Loss of osteoprotegerin expression in the inner ear causes degeneration of the cochlear nerve and sensorineural hearing loss.
Paget's disease of bone (PDB) is a common disorder in which focal abnormalities of increased bone turnover lead to complications such as bone pain, deformity, pathological fractures, and deafness. PDB has a strong genetic component and several susceptibility loci for the disease have been identified by genome-wide scans. Mutations that predispose individuals to PDB and related disorders have been identified in four genes. The rare PDB-like syndromes of familial expansile osteolysis, early-onset familial PDB, and expansile skeletal hyperphosphatasia are caused by insertion mutations in TNFRSF11A, which encodes receptor activator of nuclear factor (NF)κB (RANK)—a critical regulator of osteoclast function. Inactivating mutations in TNFRSF11B, which encodes osteoprotegerin (a decoy receptor for RANK ligand) cause idiopathic hyperphosphatasia, and polymorphisms in this gene seem to increase the risk for classical PDB. Mutations of the sequestosome 1 gene (SQSTM1), which encodes an important scaffold protein in the NFκB pathway, are a common cause of classical PDB. The rare syndrome of hereditary inclusion body myopathy, PDB, and fronto-temporal dementia is caused by mutations in the valosin-containing protein (VCP) gene. This gene encodes VCP, which has a role in targeting the inhibitor of NFκB for degradation by the proteasome. Several additional genes for PDB remain to be discovered, and it seems likely that they will also involve the RANK−NFκB signaling pathway or components of the proteasomal processing of this pathway, underscoring the critical importance of this signaling pathway in bone metabolism and bone disease.