Certain mutations, however, affected trafficking of the PHEX protein inside cells, supporting the role of protein location on disease development and its potential as a therapeutic target, researchers said.
The study, “Functional Characterization of PHEX Gene Variants in Children with X- linked Hypophosphatemic Rickets Shows no Evidence of Genotype- Phenotype Correlation,” was published in the Journal of Bone and Mineral Research.
XLH is the most common type of hypophosphatemic rickets (HLXR). It is characterized by defective absorption of phosphate by the kidneys and low to normal levels of vitamin D3 in the blood. Insufficient phosphate leads to growth stagnation, as well as rickets and osteomalacia (soft and weak bones).
Although PHEX mutations have been identified as the main cause of XLH, scientists are still trying to understand how different genetic alterations affect disease development and severity.
More than 450 genetic alterations have been described in the PHEX gene, but less than 10 have been studied for their relation to disease symptoms.
Researchers at Nanjing Medical University, in China, investigated how a particular type of mutations — called truncating variants — affect XLHR development.
Notably, truncating mutations cause an early stop on protein production, leading to a shorter protein often with altered function.
The study included 53 children with XLH, 24 of whom were hereditary and 29 were sporadic (not inherited) cases. Median age at diagnosis was 28 months.
A total of 35 children were treated with oral phosphate and active vitamin D (calcitrol). Six of 16 patients who received continuous treatment for more than two years presented significantly increased height, suggesting that early therapy initiation promoted growth, the investigators said.
Analysis of the PHEX gene revealed 47 genetic variants, 27 of which had not been described previously. Most of the identified genetic variants (39, nearly 72%) were predicted to be truncating mutations.
No obvious association was found at diagnosis between type of mutation and bone involvement, height, phosphate levels in the blood, kidney function (as measured by renal reabsorption of phosphate) and liver function — as assessed by levels of the alkaline phosphatase enzyme.
Next, researchers evaluated how different mutations affected the PHEX protein by checking the protein’s levels and location in cells.
Results showed that proteins coded from six non-truncating mutations (p.Cys77Tyr, p.Cys85Ser, p.Ile281Lys, p.Ile333del, p.Ala514Pro, and p.Gly572Ser) were not secreted to the extracelullar space, and instead remained inside the cell in an immature form. In turn, three truncating mutations (p.Arg567*, p.Gln714* and p.Arg747*) led to a shorter protein with altered trafficking in cells.
As found in normal PHEX protein, the protein generated from a mutation known as p.Gly553Glu was found at the cell surface and was released by the cell. However, its activity was only 13% of normal, indicating impaired function.
Glycosylation, a process that adds sugar molecules to proteins and helps them function properly, was impaired in some mutants. Such alteration prolonged the immature state of PHEX proteins and their retention inside the cell.
“This is the largest genetic report of a case series [in] pediatric patients with XLHR; concurrently, it is the first study of a comprehensive functional characterization of PHEX gene variants identified in XLHR patients,” the researchers wrote.
As their study showed no clear differences between truncating and no-truncating variants, the investigators added that “there is no correlation between disease severity and the type of PHEX mutation.”
Yet, “the impaired trafficking portrait of PHEX variants suggests a future therapeutic strategy (…) that enhances the cell surface localization of some proteins,” they wrote.
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