ESPE Abstracts

X-linked hypophosphatemia (XLH, MIM#307800) is a rare, dominantly inherited skeletal disorder characterized by excessive production and elevated circulating levels of fibroblast growth factor 23 (FGF23) resulting in renal phosphate wasting, hypophosphatemia, and defective bone mineralization. This leads to rickets, osteomalacia, pain, skeletal deformities, short stature, and reduced physical function. Recently, a monoclonal FGF23 antibody (burosumab) has become available as a novel and effective treatment in patients with PHEX mutations. We report on a five-generation family with a delayed genetic diagnosis of a novel deep intronic PHEX mutation, which could not be detected by exome sequencing analysis. The index patient is a now 3-year-old male who presented with hypophosphatemia, elevated alkaline phosphatase and parathormone levels within the first year of life. Family history revealed that the mother was diagnosed with hypophosphatemic rickets from the age of 6 months. She had disproportionate short stature, skeletal deformities, gait disturbances and recurrent dental abscesses and has been treated with phosphate and active vitamin D. Four other females and two males are affected in this family. Previous attempts to identify the causative genetic variant including multi-panel and exome analyses were unsuccessful. The index patient was commenced on phosphate and active vitamin D at nine months of age, but this never fully corrected the biochemical features, radiographic features of rickets and pronounced genua vara deformity. Finally, we undertook a whole genome sequencing which revealed a novel deep intronic variant in PHEX (NM_000444.6:c.1646-1982T>G) in intron 15 that segregated with the disease. SpliceAI, a deep learning based splice variant identification tool, predicted an increased likelihood of alternative splicing due to this variant with the creation of a new splice acceptor site. Using reverse transcription of the mRNA followed by PCR and sequencing, we show that the new splice site leads to an alternative splicing event with insertion of a 63 base pair pseudo-exon, suspecting the pathogenic character of this intronic variant. This finding allowed us to switch all affected family members, including the index patient, from conventional therapy to burosumab treatment. The index patient was 2 years old when burosumab was initiated. Seven months later, this treatment resulted in the prompt correction of hypophosphatemia, decrease of parathormone levels and improved linear growth. In conclusion, in view of the delayed diagnosis of a rare intronic PHEX mutation we recommend the immediate performance of a whole genome sequencing in families with suspected XLH, with special attention to intronic variants.