Skeletal dysplasia is a group of rare diseases that afflict skeletal growth through abnormalities in bone and cartilage. Its onset hits at the fetal stage and is caused by genetic mutations. A mutation in the gene encoding fibroblast growth factor receptor 3 (FGFR3) has been associated with two types of skeletal dysplasia, thanatophoric dysplasia (TD), a skeletal dysplasia that cause serious respiratory problems at birth and is often lethal, and achondroplasia (ACH), which causes stunted growth and other complications throughout life. Several experimental treatments have been considered, but none are commercially available.
The need for new drug compounds that can combat skeletal dysplasia has led the Noriyuki Tsumaki group at CiRA, Kyoto University, to consider iPS cell technology. In a joint study with Associate Professor Hideaki Sawai of Hyogo College of Medicine and Team Leader Shiro Ikegawa of RIKEN, Professor Tsumaki's team screened molecules based on their ability to rescue TD-iPSCs from degraded cartilage. Molecules known to affect FGFR3 signaling and/or the metabolism of chondrocytes, the cells responsible for growing cartilage, were identified as good candidates. More importantly, so too were statins, a class of drugs renown for their action against cholesterol and investigated because they have anabolic and protective effects on chondrocytes.
The authors used iPS cells generated from the fibroblasts of both healthy individuals (WT-iPSC) and TD patients (TD-iPSC). Chondrocytes differentiated from TD-iPSC produced less cartilage than those from WT-iPSC and also had a lower proliferation rate and greater apoptosis, properties that were attributed to a gain of function by the mutated FGFR3. Adding statin recovered the cartilage formation in TD-iPSC and increased the proliferation rate. Coincidently, the group observed increased expressions of SOX9, a chondrocytic transcription factor, and of COL2A1 and ACAN, two cartilage extracellular components, all of which are down-regulated in TD patients. Moreover, statin treatment was found to accelerate the degradation of the FGFR3 protein in chondrogenically differentiated TD-iPSC, a process inhibited in TD cases.