Summary Cherubism is an autosomal dominant syndrome characterized by inflammatory destructive bony lesions resulting in symmetrical deformities of the facial bones. Cherubism is caused by mutations in Sh3bp2, the gene that encodes the adaptor protein 3BP2. Most identified mutations in 3BP2 lie within the peptide sequence RSPPDG. A mouse model of cherubism develops hyperactive bone remodelling osteoclasts and systemic inflammation characterized by expansion of the myelomonocytic lineage. The mechanism by which cherubism mutations alter 3BP2 function has remained obscure. Here we show that Tankyrase, a member of the poly(ADP-ribose)polymerase (PARP) family, regulates 3BP2 stability through ADP-ribosylation and subsequent ubiquitylation by the E3-ubiquitin ligase RNF146 in osteoclasts. Cherubism mutations uncouple 3BP2 from Tankyrase-mediated protein destruction, which results in its stabilization and subsequent hyperactivation of the SRC, SYK and VAV signalling pathways.

A fine balance between bone resorption by osteoclasts and bone formation by osteoblasts maintains bone homeostasis. In patients with cherubism, gain-of-function mutations in 3BP2, which is encoded by SH3-domain binding protein 2 (SH3BP2), cause cystic lesions with activated osteoclasts that lead to craniofacial abnormalities. However, little is known about the function of wild-type 3BP2 in regulating bone homeostasis. Here we have shown that 3BP2 is required for the normal function of both osteoblasts and osteoclasts. Initial analysis showed that Sh3bp2 -/-mice developed osteoporosis as a result of reduced bone formation despite the fact that bone resorption was impaired. We demonstrated using reciprocal bone marrow chimeras, a cellintrinsic defect of the osteoblast and osteoclast compartments in vivo. Further, Sh3bp2 -/-osteoblasts failed to mature and form mineralized nodules in vitro, while Sh3bp2 -/-osteoclasts spread poorly and were unable to effectively degrade dentine matrix in vitro. Finally, we showed that 3BP2 was required for Abl activation in osteoblasts and Src activation in osteoclasts, and demonstrated that the in vitro defect of each cell type was restored by the respective expression of activated forms of these kinases. These findings reveal an unanticipated role for the 3BP2 adapter protein in osteoblast function and in coordinating bone homeostatic signals in both osteoclast and osteoblast lineages. Introduction3BP2 is an adapter protein that contains an N-terminal pleckstrin homology (PH) domain, a proline-rich stretch that binds to Src homology 3 (SH3) domain-containing proteins, and a C-terminal SH2 domain that binds to phosphotyrosine residues (1). 3BP2 was initially identified as a binding protein of the tyrosine kinase Abl (2). Work from our lab and others has identified the Src family kinases (SFKs), Syk, and the Vav family of Rho guanine nucleotide exchange factors (GEFs) as 3BP2-binding partners (1), all of which are known to play important roles in osteoclast function (3-5). Cherubism is a dominantly inherited syndrome characterized by excessive maxillary and mandibular bone resorption that is associated with activated osteoclasts and inflammatory cells creating interosseous cystic lesions (6). Single missense mutations in the gene encoding the adapter protein 3BP2 result in a gain-of-function alteration in the protein and is associated with the majority of cherubism patients (7). A mouse model that harbors 2 copies of a cherubism allele develops severe osteoporosis associated with highly activated osteoclasts (8).In order to elucidate the role of the wild-type form of 3BP2 in bone homeostasis, we analyzed loss-of-function mutant mice. As distinct from the osteoporotic phenotype of mice expressing the cherubism gain-of-function form of 3BP2 associated with active osteoclasts, we uncovered a complex bone phenotype in mice lacking 3BP2 characterized by loss-of-function in both the osteoclast and osteoblast lineages resulting in net decreased bone mineral density and reduced mechanical ...

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