Abstract
<b><i>Introduction:</i></b> Childhood hypophosphatemia is a rare condition and may be caused by malabsorption, malignancies, or genetic factors. Prolonged hypophosphatemia leads to impaired growth and radiographic signs of rickets. <b><i>Methods:</i></b> We performed a detailed clinical and genetic evaluation of an adolescent boy with repeatedly low plasma phosphate concentrations (below 0.60 mmol/L) and growth failure. <b><i>Results:</i></b> At 14 years, the patient presented with decelerating growth and delayed puberty. Biochemistry showed hypophosphatemia due to increased urinary phosphate loss; kidney function and vitamin D status were normal. Radiographs showed mild metaphyseal changes. A gene panel for known genetic hypophosphatemia was negative. Trio exome analysis followed by Sanger sequencing identified a pathogenic heterozygous de novo stop-gain variant in <i>PRPF8</i> gene, c.5548C>T p.(Arg1850*), in the conserved RNase H homology domain. <i>PRPF8</i> encodes the pre-RNA protein 8, which has a role in RNA processing. Heterozygous <i>PRPF8</i> variants have been associated with retinitis pigmentosa and neurodevelopmental disorders but not with phosphate metabolism. The patient underwent growth hormone (GH) stimulation tests which confirmed GH deficiency. Head MRI indicated partially empty sella. GH treatment was started at 15 years. Surprisingly, phosphate metabolism normalized during GH treatment, suggesting that hypophosphatemia was at least partly secondary to GH deficiency. <b><i>Conclusion:</i></b> The evaluation of an adolescent with profound long-term hypophosphatemia revealed a pituitary developmental defect associated with a stop-gain variant in <i>PRPF8.</i> Hypophosphatemia alleviated with GH treatment. The pathological <i>PRPF8</i> variant may contribute to abnormal pituitary development; however, its role in phosphate metabolism remains uncertain.