Mutation in the CCAL1 locus accounts for bidirectional process of human subchondral bone turnover and cartilage mineralization

Objectives: To study the mechanism by which the readthrough mutation in TNFRSF11B encoding OPG-XL, previously shown to affect binding between OPG and heparan sulphate (HS) causes the characteristic bidirectional phenotype of subchondral bone turnover accompanied by cartilage mineralization in CCAL1 chondrocalcinosis patients.

Methods: OPG-XL was studied by human induced pluripotent stem cells expressing OPG-XL and two isogenic CRISPR/Cas9-corrected controls in cartilage and bone organoids. Osteoclastogenesis was studied with monocytes from OPG-XL carriers and matched healthy controls followed by gene expression characterization. DEXA scans, and MRI analyses were used to characterize the phenotype of carriers and non-carriers of the mutation.

Results: Human OPG-XL carriers relative to sex- and age-matched controls showed, after an initial delay, large active osteoclasts with high number of nuclei. By employing hiPSCs expressing OPG-XL and isogenic CRISPR/Cas9-corrected controls to established cartilage and bone organoids, we demonstrated that expression of OPG-XL resulted in excessive fibrosis in cartilage and high mineralization in bone accompanied by marked downregulation of MGP and upregulation of DIO2 gene expression, respectively.

Conclusions: The readthrough mutation at CCAL1 locus in TNFRSF11B identifies a unknown role for OPG-XL in subchondral bone turnover and cartilage mineralization in humans via DIO2 and MGP functions. Effects may be triggered by interference with RANKL-HS-OPG causing deficiency in immobilized OPG. Since the characteristic bidirectional pathophysiology of articular cartilage calcification accompanied by low subchondral bone mineralization is also a hallmark of OA pathophysiology, our results are likely extrapolated to common arthropathies.