Effect of Mutation Type on Ectopic Ossification Among Adult Patients With X-Linked Hypophosphatemia.

Context: Causative factors for ectopic ossifications in X-linked hypophosphatemia (XLH) remain to be elucidated.

Objective: This work aimed to investigate the genotype-phenotype correlations between the phosphate-regulating endopeptidase homologue, X-linked gene (PHEX) and ectopic ossifications in XLH.

Methods: Biochemical data, spinal computed tomography scans, and x-rays of hip/knee joints were retrospectively reviewed. Genetic analysis and the measurement of plasma inorganic pyrophosphate (PP_i)-a potent inhibitor of tissue calcification-were performed. The effect of PHEX mutations on protein function was predicted using nonsense-mediated decay (NMD) and 3-dimensional structure modeling. The index of ossification of the anterior/posterior longitudinal ligament and yellow ligament (OA/OP/OY index) and the sum of the OA/OP/OY index (OS index) were used to quantify the severity of spinal ligament ossification. The severity of the hip/knee osteoarthritis was evaluated by the Kellgren-Lawrence classification.

Results: We examined 24 distinct pathogenic PHEX variants in 28 patients from a study population of 33 individuals in 27 unrelated, nonconsanguineous families. Among the 31 patients whose plasma samples were analyzed for PP_i, 14 patients (45%) showed decreased plasma PP_i concentrations; however, PP_i concentrations did not correlate with mutation type or ectopic ossification. Fibroblast growth factor 23 levels in women with NMD-insensitive mutations trended lower than in men with NMD-sensitive mutations but failed to reach statistical significance. Both models revealed no correlations between PHEX pathogenic variant and ectopic ossification.

Conclusion: Neither modeling found correlates between PHEX pathogenic variants and ectopic ossification. The effects of PP_i on ectopic ossifications in adults with XLH revealed trends that should be investigated with a large sample size.