Understanding skeletal diseases using human induced pluripotent stem cells

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Multiple epiphyseal dysplasia (MED) and pseudoachondroplasia (PSACH) are clinically similar skeletal diseases, involving short stature and osteoarthritis at a young age. MED and PSACH can be caused by mutations in two extracellular matrix proteins matrilin-3 and cartilage oligomeric matrix protein (COMP). We made stem cells by reprogramming blood cells from patients with MED and PSACH, and developed a method to produce cartilage from these stem cells in a dish. Thus we produced a tractable model for analysis of the mechanisms underlying the pathology in these diseases which can be compared with cartilage from people with other skeletal diseases or osteoarthritis. We have found that the chondrocytes (cells within cartilage) produced from patients with skeletal disease respond differently to those produced from healthy people. Our aim is to now build on these data to understand the molecular mechanisms underlying these skeletal diseases and to identify potential therapeutic drugs. In this project, we will generate additional stem cells [known as induced pluripotent stem cells (iPSCs)] from other patients with MED and PSACH and as controls their healthy relatives. We need to look at several different families to make sure the symptoms are not caused by other genetic factors than the diagnosed mutation. We will also use a technique called gene editing to generate the same disease causing mutations in healthy iPSCs also allowing us to determine that the differences we see are caused by the mutation. Further, we will correct mutations in disease iPSCs, after which the formation of cartilage by those cells should be the same as cartilage from healthy stem cells. We will then perform a deep analysis of the molecules in chondrocytes and cartilage produced by both mutant and healthy stem cells. This will include looking at all the RNA molecules that code for protein (by a technique called RNA-Seq) and evaluating different regions of the cartilage pellets from the disease iPSC-chondrocytes, and comparing these to healthy cartilage produced in our culture system and to adult cartilage. Advanced ultrastructural analysis will be used to reveal structural changes in cartilage proteins contributing to the differences seen in the patients. We will also identify and quantify how protein interactions, particularly that of a small protein which promotes cartilage formation (BMP-2), differ between healthy and mutant Matrilin-3, for which we have preliminary data and find out if this is also the case for COMP mutation. Based on our findings, we will select drugs to correct the disease phenotype of the mutant cells and experimentally induce it in healthy stem cell derived cartilage to confirm the identified mechanism(s) of pathology. This structural and mechanistic analysis will increase general understanding of cartilage development and disease, and help identify new drug targets for MED and PSACH and potentially for a subset of patients with osteoarthritis.