Effects of the TNFRSF11B Mutation Associated With Calcium Pyrophosphate Deposition Disease in Osteoclastogenesis in a Murine Model

Calcium Pyrophosphate Crystal Formation and Deposition: Where Do we Stand and What Does the Future hold?

PURPOSE OF THE REVIEW

Although calcium pyrophosphate deposition (CPPD) has been known since the 1960s, our understanding of its pathogenesis remains rudimentary. This review aims to illustrate the known mechanisms underlying calcium pyrophosphate (CPP) crystal formation and deposition and explore future directions in research. By examining various perspectives, from basic research to clinical and imaging assessments, as well as new emerging methodologies, we can establish a starting point for a deeper understanding of CPPD pathogenesis.

RECENT FINDINGS

Recent years have seen significant advances in CPPD research, particularly in the clinical field with the development of the 2023 ACR/EULAR classification criteria for CPPD disease, and in imaging with the introduction of the OMERACT ultrasonographic definitions and scoring system. However, progress in basic research has been slower. New laboratory approaches, such as Raman spectroscopy and omics sciences, offer promising insights that may help piece together the puzzle of CPPD. CPPD is a common yet understudied condition. As the population ages and CPPD becomes more prevalent, there is an urgent need to better understand the disease and the mechanisms involved in crystal formation and deposition, in order to improve diagnosis and therapeutic approaches.

Calcium pyrophosphate deposition disease

Calcium pyrophosphate deposition (CPPD) disease is a consequence of the immune response to the pathological presence of calcium pyrophosphate (CPP) crystals inside joints, which causes acute or chronic inflammatory arthritis. CPPD is strongly associated with cartilage degradation and osteoarthritis, although the direction of causality is unclear. This clinical presentation is called CPPD with osteoarthritis. Although direct evidence is scarce, CPPD disease might be the most common cause of inflammatory arthritis in older people (aged >60 years). CPPD is caused by elevated extracellular-pyrophosphate concentrations in the cartilage and causes inflammation by activation of the NLRP3 inflammasome. Common risk factors for CPPD disease include ageing and previous joint injury. It is uncommonly associated with metabolic conditions (eg, hyperparathyroidism, haemochromatosis, hypomagnesaemia, and hypophosphatasia) and genetic variants (eg, in the ANKH and osteoprotegerin genes). Apart from the detection of CPP crystals in synovial fluid, imaging evidence of CPPD in joints by mainly conventional radiography, and increasingly ultrasonography, has a central role in the diagnosis of CPPD disease. CT is useful in showing calcification in axial joints such as in patients with crowned dens syndrome. To date, no treatment is effective in dissolving CPP crystals, which explains why control of inflammation is currently the main focus of therapeutic strategies. Prednisone might provide the best benefit-risk ratio for the treatment of acute CPP-crystal arthritis, but low-dose colchicine is also effective with a risk of mild diarrhoea. Limited evidence suggests that colchicine, low-dose weekly methotrexate, and hydroxychloroquine might be effective in the prophylaxis of recurrent flares and in the management of persistent CPP-crystal inflammatory arthritis. Additionally, biologics inhibiting IL-1 and IL-6 might have a role in the management of refractory disease.

Fractures in Patients With Acute Calcium Pyrophosphate Crystal Arthritis Versus Matched Comparators in a Large Cohort Study

OBJECTIVE

Calcium pyrophosphate deposition (CPPD) disease was associated with osteopenia in two cross-sectional studies. We compared fracture risks in patients with acute calcium pyrophosphate (CPP) crystal arthritis versus matched comparators.

METHODS

We performed a longitudinal cohort study using electronic health record data from a single large academic health system, with data from 1991 to 2023. Patients with one or more episodes of acute CPP crystal arthritis were matched to comparators on the index date (first documentation of "pseudogout" or synovial fluid CPP crystals or matched encounter) and first encounter in the health system. The primary outcome was first fracture at the humerus, wrist, hip, or pelvis. We excluded patients with fracture before the index date. Covariates included demographics, body mass index, smoking, comorbidities, health care use, glucocorticoids, and osteoporosis treatments. We estimated incidence rates and adjusted hazard ratios for fracture. Sensitivity analyses excluded patients prescribed glucocorticoids, patients prescribed osteoporosis treatments, or patients with rheumatoid arthritis and additionally adjusted for chronic kidney disease.

RESULTS

We identified 1,148 patients with acute CPP crystal arthritis matched to 3,730 comparators, with a mean age of 73 years. Glucocorticoids and osteoporosis treatments were more frequent in the acute CPP crystal arthritis cohort. Fracture incidence rates were twice as high in the acute CPP crystal arthritis cohort (11.7 per 1,000 person-years) versus comparators (5.5 per 1,000 person-years). After multivariable adjustment, fracture relative risk was twice as high in the acute CPP crystal arthritis cohort (hazard ratio 1.8 [95% confidence interval 1.3-2.3]); results were similar in sensitivity analyses.

CONCLUSION

In this first published study of fractures and CPPD, fracture risk was nearly doubled in patients with acute CPP crystal arthritis.

Biochemical characterization of a disease-causing human osteoprotegerin variant

Recently, a human mutation of OPG was identified to be associated with familial forms of osteoarthritis. This missense mutation (c.1205A =  > T; p.Stop402Leu) occurs on the stop codon of OPG, which results in a 19-residue appendage to the C-terminus (OPG+19). The biochemical consequence of this unusual sequence alteration remains unknown. Here we expressed OPG+19 in 293 cells and the mutant OPG was purified to homogeneity by heparin affinity chromatography and size exclusion chromatography. We found that in sharp contrast to wildtype OPG, which mainly exists in dimeric form, OPG+19 had a strong tendency to form higher-order oligomers. To our surprise, the hyper-oligomerization of OPG+19 had no impact on how it binds cell surface heparan sulfate, how it inhibits RANKL-induced osteoclastogenesis and TRAIL-induced chondrocytes apoptosis. Our data suggest that in biological contexts where OPG is known to play a role, OPG+19 functions equivalently as wildtype OPG. The disease-causing mechanism of OPG+19 likely involves an unknown function of OPG in cartilage homeostasis and mineralization. By demonstrating the biochemical nature of this disease-causing OPG mutant, our study will likely help elucidating the biological roles of OPG in cartilage biology.

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 osteoprotegerin (OPG) with additional 19 amino acids at its C-terminus (OPG-XL), causes the characteristic bidirectional phenotype of subchondral bone turnover accompanied by cartilage mineralization in 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. Dual energy X-ray absorptiometry 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, encoding matrix Gla protein, and upregulation of DIO2, encoding type 2 deiodinase, gene expression, respectively.

CONCLUSIONS

The readthrough mutation at CCAL1 locus in TNFRSF11B identifies an unknown role for OPG-XL in subchondral bone turnover and cartilage mineralization in humans via DIO2 and MGP functions. Previously, OPG-XL was shown to affect binding between RANKL and heparan sulphate (HS) resulting in loss of immobilized OPG-XL. Therefore, effects may be triggered by deficiency in the immobilization of OPG-XL 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.

Identification of Common Pathogenic Pathways Involved in Hemochromatosis Arthritis and Calcium Pyrophosphate Deposition Disease: a Review

OBJECTIVES

Arthritis is a common clinical manifestation of hereditary hemochromatosis (HH), and HH is one of a handful of conditions linked to calcium pyrophosphate deposition (CPPD) in joints. The connection between these two types of arthritis has not yet been fully elucidated. In light of new pathogenic pathways recently implicated in CPPD involving bone, we reviewed the literature on the etiology of hemochromatosis arthropathy (HHA) seeking shared pathogenic mechanisms.

RESULTS

Clinical observations reinforce striking similarities between HHA and CPPD even in the absence of CPP crystals. They share a similar joint distribution, low grade synovial inflammation, and generalized bone loss. Excess iron damages chondrocytes and bone cells in vitro. While direct effects of iron on cartilage are not consistently seen in animal models of HH, there is decreased osteoblast alkaline phosphatase activity, and increased osteoclastogenesis. These abnormalities are also seen in CPPD. Joint repair processes may also be impaired in both CPPD and HHA.

CONCLUSIONS

Possible shared pathogenic pathways relate more to bone and abnormal damage/repair mechanisms than direct damage to articular cartilage. While additional work is necessary to fully understand the pathogenesis of arthritis in HH and to firmly establish causal links with CPPD, this review provides some plausible hypotheses explaining the overlap of these two forms of arthritis.

Pathogenesis of calcium pyrophosphate deposition disease

Calcium pyrophosphate deposition disease is defined by the presence of calcium pyrophosphate (CPP) crystals in articular cartilage and is the fourth most common type of arthritis in adults. Despite its high prevalence, the etiology of CPPD disease remains unclear and no specific therapies currently exist. It has been known for several decades that abnormalities of cartilage pyrophosphate metabolism are common in patients with CPPD disease, and this classic work will be reviewed here. Recent studies of rare familial forms of CPPD disease have provided additional novel information about its pathophysiology. This work suggests that CPPD disease occurs through at least two unique and potentially intertwined biomolecular pathways. We are hopeful that a detailed understanding of the components and regulation of these pathways will lead to improved therapies for this common disease.