Defective joint development and maintenance in GDF6-related multiple synostoses syndrome.

Multiple synostoses syndromes (SYNS) are a group of rare genetic bone disorders characterized by multiple joint fusions. We previously reported an SYNS4-causing GDF6 c.1330T>A (p.Tyr444Asn) mutation, which reduced Noggin-induced GDF6 inhibition and enhanced SMAD1/5/8 signaling. However, the mechanisms by which GDF6 gain-of-function mutation alters joint formation and the comprehensive molecular portraits of SYNS4 remain unclear. Herein, we introduced the p.Tyr443Asn (orthologous to the human GDF6 p.Tyr444Asn) mutation into the mouse Gdf6 locus and performed extensive phenotype analysis, joint development investigation, and transcriptome profiling of Gdf6 p.Tyr443Asn limb buds. Gdf6 p.Tyr443Asn knock-in mice recapitulated the morphological features of human SYNS4, showing joint fusion in the wrists, ankles, phalanges, and auditory ossicles. Analysis of mouse embryonic forelimbs demonstrated joint interzone formation defects and excess chondrogenesis in Gdf6 p.Tyr443Asn knock-in mice. Further RNA sequencing of forelimb buds revealed enhanced bone formation and upregulated bone morphogenetic protein (BMP) signaling in mice carrying the Gdf6 p.Tyr443Asn mutation. As tightly regulated BMP signaling is critical for skeletal development and joint morphogenesis, our study shows that enhancing GDF6 activity has a significant impact in both prenatal joint development and postnatal joint maintenance. This article is protected by copyright. All rights reserved.