Insight into the function of DIO2, a susceptibility gene in human osteoarthritis, as an inducer of cartilage damage in a rat model: is there a role for chondrocyte hypertrophy?

Transcriptomic analyses and machine-learning methods reveal dysregulated key genes and potential pathogenesis in human osteoarthritic cartilage

Aims

This study aimed to explore the biological and clinical importance of dysregulated key genes in osteoarthritis (OA) patients at the cartilage level to find potential biomarkers and targets for diagnosing and treating OA.

Methods

Six sets of gene expression profiles were obtained from the Gene Expression Omnibus database. Differential expression analysis, weighted gene coexpression network analysis (WGCNA), and multiple machine-learning algorithms were used to screen crucial genes in osteoarthritic cartilage, and genome enrichment and functional annotation analyses were used to decipher the related categories of gene function. Single-sample gene set enrichment analysis was performed to analyze immune cell infiltration. Correlation analysis was used to explore the relationship among the hub genes and immune cells, as well as markers related to articular cartilage degradation and bone mineralization.

Results

A total of 46 genes were obtained from the intersection of significantly upregulated genes in osteoarthritic cartilage and the key module genes screened by WGCNA. Functional annotation analysis revealed that these genes were closely related to pathological responses associated with OA, such as inflammation and immunity. Four key dysregulated genes (cartilage acidic protein 1 (CRTAC1), iodothyronine deiodinase 2 (DIO2), angiopoietin-related protein 2 (ANGPTL2), and MAGE family member D1 (MAGED1)) were identified after using machine-learning algorithms. These genes had high diagnostic value in both the training cohort and external validation cohort (receiver operating characteristic > 0.8). The upregulated expression of these hub genes in osteoarthritic cartilage signified higher levels of immune infiltration as well as the expression of metalloproteinases and mineralization markers, suggesting harmful biological alterations and indicating that these hub genes play an important role in the pathogenesis of OA. A competing endogenous RNA network was constructed to reveal the underlying post-transcriptional regulatory mechanisms.

Conclusion

The current study explores and validates a dysregulated key gene set in osteoarthritic cartilage that is capable of accurately diagnosing OA and characterizing the biological alterations in osteoarthritic cartilage; this may become a promising indicator in clinical decision-making. This study indicates that dysregulated key genes play an important role in the development and progression of OA, and may be potential therapeutic targets.

Association of urinary phthalate metabolites with osteoarthritis in American adults: Results from the national health and nutrition examination survey 2003-2014

Phthalates have extensive existence in the living environment of human, probably tightly associated with multiple human diseases. The present study aimed to exploratorily investigate the association of urinary phthalate metabolites with osteoarthritis (OA) in American adults by exploiting the data extracted from National Health and Nutrition Examination Survey (NHANES) 2003-2014 with levels of eleven urinary phthalate metabolites as exposure. The multivariable logistic regression models were performed after controlling for urinary creatinine, age, gender, race/ethnicity, education level, marital status, smoking, body mass index, physical activity in recreational time, family poverty income ratio, diabetes, hypertension, as well as survey cycle. Compared with those in the lowest quantile, we observed higher prevalence of OA in the maximal quantile of MCOP (OR = 1.55, 95% CI = 1.06-2.27) in adjusted model. A one-unit increase in log-transformed phthalate metabolites was significantly associated with higher OA prevalence, including MCOP (OR = 1.13, 95% CI = 1.02-1.26) and MBzP (OR = 1.12, 95% CI = 1.00-1.26) in adjusted model. In subgroup analysis, the positive associations between phthalate metabolites and OA prevalence remained robust both in males and females. In brief, this study first presented positive evidence for the association of urinary level of phthalate metabolites with OA prevalence in American adults. Additional causal research is required to confirm the finding from our analysis and elucidate the potential underlying mechanisms of phthalates exposure on OA.

Associations of urinary levels of phenols and parabens with osteoarthritis among US adults in NHANES 2005-2014

Phenols and parabens are two major classes of endocrine-disrupting compounds (EDCs) that may be related to multiple human diseases. However, there has been no studies examining the association between phenols as well as parabens and osteoarthritis (OA). We assessed the link between urinary concentrations of triclosan (TCS), benzophenone-3 (BP-3), bisphenol A (BPA), and parabens with OA based on the data collected from National Health and Nutrition Examination Survey in multivariable logistic regression models. Among all the 7114 participants included, the weighted percentage of OA was 12.11% (n = 807). Compared with participants at tertile 1, those at tertile 2 of urinary BP-3, and tertile 3 of urinary BP-3 were more likely to show increased OA prevalence in a fully adjusted model, with odd ratio (OR) as 1.34 [95% confidence interval (CI): 1.01-1.78], 1.55 (95 CI%: 1.17-2.06), and 1.66 (95 CI%: 1.23-2.24), respectively. In subgroup analyses stratified by potential confounders, various subgroups remained to show statistically significant positive association between urinary BP-3 and OA prevalence. Otherwise, we observed no statistically significant associations between urinary TCS, BPA or parabens with OA. In conclusion, this serves as the first study in which we found that the urinary concentration of BP-3 was positively correlated to prevalence of OA among the US population.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Inhibition of mutant IDH1 decreases D-2-HG levels without affecting tumorigenic properties of chondrosarcoma cell lines

Mutations in isocitrate dehydrogenase 1 (IDH1) and IDH2 are found in a subset of benign and malignant cartilage tumors, gliomas and leukaemias. The mutant enzyme causes the production of D-2-hydroxyglutarate (D-2-HG), affecting CpG island and histone methylation. While mutations in IDH1/2 are early events in benign cartilage tumors, we evaluated whether these mutations play a role in malignant chondrosarcomas. Compared to IDH1/2 wildtype cell lines, chondrosarcoma cell lines harboring an endogenous IDH1 (n=3) or IDH2 mutation (n=2) showed up to a 100-fold increase in intracellular and extracellular D-2-HG levels. Specific inhibition of mutant IDH1 using AGI-5198 decreased levels of D-2-HG in a dose dependent manner. After 72 hours of treatment one out of three mutant IDH1 cell lines showed a moderate decrease in viability , while D-2-HG levels decreased >90%. Likewise, prolonged treatment (up to 20 passages) did not affect proliferation and migration. Furthermore, global gene expression, CpG island methylation as well as histone H3K4, -9, and -27 trimethylation levels remained unchanged. Thus, while IDH1/2 mutations cause enchondroma, malignant progression towards central chondrosarcoma renders chondrosarcoma growth independent of these mutations. Thus, monotherapy based on inhibition of mutant IDH1 appears insufficient for treatment of inoperable or metastasized chondrosarcoma patients.

Translating genomics into mechanisms of disease: Osteoarthritis

Osteoarthritis (OA) is the most common age-related arthritic disorder and is characterized by aberrant extracellular matrix (ECM) content and surface disruptions that range from fibrillation, clefting and delamination, leading to articular surface erosion. Worldwide, over 20% of the population is affected with OA and 80% of these patients have limitations in movement, whereas 25% experience inhibition in major daily activities of life. OA is the most common disabling arthritic disease; nevertheless, no disease-modifying treatment is available except for the expensive total joint replacement surgery at end-stage disease. Lack of insight into the underlying pathophysiological mechanisms of OA has considerably contributed to the inability of the scientific community to develop disease-modifying drugs. To overcome this critical barrier, focus should be on translation of identified robust gene deviations towards the underlying biological mechanisms.

Chondrocyte hypertrophy in skeletal development, growth, and disease

Most of our bones form through the process of endochondral ossification, which is tightly regulated by the activity of the cartilage growth plate. Chondrocyte maturation through the various stages of growth plate physiology ultimately results in hypertrophy. Chondrocyte hypertrophy is an essential contributor to longitudinal bone growth, but recent data suggest that these cells also play fundamental roles in signaling to other skeletal cells, thus coordinating endochondral ossification. On the other hand, ectopic hypertrophy of articular chondrocytes has been implicated in the pathogenesis of osteoarthritis. Thus, a better understanding of the processes that control chondrocyte hypertrophy in the growth plate as well as in articular cartilage is required for improved management of both skeletal growth disorders and osteoarthritis. This review summarizes recent findings on the regulation of hypertrophic chondrocyte differentiation, the cellular mechanisms involved in hypertrophy, and the role of chondrocyte hypertrophy in skeletal physiology and pathophysiology.

Cartilage tissue engineering: recent advances and perspectives from gene regulation/therapy

Diseases in articular cartilages affect millions of people. Despite the relatively simple biochemical and cellular composition of articular cartilages, the self-repair ability of cartilage is limited. Successful cartilage tissue engineering requires intricately coordinated interactions between matrerials, cells, biological factors, and phycial/mechanical factors, and still faces a multitude of challenges. This article presents an overview of the cartilage biology, current treatments, recent advances in the materials, biological factors, and cells used in cartilage tissue engineering/regeneration, with strong emphasis on the perspectives of gene regulation (e.g., microRNA) and gene therapy.

Adult mice lacking the type 2 iodothyronine deiodinase have increased subchondral bone but normal articular cartilage

BACKGROUND

Although osteoarthritis (OA) is the commonest joint disorder and has a rising prevalence as the population ages, no drugs are available that prevent or delay the onset and progression of disease. Recent studies identified the DIO2 gene encoding type 2 deiodinase (D2) as a susceptibility locus for OA, and further data suggest deiodinase-regulated local availability of triiodothyronine (T3) in the joint plays an important role in cartilage maintenance and repair. To investigate the hypothesis that reduced tissue T3 availability protects joints from development of OA, the joint phenotypes of adult mice lacking D2 (D2KO) or lacking both D1 and D2 (D1D2KO), the only enzymes that catalyze conversion of the prohormone thyroxine to active T3, were determined.

METHODS

Knee joints were prepared from male 16-week-old adult wild type (WT; n=9), D2KO (n=5), and D1D2KO (n=3) mice. Articular cartilage pathology was scored using the Osteoarthritis Research Society International (OARSI) histopathology scale for murine OA to determine the severity and extent of disease. Digital X-ray microradiography was used to determine the area and mineral content of subchondral bone immediately beneath the articular cartilage surface.

RESULTS

There were no differences in maximum and standardized OA scores, cartilage erosion indices, or articular cartilage cellularity among WT, D2KO, and D1D2KO mice. Subchondral bone area did not differ among genotypes, but mineral content was markedly increased in both D2KO and D1D2KO mice compared to WT.

CONCLUSIONS

Although adult D2KO mice have normal articular cartilage and no other features of spontaneous joint damage, they exhibit increased subchondral bone mineral content.