Biomarkers in Early and Post-traumatic Osteoarthritis
Acute traumatic knee joint injuries common in athletes and a physically active population have been identified as risk factors for subsequent post-traumatic osteoarthritis. Approximately 12% of the overall burden of osteoarthritis is attributed to joint trauma.
Early onset of osteoarthritis (OA) can occur within 10 years after injury, indicating that patients with post-traumatic osteoarthritis (PTOA) are much younger (18 years to 44 years) than patients with idiopathic OA. In the United States alone, there are approximately 5.6 million people PTOA and the annual health care costs associated with PTOA are estimated to be nearly $12 billion.
It is increasingly believed the development of PTOA in injured joints is triggered by the damage in bone and cartilage caused by traumatic and inflammatory events coupled with possible long-term changes to joint loading. Injury leads to a biological response dominated by catabolic activity likely caused by initial trauma, as well as subsequent abnormal joint motion and loading. However, unlike primary OA, PTOA is initiated at a known time of onset, namely the time of joint injury. This makes it a more clearly defined target for early intervention than primary OA, the onset of which is more subtle.
The diagnosis of primary OA is currently based mainly on radiographic and clinical criteria that are flawed with low interrater reliability. However, once radiographic evidence becomes apparent, significant structural joint damage has already occurred. Further, the sensitivity of current imaging techniques is unable to detect early, periarticular events initiated by acute joint injuries. Similarly, the time course in which clinically measurable PTOA develops is highly variable, ranging from as few as 2 years in articular fractures to decades for less severe joint injuries. Widespread use of MRI may be limited by cost, availability and the absence of a validated OA score. Therefore, the limitations of imaging have led to the investigation of alternative measures of early osteochondral damage that could serve as molecular biomarkers for disease diagnosis and monitoring, drug development and assessment of the efficacy of targeted therapy.
Definition of Biomarkers
The WHO defines a “biomarker” as “any substance, structure or process that can be measured in the body or its products and that influences or predicts the incidence or outcome of disease.” The critical need in the diagnosis of chondral injuries and early OA is to identify cytokines, soluble proteins or breakdown products that appear shortly after the initial injury, at the initiation of the disease process. These biomarkers need to be valid, reliable, feasible and practical. They must be cost-effective, quantifiable, and accessible in biological fluids (serum, urine and synovial fluid). Currently, most research is dedicated to investigating idiopathic OA, but the focus of this review is on the importance and use of biomarkers for PTOA. Interventions that prevent post-traumatic cartilage degeneration and loss of joint homeostasis would be valuable to those at risk of PTOA. Evidence suggests this may be an attainable goal.
Significance of Biomarkers
The importance of biomarkers (Table) at early stages of OA and PTOA was recognized as a research priority at the American Orthopaedic Society for Sports Medicine/National Institutes of Health (AOSSM/NIH) U-13 Post-Joint Injury Osteoarthritis Conference II, especially “improved staging of early, pre-radiographic disease through identification and validation of sensitive and predictive biomarkers.” Advantages of cartilage metabolism biomarkers include easy accessibility, potentially increased sensitivity and lower cost than imaging. Following acute trauma, an initial increase in proinflammatory cytokines similar to that seen in wound healing occurs. These include tumor necrosis factor-alpha (TNF-), interleukin-1beta (IL-1), IL-6, IL-8, IL-1 receptor antagonist and IL-10. The release of proteoglycan and collagen fragments signifying cartilage damage after ACL rupture has been found to peak in the first few weeks after injury, but can persist at a significantly elevated level in synovial fluid for decades. Other matrix molecules, such as matrix metalloproteinase (MMP)-3, tissue inhibitor of metalloproteinase (TIMP)-1 and cartilage oligomeric matrix protein (COMP), also may have persistently elevated concentrations in synovial fluid following an ACL injury. Thus, biomarkers have the potential to not only predict the risk for progression to PTOA based on specific joint tissue extracellular matrix components, but also be useful for targeted drug development and monitoring therapeutic efficacy.
Preclinical and Clinical Studies
Acute joint trauma, especially ACL and meniscus tears, can be defined by three overlapping phases of initial biologic response: an early phase, characterized by cell death/apoptosis and inflammation (eg, elevation of caspases, proinflammatory cytokines, C-reactive protein, nitric oxide, reactive-oxygen species and basic fibroblast growth factor) leading to activation of proteolytic enzymes (aggrecanases and MMPs), which results in the degradation of main cartilage extracellular matrix components, aggrecan and collagen type II; an intermediate phase, characterized by a potential balance of catabolic and anabolic responses; and a late phase, characterized by limited or aberrant repair and matrix remodeling and formation. An understanding of these phases provides a platform for identifying potential biological targets for therapeutic and preventive interventions.
Proinflammatory cytokines, such as IL-1 and TNF-, have been documented to stimulate the production of MMPs and to suppress chondrocytic synthesis of aggrecan and type II collagen, necessary for the maintenance and repair of the extracellular matrix. Changes in the concentration of chondroitin sulfate (CS) and keratan sulfate (KS), appearance of aggrecan neo-epitopes and increased cleavage and denaturation of type II collagen by collagenases (detected by C2C; C1,2C; CTX-II; or CTX-I assays) have been observed in the articular cartilage of patients who have joint injuries or have developed OA. Yoshida and colleagues demonstrated that a combination of two biomarkers (C2C and KS) is associated with the presence of high-grade chondral lesions in patients after ACL rupture, independent of patient age or length of injury. Svoboda and colleagues observed significant decreases in serum C1, 2C, and C2C concentrations, changes in serum CS846 levels and differences in the changes of the ratio of type II collagen degradation to synthesis (eg, C2C:CPII ratio and C1,C2:CPII ratio) with time between ACL-injured cases and uninjured controls. Importantly, this study demonstrated preinjury baseline differences for C1, C2, C2C, CS846, and CPII in the ACL-injured cases and uninjured controls, thus suggesting these serum biomarkers may be used to predict which patients may be more prone to injury. Similarly, Catterall and colleagues reported decreases in the serum concentrations of CPII, C2C and C1,2C from baseline to follow-up after an acute ACL injury. The authors also found a significant inverse relationship between serum and synovial fluid concentrations of C2C, thereby suggesting higher concentrations of C2C in the affected joints were associated with lower concentrations of C2C in serum. Chmielewski and colleagues observed urine CTX-II concentrations are elevated after and improved with time following ACL reconstruction, and may be associated with changes in knee symptoms and function.
Another biomarker of interest is COMP, a pentameric noncollagenous glycoprotein that can bind to collagen type I, II and IX. By binding five collagen molecules, COMP maintains a complex collagen network and facilitates collagen-collagen interactions and microfibril formation. Several studies suggest articular chondrocytes produce COMP and the level of COMP in synovial fluid and serum may be related to cartilage damage. Verma and Dalal found elevated concentrations of COMP in the early stages of knee OA development as an indicator of high metabolism of articular chondrocytes. The study also demonstrated a positive and significant correlation between COMP levels and IL-1. The latter is consistent with the observations of Joosten and colleagues, who demonstrated a decrease in serum COMP levels with anti-IL-1 and IL-1 antibody treatment. Therefore, it is possible to conclude that serum COMP level may serve as a diagnostic, prognostic and therapeutic marker of PTOA.
Multiple studies reported changes in COMP levels after single episodes of physical activity, including moderate walking. Hoch and colleagues documented significant increases in COMP levels as the athletes reported an increased level of function with time (measured by Lysholm and International Knee Documentation Committee [IKDC] activities scores). However, whether these increases in COMP levels negatively influenced the overall joint health remains unclear.
The FDA and the European Medicines Agency have yet to accept MRI as an imaging endpoint for OA in clinical trials. Consequently, the development of disease-modifying therapy has been slowed secondary to the limited capacity of plain radiography to detect clinically significant changes in joint morphology that are associated with disease progression. Nevertheless, PTOA overcomes this barrier with a clear precipitating event that provides a unique opportunity for early intervention in the acute post-traumatic period.
Intervention strategies may be successful when targeting specific mechanisms or biomarkers of disease development and progression. For example, biologic interventions can decrease chondrocyte death by apoptosis inflicted by mechanical stress, thereby suggesting limiting progressive chondrocyte damage after joint injury is plausible. D’Lima and colleagues demonstrated broad-spectrum caspase (proteinases responsible for apoptosis) inhibitor z-Vad-fmk decreased mechanically induced chondrocyte apoptosis, while Rundell and colleagues reported intra-articular injections of P188 surfactant can limit chondrocyte necrosis after impact loading. The effect of P188 on human chondrocytes has been determined, in addition to its underlying mechanism of its action. Furthermore, Martin and Buckwalter identified that the application of antioxidants within hours of injury in vitro prevents progressive mechanically induced chondrocyte damage and matrix degradation. In addition, in vitro studies have shown that by using synthetic inhibitors of mitogen-activated protein kinases it was possible to inhibit the effects of the fibronectin pathways activated by cartilage impact injury. This prevents progressive cell damage and matrix degradation. Interventions aimed at these early procatabolic/proinflammatory responses of injured articular surfaces (eg, anti-inflammatory therapy or limitation of weight-bearing) may be beneficial and minimizes the extent of tissue damage and accelerates healing.
Comprehensive profiling of soluble biomarkers could potentially predict the risk of knee joint injury, indicate the risk of PTOA onset and progression, and could be used to monitor the efficacy of disease-modifying treatments. Basic and clinical research has led to an understanding of the molecular mechanisms involved in the early cellular response to injury. However, studies are needed for identification of biomarkers that are specific to early PTOA and distinct from idiopathic OA. Large cohort studies are necessary to yield new clinical approaches to PTOA. Today, we are perched in a spot where the molecular basis to understand the significance of biomarkers is established and sufficient knowledge exists to recruit and maintain large patient cohorts in clinical trials. It is now important to combine both of these assets to create a meaningful database that can predict disease onset and monitor disease progression for successful diagnosis and treatment of patients with PTOA.
Cole BJ, Harris JD, eds. Biologic Knee Reconstruction: A Surgeon’s Guide (pp 41-45). © 2015 SLACK Incorporated.
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