FAM20A pertains to the family with sequence similarities 201 important for mineralization as suggested by the FAM20A2,3 and C4 gene mutation phenotypes. The canonic FAM20C phosphorylates secreted proteins5 while FAM20A has lost aminoacids of significance in its catalytic site.The impact of FAM20A gene mutation on mineral composition is explored here in vivo and in vitro.
9 patients with recessive FAM20A gene mutations were recruited at the CRMRMAFACE Center and the study, performed under their informed consent, ethical guidelines and reglementation. Patient phenotypes were scored. Teeth and gingival samples (+ 4 control patients) collected during oral treatment were studied 2. Gingival fibroblasts from 4 FAM20A/4 control patients were cultured. Histological, von Kossa and alizarin red staining as well as ultrastructural characterization (SEM and TEM + mineral electron diffraction) was performed. FAM20A and C expression was explored by RT-qPCR and in situ hybrization.
FAM20A and C were co-expressed in the epithelium and mesenchyme of dental, gingival and kidney samples. Patients with FAM20A gene mutations harboured distinctive oral (amelogenesis imperfecta, gingival hyperplasia with ectopic minerals) and renal phenotypes with varying degrees according to specific FAM20A gene mutations and no bone phenotype. Enamel was hypomineralized and desorganized. Gingival mineral deposits were calcium phosphates, with variable cristallinity depending on their (extremely diverse) size both in vivo. They were reproduced in fibroblast cultures Our data show that recessive FAM20A mutations lead one unique disease, albeit with variable grades of altered biological (enamel) and ectopic (gingiva, kidney) mineralization. The phenotype is less extensive than Raine syndrome profile relied to FAM20C gene mutations which is often lethal and with important bone and systemic disorders 4. Our data suggest that FAM20A intervenes in a limited set of cells in which FAM20A and C are coexpressed. FAM20A might be a co-factor of FAM20C which phosphorylates peptides behaving as mineralization enhancers and inhibitors in enamel and gingival mucosa, respectively. Our in vitro model delineates FAM20A-related ectopic mineralization process, shown here to result from a sequence of nucleation, fusion of independent extracellular foci and successive generation of amorphous calcium-phosphate and apatite minerals.