Identification of a novel loss-of-function PHEX mutation, Ala720Ser, in a sporadic case of adult-onset hypophosphatemic osteomalacia

Adults presenting with sporadic hypophosphatemia and elevations in circulating fibroblast growth factor-23 (FGF23) concentrations are usually investigated for an acquired disorder of FGF23 excess such as tumor induced osteomalacia (TIO). However, in some cases the underlying tumor is not detected, and such patients may harbor other causes of FGF23 excess. Indeed, coding-region and 3’UTR mutations of phosphate-regulating neutral endopeptidase (PHEX), which encodes a cell-surface protein that regulates circulating FGF23 concentrations, can lead to alterations in phosphate homeostasis, which are not detected until adulthood. Here, we report an adult female who presented with hypophosphatemic osteomalacia and raised serum FGF23 concentrations. The patient and her parents, who were her only first-degree relatives, had no history of rickets. The patient was thus suspected of having TIO. However, no tumor had been identified following extensive localization studies. Mutational analysis of the PHEX coding-region and 3’UTR was undertaken, and this revealed the patient to be heterozygous for a novel germline PHEX mutation (c.2158G>T; p.Ala720Ser). In vitro studies involving the expression of WT and mutant PHEX proteins in HEK293 cells demonstrated the Ala720Ser mutation to impair trafficking of PHEX, with 80% cell surface expression for WT PHEX (p<0.05). Thus, our studies have identified a pathogenic PHEX mutation in a sporadic case of adult-onset hypophosphatemic osteomalacia, and these findings highlight a role for PHEX gene analysis in some cases of suspected TIO, particularly when no tumor has been identified

A second report of recessive type COL4A1-related disorder: a novel homozygous missense variant.

Not heldPublished version, accepted version (12 month embargo

Missense Mutations of the Pro65 Residue of PCGF2 Cause a Recognizable Syndrome Associated with Craniofacial, Neurological, Cardiovascular, and Skeletal Features.

PCGF2 encodes the polycomb group ring finger 2 protein, a transcriptional repressor involved in cell proliferation, differentiation, and embryogenesis. PCGF2 is a component of the polycomb repressive complex 1 (PRC1), a multiprotein complex which controls gene silencing through histone modification and chromatin remodelling. We report the phenotypic characterization of 13 patients (11 unrelated individuals and a pair of monozygotic twins) with missense mutations in PCGF2. All the mutations affected the same highly conserved proline in PCGF2 and were de novo, excepting maternal mosaicism in one. The patients demonstrated a recognizable facial gestalt, intellectual disability, feeding problems, impaired growth, and a range of brain, cardiovascular, and skeletal abnormalities. Computer structural modeling suggests the substitutions alter an N-terminal loop of PCGF2 critical for histone biding. Mutant PCGF2 may have dominant-negative effects, sequestering PRC1 components into complexes that lack the ability to interact efficiently with histones. These findings demonstrate the important role of PCGF2 in human development and confirm that heterozygous substitutions of the Pro65 residue of PCGF2 cause a recognizable syndrome characterized by distinctive craniofacial, neurological, cardiovascular, and skeletal features