The Veterinary Journal 185 (2010) 95–97

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Identification, validation and qualification of biomarkers for osteoarthritisin humans and companion animals: Mission for the next decade

Ali Mobasheri a,*, Yves Henrotin b

a Musculoskeletal Research Group, Division of Veterinary Medicine, School of Veterinary Medicine and Science, Faculty of Medicine and Health Sciences,University of Nottingham Sutton Bonington Campus, Leicestershire, UKb Bone and Cartilage Research Unit, University of Liège, Institute of Pathology, Sart-Tilman, 4000 Liège, Belgium

Articular cartilage is an avascular, aneural and alymphatic con-nective tissue designed to distribute mechanical load and provide awear-resistance surface to articulating joints (Buckwalter andMankin, 1998). The tissue is made up of a tough and mechanicallyresilient extracellular matrix (ECM) consisting mainly of the carti-lage-specific type II collagen (approximately 85–90% of the totalcollagen content) and aggregating proteoglycans (predominantlyaggrecan) that occupies more than 90% of the total volume of thetissue (Carney and Muir, 1988; Muir, 1995). The chondrocyte isthe only cell type that resides within cartilage and is solely respon-sible for the synthesis and turnover of the ECM (Archer and Fran-cis-West, 2003).

Despite its durability, cartilage has a very limited self-maintain-ing capability and is vulnerable to mechanical injury and prone tostructural damage and degradation. Osteoarthritis (OA) is one ofthe most common forms of arthritis affecting synovial joints in hu-mans and companion animals, namely, the horse, dog and cat. It isestimated that at least 80% of cases of lameness and joint diseasesin companion animals are classified as OA (Lees, 2003). OA isknown by several other names, including degenerative joint dis-ease (DJD), osteoarthrosis, hypertrophic arthritis and degenerativearthritis.1,2

In humans, joint inflammation and the degeneration of articularcartilage are integral components of the clinical syndrome of OA,which is one of the most common causes of pain and disabilityin the ageing population (Buckwalter and Mankin, 1998). Thereis a strong correlation between increasing age and the prevalenceof OA and recent evidence of age-related changes in the functionof chondrocytes, suggest that such changes in articular cartilagecontribute to the development and progression of OA (Buckwalteret al., 2000, 2005). In some of the literature, OA has been incor-rectly described as ‘idiopathic’. In reality, it is a progressive disor-der characterised by destruction of articular cartilage andsubchondral bone and by synovial changes. Synovial inflammationor synovitis is a frequently observed phenomenon in OA joints andcontributes to disease pathogenesis through formation of variouscatabolic and pro-inflammatory mediators altering the balance ofcartilage matrix degradation and repair (Sutton et al., 2009).

1090-0233/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.tvjl.2010.05.026

* Corresponding author. Tel.: +44 115 951 6449; fax: +44 115 951 6440.E-mail address: ali.mobasheri@nottingham.ac.uk (A. Mobasheri).

1 See: http://www.arthritis.org/disease-center.php?disease_id = 32.2 See: http://www.nlm.nih.gov/medlineplus/osteoarthritis.html.

Several drug classes are available for treating musculoskeletaldisease in companion animals; these include non-steroidal anti-inflammatory drugs (NSAIDs) and corticosteroids. The manage-ment of OA in pets is largely palliative, focusing on the alleviationof symptoms, mainly pain. The most appropriate first line treat-ment is the use of mild analgesia. The next escalation of treatmentconsists of the lowest effective dose of NSAIDs for the shortest per-iod of time. When other pharmacological agents have been ineffec-tive (or are contraindicated) the use of weak opioids may beconsidered for the treatment of refractory pain in canine patientswith limb OA. For refractory cases, full dose NSAIDs along with lo-cal intra-articular injections of glucocorticoids or hyaluronic acidsupplementation may be used while awaiting assessment of suit-ability for joint replacement. Glucosamine sulfate/hydrochlorideand chondroitin sulfate may provide symptomatic benefit, but ifno response is apparent within 6 months of treatments, theyshould be discontinued according to the guidelines provided byACR,3 EULAR4 and OARSI.5

No current pharmacotherapy can be considered to be an ap-proved structural/disease modifying therapy for OA. A recent sys-tematic review of the literature has provided evidence for theefficacy of NSAIDs, supporting longer-term use of these agentsfor increased clinical effect (Innes et al., 2010). However, thelong-term use of NSAIDs is highly controversial, especially in viewof the adverse gastrointestinal side effects of many of these sub-stances. Complementary therapies such as oral nutraceuticalsand dietary supplements, used in conjunction with NSAIDs, mayoffer significant benefits to companion animals with joint disorderssuch as OA (Goggs et al., 2005; Henrotin et al., 2005). AlthoughNSAIDs and nutraceuticals may slow down the pace of disease pro-gression they cannot stop or reverse the cartilage degradation andsynovial inflammation that occurs in OA. There are currently nodrugs on the market capable of protecting articular cartilage fromfurther damage or affect the pathways of disease progression.

Recent disappointments in late-stage clinical development ofdisease-modifying osteoarthritic drugs (DMOADs) have refocusedefforts on OA biomarker discovery. The field of OA biomarkers israpidly expanding. Biomarker discovery and validation for OA has

3 The American College of Radiology; see: www.acr.org.4 The European League Against Rheumatism; see: www.eular.org.5 The Osteoarthritis Research Society International; see: www.oarsi.org.

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accelerated significantly as we have increased our understanding ofjoint tissue molecules and their complex interactions (Kraus, 2005).One of the main issues responsible for driving this agenda has beenthe acute need for improved OA outcome measures in clinical trials(Kraus, 2005; Hunter et al., 2010). The diagnosis of OA is generallybased on clinical and radiographic changes, which occur fairly lateand have poor sensitivity for monitoring disease progression (Rous-seau and Delmas, 2007).

Currently, the assessment of the inter-bone distance and loss ofjoint space on a radiograph of the affected joint remains the ‘goldstandard’. Unfortunately, the limitations of joint space narrowingas an outcome are considerable and have hampered the qualifica-tion of biomarkers as a surrogate endpoint in drug discovery. Weneed to identify biomarkers that may be useful for characterisingthe burden of the disease, diagnosis, prognosis and efficacy of treat-ment in OA. Current research in this area is aimed at developing ananalytical toolbox with the potential to improve the clinical devel-opment process (Qvist et al., 2010). It has been suggested that com-bining existing biomarkers may improve their prognostic accuracyand help identify at-risk patients (Williams, 2009). The challengenow is to identify sensitive and reliable biomarkers that can beaccurately measured in blood or urine. This is especially critical inthe early phases of disease so that these treatments can be startedas soon as possible to slow down progression of the disease.

Several such biomarkers of OA already exist. The earliest ones tobe identified were the ‘neo-epitopes’ created by enzymatic degra-dation of collagen type II by matrix metalloproteinases (reviewedby Lohmander (2004)). The best-known examples of this are thetype II collagen C-telopeptide fragments (CTX-II) (Christgau et al.,2001; Poole et al., 2004) the cleavage neo-epitope (C-2-C) andthe denaturation epitope (Coll2-1) and its nitrated form Coll2-1NO2. Coll2-1NO2 can be considered as an indicator of the oxida-tive related type II collagen network degradation (Henrotin et al.,2007). There are also non-collagenous biomarkers of cartilage deg-radation. The most promising so far is cartilage oligomeric matrixprotein (COMP), which has shown promise as a diagnostic andprognostic indicator and as a marker of the disease severity andthe effect of treatment (Jordan, 2004; Tseng et al., 2009). Anotherinteresting candidate is YKL-40, which may provide a snapshot ofcatabolic events in joint tissues, potentially allowing rapid assess-ment of pharmacotherapy (Huang and Wu, 2009).

In a recent Special Issue of The Veterinary Journal Dr. ElaineGarvican and her colleagues review the literature and discuss bio-markers of cartilage turnover in two separate articles (Garvicanet al., 2010a,b). The first review sets the scene by describing theneed for accurate and reliable information about collagen turnoverbefore outlining the molecular processes by which the so-called‘neo-epitopes’ are generated by the action of matrix metallopro-teinases (Garvican et al., 2010a). The authors considered the appli-cation of ‘neo-epitopes’ as biomarkers for studying healthy anddiseased cartilage with particular emphasis on veterinary species.The second review focused on non-collagenous and non-proteogly-can components of cartilage (Garvican et al., 2010b). These mole-cules can also be detected following their release from cartilageECM as a result of altered turnover in OA. The authors addressedrecent post-genomic strategies that have been used to distinguishpopulations with OA from normal populations. This second articledescribes the application of a metabolomic fingerprint of OA andsummarises some of the techniques that can be used to measurethe concentrations of some of non-collagenous biomarkers in jointdisease. These two reviews provide the readers of The VeterinaryJournal with an up-to-date summary of the research progress inthe area of OA biomarkers in companion animals.

We would like to offer a few cautionary notes to readers. In bio-marker research it is usually the very low abundance proteins thatare the interesting ones – unusual or apparently irrelevant proteins

that may transiently appear or disappear. The ideal biomarkers forOA are unlikely to be the fragments or degraded forms of essentialECM proteins. By the time the major collagenous and non-collage-nous components of the ECM are detectable in urine, blood orsynovial fluid too much damage and inflammation has already oc-curred in the synovial joint. Although systemic (serum or urine)biomarkers offer a potential alternative method of quantifying to-tal body burden of disease, no OA-related biomarker has ever beenstringently qualified (Kraus et al., 2010).

Serum proteins and acute phase proteins (APPs) have also beenproposed as disease biomarkers (Sipe, 1995). However, these areunlikely to be clinically useful biomarkers for OA, not least as thereare numerous APPs that vary widely in disease states and there isconsiderable debate as to which ones actually participate in dis-ease processes. The abundance of APPs and their non-specificupregulation in response to inflammatory diseases means thatthey cannot (by definition) be used as OA biomarkers. Neverthe-less, the future is bright for OA research and efforts in this areamay well identify panels of biomarkers that may be used as non-invasive and reliable diagnostic and prognostic indicators of dis-ease severity and response to pharmacotherapy.

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