A Child with Cutaneous-Skeletal Hypophosphatemia Syndrome Caused by a Mosaic HRAS Mutation: Outcome of Treatment with Anti-FGF23 Antibody.

Cutaneous-skeletal hypophosphatemia syndrome (CSHS) is a rare mosaic disorder that causes bone abnormalities due to hypophosphatemic rickets and skeletal dysplasia and is a significant health comorbidity. Conventional therapy involving multiple daily oral doses of phosphate and calcitriol for CSHS patients has limited effectiveness. We report the clinical features and therapeutic outcomes of the first Chinese child diagnosed with CSHS, who presented with bone fractures, a history of seizures, and recurrent gastrointestinal manifestations, including diarrhoea and bowel obstruction. The effectiveness of conventional therapy and an anti-FGF23 antibody (burosumab) was evaluated. Ultra-deep sequencing was performed on the patient's blood DNA, skin tissue, oral mucosa, and hair follicles to identify causative mutations. The child had a somatic mutation in the HRAS (p.G13R) gene, which was identified at low variant allele frequencies. We analysed the sequencing results from reported cases and determined that the sequencing of lesional tissues, such as skin and bone, is preferable to that of oral mucosa or potentially affected hair follicles for establishing a definitive diagnosis of CSHS. Compared with conventional phosphate therapy, burosumab resulted in a steady increase in blood phosphorus levels and significant improvements in patient mobility, pain outcomes and skeletal radiography. It is suggested that younger children may receive a higher initial dosage of burosumab for better outcomes. However, long-term follow-up is still necessary to confirm its efficacy and safety.