Identifying Mechanisms of Cartilage Homeostasis and Degeneration Through the Use of Patient Specific iPSCs

Key Words: Joint Degeneration, Musculoskeletal Disease, Cartilage, Pluripotent Stem Cells

Novel strategies are needed when standard approaches fail to uncover the mechanism by which a mutation causes disease. This is the case for the severe, degenerative joint disease that occurs in children with Progressive Pseudorheumatoid Arthropathy of Childhood (PPAC), caused by loss of function mutations in WISP3. PPAC becomes symptomatic between the ages of 3 and 8, and then rapidly progresses to end-stage articular cartilage failure. PPAC children thus require total hip and knee replacement surgeries in their teenage years.

Researchers have failed to determine why WISP3 deficient cartilage fails precociously because patient samples collected at the time of arthroplasty resemble end-stage osteoarthritis, and mice lacking Wisp3 have no disease phenotype. In vitro cell culture studies have suggested multiple biologic activities for WISP3; yet the cartilage-specific relevance of these findings is uncertain. Therefore, to better understand PPAC, we have generated induced pluripotent stem cells (iPSCs) from 5 patients with PPAC and will now differentiate these iPSCs into two different cartilage lineages, articular and growth plate cartilage. In an unbiased approach, we will compare RNA, histologic, cell biologic, and biochemical profiles of these in vitro cartilages to those generated from wild-type iPSCs, PPAC iPSCs that had their WISP3 mutation corrected using Crispr/Cas9, and wild-type iPSCs that had WISP3 inactivated using Crispr/Cas9. Because cartilage degeneration occurs during childhood and is not present at birth, we hypothesize that mechanical stress can contribute to disease pathogenesis. Thus we will characterize the response of PPAC cartilages to mechanical and environmental stresses to identify mechanisms by which cartilage homeostasis is perturbed.

The knowledge we gain about mechanisms that lead to cartilage failure will benefit patients with PPAC, and could also point to new approaches for protecting cartilage from damage associated with other skeletal dysplasias and more common forms of degenerative joint disease.