Description of a novel SLC34A3.c.671delT mutation causing hereditary hypophosphatemic rickets with hypercalciuria in two adolescent boys and response to recombinant human growth hormone

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.

Diagnosis, treatment, and management of rickets: a position statement from the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology

Rickets results from impaired mineralization of growing bone due to alterations in calcium and phosphate homeostasis. Clinical signs of rickets are related to the age of the patient, the duration of the disease, and the underlying disorder. The most common signs of rickets are swelling of the wrists, knees or ankles, bowing of the legs (knock-knees, outward bowing, or both) and inability to walk. However, clinical features alone cannot differentiate between the various forms of rickets. Rickets includes a heterogeneous group of acquired and inherited diseases. Nutritional rickets is due to a deficiency of vitamin D, dietary calcium or phosphate. Mutations in genes responsible for vitamin D metabolism or function, the production or breakdown of fibroblast growth factor 23, renal phosphate regulation, or bone mineralization can lead to the hereditary form of rickets. This position paper reviews the relevant literature and presents the expertise of the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology (SIEDP). The aim of this document is to provide practical guidance to specialists and healthcare professionals on the main criteria for diagnosis, treatment, and management of patients with rickets. The various forms of rickets are discussed, and detailed references for the discussion of each form are provided. Algorithms to guide the diagnostic approach and recommendations to manage patients with rare forms of hereditary rickets are proposed.

Hereditary Hypophosphatemic Rickets with Hypercalciuria Presenting with Enthesopathy, Renal Cysts, and High Serum c-Terminal FGF23: Single-Center Experience and Systematic Review

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare disorder of phosphate homeostasis. We describe a single-center experience of genetically proven HHRH families and perform systematic review phenotype-genotype correlation in reported biallelic probands and their monoallelic relatives. Detailed clinical, biochemical, radiological, and genetic data were retrieved from our center and a systematic review of Pub-Med and Embase databases for patients and relatives who were genetically proven. Total of nine subjects (probands:5) carrying biallelic SLC34A3 mutations (novel:2) from our center had a spectrum from rickets/osteomalacia to normal BMD, with hypophosphatemia and hypercalciuria in all. We describe the first case of genetically proven HHRH with enthesopathy. Elevated FGF23 in another patient with hypophosphatemia, iron deficiency anemia, and noncirrhotic periportal fibrosis led to initial misdiagnosis as tumoral osteomalacia. On systematic review of 58 probands (with biallelic SLC34A3 mutations; 35 males), early-onset HHRH and renal calcification were present in ~ 70% and late-onset HHRH in 10%. c.575C > T p.(Ser192Leu) variant occurred in 53% of probands without skeletal involvement. Among 110 relatives harboring monoallelic SLC34A3 mutation at median age 38 years, renal calcification, hypophosphatemia, high 1,25(OH)2D, and hypercalciuria were observed in ~30%, 22.3%, 40%, and 38.8%, respectively. Renal calcifications correlated with age but were similar across truncating and non-truncating variants. Although most relatives were asymptomatic for bone involvement, 6/12(50%) had low bone mineral density. We describe the first monocentric HHRH case series from India with varied phenotypes. In a systematic review, frequent renal calcifications and low BMD in relatives with monoallelic variants (HHRH trait) merit identification.

Growth hormone therapy in HHRH

Background

Hereditary Hypophosphatemic Rickets with Hypercalciuria (HHRH) (SLC34A3 gene, OMIM 241530) is an autosomal recessive disorder that results in a loss of function of the sodium-phosphate NPT2c channel at the proximal tubule. Phosphate supplementation rarely improves serum phosphate, hypercalciuria, nephrocalcinosis, 1,25(OH)2 vitamin D (1,25(OH)2D) levels or short stature.

Methods

We describe 23Na MRI and the successful use of recombinant human growth hormone (rhGH) and Fluconazole to improve growth (possibly confounded by puberty) and hypercalciuria in a now 12-year-old male with HHRH (novel homozygous SLC34A3 mutation, c.835_846 + 10del.T).

Results

The patient had chronic bone pain, hypophosphatemia (0.65 mmol/L[reference interval 1.1-1.9]), pathological fractures and medullary nephrocalcinosis/hypercalciuria (urinary calcium/creatinine ratio 1.66 mol/mmol[<0.6]). TmP/GFR was 0.65 mmol/L[0.97-1.64]; 1,25(OH)2D was >480 pmol/L[60-208]. Rickets Severity Score was 4. Treatment with 65 mg/kg/day of sodium phosphate and potassium citrate 10 mmol TID failed to correct the abnormalities.Adding rhGH at 0.35 mg/kg/week to the phosphate therapy, improved bone pain, height z-score from -2.09 to -1.42 over 6 months, without a sustained effect on TmP/GFR. Fluconazole was titrated to 100 mg once daily, resulting for the first time in a reduction of the 1,25(OH)2D to 462 and 426 pmol/L; serum phosphate 0.87 mmol/L, and calcium/creatinine ratio of 0.73.23Na MRI showed normal skin (z-score + 0.68) and triceps surae muscle (z-score + 1.5) Na+ levels; despite a defect in a sodium transporter, hence providing a rationale for a low sodium diet to improve hypercalciuria.

Conclusions

The addition of rhGH, Fluconazole and salt restriction to phosphate/potassium supplementation improved the conventional therapy. Larger studies are needed to confirm our findings.

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.