Degenerative joint disease, also known as osteoarthritis, is the most common joint disorder in human beings. The molecular mechanism underlying this disease is not fully understood. Here, we report that disruption of mitogen-inducible gene 6 (Mig-6) in mice by homologous recombination leads to early onset degenerative joint disease, which is revealed by simultaneous enlargement and deformity of multiple joints, degradation of articular cartilage, and the development of bony outgrowths or osteophyte formation within joint space. The osteophyte formation appears to be derived from proliferation of mesenchymal progenitor cells followed by differentiation into chondrocytes. Absence of the Rag2 gene does not rescue the joint phenotype, excluding a role for the acquired immune system in the development of this disease. Our results provide insight into the mechanism of osteoarthritis by showing that loss of Mig-6 leads to early onset of this disease, implying that this gene or its pathway is important in normal joint maintenance. Because of the striking similarity of osteoarthritis in humans and mice, the Mig-6 mutant mouse should provide a useful animal model for studying the mechanism of this disease and for testing drugs or therapies for treating osteoarthritis.animal model ͉ mutation ͉ osteoarthritis ͉ bony outgrowth ͉ signaling pathway D egenerative joint disease, or osteoarthritis, affects nearly 12% of the population between ages 25 and 74 in the United States (1) and greatly interferes with quality of life by causing acute and chronic pain and disability (2, 3). The characteristic features of this disease are joint pain, stiffness, joint enlargement and malalignment, damage of articular cartilage, and formation of osteophytes or bony outgrowths at the margin of the synovial and cartilage junctions (2, 3). Currently, therapy is directed toward controlling symptoms, and no disease-modifying or chondroprotective treatment is available (2, 3). In addition, the costs for pain relief medication are astronomical. Although several genetic and biomechanical factors, including heredity, obesity, injury and joint overuse, are thought to contribute to the development of osteoarthritis (2, 3), the molecular mechanism underlying this disease is still elusive.Mitogen-inducible gene 6 (Mig-6), also known as gene 33, is an immediate early response gene that can be induced by many growth factors or stressful stimuli (4, 5). It has also been shown to act as a negative feedback inhibitor in EGF receptor signaling through direct interaction with EGF receptor family (5-7), and, similarly, the gene is induced by hepatocyte growth factor͞ scatter factor by means of the receptor tyrosine kinase Met (data not shown). To understand its role during mouse development and homeostasis, we generated Mig-6-deficient mice and demonstrated that Mig-6 is essential for normal joint maintenance and that loss of Mig-6 leads to early onset degenerative joint disease.
Materials and MethodsMice and Genotyping. To generate Mig-6 knockout mice, we constr...