Clinical and cellular phenotypes associated with sequestosome 1 (SQSTM1) mutations.

Familial Paget's disease of bone has been shown to be associated with mutations in the ubiquitin-associated (UBA) domain of the sequestosome 1 (SQSTM1) gene. We have clinical findings on five families with diverse racial and ethnic backgrounds who all harbor SQSTM1 UBA domain mutations (P387L, P392L, D391fsX394, P392fsX394). Intrafamilial expressivity was highly variable. The probands in two of the families had early-onset disease involving a large number of bones and highly elevated prediagnostic levels of serum alkaline phosphatase. Affected siblings in these same families had limited bone involvement and were only diagnosed by technetium-99m methylene diphosphonate (MDP) bone scans. Furthermore, there was at least one subject in each family with no evidence of Paget's disease, although they carried one mutated copy of the SQSTM1 gene. A total of 18 such individuals were identified across the five kindreds. Thus, the gene seems to have highly variable expressivity, as well as incomplete penetrance, supporting the role of this gene as a predisposition gene for familial Paget's disease of the bone. Molecular studies of the SQSTM1 protein showed different cellular aggregation phenotypes depending on the nature of the mutation. In general, the point mutations formed larger cytoplasmic aggregates than the wildtype or truncation mutations. This aggregation phenotype was not altered on removal of the N-terminal PB1 dimerization domain, implying that aggregate formation is not wholly mediated by interaction through the PB1 domain. Although there was a genotype/phenotype correlation on the cellular level, this was not apparent on the clinical level. This supports the argument that other nongenetic factors play an important role in the pathogenesis of the disease.

Three novel mutations in SQSTM1 identified in familial Paget's disease of bone.

UNLABELLED: Mutations in Sequestosome 1 (SQSTM1) have been shown to segregate with familial Paget's disease of bone (PDB). We examined the coding sequence of SQSTM1 in five PDB pedigrees and found three novel mutations clustered around the C-terminal ubiquitin associated domain. Disruptions of the C-terminal domain of SQSTM1 seem to be a leading cause of familial PDB. INTRODUCTION: The characteristic features of Paget's disease of bone (PDB) are caused by focal areas of excessive and uncoordinated bone remodeling. A total of seven genetic loci (PDB1-PDB7) have been reported to be associated with the disease. The gene for Sequestosome 1 (p62; SQSTM1) has been identified as the causative gene for PDB3 in numerous French-Canadian families and families predominantly of British descent. To date, a total of three mutations, all affecting the ubiquitin-associated domain of SQSTM1, have been identified: a single 1215 C to T (P392L) transversion in exon 8, a T insertion in exon 8 (E396X), and a G to A mutation at the splice junction of exon 7 (IVS7 + 1). MATERIALS AND METHODS: DNA was isolated from blood collected from the members of five U.S. PDB pedigrees. Mutation analysis of the coding sequence of the SQSTM1 gene was performed on the proband and other key individuals in the pedigrees. RESULTS: Four of the five families had SQSTM1 mutations. Three of these mutations were novel: a single base deletion in exon 8 at position 1210 (1210delT) resulting in a premature stop codon at amino acid 394, a single C deletion in exon 8 at position 1215 (1215delC) also resulting in a premature stop codon at amino acid 394, and a single 1200 C to T (P387L) transversion in exon 7. CONCLUSION: Noteworthy is the fact that these three SQSTM1 mutations, in addition to the three previously described mutations, are clustered near the C-terminal of the protein. These mutations may be acting in a dominant-negative fashion to disrupt the ubiquitin-binding function, which could result in abnormal activation of the NF-kappaB pathway and the subsequent activation of the osteoclasts. These findings imply that SQSTM1 mutations may play a role in the majority of familial PDB in the United States.