Functional analysis of mutant FAM20C in Raine syndrome with FGF23-related hypophosphatemia.

Raine syndrome is an autosomal recessive disorder characterized by generalized osteosclerosis with periosteal bone formation and a distinctive facial phenotype. Either homozygous or compound heterozygous mutations in family with sequence similarity 20, member C (FAM20C) have been reported to cause this syndrome. Recently, it was reported that fibroblast growth factor 23 (FGF23)-related hypophosphatemia was found in patients with non-lethal Raine syndrome, and Fam20c conditional knockout mice presented Fgf23-related hypophosphatemic rickets. To clarify the mechanism of how FAM20C regulates FGF23, we performed functional analysis of mutant FAM20C proteins reported in Raine syndrome. We analyzed 6 mutant FAM20C proteins (T268M, P328S, R408W, D451N, D478A, and R549W) for their distributions, kinase activities, and effects on dentin matrix protein (DMP1) promoter activity. We also analyzed the effect of Fam20c knockdown on Dmp1 and Fgf23 mRNA levels in UMR-106 cells. As a result, all the mutant FAM20C proteins showed decreased kinase activities compared to wild-type (WT) FAM20C, and most of them also showed impaired secretion. Overexpression of WT FAM20C increased DMP1 promoter activity in Saos-2 cells while mutant FAM20C did not. Fam20c knockdown decreased Dmp1 mRNA and increased Fgf23 mRNA in UMR-106 cells. In conclusion, our results suggest that FAM20C suppresses FGF23 production by enhancing DMP1 expression, and inactivating mutations in FAM20C cause FGF23-related hypophosphatemia by decreasing transcription of DMP1.