Sclerosteosis is a progressive sclerosing bone dysplasia with an autosomal recessive mode of inheritance. Radiologically, it is characterized by a generalized hyperostosis and sclerosis leading to a markedly thickened and sclerotic skull, with mandible, ribs, clavicles and all long bones also being affected. Due to narrowing of the foramina of the cranial nerves, facial nerve palsy, hearing loss and atrophy of the optic nerves can occur. Sclerosteosis is clinically and radiologically very similar to van Buchem disease, mainly differentiated by hand malformations and a large stature in sclerosteosis patients. By linkage analysis in one extended van Buchem family and two consanguineous sclerosteosis families we previously mapped both disease genes to the same chromosomal 17q12-q21 region, supporting the hypothesis that both conditions are caused by mutations in the same gene. After reducing the disease critical region to approximately 1 Mb, we used the positional cloning strategy to identify the SOST gene, which is mutated in sclerosteosis patients. This new gene encodes a protein with a signal peptide for secretion and a cysteine-knot motif. Two nonsense mutations and one splice site mutation were identified in sclerosteosis patients, but no mutations were found in a fourth sclerosteosis patient nor in the patients from the van Buchem family. As the three disease-causing mutations lead to loss of function of the SOST protein resulting in the formation of massive amounts of normal bone throughout life, the physiological role of SOST is most likely the suppression of bone formation. Therefore, this gene might become an important tool in the development of therapeutic strategies for osteoporosis.

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Paget's disease of bone (PDB) is a common disorder characterized by focal abnormalities of increased and disorganized bone turnover. Genetic factors are important in the pathogenesis of PDB, and in previous studies, we and others identified a locus for familial PDB by genome-wide search on 5q35-qter (PDB3). The gene encoding sequestosome 1 (SQSTM1/p62) maps to within the PDB3 critical region, and recent studies have identified a proline-leucine amino acid change at codon 392 of SQSTM1 (P392L) in French-Canadian patients with PDB. We conducted mutation screening of positional candidate genes in the PDB3 locus in patients with PDB, and also identified mutations in the gene encoding SQSTM1 as a common cause of familial and sporadic PDB. Three different mutations were found, all affecting the highly conserved ubiquitin-binding domain. The most common mutation was the P392L change in exon 8, which was found in 13 of 68 families (19.1%). Another mutation-a T insertion that introduces a stop codon at position 396 in exon 8-was found in four (5.8%) families. A third mutation affecting the splice donor site in intron 7 was found in one (1.5%) family. The P392L mutation was also found in 15 of 168 (8.9%) of patients with sporadic PDB and 0 of 160 of age- and sex-matched controls (P<0.0001). These studies confirm that mutations affecting the ubiquitin-binding domain of SQSTM1 are a common cause of familial and sporadic Paget's disease of bone.

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Mutations in GJB2, the gene encoding connexin-26 at the DFNB1 locus on 13q12, are found in as many as 50% of subjects with autosomal recessive, nonsyndromic prelingual hearing impairment. However, genetic diagnosis is complicated by the fact that 10%-50% of affected subjects with GJB2 mutations carry only one mutant allele. Recently, a deletion truncating the GJB6 gene (encoding connexin-30), near GJB2 on 13q12, was shown to be the accompanying mutation in approximately 50% of these deaf GJB2 heterozygotes in a cohort of Spanish patients, thus becoming second only to 35delG at GJB2 as the most frequent mutation causing prelingual hearing impairment in Spain. Here, we present data from a multicenter study in nine countries that shows that the deletion is present in most of the screened populations, with higher frequencies in France, Spain, and Israel, where the percentages of unexplained GJB2 heterozygotes fell to 16.0%-20.9% after screening for the del(GJB6-D13S1830) mutation. Our results also suggest that additional mutations remain to be identified, either in DFNB1 or in other unlinked genes involved in epistatic interactions with GJB2. Analysis of haplotypes associated with the deletion revealed a founder effect in Ashkenazi Jews and also suggested a common founder for countries in Western Europe. These results have important implications for the diagnosis and counseling of families with DFNB1 deafness.