Inactiviation of FAM20C (DMP4) Causes Hypophosphatemic Rickets and Dental Defects

Objectives: Family with sequence similarity 20-member C (FAM20C), also known as DMP4, is an evolutionaryly conserved molecule that is highly expressed in the mineralized tissues of mammals. Genetic studies showed that the loss-of-function mutations in FAM20C were associated with human lethal osteosclerotic bone dysplasia (Raine Syndrome), indicating that this molecule may act as an inhibitor in the formation of bone. However, in vitro gain- and loss-of-function studies have suggested that FAM20C promotes the differentiation and mineralization of mouse mesenchymal cells and odontoblasts.

Methods: We generated Fam20C conditional knockout (cKO) mice in which Fam20C was ubiquitously inactivated (by crossbreeding with Sox2 promoter-Cre mice) or inactivated specifically in the mineralized tissues (by crossbreeding with 3.6 kb Col 1a1 promoter-Cre mice). Recombinant FAM20C protein was generated using insect cells. Fam20C transgenic mice were also generated and crossbred with Fam20C cKO mice to introduce the transgene in the Fam20C knockout background.

Results: Both ubiquitous and mineralized-tissue-specific inactivation of Fam20C leads to hypophosphatemic rickets (but not osteosclerosis) along with a significant downregulation of osteoblast differentiation markers and a dramatic elevation of fibroblast growth factor 23 (FGF23) in the serum and bone. The tooth of Fam20C cKO mice showed severe defects including dramatically thinner dentin, underdeveloped roots, severe defects of enamel and significant downregulation of odontoblast differentiation markers. The recombinant FAM20C protein promoted the differentiation and mineralization of MC3T3-E1 preosteoblasts in a dose-dependent manner. Transgenic mice in the wild type background showed no abnormalities in the skeleton, but fully rescued the skeletal defects of the Fam20C cKO mice.

Conclusions: These results indicate that FAM20C is essential to osteogenesis and odontogenesis; it is a “promoter” for these processes. FAM20C may regulate osteogenesis and odontogenesis through its direct role in accelerating cell differentiation and its systemic regulation of phosphate homeostasis via the mediation of FGF23.