#1558 Mutational spectrum of X-linked hypophosphatemia (XLH) in Hungarian patients

Abstract Background and Aims X-linked hypophosphatemia (XLH), the most common form of hypophosphatemic rickets, is caused by genetic alterations in the PHEX gene. Prevalence of the X-linked disease is ∼1/20, 000. The PHEX gene has 22 exons and encodes a 749 amino acid protein with homology to zinc metallopeptidases. The physiological protein target for PHEX cleavage is unknown. PHEX promotes mineralization by regulating the expression of FGF23. To date, more than 800 disease-causing variants in the PHEX gene have been described in XLH patients. These variants might occur across the entire gene and include missense, nonsense, frameshift, splice site, copy-number variants with a consequence of loss of PHEX protein function. Genetic confirmation of XLH has significant clinical importance from risk prediction to modern treatment options. Here we describe the genetic results of 20 mutation-positive patients from 18 unrelated families, the first such report from Hungary. Method The cohort consisted of 8 male and 12 female patients (mean age at genetic diagnosis (range): 8. 25 years (3-17 years) and 12 years (2-32 years) ). Genomic DNA was isolated from peripheral blood leukocytes and in the mosaic case also from mouthwash sample. PHEX sequencing was performed using next generation sequencing with a custom designed library preparation kit (Twist Biosciences) on an Illumina MiSeq instrument. For the bioinformatic analysis, NextGene software was used. All small-scale mutations were confirmed by Sanger sequencing. For copy number variation (CNV) analysis, a coverage-based pipeline was used. Detected CNVs were confirmed using multiplex ligation-dependent probe amplification (MLPA). Mutation classification was performed according to the ACMG guidelines. Results All female patients were heterozygous, while all but one male patients were hemizygous. In case of one male patient, a mosaic deletion of exon 10 was observed. Most common mutation type was nonsense (n = 7) followed by small deletions/insertions (n = 6) and splicing alterations (n = 3). One missense mutation was detected. CNVs were responsible in three cases (two large deletions and one large duplication). From the 11 most common PHEX alleles known from the literature (Hum Mutat 2022;43: 143-57), three (c. 2104C T, c. 2239C T, c. 871C T) were detected in our cohort. Nine novel, previously undescribed alterations were found: c. 264G A, c. 1073₁074delGA, c. 1148dupA, c. 1645+1G C, c. 1682G A, c. 1701-1G C, c. 2150T G, c. 2172delT, e6-15dup. Conclusion Our data shed more light on the distribution of the PHEX causative variations in the Hungarian XLH patients and opens the possibility of the state-of-the-art treatment in many of them.