ESPE Abstracts

X-linked hypophosphatemia (XLH) is the most common genetic disorder of phosphate homeostasis. It is caused by inactivating mutations in the PHEX gene, which encodes a phosphate regulating endopeptidase predominantly expressed in osteoblasts, osteocytes, and odontoblasts.

In children there is a broad phenotypic spectrum of XLH ranging from isolated hypophosphatemia without clinical signs up to severe symptoms, such as rickets, extreme lower limb deformities, distinct tooth problems, and a disproportionate short stature. In adults suffering from XLH further symptoms occur, such as osteomalacia, arthritis, pseudo fractures, and low final height.

Since XLH is a rare disease, the diagnosis is frequently delayed, which has a significant impact on patient outcomes. In the past, molecular genetic testing included single exon/gene testing by Sanger sequencing analysis, which has been expensive and very time-consuming because of the largeness of the PHEX gene, including twenty-two exons.

A new NGS panel tool was designed, investigating not only PHEX, but also ten other genes, being candidate genes for other phosphate wasting disorders. For validation of the panel we involved fifty-two DNA samples from patients that had been sent to the University of Luebeck, Germany, for molecular genetic testing from December 2007 until today. All patients had the clinical and/or biochemical diagnosis of XLH and all (but three) had a proven molecular genetic change in PHEX revealed by Sanger sequencing in our laboratory. We included the samples of thirty-one females and twenty-one males of unrelated individuals, ranging from two years and ten months until forty-five years and nine months of age at the time of diagnosis.

The DNA samples were sent anonymous and blinded to our partner company "Bioglobe". The new developed NGS tool revealed in all patient samples with known changes in PHEX alterations, including different missense-, nonsense- and frameshift mutations, one splice site mutation, and in addition a duplication and a deletion. The molecular changes were spread over the whole gene. By using the new NGS tool, we confirmed in all cases the formerly made molecular diagnosis. The coverage of the PHEX gene was about 100%.

This new panel will be not only a valuable tool for a reliable and faster diagnosis of XLH, but also for other disorders of renal phosphate wasting having a similar phenotype, but a different molecular genetic change in one of the other candidate genes.