Osteoarthritis (OA) is the most common degenerative joint disorder in humans and companion animals (Mele, 2007; Martel-Pelletier et al., 2016; Cimino Brown, 2017). The Lancet Commission on Osteoarthritis reported that more than 500 million people worldwide were affected by this complex disease in 2020 (Hunter et al.,2020). The etiology involves metabolic disruption of the articular cartilage, subchondral bone, ligaments, capsule and synovial membrane leading to articular cartilage loss, subchondral bone sclerosis, and inflammation. Progression of these structural alterations leads to joint failure, loss of mobility, pain, and decreased quality of life (Zheng, 2005; Martel-Pelletier et al., 2016). Risk factors associated with OA are heterogenic, for instance, age, gender, obesity, joint biomechanics and genetic background are the most described (Martel-Pelletier et al., 2016; Abramoff and Caldera, 2020). OA commonly appears secondary to hip dysplasia, knee cruciate ligament rupture and avascular necrosis of the femoral (Jacobsen and Sonne-Holm, 2005; Kim, 2012; Simon et al., 2015).
Canine forms of OA are very similar to those in humans and represent a welfare problem in the world dog population. In the UK, an estimated annual period prevalence of 2.5% for appendicular OA has been reported, based on primary-care data. This equates to around 200,000 UK affected dogs annually (Anderson et al., 2018). The most common locations of OA in dogs include the stifle (cranial cruciate ligament rupture and medial patellar luxation), hip (hip dysplasia), elbow (fragmentation of the medial process) and shoulder (osteochondrosis dissecans). While risk factors are similar between owners and pets, inherited defects related to skeletal conformation of certain dog breeds have been associated with OA developing (Anderson et al., 2018). Medium to large breeds such as Border Collie, Bull Mastif, Dogue de Bordeaux, German Pointer, German Shepherd, Golden Retriever, Labrador Retriever, Old English Sheepdog, Rottweiler, Scottish Collie and Springer Spaniel have shown higher odds of OA diagnosis than small and crossbreeds, even in early life (Anderson et al., 2018; O'Neill et al., 2020).
The diagnosis of OA in human and veterinary medicine is usually made by clinical examination and plain radiography. In some instances, magnetic resonance imaging (MRI) and computed tomography (CT) scans are extremely useful to identify early chondral damage and thus predisposing factors to OA, such as meniscal and cruciate ligament injuries (Abramoff and Caldera, 2020). Different from humans, diagnosis of OA in dogs cannot be assessed based on patient’s symptoms and usually by the time owners detect limb movement imbalance and pain in their pets, the disease stage is already advanced (Cimino Brown, 2017; Meeson et al., 2019). It has been reported that diagnosis of OA was usually made when dogs were older. In cases with available dates of diagnosis and death, mean proportion of lifespan affected by OA was 11% (Anderson et al., 2018). Thus, identifying early articular degradation is the most important challenge in OA research and clinical care.
During the last decade, the use of biomarkers in the diagnosis of OA has gained interest. The rapid development of high-throughput sequencing technologies has enabled the identification of gene expression profiles and networks correlated with OA etiopathology (Mobasheri and Henrotin, 2010; Munjal et al., 2019). Interleukins, matrix metalloproteinases, collagen family members, TGF-β pathway-related genes, long noncoding RNA (lncRNA) and microRNAs (miRNA) are some of the most identified in OA gene expression analysis (Reynard and Barter,2020). In dogs, patients and experimentally induced OA have been broadly used to identify OA associated biomarkers. It is difficult to study patients with naturally occurring OA since many factors including breed, age, activity level and severity and duration of the disease are not standardized (de Bakker et al., 2017). However, as ethical regulations in animal research have become more important, patients represent the most feasible model (The NC3Rs, https://www.nc3rs.org.uk/). The most used tissues and bio-fluids in OA biomarkers identification are cartilage, synovial fluid, blood and urine (Munjal et al., 2019). For instance, in synovial fluid, cytokines, C-reactive protein and matrix metalloproteinase enzymes have been correlated with the early phase of inflammation or tissue destruction. Whereas in cartilage, collagen type II synthesis and degradation products, proteoglycans, hyaluronic acid and cartilage oligomeric matrix protein are associated with long stage joint damage (de Bakker et al., 2017). Systemic biomarkers (serum or urine) offer a potential alternative method of quantifying total body burden of OA (Kraus et al., 2010), being blood the most suitable due to its easy collection in patients and healthy controls, and role in many of the metabolic pathways, including osteoclastogenesis and bone resorption (Munjal et al., 2019).
So far, given the heterogeneous nature of the disease and the lack of sufficiently clinical validation, no single canine OA biomarker stands out as the gold standard (de Bakker et al., 2017). Therefore, there is an increasing focus on the development of a panel of biomarkers, which could cover a range of patho-physiological effects, such as cartilage synthesis and degradation, synovitis and inflammation. Combining existing biomarkers may improve their prognostic accuracy and help identify at-risk patients (Williams, 2009). Thus, identification of biomarkers in peripheral blood, together with the established diagnostic imaging, would enable a more accurate diagnosis of OA, improving canine welfare and avoiding economic losses in veterinary medicine. In this study, we investigated the transcriptomic profile of peripheral blood in German Shepherd patients with OA in order to identify new putative OA biomarkers.