SAT-069 Advantages of Next Generation Sequencing (NGS) in Hypophosphatemic Disorders Diagnosis. First Case of SLC9A3R1 Gene Pathogenic Variant Detected in a Pediatric Patient

Abstract

Background: Hereditary hypophosphatemic rickets (HHR) is a group of inherited disorders characterized by hypophosphatemia due to renal-phosphate wasting and impairment of vitamin D metabolism, rickets and disproportioned short stature. Different genetic defects are known to cause HHR, but similar clinical and biochemical features were reported. Dominant-X-linked HR (XLHR) is the most frequent form, with an incidence of 1 in 20.000, although dominant and recessive autosomal forms are also described. XLHR is caused by inactivating mutations in the PHEX gene (located at Xp22.1), encoding a endopeptidase which regulates the phosphaturic secretion. Affected individuals present with a broad phenotypic spectrum, ranging from isolated hypophosphatemia up to severe symptoms of rickets. Therefore NGS studies represent an useful tool for molecular diagnosis characterization Aim: to develop a reliable NGS diagnostic tool for HHR and related disorders. Patients and Methods: we develop a NGS panel including 13 genes related with HHR or other hypophosphatemic disorders, using Illumina TruSeq Custom Amplicon technology.We analyzed 12 patients which have been sent to our laboratory for molecular genetic testing under suspicion of HHR based on clinical phenotype and laboratory studies but with no proven mutation in PHEX gene by Sanger sequencing or MLPA analysis or other hypophosphatemic disorder. Results: A previously reported pathogenic variant (p.Arg153Gln) was found in SLC9A3R1 gene encoding NHERF1cotransporter, which interact with phosphate and sodium renal transporter NaPi2a in a 13 months old girl. There are only 5 reported cases with alterations in this gene and all of them were adult patients with nephrolithiasis. The patient was referred to our hospital due to hypercalcemia. She had poor weight gain and laboratory findings showed high serum calcium (16,6 mg/dl), mild serum phosphate (3.9 mg/dl), very low parathyroid hormone (PTH) (< 3 ng/ml), normal 25OHvit D (40 ng/ml) levels, and elevated urinary calcium/ creatinine rate (2), and low phosphate tubular reabsorption (85%). Ultrasound showed nephrolithiasis. Since she had hypophosphatemia and renal phosphate wasting with symptomatic severe PTH independent hypercalcemia probably secondary to excessive calcitriol production with hypercalciuria, a molecular alteration of CYP24A1 or SLC34A1genes was suspected. Conclusion: NGS allowed to report for the first time the identification of a mutation in the SLC9A3R1 gene in a pediatric patient. An early diagnosis might improve long term outcome starting the right therapy to avoid progression of nephrolithiasis and nephrocalcinosis and chronic renal failure.