The utility of animal models in understanding osteoarthritis (OA) pathogenesis - an update on the impact of genetically modified mice.

Osteoarthritis (OA) is one of the most common health conditions worldwide leading to immense individual and societal burden. Current treatments for OA are inadequate with no approved structural disease modifying therapies, and existing options for chronic pain only moderately successful long-term. Improving this bleak picture requires a better understanding of OA molecular pathophysiology, how this differs between individuals and over time. Critical in this goal are animal models. There have been four key advancements in this field that have dramatically improved OA pathophysiology discovery research: (1) initial studies showing mouse OA-risk is modified by the same factors as humans-age, sex/sex-hormones, diet and genetics (1952-65); (2) first studies of naturally-occurring OA in mice with spontaneous (1972-81) and induced (1993) genetic mutations (GMs); (3) developing reproducible inducible models with good structural and symptomatic fidelity to human OA (1990-2005); and (4) using inducible and spontaneous OA-models in GM-mice to show disease and symptom modification and define molecular causality (1999-present). These milestones revolutionized OA pathophysiology research, such that there are now >500 unique genes/gene-products identified as having significant effects on OA (beneficial or detrimental). Studies in different mouse OA-models have underpinned the concept of OA-phenotypes, and more particularly endotypes and theratypes, with ~35% of tested molecular targets having different effects on post-traumatic (pt)OA versus spontaneous/age-associated-OA. Deciphering and translating the enormous and growing data from animal-models into effective therapeutics for people remains challenging. This will require better identification and stratification of patients with different OA pheno/endotypes, and improved collaboration between clinical and pre-clinical researchers.