variants are located in exons and have been classified as missense mutations. The functional consequences of some of these claudin-16 mutant proteins have been analysed after heterologous expression showing indeed a significant loss of function compared to the wild-type claudin-16. We hypothesize that a number of CLDN16 exonic mutations can be responsible for the disease phenotype by disrupting the pre-mRNA splicing process.

METHODS

We selected 12 previously described presumed CLDN16 missense mutations and analysed their potential effect on pre-mRNA splicing using a minigene assay.

RESULTS

Our results indicate that five of these mutations induce significant splicing alterations. Mutations c.453G > T and c.446G > T seem to inactivate exonic splicing enhancers and promote the use of an internal cryptic acceptor splice site resulting in inclusion of a truncated exon 3 in the mature mRNA. Mutation c.571G > A affects an exonic splicing enhancer resulting in partial skipping of exon 3. Mutations c.593G > C and c.593G > A disturb the acceptor splice site of intron 3 and cause complete exon 4 skipping.

CONCLUSIONS

To our knowledge, this is the first report of CLDN16 exonic mutations producing alterations in splicing. We suggest that in the absence of patients RNA samples, splicing functional assays with minigenes could be valuable for evaluating the effect of exonic CLDN16 mutations on pre-mRNA splicing.

Establishment of urinary exosome-like vesicles isolation protocol for FHHNC patients and evaluation of different exosomal RNA extraction methods

BACKGROUND

Molecular and cellular pathophysiological events occurring in the majority of rare kidney diseases remain to be elucidated. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessive disorder caused by mutations in either CLDN16 or CLDN19 genes. This disease is characterized by massive urinary wasting of magnesium and calcium, osmosis deregulation and polyuria. Patients with p.G20D homozygous mutation in CLDN19 gene exhibit different progression to kidney failure suggesting that beyond the pathogenic mutation itself, other molecular events are favoring disease progression. Due to the fact that biopsy is not clinically indicated in these patients, urinary exosome-like vesicles (uEVs) can be envisioned as a valuable non-invasive source of information of events occurring in the kidney. Exosome research has increased notably to identify novel disease biomarkers but there is no consensus standardized protocols for uEVs isolation in patients with polyuria. For this reason, this work was aimed to evaluate and refine different uEVs isolation methods based on differential centrifugation, the gold standard method.

RESULTS

Characterization by NTA, cryo-TEM and immunoblotting techniques identified the most appropriate protocol to obtain the highest yield and purest uEVs enriched fraction possible from urine control samples and FHHNC patients. Moreover, we tested five different RNA extraction methods and evaluated the miRNA expression pattern by qRT-PCR.

CONCLUSIONS

In summary, we have standardized the conditions to proceed with the identification of differentially expressed miRNAs in uEVs of FHHNC patients, or other renal diseases characterized by polyuria.

[Genetic and biochemical features of the monogenic hereditary urolithiasis]

Urolithiasis is a common urological problem. In most cases, this multifactorial pathology develops due to the combination of inherited low-penetrance gene variants and environment factors such as urinary tract infections and unbalanced diet. However, some cases are monogenic. These hereditary forms of urolithiasis manifest in childhood, and are characterized by multiple, bilateral and recurrent kidney stones and progress to chronic renal failure relatively early. Due to widening acceptance of exome and gene panel sequencing, substantially larger percentages of urolithiasis cases are now attributed to hereditary causes, up to 20% among patients of 18 years old or younger. Here we review genetic and biochemical mechanisms of urolithiasis, with an emphasis on its hereditary forms, including fermentopathies (primary hyperoxaluria, adenine phosphorobosyltransferase deficiency, phosphoribosyl-pyrophosphate-synthetase deficiency, xanthinuria, Lesch-Nihan syndrome) and these caused by membrane transport alterations (Dent's disease, familial hypomagnesia with hypercalciuria and nephrocalcinosis, hypophosphatemic urolithiasis, distal tubular acidosis, cystinuria, Bartter's syndrome). We suggest a comprehensive gene panel for NGS diagnostics of the hereditary urolithiasis. It is expected that accurate and timely diagnosis of hereditary forms of urolithiasis would enable the counselling of the carriers in affected families, and ensure personalized management of the patients with these conditions.

Familial hypomagnesaemia, Hypercalciuria and Nephrocalcinosis associated with a novel mutation of the highly conserved leucine residue 116 of Claudin 16 in a Chinese patient with a delayed diagnosis: a case report

Background

Sixty mutations of claudin 16 coding gene have been reported in familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) patients. Recent investigations revealed that a highly conserved glycine-leucine-tryptophan (¹¹⁵G-L-W¹¹⁷) motif in the first extracellular segment (ESC1) of claudin 16 might be essential for stabilization of the appropriately folded ECS1 structure and conservation of normal claudin 16 function. However, neither missense nor nonsense mutation has ever been described in this motif. Our study aimed at identifying mutations in a Chinese patient with FHHNC and exploring the association between genotype and phenotype.

Case presentation

A 33-year-old female presented with 4 years history of recurrent acute pyelonephritis without other notable past medical history. Her healthy parents, who aged 56 and 53 respectively, were second cousins, and her only sibling died from renal failure without definite cause at age 25. Renal ultrasound imaging demonstrated atrophic kidneys and bilateral nephrocalcinosis. The laboratory workup revealed impaired renal function (Stage CKD IV), hypocalcemia and mild hypomagnesemia, accompanied with marked renal loss of magnesium and hypercalciuria. During the follow-up, treatment with calcitriol and calcium but not with magnesium was difficult to achieve normal serum calcium levels, whereas her serum magnesium concentration fluctuated within normal ranges. In the end, the patient unavoidably reached ESRD at 36 years old. The clinical features and family history suggested the diagnosis of FHHNC. To make a definite diagnosis, we use whole-exome sequencing to identify the disease-causing mutations and Sanger sequencing to confirm the mutation co-segregation in the family. As a result, a novel homozygous mutation (c.346C > G, p.Leu116Val) in ¹¹⁵G-L-W¹¹⁷ motif of claudin 16 was identified. Her parents, grandmother and one of her cousins carried heterozygous p.Leu116Val, whereas 200 unrelated controls did not carry this mutation.

Conclusions

We described a delayed diagnosis patient with FHHNC in the Chinese population and identified a novel missense mutation in the highly conserved ¹¹⁵G-L-W¹¹⁷ motif of claudin 16 for the first time. According to the reported data and the information deduced from 3D modeling, we speculate that this mutation probably reserve partial residual function which might be related to the slight phenotype of the patient.

Electronic supplementary material

The online version of this article (10.1186/s12882-018-0979-1) contains supplementary material, which is available to authorized users.

The importance of the thick ascending limb of Henle's loop in renal physiology and pathophysiology

The thick ascending limb (TAL) of Henle’s loop is a crucial segment for many tasks of the nephron. Indeed, the TAL is not only a mainstay for reabsorption of sodium (Na⁺), potassium (K⁺), and divalent cations such as calcium (Ca²⁺) and magnesium (Mg²⁺) from the luminal fluid, but also has an important role in urine concentration, overall acid–base homeostasis, and ammonia cycle. Transcellular Na⁺ transport along the TAL is a prerequisite for Na⁺, K⁺, Ca²⁺, Mg²⁺ homeostasis, and water reabsorption, the latter through its contribution in the generation of the cortico-medullar osmotic gradient. The role of this nephron site in acid–base balance, via bicarbonate reabsorption and acid secretion, is sometimes misunderstood by clinicians. This review describes in detail these functions, reporting in addition to the well-known molecular mechanisms, some novel findings from the current literature; moreover, the pathophysiology and the clinical relevance of primary or acquired conditions caused by TAL dysfunction are discussed. Knowing the physiology of the TAL is fundamental for clinicians, for a better understanding and management of rare and common conditions, such as tubulopathies, hypertension, and loop diuretics abuse.

Mechanisms and causes of hypomagnesemia

PURPOSE OF REVIEW

Identification of the mechanisms of magnesium absorption and reabsorption has markedly enhanced our understanding of the causes of hypomagnesemia.

RECENT FINDINGS

New gastrointestinal and renal causes of hypomagnesemia have been recently documented.

SUMMARY

The recognition of new mechanisms and causes of magnesium absorption and reabsorption should enhance the ability to monitor patients at risk for hypomagnesemia and improve our ability to mitigate the serious symptoms associated with this disorder.

Nephrolithiasis and Nephrocalcinosis in Childhood-Risk Factor-Related Current and Future Treatment Options

Nephrolithiasis, urolithiasis, and nephrocalcinosis (NC) have become common causes of hospitalization and referral to pediatric outpatient clinics. It is of utmost importance to start with diagnostic evaluation directly after the first passage of a kidney stone, or if NC is diagnosed, in each pediatric patient. This is necessary, as in about 80% of children a metabolic reason for stone disease is detected. Current treatment options are scarce and mainly include general measures like an increased fluid intake or elevating the solubility of a lithogenic substance. According to the given lithogenic risk factor(s), specific treatment options are available and are being summarized in this review. Furthermore, an outlook on potential future treatment options, including innovative strategies such as mRNA-based or recombinant enzyme substitution therapy, is given.

Genetics of Magnesium Disorders

Background

Magnesium (Mg²⁺), the second most abundant cation in the cell, is woven into a multitude of cellular functions. Dysmagnesemia is associated with multiple diseases and, when severe, can be life-threatening.

Summary

This review discusses Mg²⁺ homeostasis and function with specific focus on renal Mg²⁺ handling. Intrarenal channels and transporters related to Mg²⁺ absorption are discussed. Unraveling the rare genetic diseases with manifestations of dysmagnesemia has greatly increased our understanding of the complex and intricate regulatory network in the kidney, specifically, functions of tight junction proteins including claudin-14, -16, -19, and -10; apical ion channels including: TRPM6, K_v1.1, and ROMK; small regulatory proteins including AC3 and ANK3; and basolateral proteins including EGF receptor, γ-subunit (FXYD2) of Na-K-ATPase, K_ir4.1, CaSR, CNNM2, and SLC41A. Although our understanding of Mg²⁺ handling of the kidney has expanded considerably in the last two decades, many questions remain. Future studies are needed to elucidate a multitude of unknown aspects of Mg²⁺ handling in the kidney.

Key Message

Understanding rare and genetic diseases of Mg²⁺ dysregulation has expanded our knowledge and furthers the development of strategies for preventing and managing dysmagnesemia.

Inherited and acquired disorders of magnesium homeostasis

PURPOSE OF REVIEW

Magnesium (Mg) imbalances are frequently overlooked. Hypermagnesemia usually occurs in preeclamptic women after Mg therapy or in end-stage renal disease patients, whereas hypomagnesemia is more common with a prevalence of up to 15% in the general population. Increasing evidence points toward a role for mild-to-moderate chronic hypomagnesemia in the pathogenesis of hypertension, type 2 diabetes mellitus, and metabolic syndrome.

RECENT FINDINGS

The kidneys are the major regulator of total body Mg homeostasis. Over the last decade, the identification of the responsible genes in rare genetic disorders has enhanced our understanding of how the kidney handles Mg. The different genetic disorders and medications contributing to abnormal Mg homeostasis are reviewed.

SUMMARY

As dysfunctional Mg homeostasis contributes to the development of many common human disorders, serum Mg deserves closer monitoring. Hypomagnesemic patients may be asymptomatic or may have mild symptoms. In severe hypomagnesemia, patients may present with neurological symptoms such as seizures, spasms, or cramps. Renal symptoms include nephrocalcinosis and impaired renal function. Most conditions affect tubular Mg reabsorption by disturbing the lumen-positive potential in the thick ascending limb or the negative membrane potential in the distal convoluted tubule.

Urinary Calcium Excretion and Risk of Chronic Kidney Disease in the General Population

Introduction

High urinary calcium excretion (UCaE) has been shown to lead to accelerated renal function decline in individuals with renal tubular diseases. It is not known whether this association also exists in the general population. Therefore, we investigated whether high UCaE is associated with risk of developing chronic kidney disease (CKD) in community-dwelling subjects.

Methods

Urine samples of 5491 subjects who were free of CKD at baseline and participated in the Prevention of Renal and Vascular End-Stage Disease study (a prospective, observational, general population-based cohort of Dutch men and women aged 28–75 years) were examined for UCaE. UCa concentration was measured in two 24-hour urine samples at baseline (1997–1998) by indirect potentiometry. UCaE was treated as a continuous variable and a categorical variable grouped according to sex-specific quintiles for UCaE. UCaE was compared with de novo development of estimated glomerular filtration rate <60 ml/min per 1.73 m² and/or albuminuria >30 mg/24 h.

Results

Baseline median UCaE was 4.13 mmol/24 h for men and 3.52 mmol/24 h for women. During a median follow-up of 10.3 years, 899 subjects developed CKD. After multivariable adjustment, every 1 mmol/24 h higher baseline UCaE was associated with a 6% lower risk for incident CKD during follow-up (hazard ratio: 0.94 [0.88–0.99], P = 0.02). The association was shown to be significantly nonlinear, with highest risk of CKD in the lowest quintile for UCaE (hazard ratio: 1.28 [0.97–1.68], P = 0.09). There was no association between UCaE and mortality or cardiovascular health during follow-up, suggesting that this association was not a reflection of poor nutritional intake due to bad health.

Discussion

These findings indicate that high UCaE does not increase risk of CKD, but rather that low UCaE may be harmful.

Amelogenesis imperfecta in familial hypomagnesaemia and hypercalciuria with nephrocalcinosis caused by CLDN19 gene mutations

BACKGROUND

Amelogenesis imperfecta (AI) is a group of genetic diseases characterised by tooth enamel defects. AI was recently described in patients with familial hypercalciuria and hypomagnesaemia with nephrocalcinosis (FHHNC) caused by CLDN16 mutations. In the kidney, claudin-16 interacts with claudin-19 to control the paracellular passage of calcium and magnesium. FHHNC can be linked to mutations in both genes. Claudin-16 was shown to be expressed during amelogenesis; however, no data are available on claudin-19. Moreover, the enamel phenotype of patients with CLDN19 mutations has never been described. In this study, we describe the clinical and genetic features of nine patients with FHHNC carrying CLDN19 mutations and the claudin-19 expression profile in rat ameloblasts.

METHODS

Six FHHNC Brazilian patients were subjected to mutational analysis. Three additional French patients were recruited for orodental characterisation. The expression profile of claudin-19 was evaluated by RT-qPCR and immunofluorescence using enamel epithelium from rat incisors.

RESULTS

All patients presented AI at different degrees of severity. Two new likely pathogenic variations in CLDN19 were found: p.Arg200Gln and p.Leu90Arg. RT-qPCR revealed low Cldn19 expression in ameloblasts. Confocal analysis indicated that claudin-19 was immunolocalised at the distal poles of secretory and maturing ameloblasts.

CONCLUSIONS

For the first time, it was demonstrated that AI is associated with FHHNC in patients carrying CLDN19 mutations. The data suggest claudin-19 as an additional determinant in enamel formation. Indeed, the coexistence of hypoplastic and hypomineralised AI in the patients was consistent with claudin-19 expression in both secretory and maturation stages. Additional indirect systemic effects cannot be excluded.

A novel mutation in CLDN16 results in rare familial hypomagnesaemia with hypercalciuria and nephrocalcinosis in a Chinese family

BACKGROUND

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessively inherited disease characterized by excessive wasting of renal tubular magnesium and calcium. FHHNC is associated with various mutations in CLDN16 and CLDN19.

CASES

Two children from a consanguineous family of Chinese Han origin demonstrated manifestations of rickets, polyuria, polydipsia, hematuria and failure to thrive. Hypomagnesaemia (0.49-0.50mmol/L), hypercalciuria or a trend to hypercalciuria (24hour urine calcium: 3.8-5.1mg/kg/day), and secondary hyperparathyroidism (serum PTH level: 94.7-200pg/mL) were revealed upon laboratory examination. Using targeted next-generation sequencing and subsequent confirmation by Sanger sequencing, a novel homozygous mutation was identified in the CLDN16 gene of both FHHNC patients. This specific mutation, a 16bp deletion followed by a 23bp insertion in exon 3, led to the generation of a premature termination codon. The parents and an unaffected sister were all heterozygous carriers of this mutation.

CONCLUSIONS

We detected a novel mutation in CLDN16 for the first time. The clinical and genetic findings from this study will help to expand the understanding of this rare disease, FHHNC.