Genetic and Molecular Insights into Cartilage Regeneration, Primary and Posttraumatic Osteoarthritis Background and Rationale: This proposal is from Dr. M. Farooq Rai, a basic science investigator in the field of cartilage biology and osteoarthritis (OA). The mentored-phase will be completed under the mentorship of i) Dr. Linda J. Sandell, a cartilage biologist in the Departments of Orthopaedic Surgery and Cell Biology, ii) Dr. James M. Cheverud, a population geneticist and morphologist in the Department of Anatomy and Neurobiology, iii) Dr. Mathew J. Silva, a biomechanical engineer and bone biologist in the Department of Orthopaedic Surgery and iv) Dr. Ingrid Borecki, a biostatistician at the Division of Statistical Genomics. Using inbred strains related to the super healing mouse, we have defined phenotypes of ear wound healing and articular cartilage regeneration and have established a strong positive correlation between these two phenotypes. Furthermore, proof-of-concept studies strongly suggest that the ability to heal articular cartilage is positively correlaed with protection from posttraumatic OA. This unique genetic resource (i.e. recombinant inbred lines) will be used to systematically identify genes that contribute to cartilage regeneration and thus to age-related primary OA and posttraumatic OA. The methods used to undertake these specific aims rely on the availability of recombinant inbred strains of LG/J and SM/J with different abilities to regenerate cartilage and ear tissue combined with mouse surgical techniques for articular cartilage injury (full thickness) and a model of posttraumatic OA, destabilization of the medial meniscus (DMM - an OA model). Functional analyses of chondrocytes and stem cells will be performed in the strains that are extremes of healing. The outcome of this project will be the identification of genes affecting OA development and articular cartilage regeneration in mice, providing high-quality candidates for probing the human genome. These recombinant inbred lines were established by one of my mentors Dr. James Cheverud and have been extensively used to study other complex diseases such as obesity and diabetes in addition to tissue regeneration and long bone growth. Over the last 3.5 years, in collaboration, we have established the phenotype of some of these strains for cartilage regeneration and OA. Hypothesis: Gene variants can be identified in recombinant inbred mouse strains that will ultimately correlate with susceptibility to and protection from age-related primary OA and posttraumatic OA. We have developed the hypothesis that the ability to regenerate articular cartilage is positively genetically correlated with the ability to regenerate ear tissues (Rai et a., Arthritis Rheum 64:2300-10. 2012). We also began a study of the relationship between the ability to regenerate articular cartilage and susceptibility to OA, finding an inverse correlation between articular cartilage regeneration and OA (Hashimoto et al., Osteoarthritis Cartilage 20:562-71. 2012). We have also analyzed expression of candidate genes through branched-chain DNA technology on tissue lysates obtained from histological sections of eight strains. The expression of several genes was significantly heritable among strains. Four genes representing DNA repair (Xrcc2, Pcna) and Wnt signaling (Axin2, Wnt16) pathways were significantly positively correlated with both phenotypes suggesting a common genetic basis of tissue healing (under review in G3). The guiding hypothesis for this proposal is that genes can be identified that are involved in cartilage repair that will also be involved in posttraumatic OA. We propose to follow up these exciting results by gene mapping, analysis of cell function, and identification of specific genes involved in cartilage repair and OA. Study Design: The study contains four specific aims: (1) Determine phenotypic differences in young and old LGXSM recombinant inbred lines for articular cartilage regeneration and development of primary OA and posttraumatic OA. (2) Map genetic variation in young and old LGXSM recombinant inbred lines for articular cartilage regeneration and development of primary OA and posttraumatic OA, mapping QTLs for all traits to 10 cM genomic intervals. (3) Delineate molecular differences in LGXSM recombinant inbred lines for articular cartilage regeneration and development of primary OA and posttraumatic OA. (4) Monitor intrinsic functional differences in chondrocytes and mesenchymal stem cells in selected LGXSM recombinant inbred lines. Significance of Outcomes: We will be able to identify genes or groups of genes that govern tissue regeneration (ear wound and knee cartilage and protection from getting age-related primary and posttraumatic OA. Genes that playa a role in OA are unknown and studies in human address only primary OA, even then, have yielded little information, primarily because there will be many genes with small effects. This limitation can be addressed with our genetic mouse models. Finally, the cellular and molecular differences between healer and non-healer strains will provide novel insights into the mechanisms of regeneration and degermation of cartilage. Applied to human, these findings could facilitate better stratification of patient risk for progression of OA and identify possible targets for therapeutic intervention.
It has been suspected that humans differ in their ability to repair cartilage over time and to withstand cartilage degeneration due to osteoarthritis (OA). While correlating genes with human OA has been difficult, it has recently been demonstrated by three laboratories including ours that certain strains of mice (MRL/MpJ and DBA) are better able to repair cartilage and are protected from post-traumatic arthritis in young animals and age-related osteoarthritis in older animals. We propose to use the intercross of genetic strains also known to vary dramatically in their wound healing capacity, as models of genetic variation in cartilage repair and OA. We predict that the animals that are better able to heal a wound in their ear, the classic test for wound healing, are also better able to repair damaged articular cartilage and, therefore, more resistant to both age-related and posttraumatic OA.