Clinical and molecular heterogeneity in a large series of patients with hypophosphatemic rickets

Hypophosphatemic rickets (HR) is a rare disease that includes a group of hereditary and sporadic conditions characterized by renal phosphate loss associated with normal to low vitamin D serum concentration. The most common form is the X-linked hypophosphatemic rickets, with an incidence of 1:20,000. Several mutations have recently been identified in the PHEX, FGF23, DMP1 and ENPP1 genes in patients with HR. Moreover, in vitro and in vivo studies suggested an involvement of MEPE for defective mineralization in HR.

The present case series describes the clinical features and the analysis of genes implicated in HR in a cohort of 26 Italian HR patients.

All patients were analyzed for the PHEX and FGF23 genes by direct sequencing. When no mutations were detected, Multiplex Ligation-dependent Probe Amplification (MLPA) analysis was performed. The negative patients were screened for the DMP1, MEPE and ENPP1 genes by direct sequencing.

Twenty-two patients (84%) harbored mutations in the PHEX gene. In particular, we detected 19 different mutations, 15 of which were novel. One patient presented a novel splice variation in the ENPP1 gene while no alterations were identified in the FGF23, DMP1 and MEPE genes. The genetic study of the families showed that 11 patients (55%) had de novo mutations. Clinical presentation and disease severity did not show an evident correlation between the mutation types.

This report represents the first large familial study performed on Italian patients. It confirms that mutations in PHEX are the most frequent cause of HR. Furthermore, the variety of clinical manifestations identified in our HR patients underlines the extreme clinical and genetic heterogeneity of this disease.