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

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is caused by mutations in SLC34A3, the gene encoding the renal sodium-phosphate co-transporter NaPi-IIc. Despite increased urinary calcium excretion, HHRH is typically not associated with kidney stones prior to treatment. However, here we describe two sisters, who displayed nephrolithiasis or nephrocalcinosis upon presentation. The index patient, II-4, presented with short stature, bone pain, and knee X-rays suggestive of mild rickets at age 8.5 years. Laboratory evaluation showed hypophosphatemia, elevated 1,25(OH) (2) vitamin D levels, and hypercalciuria, later also developing vitamin D deficiency. Her sister, II-6, had a low normal serum phosphorous level, biochemically vitamin D deficiency and no evidence for osteomalacia, but had undergone left nephro-ureterectomy at age 17 because of ureteral stricture secondary to renal calculi. Nucleotide sequence analysis of DNA from II-4 and II-6 revealed a homozygous missense mutation c.586G>A (p.G196R) in SLC34A3/NaPi-IIc. Ultrasonographic examinations prior to treatment showed grade I nephrocalcinosis for II-4, while II-6 had grade I-II nephrocalcinosis in her remaining kidney. Four siblings and the mother were heterozygous carriers of the mutation, but showed no biochemical abnormalities. With oral phosphate supplements, hypophosphatemia and hypercalciuria improved in both homozygous individuals. Renal calcifications that are presumably due to increased urinary calcium excretion can be the presenting finding in homozygous carriers of G196R in SLC34A3/NaPi-IIc, and some or all laboratory features of HHRH may be masked by vitamin D deficiency.