Genome-wide association and functional studies identify the DOT1L gene to be involved in cartilage thickness and hip osteoarthritis.

Hip osteoarthritis (HOA) is one of the most disabling and common joint disorders with a large genetic component that is, however, still ill-defined. To date, genome-wide association studies (GWAS) in osteoarthritis (OA) and specifically in HOA have yielded only few loci, which is partly explained by heterogeneity in the OA definition. Therefore, we here focused on radiographically measured joint-space width (JSW), a proxy for cartilage thickness and an important underlying intermediate trait for HOA. In a GWAS of 6,523 individuals on hip-JSW, we identified the G allele of rs12982744 on chromosome 19p13.3 to be associated with a 5% larger JSW (P = 4.8 × 10(-10)). The association was replicated in 4,442 individuals from three United Kingdom cohorts with an overall meta-analysis P value of 1.1 × 10(-11). The SNP was also strongly associated with a 12% reduced risk for HOA (P = 1 × 10(-4)). The SNP is located in the DOT1L gene, which is an evolutionarily conserved histone methyltransferase, recently identified as a potentially dedicated enzyme for Wnt target-gene activation in leukemia. Immunohistochemical staining of the DOT1L protein in mouse limbs supports a role for DOT1L in chondrogenic differentiation and adult articular cartilage. DOT1L is also expressed in OA articular chondrocytes. Silencing of Dot1l inhibited chondrogenesis in vitro. Dot1l knockdown reduces proteoglycan and collagen content, and mineralization during chondrogenesis. In the ATDC5 chondrogenesis model system, DOT1L interacts with TCF and Wnt signaling. These data are a further step to better understand the role of Wnt-signaling during chondrogenesis and cartilage homeostasis. DOT1L may represent a therapeutic target for OA.

Blocking p38 signalling inhibits chondrogenesis in vitro but not ankylosis in a model of ankylosing spondylitis in vivo.

OBJECTIVES: To investigate p38 mitogen activated protein kinase (MAPK) signalling in an in vitro model of bone morphogenetic protein (BMP) and transforming growth factor ß (TGFß)-induced chondrogenesis and in vivo, with specific attention to its potential role in ankylosing enthesitis. METHODS: Human periosteum-derived cells (hPDCs) were cultured in pellets and stimulated with BMP2 or TGFß1 in the presence or absence of a p38 inhibitor SB203580 or proinflammatory cytokines. Chondrogenic differentiation was evaluated using quantitative PCR. Male DBA/1 mice from different litters were caged together at the age of 8 weeks and treated with SB203580 in both a preventive and therapeutic strategy. The mice were evaluated for prospective signs of arthritis and the toe joints were analysed histologically to assess disease severity. RESULTS: p38 inhibition by SB203580 and proinflammatory cytokines downregulated chondrogenic markers in pellet cultures stimulated by BMP2 or TGFß1. In contrast, the in vivo experiments resulted in an increased clinical incidence of arthritis and pathology severity score, reflecting progression towards ankylosis in mice given SB203580. CONCLUSION: Inhibition of p38 inhibited chondrogenic differentiation of progenitor cells, showing that not only the SMAD signalling pathways and also alternative activation of MAPKs including p38 contribute to chondrogenesis. Such an inhibitory effect is not found in an in vivo model of joint ankylosis and spondyloarthritis. Increased incidence and severity of disease in preventive experiments and shifts in disease stages in a therapeutic experimental set-up suggest that specific inhibition of p38 may have deleterious rather than beneficial effects.

GDF5 deficiency in mice is associated with instability-driven joint damage, gait and subchondral bone changes.

OBJECTIVES: A functional polymorphism leading to reduced levels of growth and differentiation factor 5 (GDF5) was recently identified as a susceptibility factor for osteoarthritis. The authors studied the potential mechanisms of GDF5 involvement in osteoarthritis using haploinsufficient Gdf5(Bp-J/+) mice. METHODS: Gdf5(Bp-J/+) mice were challenged in the collagenase-induced arthritis model, the medial meniscus destabilisation model, the papain-induced arthritis model and a treadmill running model. Bone density and subchondral bone parameters were determined using dual energy x-ray absorptiometry and peripheral quantitative CT. Additional in-vitro and ex-vivo analyses studied cartilage metabolism, gait and collagen characteristics. RESULTS: Gdf5(Bp-J/+) mice appeared phenotypically normal. No difference in osteoarthritis severity was found in the different models, with the exception of increased synovial hyperplasia in the joints of Gdf5(Bp-J/+) mice in the treadmill model. However, in the collagenase-induced model severe joint damage was found in the contralateral joints of Gdf5(Bp-J/+) mice. Gait analysis demonstrated an aberrant walking pattern in Gdf5(Bp-J/+) mice. In addition, Gdf5(Bp-J/+) mice have a decreased subchondral bone density and a distorted arrangement of collagen fibres in bone. CONCLUSIONS: These data suggest that decreased GDF5 levels in mice can contribute to osteoarthritis development by different mechanisms including altered loading and subchondral bone changes. This highlights the importance of the joint as an organ with different tissues involved in joint disease.

WNT Signaling in osteoarthritis and osteoporosis: what is the biological significance for the clinician?

Osteoporosis and osteoarthritis are common musculoskeletal disorders in which cause and outcome are determined by genetic and environmental factors. The WNT signaling pathway plays an important role in skeletal development and growth. Polymorphisms in a number of genes that belong to this pathway are associated with osteoarthritis and/or osteoporosis. This suggests a role for this molecular signaling pathway in postnatal joint and bone homeostasis and pathology. Increased activity of WNT signaling strengthens the bone but may have adverse effects on the articular cartilage. Frizzled related protein (FRZB) plays a role in both bone and cartilage. Better understanding of the WNT pathway and its modulators may lead to specific therapeutics for both osteoarthritis and osteoporosis. This review focuses on recent studies in human genetics and animal models and highlights the potential clinical relevance of this rapidly evolving field of research.

Modulation of bone morphogenetic protein signaling inhibits the onset and progression of ankylosing enthesitis.

Joint ankylosis is a major cause of disability in the human spondyloarthropathies. Here we report that this process partially recapitulates embryonic endochondral bone formation in a spontaneous model of arthritis in DBA/1 mice. Bone morphogenetic protein (BMP) signaling appears to be a key molecular pathway involved in this pathological cascade. Systemic gene transfer of noggin, a BMP antagonist, is effective both as a preventive and a therapeutic strategy in the mouse model, mechanistically interfering with enthesial progenitor cell proliferation in early stages of the disease process. Immunohistochemical staining for phosphorylated smad1/5 in enthesial biopsies of patients with spondyloarthropathy reveals active BMP signaling in similar target cells. Our data suggest that BMP signaling is an attractive therapeutic target for interfering with structural changes in spondyloarthropathy either as an alternative or complementary approach to current antiinflammatory treatments.

Joint homeostasis, restoration, and remodeling in osteoarthritis.

Osteoarthritis is the major cause of joint failure. The outcome of the disease process is determined by complex interactions between cells and molecules steering homeostasis, destruction, restoration, and remodeling. The articular cartilage has a limited restoration and repair capacity. Genetic studies in humans and the development of mouse models have identified the role of signaling pathways that are important for skeletal development in the postnatal biology and pathology of articular cartilage. These include bone morphogenetic protein, transforming growth factor beta, fibroblast growth factor, wingless-type signaling, and their respective antagonists such as noggin and frizzled related protein. The synovium is prone to inflammation and emerging evidence suggests that innate and adaptive immune responses are important. Bone and cartilage form a biomechanical unit; stiffer bones might impair cartilage homeostasis. The biology of frizzled related protein provides a basis for the hypothesized inverse relationship between osteoarthritis and osteoporosis.

Remission in psoriatic arthritis.

Psoriatic arthritis (PsA) is a chronic inflammatory joint disease associated with psoriasis that can lead to joint deformity and disability. PsA has a low likelihood of clinical remission and cure. For a long time, therapeutic options for PsA patients have been unsatisfactory, but the introduction of anti-tumor necrosis factor agents have markedly improved articular and cutaneous signs and symptoms. The efficacy of tumor necrosis factor blockers also raised the bar for treatment goals in PsA. Remission may now be an attainable outcome in the treatment paradigm. At this time, specific tools to define PsA remission are not available. New assessments to define remission must be developed and incorporated into clinical trials and longitudinal registries.

Bone morphogenetic protein signaling in joint homeostasis and disease.

Despite advances in therapies that target inflammation and tissue destruction in chronic arthritis, stimulation of tissue repair and restoration of joint function, the ultimate goal of treatment, is far from achieved. We introduce a new paradigm that may help to improve our understanding and management of chronic arthritis. The presence or absence of tissue responses distinguishes destructive arthritis, steady-state arthritis and remodeling arthritis. Increasing evidence suggests that reactivation of embryonic molecular pathways is an important mechanism to stimulate postnatal tissue repair. Bone Morphogenetic Proteins (BMPs) have critical roles in skeletal development and joint morphogenesis, but also in postnatal joint homeostasis and joint tissue remodeling. Therefore, modulation of BMP signaling may be an attractive therapeutic target in chronic arthritis to restore homeostasis and function of synovial joints.

Contemporary concepts of inflammation, damage and repair in rheumatic diseases.

Chronic arthritis has been regarded as a disease resulting from a disequilibrium in pro- and anti-inflammatory cytokines. Restoration of this imbalance by using blocking antibodies or soluble receptors against a variety of inflammatory components has been the focus of most therapeutic interventions so far. More recently, other destructive mechanisms partially independent of inflammation have been elucidated, including osteoclast mediated bone resorption driven by the RANKL/RANK system. Despite efficient control of inflammation and destruction, little joint tissue repair has been observed. In addition, abnormal tissue responses such as cartilage calcification and ankylosis may contribute to disease progression and loss of joint function. We propose that 'true' disease remission may only be achieved with appropriate activation of local joint tissue responses leading to restoration of joint homeostasis and recovery of joint function. Understanding the molecular networks of joint homeostasis, repair and remodelling will be required to achieve this goal. Defining and validating clinical outcomes evaluating remission remain a challenge.

Aberrant Calreticulin Expression in Articular Cartilage of Dio2 Deficient Mice.

OBJECTIVE: To identify intrinsic differences in cartilage gene expression profiles between wild-type- and Dio2-/--mice, as a mechanism to investigate factors that contribute to prolonged healthy tissue homeostasis. METHODS: Previously generated microarray-data (Illumina MouseWG-6 v2) of knee cartilage of wild-type and Dio2 -/- -mice were re-analyzed to identify differential expressed genes independent of mechanical loading conditions by forced treadmill-running. RT-qPCR and western blot analyses of overexpression and knockdown of Calr in mouse chondro-progenitor cells (ATDC5) were applied to assess the direct effect of differential Calr expression on cartilage deposition. RESULTS: Differential expression analyses of articular cartilage of Dio2-/- (N = 9) and wild-type-mice (N = 11) while applying a cutoff threshold (P < 0.05 (FDR) and FC > |1,5|) resulted in 1 probe located in Calreticulin (Calr) that was found significantly downregulated in Dio2-/- mice (FC = -1.731; P = 0.044). Furthermore, overexpression of Calr during early chondrogenesis in ATDC5 cells leads to decreased proteoglycan deposition and corresponding lower Aggrecan expression, whereas knocking down Calr expression does not lead to histological differences of matrix composition. CONCLUSION: We here demonstrate that the beneficial homeostatic state of articular cartilage in Dio2-/- mice is accompanied with significant lower expression of Calr. Functional analyses further showed that upregulation of Calr expression could act as an initiator of cartilage destruction. The consistent association between Calr and Dio2 expression suggests that enhanced expression of these genes facilitate detrimental effects on cartilage integrity.

Targets, models and challenges in osteoarthritis research.

Osteoarthritis is a chronic degenerative disorder of the joint and represents one of the most common diseases worldwide. Its prevalence and severity are increasing owing to aging of the population, but treatment options remain largely limited to painkillers and anti-inflammatory drugs, which only provide symptomatic relief. In the late stages of the disease, surgical interventions are often necessary to partially restore joint function. Although the focus of osteoarthritis research has been originally on the articular cartilage, novel findings are now pointing to osteoarthritis as a disease of the whole joint, in which failure of different joint components can occur. In this Review, we summarize recent progress in the field, including data from novel 'omics' technologies and from a number of preclinical and clinical trials. We describe different in vitro and in vivo systems that can be used to study molecules, pathways and cells that are involved in osteoarthritis. We illustrate that a comprehensive and multisystem approach is necessary to understand the complexity and heterogeneity of the disease and to better guide the development of novel therapeutic strategies for osteoarthritis.

Pathophysiology of new bone formation and ankylosis in spondyloarthritis.

The outcome of patients suffering from spondyloarthritis is determined by chronic inflammation and new bone formation leading to ankylosis. The latter process manifests by new cartilage and bone formation leading to joint or spine fusion. This article discusses the main mechanisms of new bone formation in spondyloarthritis. It reviews the key molecules and concepts of new bone formation and ankylosis in animal models of disease and translates these findings to human disease. In addition, proposed biomarkers of new bone formation are evaluated and the translational current and future challenges are discussed with regards to new bone formation in spondyloarthritis.

Osteoarthritis, a disease bridging development and regeneration.

The osteoarthritic diseases are common disorders characterized by progressive destruction of the articular cartilage in the joints, and associated with remodeling of the subchondral bone, synovitis and the formation of bone outgrowths at the joint margins, osteophytes. From the clinical perspective, osteoarthritis leads to joint pain and loss of function. Osteoarthritis is the leading cause of progressive disability. New data from genetic, translational and basic research have demonstrated that pathways with essential roles in joint and bone development also contribute to the postnatal homeostasis of the articular cartilage and are involved in osteoarthritis, making these potential therapeutic targets. Other systems of interest are the tissue-destructive enzymes that break down the extracellular matrix of the cartilage as well as mediators of inflammation that contribute to synovitis. However, the perspective of a durable treatment over years to decades highlights the need for a personalized medicine approach encompassing a global view on the disease and its management, thereby including nonpharmaceutical approaches such as physiotherapy and advanced surgical methods. Integration of novel strategies based on their efficacy and safety with the identification of individuals at risk and optimal individual rehabilitation management remains a major challenge for the medical community in particular, as the incidence of osteoarthritis is likely to further increase with the overall aging of the population.

Are current available therapies disease-modifying in spondyloarthritis?

Disease modification in spondyloarthritis should target the improvement of symptoms and preservation of function. Therefore, inhibition of structural damage caused by the disease processes appears essential. In spondyloarthritis, structural damage results mainly in progressive ankylosis of the spine and peripheral joint destruction. Currently available therapies for the treatment of spondyloarthritis appear effective at inhibiting tissue destruction but, with the exception of celecoxib, do not appear to affect new tissue formation leading to ankylosis. In this article, we discuss clinical and pathophysiological concepts of disease modification in spondyloarthritis, challenges in its evaluation, recent clinical data and new concepts that may help explain structural damage as well as the onset and progression of disease.

Bone morphogenetic protein signaling and arthritis.

Chronic joint diseases have a major impact on society as patients suffer from pain and disability. The spectrum of arthritic disorders is wide including autoimmune and autoinflammatory diseases such as rheumatoid arthritis, ankylosing spondylitis and related spondyloarthritides but also the more prevalent osteoarthritic diseases. The latter appear to be mainly the consequence of injury, strain and aging in a predisposing genetic background. The therapeutic options for chronic inflammatory and immune joint diseases have greatly increased over the last decade by the use of targeted anti-cytokine or anti-immune cell drugs. However, such a shift towards successful treatment has not been achieved for osteoarthritis. In addition, control of inflammation does not equal cure of the disease as relapse occurs as soon as the treatment is interrupted, and only limited tissue repair has been observed. Bone morphogenetic proteins are potent regulators of cell proliferation, differentiation and apoptosis and they have come into the spotlight in arthritis research. Here, we summarize the recent data on the role of bone morphogenetic proteins in joint protection and repair and but also their potential disease promoting or controlling roles. These data are presented in the context of a systems biology view of joint diseases based on their histomorphological phenotype rather than on existing clinical classifications.

Progress in spondylarthritis. Mechanisms of new bone formation in spondyloarthritis.

Targeted therapies that neutralize tumour necrosis factor are often able to control the signs and symptoms of spondyloarthritis. However, recent animal model data and clinical observations indicate that control of inflammation may not be sufficient to impede disease progression toward ankylosis in these patients. Bone morphogenetic proteins and WNTs (wingless-type like) are likely to play an important role in ankylosis and could be therapeutic targets. The relationship between inflammation and new bone formation is still unclear. This review summarizes progress made in our understanding of ankylosis and offers an alternative view of the relationship between inflammation and ankylosis.

Dynamic activation of bone morphogenetic protein signaling in collagen-induced arthritis supports their role in joint homeostasis and disease.

INTRODUCTION: Rheumatoid arthritis is a chronic systemic autoimmune disease affecting peripheral joints and leading to loss of joint function. The severity and outcome of disease are dependent on the balance between inflammatory/destructive and homeostatic or repair pathways. Increasing evidence suggests a role for bone morphogenetic protein (BMP) signaling in joint homeostasis and disease. METHODS: Activation of BMP signaling in collagen-induced arthritis as a model of rheumatoid arthritis was studied by immunohistochemistry and Western blot for phosphorylated SMAD1/5 at different time points. Expression of different BMP ligands and noggin, a BMP antagonist, was determined on synovium and cartilage extracts of arthritic knees, at different time points, with quantitative polymerase chain reaction. At the protein level, BMP2 and BMP7 were studied with immunohistochemistry. Finally, the effect of anti-tumor necrosis factor-alpha (TNFalpha) treatment on the expression of BMP2, BMP7, and growth and differentiation factor-5 (GDF5) in synovium and cartilage of arthritic knees was investigated. RESULTS: A time-dependent activation of the BMP signaling pathway in collagen-induced arthritis was demonstrated with a dynamic and characteristic expression pattern of different BMP subfamily members in synovium and cartilage of arthritic knees. As severity increases, the activation of BMP signaling becomes more prominent in the invasive pannus tissue. BMP2 is present in cartilage and the hyperplastic lining layer. BMP7 is found in the sublining zone and inflammatory infiltrate. Treatment with etanercept slowed down progression of disease, but no change in expression of GDF5, BMP2, and BMP7 in synovium was found; in the cartilage, however, blocking of TNFalpha increased the expression of BMP7. CONCLUSIONS: BMP signaling is dynamically activated in collagen-induced arthritis and is partly TNFalpha-independent. TNFalpha blocking increased the expression of BMP7 in the articular cartilage, possibly enhancing anabolic mechanisms. Different types of source and target cells are recognized. These data further support a role for BMP signaling in arthritis.

Bone morphogenetic proteins in destructive and remodeling arthritis.

Joint destruction and tissue responses determine the outcome of chronic arthritis. Joint inflammation and damage are often the dominant clinical presentation. However, in some arthritic diseases, in particular the spondyloarthritides, joint remodeling is a prominent feature, with new cartilage and bone formation leading to ankylosis and contributing to loss of function. A role for bone morphogenetic proteins in joint remodeling has been demonstrated in the formation of both enthesophytes and osteophytes. Data from genetic models support a role for bone morphogenetic protein signaling in cartilage homeostasis. Finally, this signaling pathway is likely to play a steering role in the synovium.