ClinVar Genomic variation as it relates to human health

The identified PHEX mutation is the likely genetic cause for the hypophosphatemic rickets observed in the patient.

The G579R missense variant in the PHEX gene has been reported previously as de novo and in association with X-linked hypophosphatemic rickets (Rowe et al., 1997; Durmaz et al., 2013; Radlovic et al., 2014). The variant is not observed in large population cohorts (Lek et al., 2016; 1000 Genomes Consortium et al., 2015; Exome Variant Server). G579R is a non-conservative amino acid substitution, which is likely to impact secondary protein structure as these residues differ in polarity, charge, size and/or other properties. This substitution occurs at a position that is conserved across species and in silico analysis predicts this variant is probably damaging to the protein structure/function. In addition, another nucelotide change at the same codon (c.1735 G>C) also leading to the G579R missense change, a missense variant in at the same residue (G579V), and missense variants in nearby residues (H580P, F582S, H584P) have all been reported in the Human Gene Mutation Database in association with PHEX-related disorders (Stenson et al., 2014), supporting the functional importance of this region of the protein. Furthermore, functional studies of the G579R variant have shown that it results in incomplete glycosylation of the protein leading to failed expression at the plasma membrane and degradation within the endoplasmic reticulum (Sabbagh et al., 2001; Sabbagh et al., 2003).

This variant is interpreted as a Pathogenic, for Hypophosphatemic rickets, X-linked dominant, in X-linked Dominant manner. The following ACMG Tag(s) were applied: PP3 => Multiple lines of computational evidence support a deleterious effect on the gene or gene product. PM2 => Absent from controls (or at extremely low frequency if recessive) in Exome Sequencing Project, 1000 Genomes Project, or Exome Aggregation Consortium. PS4-Moderate => Recurrent mutation found in unrelated patients. (PMID:9199930). PS3 => Well-established functional studies show a deleterious effect (PMID:11468271,12727977). PM6 => Assumed de novo, but without confirmation of paternity and maternity (PMID:24684036).

Same nucleotide change resulting in same amino acid change has been previously reported as pathogenic/likely pathogenic with strong evidence (ClinVar ID: VCV000226119,VCV000438542, PS1_S). The variant is located in a well-established functional domain or exonic hotspot, where pathogenic variants have frequently reported (PM1_M).Different missense changes at the same codon (p.Gly579Val, p.Gly579Glu) have been reported as pathogenic/likely pathogenic with strong evidence (ClinVar ID: VCV000372777,VCV000378359, PMID:9097956, PM5_M). In silico tool predictions suggest damaging effect of the variant on gene or gene product (REVEL: 0.931, 3CNET: 0.997, PP3_P). It is not observed in the gnomAD v2.1.1 dataset (PM2_M). Therefore, this variant is classified as pathogenic according to the recommendation of ACMG/AMP guideline.

This sequence change replaces glycine, which is neutral and non-polar, with arginine, which is basic and polar, at codon 579 of the PHEX protein (p.Gly579Arg). This variant is not present in population databases (gnomAD no frequency). This missense change has been observed in individuals with hypophosphatemic rickets (PMID: 9097956, 9199930, 18625346, 29858904). ClinVar contains an entry for this variant (Variation ID: 226119). Advanced modeling of protein sequence and biophysical properties (such as structural, functional, and spatial information, amino acid conservation, physicochemical variation, residue mobility, and thermodynamic stability) performed at Invitae indicates that this missense variant is expected to disrupt PHEX protein function. Experimental studies have shown that this missense change affects PHEX function (PMID: 11468271). Algorithms developed to predict the effect of sequence changes on RNA splicing suggest that this variant may disrupt the consensus splice site. For these reasons, this variant has been classified as Pathogenic.