New insights into the molecular mechanism of multiple synostoses syndrome (SYNS): Mutation within the GDF5 knuckle epitope causes noggin-resistance

Growth and differentiation factor 5 (GDF5), a member of the bone morphogenetic protein (BMP) family, is essential for cartilage, bone, and joint formation. Antagonists such as noggin counteract BMP signaling by covering the ligand's BMP type I (BMPRI) and type II (BMPRII, ActRII, ActRIIB) interaction sites. The mutation GDF5‐S94N is located within the BMPRII interaction site, the so‐called knuckle epitope, and was identified in patients suffering from multiple synostoses syndrome (SYNS). SYNS is characterized by progressive symphalangism, carpal/tarsal fusions, deafness and mild facial dysmorphism. Here we present a novel molecular mechanism of a GDF5 mutation affecting chondrogenesis and osteogenesis. GDF5‐S94N exhibits impaired binding to BMPRII causing alleviated Smad and non‐Smad signaling and reduced chondrogenic differentiation of ATDC5 cells. Surprisingly, chondrogenesis in mouse micromass cultures was strongly enhanced by GDF5‐S94N. By using quantitative techniques (SPR, reporter gene assay, ALP assay, qPCR), we uncovered that this gain of function is caused by strongly reduced affinity of GDF5‐S94N to the BMP/GDF antagonist noggin and the consequential lack of noggin inhibition. Thus, since noggin is upregulated during chondrogenic differentiation, GDF5‐S94N exceeds the GDF5 action, which results in the phenotypic outcome of SYNS. The detailed molecular characterization of GDF5‐S94N as a noggin‐resistant growth factor illustrates the potential of GDF5 mutants in applications with defined therapeutical needs. © 2012 American Society for Bone and Mineral Research