Involvement of reduced ENPP1 function in the pathogenesis of ossification of the posterior longitudinal ligament

Ossification of the posterior longitudinal ligament (OPLL) is an ectopic ossification disorder with incompletely understood pathophysiology. In a previously reported 50-patient OPLL cohort, ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) exonic heterozygous variants were identified in 7 patients (14%), and plasma inorganic pyrophosphate (PPi), a biochemical marker reflecting ENPP1 function, was lower in patients with OPLL than in cervical osteoarthritis controls regardless of ENPP1 variant status. However, the clinical significance of ENPP1 variants and reduced PPi in OPLL, as well as the possible presence of non-genetic ENPP1 dysfunction, remains unclear. The study included the same cohort of 50 patients with OPLL as in our previous study, 7 of whom had ENPP1 variants. Clinical, biochemical, and radiographic characteristics were compared according to ENPP1 variant status and cohort median PPi level. Anti-ENPP1 autoantibodies were evaluated in an exploratory analysis using a luciferase immunoprecipitation system (LIPS) assay in 29 patients with available serum samples. Patients with ENPP1 variants showed higher serum FGF23 levels than those without variants (median 46 vs 36 pg/mL, p = .011) and tended to have lower plasma PPi levels (1477 vs 1693 nM, p = .052), suggesting partial impairment of ENPP1 function. However, radiographic ossification indices, diffuse idiopathic skeletal hyperostosis (DISH), and surgical characteristics did not differ by variant status. In contrast, DISH was more frequent in the lower-PPi group than in the higher-PPi group (72% vs 44%, p = .045). No apparent anti-ENPP1 autoantibodies were detected using the LIPS assay. Ectonucleotide pyrophosphatase/phosphodiesterase 1 exonic heterozygous variants may contribute to OPLL through partial ENPP1 dysfunction. More broadly, reduced ENPP1 function itself, regardless of variant status, may be involved in ectopic ligament ossification in OPLL, although anti-ENPP1 autoantibodies were not detected in the current study. The broader involvement of ENPP1 dysfunction in OPLL may indicate a wider population that could potentially benefit from future ENPP1 replacement therapy.