Hypophosphatemic rickets with hypercalciuria due to mutation in SLC34A3/NaPi-IIc can be masked by vitamin D deficiency and can be associated with renal calcifications

An update on clinical presentation and responses to therapy of patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH)

Pathogenic variants in solute carrier family 34, member 3 (SLC34A3), the gene encoding the sodium-dependent phosphate cotransporter 2c (NPT2c), cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH). Here, we report a pooled analysis of clinical and laboratory records of 304 individuals from 145 kindreds, including 20 previously unreported HHRH kindreds, in which two novel SLC34A3 pathogenic variants were identified. Compound heterozygous/homozygous carriers show above 90% penetrance for kidney and bone phenotypes. The biochemical phenotype for heterozygous carriers is intermediate with decreased serum phosphate, tubular reabsorption of phosphate (TRP (%)), fibroblast growth factor 23, and intact parathyroid hormone, but increased serum 1,25-dihydroxy vitamin D, and urine calcium excretion causing idiopathic hypercalciuria in 38%, with bone phenotypes still observed in 23% of patients. Oral phosphate supplementation is the current standard of care, which typically normalizes serum phosphate. However, although in more than half of individuals this therapy achieves correction of hypophosphatemia it fails to resolve the other outcomes. The American College of Medical Genetics and Genomics score correlated with functional analysis of frequent SLC34A3 pathogenic variants in vitro and baseline disease severity. The number of mutant alleles and baseline TRP (%) were identified as predictors for kidney and bone phenotypes, baseline TRP (%) furthermore predicted response to therapy. Certain SLC34A3/NPT2c pathogenic variants can be identified with partial responses to therapy, whereas with some overlap, others present only with kidney phenotypes and a third group present only with bone phenotypes. Thus, our report highlights important novel clinical aspects of HHRH and heterozygous carriers, raises awareness to this rare group of disorders and can be a foundation for future studies urgently needed to guide therapy of HHRH.

Mineral Metabolism in Children: Interrelation between Vitamin D and FGF23

Fibroblast growth factor 23 (FGF23) was identified at the turn of the century as the long-sought circulating phosphatonin in human pathology. Since then, several clinical and experimental studies have investigated the metabolism of FGF23 and revealed its relevant pathogenic role in various diseases. Most of these studies have been performed in adult individuals. However, the mineral metabolism of the child is, to a large extent, different from that of the adult because, in addition to bone remodeling, the child undergoes a specific process of endochondral ossification responsible for adequate mineralization of long bones’ metaphysis and growth in height. Vitamin D metabolism is known to be deeply involved in these processes. FGF23 might have an influence on bones’ growth as well as on the high and age-dependent serum phosphate concentrations found in infancy and childhood. However, the interaction between FGF23 and vitamin D in children is largely unknown. Thus, this review focuses on the following aspects of FGF23 metabolism in the pediatric age: circulating concentrations’ reference values, as well as those of other major variables involved in mineral homeostasis, and the relationship with vitamin D metabolism in the neonatal period, in vitamin D deficiency, in chronic kidney disease (CKD) and in hypophosphatemic disorders.

Clinical Spectrum of Hereditary Hypophosphatemic Rickets With Hypercalciuria (HHRH)

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) represents an FGF23-independent disease caused by biallelic variants in the solute carrier family 34-member 3 (SLC34A3) gene. HHRH is characterized by chronic hypophosphatemia and an increased risk for nephrocalcinosis and rickets/osteomalacia, muscular weakness, and secondary limb deformity. Biochemical changes, but no relevant skeletal changes, have been reported for heterozygous SLC34A3 carriers. Therefore, we assessed the characteristics of individuals with biallelic and monoallelic SLC34A3 variants. In 8 index patients and 5 family members, genetic analysis was performed using a custom gene panel. The skeletal assessment comprised biochemical parameters, areal bone mineral density (aBMD), and bone microarchitecture. Pathogenic SLC34A3 variants were revealed in 7 of 13 individuals (2 homozygous, 5 heterozygous), whereas 3 of 13 carried monoallelic variants of unknown significance. Whereas both homozygous individuals had nephrocalcinosis, only one displayed a skeletal phenotype consistent with HHRH. Reduced to low-normal phosphate levels, decreased tubular reabsorption of phosphate (TRP), and high-normal to elevated values of 1,25-OH₂ -D₃ accompanied by normal cFGF23 levels were revealed independently of mutational status. Interestingly, individuals with nephrocalcinosis showed significantly increased calcium excretion and 1,25-OH₂ -D₃ levels but normal phosphate reabsorption. Furthermore, aBMD Z-score <-2.0 was revealed in 4 of 8 heterozygous carriers, and HR-pQCT analysis showed a moderate decrease in structural parameters. Our findings highlight the clinical relevance also of monoallelic SLC34A3 variants, including their potential skeletal impairment. Calcium excretion and 1,25-OH₂ -D₃ levels, but not TRP, were associated with nephrocalcinosis. Future studies should investigate the effects of distinct SLC34A3 variants and optimize treatment and monitoring regimens to prevent nephrocalcinosis and skeletal deterioration. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

SLC34A3 GENE MUTATION AS A RARE CAUSE OF HYPOPHOSPHATEMIA IN TWO SIBLINGS

Context

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare autosomal recessive disorder, which is characterized by renal phosphate wasting, hypercalciuria, increased 1,25-dihydroxyvitamin D, and decreased parathormone (PTH) levels.

Objective

Here we report different clinical features of two siblings with HHRH, confirmed with molecular diagnosis.

Subjects and methods

16.4 years old boy (P1), and 8.7 years old girl (P2) were referred to our outpatient clinic due to clinical suspicion of metabolic bone diseases.

Results

P1 had severe hypophosphatemia. Additionally, PTH concentration was near to the lower limit, 1,25-dihydroxyvitamin-D concentration was near to the upper limit. P2 had relatively milder clinical and laboratory findings. Bilateral renal calculi were detected on ultrasound in both of them. HHRH was suspected due to their described biochemistry and the presence of bilateral renal calculi. Molecular analysis of SLC34A3 gene revealed a homozygous variant c.756G>A (p.Gln252=) and a splice donor variant c.1335+2T>A. After oral phosphate treatment, clinical and biochemical improvements were observed. However treatment nonadherence of patients was a barrier to reach treatment goal.

Conclusion

The clinical phenotype due to the same mutation in the SLC34A3 gene may vary even among the members of the same family. An accurate diagnosis is important for the appropriate treatment.

ONCOCYTIC ADRENOCORTICAL CARCINOMA IN A YOUNG PATIENT

Context

Adrenocortical carcinoma (ACC) is a rare neoplasm with an aggressive course and poor prognosis. The worldwide incidence is about 0.5 to 2 cases per million population per year. Oncocytic adrenocortical carcinoma is a rare histopathological variant of ACC with only a few reported cases in the literature.

Case report

We report a case of an oncocytic variant of adrenocortical carcinoma in a 21-year-old male patient who presented with a left adrenal mass. Imaging studies confirmed a large left adrenal mass with involvement of the left renal vein and inferior vena cava. Endocrine workup showed mildly elevated serum cortisol levels.

Discussion

Oncocytic AAC is a rare histopathological variant of ACC, as well as a rare subgroup of oncocytic adrenal neoplasms Hormonally active or functioning adrenocortical carcinomas most commonly secrete cortisol whereas co-secretion of multiple steroid hormones is a rare phenomenon.

Conclusions

Surgery remains the mainstay of treatment, but most of the patients present late with large masses and eventually become unsuitable for curative resection.

Variable Clinical Presentation of Children with Hereditary Hypophosphatemic Rickets with Hypercalciuria: A Case Series and Review of the Literature

INTRODUCTION

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare condition of renal phosphate wasting due to SLC34A3 mutations [Am J Hum Genet. 2006;78(2):193-201]. Patients exhibit low serum phosphorus, high 1,25-dihydroxyvitamin D, and inappropriately high urine phosphate and calcium. However, symptoms vary, and little is known about specific phenotype-genotype correlations.

METHODS

We report 3 HHRH cases in unrelated 12-year-old, 9-year-old, and 14-year-old patients and perform a systematic literature review.

RESULTS

All 3 patients exhibited labs typical of HHRH. Yet, their presentations differed, and 2 novel SLC34A3 variants were identified. As found in the literature review, bone symptoms are most common (50%), followed by renal symptoms (17%), combined bone and renal symptoms (18%), and asymptomatic (9%).

CONCLUSION

These 3 cases highlight the variability of presenting signs and symptoms among individuals with HHRH. An accurate diagnosis is critical as treatment differs from other disorders of phosphate wasting, urinary stones, and mineralization defects.

The Molecular Basis of Calcium and Phosphorus Inherited Metabolic Disorders

Calcium (Ca) and Phosphorus (P) hold a leading part in many skeletal and extra-skeletal biological processes. Their tight normal range in serum mirrors their critical role in human well-being. The signalling “voyage” starts at Calcium Sensing Receptor (CaSR) localized on the surface of the parathyroid glands, which captures the “oscillations” of extracellular ionized Ca and transfers the signal downstream. Parathyroid hormone (PTH), Vitamin D, Fibroblast Growth Factor (FGF23) and other receptors or ion-transporters, work synergistically and establish a highly regulated signalling circuit between the bone, kidneys, and intestine to ensure the maintenance of Ca and P homeostasis. Any deviation from this well-orchestrated scheme may result in mild or severe pathologies expressed by biochemical and/or clinical features. Inherited disorders of Ca and P metabolism are rare. However, delayed diagnosis or misdiagnosis may cost patient’s quality of life or even life expectancy. Unravelling the thread of the molecular pathways involving Ca and P signaling, we can better understand the link between genetic alterations and biochemical and/or clinical phenotypes and help in diagnosis and early therapeutic intervention.

Congenital Conditions of Hypophosphatemia Expressed in Adults

The main congenital conditions of hypophosphatemia expressed in adulthood include several forms of hereditary hypophosphatemic rickets and a congenital disorder of vitamin D metabolism characterized by osteomalacia and hypophosphatemia in adult patients. Hypophosphatemia in adults is defined as serum phosphate concentration < 0.80 mmol/L. The principal regulators of phosphate homeostasis, as is well known, are parathyroid hormone (PTH), activated vitamin D, and Fibroblast Growth Factor 23 (FGF23). Differential diagnosis of hypophosphatemia is based on the evaluation of mechanisms leading to this alteration, such as high PTH activity, inadequate phosphate absorption from the gut, or renal phosphate wasting, either due to primary tubular defects or high FGF23 levels. The most common inherited form associated to hypophosphatemia is X-linked hypophosphatemic rickets (XLH), caused by PHEX gene mutations with enhanced secretion of the FGF23. Until now, the management of hypophosphatemia in adulthood has been poorly investigated. It is widely debated whether adult patients benefit from the conventional treatments normally used for pediatric patients. The new treatment for XLH with burosumab, a recombinant human IgG1 monoclonal antibody that binds to FGF23, blocking its activity, may change the pharmacological management of adult subjects with hypophosphatemia associated to FGF23-dependent mechanisms.

Hereditary Hypophosphatemic Rickets with Hypercalciuria (HHRH) Presenting with Genu Valgum Deformity: Treatment with Phosphate Supplementation and Surgical Correction

We describe a case of hereditary hypophosphatemic rickets with hypercalciuria (HHRH) in a 32-year-old female with short stature, chronic pathologic genu valgum deformity, and knee pain who was referred to endocrinology clinic after previous inconclusive workups. We present imaging spanning 10 years of untreated disease. Biochemical studies showed hypophosphatemia with undetectable fibroblast growth factor 23 (FGF23.) Renal ultrasound revealed bilateral medullary nephrocalcinosis despite no apparent hypercalciuria. Due to concern for HHRH, genetic testing was performed that determined this patient to be homozygous in the SLC34A3 gene for a previously described missense variant (c.1402C > T, p.Arg468Trp). There was no known family history of rickets. A bone biopsy with metabolic studies was performed for diagnostic and prognostic reasons. The histopathological findings along with tetracycline uptake studies were consistent with a diagnosis of HHRH. Treatment with phosphorous supplementation and surgical correction of her valgum deformity resulted in resolution of pain, but no change in bone histomorphometry.

HYPOPHOSPHATEMIC RICKETS WITH HYPERCALCIURIA: A NOVEL HOMOZYGOUS MUTATION IN SLC34A3 AND LITERATURE REVIEW

OBJECTIVE

Hypophosphatemic rickets with hypercalciuria (HHRH) is a rare, recessively-inherited form of rickets caused by homozygous or compound heterozygous mutations in the SLC34A3 gene that encodes the renal tubular phosphate transporter protein NaPi2c. The bone phenotype varies from severe rickets to no disease. Accurate diagnosis is important as the treatment differs from other forms of rickets.

METHODS

The patient was a 12-year-old boy from the Indian subcontinent with florid hypophosphatemic rickets. A targeted gene panel to search for mutations in genes associated with inherited forms of rickets was performed. We also completed a literature search of published cases of HHRH.

RESULTS

The targeted gene panel demonstrated a novel homozygous SLC34A3 mutation: c.1339 G>A (p.Ala447Thr). His parents were heterozygous for the mutation. In our literature review we found that people with homozygous SLC34A3 mutations were more likely to have rickets than those with compound heterozygous mutations (85% versus 45%, p<0.002) and that serum phosphate z scores were lower in those with rickets than those without (-3.3 with a standard deviation of 1.5 versus -2.1 with a standard deviation of 1.5, p<0.005).

CONCLUSION

The bone phenotype of HHRH is related to the nature of the mutation and serum phosphate levels. Targeted gene panels can aid in the accurate diagnosis of inherited forms of rickets, and facilitate correct treatment.

Hereditäre hypophosphatämische Rachitis

Die hereditären hypophosphatämischen Rachitiden (HR) sind eine genetisch und klinisch heterogene Gruppe von Erkrankungen, bedingt durch einen renal-tubulären Phosphatverlust und gekennzeichnet durch die Folgen des Verlustes an mineralisierter Knochensubstanz. Die klinischen Ausprägungen sind variabel. In der Kindheit dominieren Kleinwuchs und Beinfehlstellung durch die Verformungen der unteren Extremitäten infolge von Rachitis und Osteomalazie. Im Erwachsenenalter kommen oftmals Insuffizienzfrakturen, Bewegungseinschränkungen, Arthrosen und extraskelettale Verkalkungen hinzu. Die häufigste Form ist die X‑chromosomal vererbte hypophosphatämische Rachitis, die durch Mutationen im PHEX-Gen verursacht wird. PHEX kodiert für das „phosphatregulierende Gen mit Homologie zu Endopeptidasen“, das in die Regulation des Fibroblastenwachstumsfaktors FGF23 involviert ist. Eine durch PHEX oder andere Genmutationen bedingte Erhöhung von FGF23 führt zu einer forcierten Phosphatausscheidung über die Niere und einem konsekutiven Phosphatverlust. Die selteneren Formen der HR, die autosomal-dominant, autosomal-rezessiv oder X‑chromosomal-rezessiv vererbt werden können, werden heutzutage danach unterschieden, ob sie eine FGF23-Erhöhung hervorrufen oder FGF23-unabhängig einen Phosphatverlust induzieren. Dies ist von diagnostischer und therapeutischer Bedeutung. Während die Therapie der HR bislang durch mehrfach tägliche Phosphatgaben und aktive Vitamin-D-Metabolite erfolgte, so steht seit Kurzem mit Burosumab ein anti-FGF23 Antikörper zur Verfügung. Dieser ist allerdings zurzeit in Europa nur für die XLHR im Kindes- und Jugendalter zugelassen. Mit dieser Therapie lässt sich laborchemisch eine Normalisierung des renalen Phosphatverlusts und damit eine deutliche Verbesserung der Rachitis und Osteomalazie erreichen. Kurz- und mittelfristig ist dadurch eine deutliche klinische Verbesserung zu beobachten. Langzeitergebnisse und umfassende Daten zur Anwendung bei erwachsenen XLH-Patient*innen stehen hingegen noch aus.

Phosphate matters when investigating hypercalcemia: a mutation in SLC34A3 causing HHRH

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare, autosomal recessive disorder caused by mutations in the SLC34A3 gene that encodes the renal sodium-dependent phosphate cotransporter 2c (NaPi-IIc). It may present as intermittent mild hypercalcemia which may attract initial diagnostic attention but appreciation of concomitant hypophosphatemia is critical for consideration of the necessary diagnostic approach. A 21-year-old woman was assessed by adult endocrinology for low bone mass. She initially presented age two with short stature, nephrocalcinosis and mild intermittent hypercalcemia with hypercalciuria. She had no evidence of medullary sponge kidney or Fanconi syndrome and no bone deformities, pain or fractures. She had recurrent episodes of nephrolithiasis. In childhood, she was treated with hydrochlorothiazide to reduce urinary calcium. Upon review of prior investigations, she had persistent hypophosphatemia with phosphaturia, low PTH and a high-normal calcitriol. A diagnosis of HHRH was suspected and genetic testing confirmed a homozygous c.1483G>A (p.G495R) missense mutation of the SLC34A3 gene. She was started on oral phosphate replacement which normalized her serum phosphate, serum calcium and urine calcium levels over the subsequent 5 years. HHRH is an autosomal recessive condition that causes decreased renal reabsorption of phosphate, leading to hyperphosphaturia, hypophosphatemia and PTH-independent hypercalcemia due to the physiologic increase in calcitriol which also promotes hypercalciuria. Classically, patients present in childhood with bone pain, vitamin D-independent rickets and growth delay. This case of a SLC34A3 mutation illustrates the importance of investigating chronic hypophosphatemia even in the presence of other more common electrolyte abnormalities.

Hereditary hypophosphatemic rickets with hypercalciuria: pathophysiology, clinical presentation, diagnosis and therapy

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH; OMIM: 241530) is a rare autosomal recessive disorder with an estimated prevalence of 1:250,000 that was originally described by Tieder et al. Individuals with HHRH carry compound-heterozygous or homozygous (comp/hom) loss-of-function mutations in the sodium-phosphate co-transporter NPT2c. These mutations result in the development of urinary phosphate (Pi) wasting and hypophosphatemic rickets, bowing, and short stature, as well as appropriately elevated 1,25(OH)₂D levels, which sets this fibroblast growth factor 23 (FGF23)-independent disorder apart from the more common X-linked hypophosphatemia. The elevated 1,25(OH)₂D levels in turn result in hypercalciuria due to enhanced intestinal calcium absorption and reduced parathyroid hormone (PTH)-dependent calcium-reabsorption in the distal renal tubules, leading to the development of kidney stones and/or nephrocalcinosis in approximately half of the individuals with HHRH. Even heterozygous NPT2c mutations are frequently associated with isolated hypercalciuria (IH), which increases the risk of kidney stones or nephrocalcinosis threefold in affected individuals compared with the general population. Bone disease is generally absent in individuals with IH, in contrast to those with HHRH. Treatment of HHRH and IH consists of monotherapy with oral Pi supplements, while active vitamin D analogs are contraindicated, mainly because the endogenous 1,25(OH)₂D levels are already elevated but also to prevent further worsening of the hypercalciuria. Long-term studies to determine whether oral Pi supplementation alone is sufficient to prevent renal calcifications and bone loss, however, are lacking. It is also unknown how therapy should be monitored, whether secondary hyperparathyroidism can develop, and whether Pi requirements decrease with age, as observed in some FGF23-dependent hypophosphatemic disorders, or whether this can lead to osteoporosis.

Increased Probability of Co-Occurrence of Two Rare Diseases in Consanguineous Families and Resolution of a Complex Phenotype by Next Generation Sequencing

Massively parallel sequencing of whole genomes and exomes has facilitated a direct assessment of causative genetic variation, now enabling the identification of genetic factors involved in rare diseases (RD) with Mendelian inheritance patterns on an almost routine basis. Here, we describe the illustrative case of a single consanguineous family where this strategy suffered from the difficulty to distinguish between two etiologically distinct disorders, namely the co-occurrence of hereditary hypophosphatemic rickets (HRR) and congenital myopathies (CM), by their phenotypic manifestation alone. We used parametric linkage analysis, homozygosity mapping and whole exome-sequencing to identify mutations underlying HRR and CM. We also present an approximate approach for assessing the probability of co-occurrence of two unlinked recessive RD in a single family as a function of the degree of consanguinity and the frequency of the disease-causing alleles. Linkage analysis and homozygosity mapping yielded elusive results when assuming a single RD, but whole-exome sequencing helped to identify two mutations in two genes, namely SLC34A3 and SEPN1, that segregated independently in this family and that have previously been linked to two etiologically different diseases. We assess the increase in chance co-occurrence of rare diseases due to consanguinity, i.e. under circumstances that generally favor linkage mapping of recessive disease, and show that this probability can increase by several orders of magnitudes. We conclude that such potential co-occurrence represents an underestimated risk when analyzing rare or undefined diseases in consanguineous families and should be given more consideration in the clinical and genetic evaluation.

Mutations in SLC34A3/NPT2c are associated with kidney stones and nephrocalcinosis

Compound heterozygous and homozygous (comp/hom) mutations in solute carrier family 34, member 3 (SLC34A3), the gene encoding the sodium (Na(+))-dependent phosphate cotransporter 2c (NPT2c), cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a disorder characterized by renal phosphate wasting resulting in hypophosphatemia, correspondingly elevated 1,25(OH)2 vitamin D levels, hypercalciuria, and rickets/osteomalacia. Similar, albeit less severe, biochemical changes are observed in heterozygous (het) carriers and indistinguishable from those changes encountered in idiopathic hypercalciuria (IH). Here, we report a review of clinical and laboratory records of 133 individuals from 27 kindreds, including 5 previously unreported HHRH kindreds and two cases with IH, in which known and novel SLC34A3 mutations (c.1357delTTC [p.F453del]; c.G1369A [p.G457S]; c.367delC) were identified. Individuals with mutations affecting both SLC34A3 alleles had a significantly increased risk of kidney stone formation or medullary nephrocalcinosis, namely 46% compared with 6% observed in healthy family members carrying only the wild-type SLC34A3 allele (P=0.005) or 5.64% in the general population (P<0.001). Renal calcifications were also more frequent in het carriers (16%; P=0.003 compared with the general population) and were more likely to occur in comp/hom and het individuals with decreased serum phosphate (odds ratio [OR], 0.75, 95% confidence interval [95% CI], 0.59 to 0.96; P=0.02), decreased tubular reabsorption of phosphate (OR, 0.41; 95% CI, 0.23 to 0.72; P=0.002), and increased serum 1,25(OH)2 vitamin D (OR, 1.22; 95% CI, 1.05 to 1.41; P=0.008). Additional studies are needed to determine whether these biochemical parameters are independent of genotype and can guide therapy to prevent nephrocalcinosis, nephrolithiasis, and potentially, CKD.

A compound heterozygous mutation in SLC34A3 causes hereditary hypophosphatemic rickets with hypercalciuria in a Chinese patient

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare metabolic disorder inherited in an autosomal recessive fashion and characterized by hypophosphatemia, short stature, rickets and/or osteomalacia, and secondary absorptive hypercalciuria. HHRH was recently mapped to chromosome 9q34, which contains the gene SLC34A3 which encodes the renal proximal tubular sodium-phosphate cotransporter NaPi-IIc. Here we describe a 29-year-old man with a history of childhood rickets who presented with increased renal phosphate clearance leading to hypophosphatemia, hypercalciuria, low serum parathyroid hormone (PTH), elevated serum 1,25-dihydroxyvitamin D (1,25(OH)2D) and recurrent nephrolithiasis. We performed a mutation analysis of SLC34A3 (exons and adjacent introns) of the proband and his parents to determine if there was a genetic contribution. The proband proved to be compound heterozygous for two missense mutations in SLC34A3: one novel mutation in exon 7 c.571G>C (p.G191R) and one previously identified mutation in exon 13 c.1402C>T (p.R468W). His parents were both asymptomatic heterozygous carriers of one of these two mutations. We also performed an oral phosphate loading test and compared serum phosphate, intact PTH, and intact fibroblast growth factor 23 (iFGF23) in this patient versus patients with other forms of hypophosphatemic rickets, the results of which further revealed that the mechanism of hypophosphatemia in HHRH is independent of FGF23. This is the first report of HHRH in the Chinese population. Our findings of the novel mutation in exon 7 add to the list of more than 20 reported mutations of SLC34A3.

Fanconi-Bickel syndrome and autosomal recessive proximal tubulopathy with hypercalciuria (ARPTH) are allelic variants caused by GLUT2 mutations

CONTEXT

Many inherited disorders of calcium and phosphate homeostasis are unexplained at the molecular level.

OBJECTIVE

The objective of the study was to identify the molecular basis of phosphate and calcium abnormalities in two unrelated, consanguineous families.

PATIENTS

The affected members in family 1 presented with rickets due to profound urinary phosphate-wasting and hypophosphatemic rickets. In the previously reported family 2, patients presented with proximal renal tubulopathy and hypercalciuria yet normal or only mildly increased urinary phosphate excretion.

METHODS

Genome-wide linkage scans and direct nucleotide sequence analyses of candidate genes were performed. Transport of glucose and phosphate by glucose transporter 2 (GLUT2) was assessed using Xenopus oocytes. Renal sodium-phosphate cotransporter 2a and 2c (Npt2a and Npt2c) expressions were evaluated in transgenically rescued Glut2-null mice (tgGlut2-/-).

RESULTS

In both families, genetic mapping and sequence analysis of candidate genes led to the identification of two novel homozygous mutations (IVS4-2A>G and R124S, respectively) in GLUT2, the gene mutated in Fanconi-Bickel syndrome, a rare disease usually characterized by renal tubulopathy, impaired glucose homeostasis, and hepatomegaly. Xenopus oocytes expressing the [R124S]GLUT2 mutant showed a significant reduction in glucose transport, but neither wild-type nor mutant GLUT2 facilitated phosphate import or export; tgGlut2-/- mice demonstrated a profound reduction of Npt2c expression in the proximal renal tubules.

CONCLUSIONS

Homozygous mutations in the facilitative glucose transporter GLUT2, which cause Fanconi-Bickel syndrome, can lead to very different clinical and biochemical findings that are not limited to mild proximal renal tubulopathy but can include significant hypercalciuria and highly variable degrees of urinary phosphate-wasting and hypophosphatemia, possibly because of the impaired proximal tubular expression of Npt2c.

Novel NaPi-IIc mutations causing HHRH and idiopathic hypercalciuria in several unrelated families: long-term follow-up in one kindred

Homozygous and compound heterozygous mutations in SLC34A3, the gene encoding the sodium-dependent co-transporter NaPi-IIc, cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a disorder characterized by renal phosphate-wasting resulting in hypophosphatemia, elevated 1,25(OH)(2) vitamin D levels, hypercalciuria, rickets/osteomalacia, and frequently kidney stones or nephrocalcinosis. Similar albeit less severe biochemical changes are also observed in heterozygous carriers, which are furthermore indistinguishable from those encountered in idiopathic hypercalciuria (IH). We now searched for SLC34A3 mutations (exons and introns) in two previously not reported HHRH kindreds, which resulted in the identification of three novel mutations. The affected members of kindred A were compound heterozygous for two different mutations, c.1046_47del and the intronic mutation c.560+23_561-42del, while the index case in kindred B was homozygous for the nonsense SLC34A3 mutation c.1764C>G (p.Y588X). The patient in kindred C was diagnosed with IH because of bilateral medullary nephrocalcinosis, suppressed PTH levels, and hypercalciuria; she was found to have a novel heterozygous c.1571_1880del mutation. The HHRH patients in kindred A were treated for up to 7years with oral phosphate, which led to reversal of hypophosphatemia, hypercalciuria, and prevention or healing of the mild bone abnormalities. PTH levels were normal throughout the observation period, while 1,25(OH)(2) vitamin D levels remained elevated and may thus be helpful for assessing treatment efficacy and patient compliance in HHRH.

Clinical consequences of mutations in sodium phosphate cotransporters

Three families of sodium phosphate cotransporters have been described. Their specific roles in human health and disease have not been defined. Review of the literature reveals that the type II sodium phosphate cotransporters play a significant role in transepithelial transport in a number of tissues including kidney, intestine, salivary gland, mammary gland, and lung. The type I transporters seem to play a major role in renal urate handling and mutations in these proteins have been implicated in susceptibility to gout. The ubiquitously expressed type III transporters play a lesser role in phosphate homeostasis but contribute to cellular phosphate uptake, mineralization, and inflammation. The recognition of species differences in the expression, regulation, and function of these transport proteins suggests an urgent need to find ways to study them in humans.

FGF23 and syndromes of abnormal renal phosphate handling

Fibroblast growth factor 23 (FGF23) is part of a previously unrecognized hormonal bone-parathyroid-kidney axis, which is modulated by 1,25(OH)(2)-vitamin D (1,25(OH)(2)D), dietary and circulating phosphate and possibly PTH. FGF23 was discovered as the humoral factor in tumors that causes hypophosphatemia and osteomalacia and through the identification of a mutant form of FGF23 that leads to autosomal dominant hypophosphatemic rickets (ADHR), a rare genetic disorder. FGF23 appears to be mainly secreted by osteocytes where its expression is up-regulated by 1,25(OH)(2)D and probably by increased serum phosphate levels. Its synthesis and secretion is reduced through yet unknown mechanisms that involve the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), dentin matrix protein 1 (DMP1) and ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). Consequently, loss-of-function mutations in these genes underlie hypophosphatemic disorders that are either X-linked or autosomal recessive. Impaired O-glycosylation of FGF23 due to the lack of UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyl-transferase 3 (GALNT3) or due to certain homozygous FGF23 mutations results in reduced secretion of intact FGF23 and leads to familial hyperphosphatemic tumoral calcinosis. FGF23 acts through FGF-receptors and the coreceptor Klotho to reduce 1,25(OH)(2)D synthesis in the kidney and probably the synthesis of parathyroid hormone (PTH) by the parathyroid glands. It furthermore synergizes with PTH to increase renal phosphate excretion by reducing expression of the sodium-phosphate cotransporters NaPi-IIa and NaPi-IIc in the proximal tubules. Loss-of-function mutations in these two transporters lead to autosomal recessive Fanconi syndrome or to hereditary hypophosphatemic rickets with hypercalciuria, respectively.

Processing and stability of type IIc sodium-dependent phosphate cotransporter mutations in patients with hereditary hypophosphatemic rickets with hypercalciuria

Mutations in the apically located Na(+)-dependent phosphate (NaPi) cotransporter, SLC34A3 (NaPi-IIc), are a cause of hereditary hypophosphatemic rickets with hypercalciuria (HHRH). We have characterized the impact of several HHRH mutations on the processing and stability of human NaPi-IIc. Mutations S138F, G196R, R468W, R564C, and c.228delC in human NaPi-IIc significantly decreased the levels of NaPi cotransport activities in Xenopus oocytes. In S138F and R564C mutant proteins, this reduction is a result of a decrease in the V(max) for P(i), but not the K(m). G196R, R468W, and c.228delC mutants were not localized to oocyte membranes. In opossum kidney (OK) cells, cell surface labeling, microscopic confocal imaging, and pulse-chase experiments showed that G196R and R468W mutations resulted in an absence of cell surface expression owing to endoplasmic reticulum (ER) retention. G196R and R468W mutants could be partially stabilized by low temperature. In blue native-polyacrylamide gel electrophoresis analysis, G196R and R468W mutants were either denatured or present in an aggregation complex. In contrast, S138F and R564C mutants were trafficked to the cell surface, but more rapidly degraded than WT protein. The c.228delC mutant did not affect endogenous NaPi uptake in OK cells. Thus, G196R and R468W mutations cause ER retention, while S138F and R564C mutations stimulate degradation of human NaPi-IIc in renal epithelial cells. Together, these data suggest that the NaPi-IIc mutants in HHRH show defective processing and stability.

Sodium-dependent phosphate cotransporters: lessons from gene knockout and mutation studies

Inorganic phosphate (Pi) is an essential physiological compound, highlighted by the syndromes caused by hypo or hyperphosphatemic states. Hyperphosphatemia is associated with ectopic calcification, cardiovascular disease, and increased mortality in patients with chronic kidney disease (CKD). As phosphate control is not efficient with diet or dialysis, oral Pi binders are used in over 90% of patients with renal failure. However, achieving tight control of serum Pi is difficult, and lower levels of serum Pi (severe hypophosphatemia) do not lead to better outcomes. The inhibition of sodium-dependent Pi (NaPi) transporter would be a preferable method to control serum Pi levels in patients with CKD or patients undergoing dialysis. Three types of NaPi transporters (types I-III) have been identified: solute carrier series SLC17A1 (NPT1/NaPi-I/OATv1), SLC34 (NaPi-IIa, NaPi-IIb, NaPi-IIc), and SLC20 (PiT1, PiT2), respectively. Knockout mice have been created for types I-III NaPi transporters. In this review, we discuss the roles of the NaPi transporters in Pi homeostasis.

A novel compound heterozygous ROMK mutation presenting as late onset Bartter syndrome associated with nephrocalcinosis and elevated 1,25(OH)(2) vitamin D levels

Bartter syndrome (BS) is a rare renal tubular disorder presenting with hypokalemic metabolic alkalosis, which is classified into five types. KCNJ1 mutations usually cause the neonatal form of BS, type II BS (OMIM 241200). However, this report concerns a female patient with a novel, compound heterozygous KCNJ1 mutation that causes late-onset BS. The unique clinical findings of this case include persistently elevated 1,25(OH)(2) vitamin D levels, possibly due to increase prostaglandin E(2) levels, and medullary nephrocalcinosis. Treatment with COX-2 inhibitors resolved her hypercalciuria and improved her height and weight; renal function remains stable and there is no progression of nephrocalcinosis.

Hereditary hypophosphatemic rickets with hypercalciuria and nephrolithiasis-identification of a novel SLC34A3/NaPi-IIc mutation

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is characterized by rickets, hyperphosphaturia, hypophosphatemia, elevated 1,25-dihydroxyvitamin-D, increased gastrointestinal calcium absorption and hypercalciuria. Serum calcium, 25-hydroxyvitamin-D and PTH levels are normal. Here we describe a boy with HHRH, nephrolithiasis, and compound heterozygosity for one previously described mutation (g.4225_50del) and a novel splice mutation (g.1226G>A) in SLC34A3, the gene encoding the renal sodium-phosphate co-transporter NaPi-IIc. The patient's mother and grandmother are carriers of g.4225_50del, and both have a history of nephrolithiasis associated with hypercalciuria and elevated 1,25-dihydroxyvitamin-D. His three siblings (2-6 years old), who are also carriers of g.4225_50del, have hypercalciuria but so far their renal ultrasounds are normal. Thus, SLC34A3/NaPi-IIc mutations appear to be associated with variable phenotypic changes at presentation, which can include recurrent nephrolithiasis.

Hereditary disorders of renal phosphate wasting

Inherited diseases of renal phosphate handling lead to urinary phosphate wasting and depletion of total body phosphorus stores. Clinical sequelae of inherited disorders that are associated with increased urinary phosphate excretion are deleterious and can lead to abnormal skeletal growth and deformities. This Review describes hereditary disorders of renal phosphate wasting taking into account developments in our understanding of renal phosphate handling from the last decade. The cloning of genes involved in these disorders and further studies on their pathophysiological mechanisms have given important insights in to how phosphatonins, such as FGF-23, regulate renal phosphate reabsorption in health and disease. X-linked dominant hypophosphatemic rickets results from mutation of a metalloprotease (PHEX) that has an unidentified role in FGF-23 degradation. Mutation of an RXXR proteolytic cleavage site in FGF-23 prevents degradation and increases circulating levels of FGF-23 in autosomal dominant hypophosphatemic rickets. FGF-23 acts to remove sodium phosphate co-transporters from the luminal membrane of proximal tubular cells with resultant renal phosphate wasting. Loss of function mutations in genes encoding the transporters NaPi-IIc and NaPi-IIa also result in renal phosphate wasting and rickets.

An apical expression signal of the renal type IIc Na+-dependent phosphate cotransporter in renal epithelial cells

The type IIc Na(+)-dependent phosphate cotransporter (NaPi-IIc) is specifically targeted to, and expressed on, the apical membrane of renal proximal tubular cells and mediates phosphate transport. In the present study, we investigated the signals that determine apical expression of NaPi-IIc with a focus on the role of the N- and the C-terminal tails of mouse NaPi-IIc in renal epithelial cells [opossum kidney (OK) and Madin-Darby canine kidney cells]. Wild-type NaPi-IIc, the cotransporter NaPi-IIa, as well as several IIa-IIc chimeras and deletion mutants, were fused to enhanced green fluorescent protein (EGFP), and their cellular localization was analyzed in polarized renal epithelial cells by confocal microscopy and by cell-surface biotinylation. Fluorescent EGFP-fused NaPi-IIc transporter proteins are correctly expressed in the apical membrane of OK cells. The apical expression of N-terminal deletion mutants (deletion of N-terminal 25, 50, or 69 amino acids) was not affected by truncation. In contrast, C-terminal deletion mutants (deletion of C-terminal 45, 50, or 62 amino acids) did not have correct apical expression. A more detailed mutational analysis indicated that a domain (amino acids WLHSL) in the cytoplasmic C terminus is required for apical expression of NaPi-IIc in renal epithelial cells. We conclude that targeting of NaPi-IIc to the apical cell surface is regulated by a unique amino acid motif in the cytoplasmic C-terminal domain.

A loss-of-function mutation in NaPi-IIa and renal Fanconi's syndrome

We describe two siblings from a consanguineous family with autosomal recessive Fanconi's syndrome and hypophosphatemic rickets. Genetic analysis revealed a homozygous in-frame duplication of 21 bp in SLC34A1, which encodes the renal sodium-inorganic phosphate cotransporter NaPi-IIa, as the causative mutation. Functional studies in Xenopus laevis oocytes and in opossum kidney cells indicated complete loss of function of the mutant NaPi-IIa, resulting from failure of the transporter to reach the plasma membrane. These findings show that disruption of the human NaPi-IIa profoundly impairs overall renal phosphate reabsorption and proximal-tubule function and provide evidence of the critical role of NaPi-IIa in human renal phosphate handling.

Genetic and clinical peculiarities in a new family with hereditary hypophosphatemic rickets with hypercalciuria: a case report

Hereditary hypophosphatemic rickets with hypercalciuria is a rare autosomal recessive disorder (OMIM #241530), characterized by decreased renal phosphate reabsorption that leads to hypophosphatemia, rickets, and bone pain; hypophosphatemia is believed to stimulate 1,25 dihydroxyvitamin D synthesis which, in turn, results in hypercalciuria. Hereditary hypophosphatemic rickets with hypercalciuria is caused by loss-of-function in the type 2c sodium phosphate cotransporter encoded by the SLC34A3 gene. This report shows a family with a non-previously identified mutation in the SLC34A3 gene and exhibiting mild and different manifestations of HHRH. The probandus had hypophosphatemia, elevated serum 1,25 dihydroxyvitamin D concentrations, high serum alkaline phosphatase levels, hypercalciuria and nephrocalcinosis. The other members of the family presented some of these alterations: the mother, hypercalciuria and high 1,25 dihydroxyvitamin D concentrations; the son, hypercalciuria, high 1,25 dihydroxyvitamin D values and elevated alkaline phosphatases; the father, high alkaline phosphatases. The genetic analysis revealed the existence of a single mutation (G78R) in heterozygosis in the SLC34A3 gene in the probandus, her mother and her brother, but not in the father. These findings suggest that he mutation in heterozygosis likely gave rise to a mild phenotype with different penetrance in the three relatives and also indicates that the elevation of 1,25 dihydroxyvitamin D does not result from hypophosphatemia. Thus, this family raises some issues on the transmission and pathophysiology of hereditary hypophosphatemic rickets with hypercalciuria.

Hypophosphatemic rickets with hypercalciuria due to mutation in SLC34A3/type IIc sodium-phosphate cotransporter: presentation as hypercalciuria and nephrolithiasis

CONTEXT

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a metabolic disorder due to homozygous loss-of-function mutations in the SLC34A3 gene encoding the renal type IIc sodium-phosphate cotransporter (NaPi-IIc). The typical presentation is severe rickets and hypophosphatemia, and hypercalciuria is often discovered later or overlooked.

OBJECTIVE

We sought to determine the genetic basis for severe hypercalciuria and nephrolithiasis/nephrocalcinosis in an adolescent male with elevated serum levels of calcitriol but normal serum levels of calcium and phosphorus.

DESIGN AND SETTING

We used PCR to analyze the SLC34A3 gene in the proband and members of his family.

RESULTS

The proband was a compound heterozygote for two SLC34A3 missense mutations, a novel c.544C-->T in exon 6 that results in replacement of arginine at position 182 by tryptophan (R182W) and c.575C-->T in exon 7 that results in replacement of serine at position 192 by leucine (S192L). The R182W and S192L alleles were inherited from the mother and father, respectively, both of whom had hypercalciuria. A clinically unaffected brother was heterozygous for S192L.

CONCLUSION

We report a novel mutation in the SLC34A3 gene in a patient with an unusual presentation of HHRH. This report emphasizes that moderate and severe hypercalciuria can be manifestations of heterozygous or homozygous loss-of-function mutations in the SLC34A3 gene, respectively, providing further evidence for a gene dosage effect in determining the phenotype. HHRH may be an underdiagnosed condition that can masquerade as idiopathic hypercalciuria or osteopenia.

The roles of Na/Pi-II transporters in phosphate metabolism

The renal type II Na/Pi cotransporters, Na/Pi-IIa and Na/Pi-IIc, are expressed in the brush border membrane (BBM) of the renal proximal tubule cells. Because it has long been thought that Na/Pi-IIa alone can regulate the reabsorption of phosphate in the proximal renal tubules, Na/Pi-IIc has not been paid much attention by the renal research community. Recent studies, however, have identified Na/Pi-IIc mutations as the defective cause of hereditary hypophosphatemic rickets with hypercalciuria (HHRH). This finding indicates that Na/Pi-IIc has a rather important role in renal Pi reabsorption and bone mineralization, and that it may be a key determinant of plasma Pi concentrations in humans. Studies of Na/Pi-IIc mice indicate that Na/Pi-IIc is necessary for normal calcium homeostasis, but its role in the regulation of Pi metabolism and bone physiology may be different from that in HHRH patients. Of note, Na/Pi-IIc KO mice display abnormal vitamin D regulation without hypophosphatemia or hyperphosphaturia. Thus, Na/Pi-IIc may be involved in regulating renal vitamin D synthesis in the proximal tubular cells. The identification of proteins that interact with Na/Pi-IIc is an important area of future research. The physiologic roles of Na/Pi-IIa and Na/Pi-IIc require future elucidation.

Disorders of phosphate homeostasis and tissue mineralisation

Phosphate is absorbed from the diet in the gut, stored as hydroxyapatite in the skeleton, and excreted with the urine. The balance between these compartments determines the circulating phosphate concentration. Fibroblast growth factor 23 (FGF23) has recently been discovered and is part of a previously unrecognised hormonal bone-kidney axis. Phosphate-regulating gene with homologies to endopeptidases on the X chromosome, and dentin matrix protein 1 regulate the expression of FGF23 in osteocytes, which then is O-glycosylated by UDP-N-acetyl-alpha-D-galactosamine: polypeptide N-acetylgalactosaminyl-transferase 3 and secreted into the circulation. FGF23 binds with high affinity to fibroblast growth factor receptor 1c in the presence of its co-receptor Klotho. It inhibits, either directly or indirectly, reabsorption of phosphate and the synthesis of 1,25-dihydroxy-vitamin-D by the renal proximal tubule and the secretion of parathyroid hormone by the parathyroid glands. Acquired or inborn errors affecting this newly discovered hormonal system can lead to abnormal phosphate homeostasis and/or tissue mineralisation. This chapter will provide an update on the current knowledge of the pathophysiology, the clinical presentation, diagnostic evaluation and therapy of the disorders of phosphate homeostasis and tissue mineralisation.