Sequestosome 1 Mutations in Paget's Disease of Bone in Australia: Prevalence,Genotype/Phenotype Correlation,and a Novel Non‐UBA Domain Mutation (P364S) Associated With Increased NF‐κB Signaling Without Loss of Ubiquitin Binding

Previously reported Sequestosome 1(SQSTM1)/p62 gene mutations associated with Paget's disease of bone (PDB) cluster in, or cause deletion of, the ubiquitin‐associated (UBA) domain. The aims of this study were to examine the prevalence of SQSTM1 mutations in Australian patients, genotype/phenotype correlations and the functional consequences of a novel point mutation (P364S) located upstream of the UBA. Mutation screening of the SQSTM1 gene was conducted on 49 kindreds with PDB. In addition, 194 subjects with apparently sporadic PDB were screened for the common P392L mutation by restriction enzyme digestion. HEK293 cells stably expressing RANK were co‐transfected with expression plasmids for SQSTM1 (wildtype or mutant) or empty vector and a NF‐κB luciferase reporter gene. GST‐SQSTM1 (wildtype and mutant) proteins were used in pull‐down assays to compare monoubiquitin‐binding ability. We identified SQSTM1 mutations in 12 of 49 families screened (24.5%), comprising 9 families with the P392L mutation and 1 family each with the following mutations: K378X, 390X, and a novel P364S mutation in exon 7, upstream of the UBA. The P392L mutation was found in 9 of 194 (4.6%) patients with sporadic disease. Subjects with SQSTM1 mutations had more extensive disease, but not earlier onset, compared with subjects without mutations. In functional studies, the P364S mutation increased NF‐κB activation compared with wildtype SQSTM1 but did not reduce ubiquitin binding. This suggests that increased NF‐κB signaling, but not the impairment of ubiquitin binding, may be essential in the pathogenesis of PDB associated with SQSTM1 mutations.     

Novel UBA domain mutations of SQSTM1 in Paget's disease of bone: genotype phenotype correlation, functional analysis, and structural consequences.

Three novel missense mutations of SQSTM1 were identified in familial PDB, all affecting the UBA domain. Functional and structural analysis showed that disease severity was related to the type of mutation but was unrelated to the polyubiquitin-binding properties of the mutant UBA domain peptides. INTRODUCTION: Mutations affecting the ubiquitin-associated (UBA) domain of Sequestosome 1 (SQSTM1) gene have recently been identified as a common cause of familial Paget's disease of bone (PDB), but the mechanisms responsible are unclear. We identified three novel SQSTM1 mutations in PDB, conducted functional and structural analyses of all PDB-causing mutations, and studied the relationship between genotype and phenotype. MATERIALS AND METHODS: Mutation screening of the SQSTM1 gene was conducted in 70 kindreds with familial PDB. We characterized the effect of the mutations on structure of the UBA domain by protein NMR, studied the effects of the mutant UBA domains on ubiquitin binding, and looked at genotype-phenotype correlations. RESULTS AND CONCLUSIONS: Three novel missense mutations affecting the SQSTM1 UBA domain were identified, including a missense mutation at codon 411 (G411S), a missense mutation at codon 404 (M404V), and a missense mutation at codon 425 (G425R). We also identified a deletion leading to a premature stop codon at 394 (L394X). None of the mutations were found in controls. Structural analysis showed that M404V and G425R involved residues on the hydrophobic surface patch implicated in ubiquitin binding, and consistent with this, the G425R and M404V mutants abolished the ability of mutant UBA domains to bind polyubiquitin chains. In contrast, the G411S and P392L mutants bound polyubiquitin chains normally. Genotype-phenotype analysis showed that patients with truncating mutations had more extensive PDB than those with missense mutations (bones involved = 6.05 +/- 2.71 versus 3.45 +/- 2.46; p < 0.0001). This work confirms the importance of UBA domain mutations of SQSTM1 as a cause of PDB but shows that there is no correlation between the ubiquitin-binding properties of the different mutant UBA domains and disease occurrence or extent. This indicates that the mechanism of action most probably involves an interaction between SQSTM1 and a hitherto unidentified protein that modulates bone turnover.     

Familial Paget Disease and <Emphasis Type="Italic">SQSTM1</Emphasis> Mutations in New Zealand

Genetic factors play an important role in the pathogenesis of Paget disease of bone (PDB). SQSTM1 is the most important disease-associated gene identified to date. We investigated the relationship of family history, phenotype, and SQSTM1 mutation status in New Zealand (a country with a high prevalence of PDB) in patients with a family history and/or a severe phenotype. We studied 61 unrelated subjects with familial PDB. Family history was subclassified into three groups according to the closeness of the relationship. We also studied a fourth group of 19 unrelated patients defined by early onset and/or severe disease but no family history. The PDB phenotype was defined according to age, alkaline phosphatase activity, and disease extent on scintiscan at the time of diagnosis. Mutations in exon 8 of SQSTM1 were detected by screening of genomic DNA. Four different mutations were identified; the ubiquitous P392L mutation and the truncating mutation E396X accounted for 89% of cases. Overall 26% of patients with familial PBD in New Zealand had disease-associated mutations in the SQSTM1 gene. Mutations were most prevalent (60%) in those with a parent or sibling and at least one other relative affected (P < 0.002). The severity of the phenotype was significantly related to SQSTM1 mutation status but not the strength of the family history (P < 0.005). SQSTM1 mutations were found in 10.5% of patients with early onset and/or severe disease but no family history.     

Mutations of SQSTM1 are associated with severity and clinical outcome in paget disease of bone

Paget disease of bone (PDB) is a common disorder characterized by increased bone turnover at one of more sites throughout the skeleton. Genetic factors play an important role in the pathogenesis of PDB, and the most important predisposing gene is SQSTM1, which is mutated in about 10% of patients. Here we investigated the relationship between SQSTM1 mutation status, disease severity, and clinical outcome in 737 patients who took part in a randomized study of two different management strategies for the disease. Mutations of SQSTM1 were detected in 80 of 737 (10.9%) patients. Mutation carriers had an earlier age at diagnosis (59.4 ±11.5 versus 65.0 ± 10.4 years, p < .0001) and a greater number of affected bones (3.2 ± 1.2 versus 2.1 ± 1.2, p < .001) and more commonly required orthopedic surgery (26.2% versus 16.1%, p = .024) and bisphosphonate therapy (86.3% versus 75.2%, p = .01) than those without mutations. Quality of life, as assessed by the short‐form‐36 (SF36) physical summary score, was significantly reduced in carriers (34.0 ± 11.3 versus 37.1 ± 11.4, p = .036). During the study, fractures were more common in carriers (12.5% versus 5.3%, p = .011), although most of these occurred in unaffected bone. This study demonstrates that SQSTM1 mutations are strongly associated with disease severity and complications of PDB. Genetic testing for SQSTM1 mutations may be of value in identifying individuals at risk of developing severe disease, but further studies will be required to determine if a program of genetic testing and early intervention in these individuals would be cost‐effective or be of benefit in preventing these complications. © 2010 American Society for Bone and Mineral Research.     

Somatic Mutations in SQSTM1 Detected in Affected Tissues From Patients With Sporadic Paget's Disease of Bone

Paget's disease of bone (PDB) is a focal disorder of bone remodeling that leads to overgrowth of affected bone, with rare progression to osteosarcoma. Extensive studies of familial PDB showed that a majority of cases harbor germline mutations in the Sequestosome1 gene (SQSTM1). In contrast, little is known about the mutational status of SQSTM1 in sporadic PDB. We hypothesized that somatic SQSTM1 mutations might occur in the affected tissues of sporadic PDB and pagetic osteosarcoma. We used laser capture microdissection to capture homogeneous populations of cells from the affected bone or tumor of patients with sporadic PDB or pagetic osteosarcoma, respectively. DNA from these samples and appropriate controls was used for sequence analysis and allelic discrimination analysis. Two of five patients with sporadic PDB had SQSTM1^C1215T mutations detected in their affected bone but not in their blood samples, indicating a somatic origin of the mutations. Samples from three of five sporadic pagetic osteosarcoma patients had the SQSTM1^C1215T mutation, whereas the normal adjacent tissue from two of these tumors clearly lacked the mutation, again indicating an occurrence of somatic events. No SQSTM1 mutations were found in primary adolescent osteosarcomas. The discovery of somatic SQSTM1 mutations in sporadic PDB and pagetic osteosarcoma shows a role for SQSTM1 in both sporadic and inherited PDB. The discovery of somatically acquired mutations in both the diseased bone and tumor samples suggests a paradigm shift in our understanding of this disease.     

Loss of Ubiquitin Binding Is a Unifying Mechanism by Which Mutations of SQSTM1 Cause Paget’s Disease of Bone

Ubiquitin-associated (UBA) domain mutations of SQSTM1 are an important cause of Paget’s disease of bone (PDB), which is a human skeletal disorder characterized by abnormal bone turnover. We previously showed that, when introduced into the full-length SQSTM1 protein, the disease-causing P392L, M404V, G411S, and G425R missense mutations and the E396X truncating mutation (representative of all of the SQSTM1 truncating mutations) cause a generalized loss of monoubiquitin binding and impaired K48-linked polyubiquitin binding at physiological temperature. Here, we show that the remaining three known PDB missense mutations, P387L, S399P, and M404T, have similar deleterious effects on monoubiquitin binding and K48-linked polyubiquitin binding by SQSTM1. The P387L mutation affects an apparently unstructured region at the N terminus of the UBA domain, some five residues from the start of the first helix, which is dispensable for polyubiquitin binding by the isolated UBA domain. Our findings support the proposal that the disease mechanism in PDB with SQSTM1 mutations involves a common loss of ubiquitin binding function of SQSTM1 and implicate a sequence extrinsic to the compact globular region of the UBA domain as a critical determinant of ubiquitin recognition by the full-length SQSTM1 protein.     

Characteristics of Early Paget's Disease in SQSTM1 Mutation Carriers: Baseline Analysis of the ZiPP Study Cohort

Mutations in SQSTM1 are strongly associated with Paget's disease of bone (PDB), but little is known about the clinical characteristics of those with early disease. Radionuclide bone scans, biochemical markers of bone turnover, and clinical characteristics were analyzed in SQSTM1 mutation carriers who took part in the Zoledronic acid in the Prevention of Paget's disease (ZiPP) study. We studied 222 individuals, of whom 54.9% were female, with mean ± SE age of 50.1 ± 0.6 years. Twelve SQSTM1 mutations were observed, including p.Pro392Leu, which was present in 141 of 222 (63.5%) subjects. Bone scan examination revealed evidence of PDB in 20 subjects (9.0%), ten of whom (50%) had a single affected site. Participants with lesions were older than those without lesions but the difference was not significant (53.6 ± 9.1 versus 49.8 ± 8.9; p = .07). The mean age of participants with lesions was not significantly different from the age at which their parents were diagnosed with PDB (55 years versus 59 years, p = .17). All individuals with lesions were asymptomatic. Serum concentrations of total alkaline phosphatase (ALP) normalized to the upper limit of normal in each center were higher in those with lesions (0.75 ± 0.69 versus 0.42 ± 0.29 arbitary units; p < .0001). Similar findings were observed for other biochemical markers of bone turnover, but the sensitivity of ALP and other markers in detecting lesions was poor. Asymptomatic PDB is present in about 9% of SQSTM1 mutation carriers by the fifth decade. Further follow-up of this cohort will provide important information on the natural history of early PDB and its response to treatment. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.     

Paget's disease of bone in the French population: novel SQSTM1 mutations, functional analysis, and genotype-phenotype correlations.

Mutation screening of the SQSTM1 gene in 94 French patients with PDB revealed two novel point-mutations (A381V and L413F) and two new compound heterozygous genotypes (P392L/A381V and P392L/A390X). Functional analysis showed an increased level of SQSTM1/p62 protein in PDB patients and truncated forms of the protein encoded by the A390X allele. Clinical data indicate that PDB patients with SQSTM1 mutation are younger at PDB diagnosis and have more extensive bone lesions. INTRODUCTION: Paget's disease of bone (PDB) is a common chronic disease of the skeleton, with a strong genetic component. A recurrent mutation (P392L) was first identified on chromosome 5, in the Sequestosome 1 (SQSTM1) gene. Several other mutations of the SQSTM1 gene have been described in PDB patients, affecting the ubiquitin-associated domain (UBA) of the SQSTM1/p62 protein. The objectives of this study were to evaluate the frequency of the SQSTM1 mutations in French PBD patients, to study the expression of the SQSTM1/p62 protein, and to search for genotype-phenotype correlations. MATERIALS AND METHODS: Blood was obtained from 94 unrelated French PDB patients and 100 controls for mutation screening of exons 7 and 8, encoding for the UBA domain of SQSTM1. Epstein-Barr virus (EBV)-immortalized B-cell lymphocytes were established from 13 patients, giving access to functional analysis of the gene and the SQSTM1/p62 expressions using real-time PCR and Western blot. RESULTS: Mutations of the SQSTM1 gene were identified in 12 of the 94 PDB patients (13%). Eight patients carried P392L. Two novel missense mutations were identified: L413F and A381V. This A381V mutation and A390X were found in distinct patients already carriers of P392L. The SQSTM1/p62 protein expression in PDB patients increased when zero, one, or two mutations were present, and SQSTM1 truncated forms were associated with the A390X mutation. The mean age of PDB diagnosis was younger in patients with the SQSTM1 mutation. PDB was more extensive in patients who carried a SQSTM1 mutation. CONCLUSIONS: Mutations of SQSTM1 are present in the French population. PDB patients with and without the SQSTM1 mutation have an increased level of SQSTM1/p62, caused by overproduction of the protein, probably involved in the pathophysiology of PDB. The presence of the SQSTM1 mutation may be a worsening factor for PDB.     

Two novel mutations at exon 8 of the Sequestosome 1 (SQSTM1) gene in an Italian series of patients affected by Paget's disease of bone (PDB).

PDB is genetically heterogeneous. Mutations of the sequestosome1 gene have been reported in sporadic and familial forms of Paget's in patients of French Canadian and British descent. Mutational analyses in different ethnic groups are needed to accurately investigate hereditary diseases. We describe two novel mutations of sequestosome1 in 62 Italian sporadic patients, confirming the role of the encoded protein in this disorder. INTRODUCTION: Paget's disease of bone (PDB) is a relatively common disease of bone metabolism reported to affect up to 3% of whites over 55 years of age. The disorder is genetically heterogeneous, and at present, there is scientific evidence that at least eight different human chromosomal loci are correlated with its pathogenesis. Mutations of the sequestosome1 (SQSTM1) gene were identified as responsible for most of the sporadic and familial forms of Paget in patients of French Canadian and British descent. Such mutations were located at exon 7 and 8 levels, encoding for the ubiquitin protein-binding domain (UBA) and representing a mutational hot spot area. MATERIALS AND METHODS: To verify the involvement of this gene in Italian subjects affected by PDB, we performed mutational analysis in 62 sporadic PDB cases. RESULTS: We described three different mutations at exon 8 level: P392L, already described in the French Canadian population and families predominantly of British descendent, and two novel mutations consisting of the amino acid substitutions M404V and G425R. No significant differences in the clinical history of PDB have been observed in patients with SQSTM1 mutations in respect to those without. CONCLUSIONS: Even though our findings suggest a minor involvement of the SQSTM1 gene in the pathogenesis of sporadic Italian Paget's cases, the identification of different significant mutations within the SQSTM1 gene in unrelated, but clinically similar individuals, offers extremely convincing evidence for a causal relationship between this gene and PDB. Longitudinal studies are needed to assess the penetrance of genotype/phenotype correlations. Our findings confirm the evidence of a clustered mutation area at this level in this disorder.     

Common susceptibility alleles and SQSTM1 mutations predict disease extent and severity in a multinational study of patients with Paget's disease

Paget's disease of bone (PDB) has a strong genetic component. Here, we investigated possible associations between genetic variants that predispose to PDB and disease severity. Allelic variants identified as predictors of PDB from genome‐wide association studies were analyzed in 1940 PDB patients from the United Kingdom, Italy, Western Australia, and Spain. A cumulative risk allele score was constructed by adding the variants together and relating this to markers of disease severity, alone and in combination with SQSTM1 mutations. In SQSTM1‐negative patients, risk allele scores in the highest tertile were associated with a 27% increase in disease extent compared with the lowest tertile (p < 0.00001) with intermediate values in the middle tertile (20% increase; p = 0.0007). The effects were similar for disease severity score, which was 15% (p = 0.01) and 25% (p < 0.00001) higher in the middle and upper tertiles, respectively. Risk allele score remained a significant predictor of extent and severity when SQSTM‐positive individuals were included, with an effect size approximately one‐third of that observed with SQSTM1 mutations. A genetic risk score was developed by combining information from both markers, which identified subgroups of individuals with low, medium, and high levels of severity with a specificity of 70% and sensitivity of 55%. Risk allele scores and SQSTM1 mutations both predict extent and severity of PDB. It is possible that with further refinement, genetic profiling may be of clinical value in identifying individuals at high risk of severe disease who might benefit from enhanced surveillance and early intervention. © 2013 American Society for Bone and Mineral Research     

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

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.     

Ubiquitin-associated domain mutations of SQSTM1 in Paget's disease of bone: evidence for a founder effect in patients of British descent.

Mutations in the UBA domain of SQSTM1 are a common cause of Paget's disease of bone. Here we show that the most common disease-causing mutation (P392L) is carried on a shared haplotype, consistent with a founder effect and a common ancestral origin. INTRODUCTION: Paget's disease of bone (PDB) is a common condition with a strong genetic component. Mutations affecting the ubiquitin-associated (UBA) domain of sequestosome 1 (SQSTM1) have recently been shown to be an important cause of PDB. The most common mutation results in a proline to leucine amino acid change at codon 392 (P392L), and evidence has been presented to suggest that there may be a recurrent mutation rather than a founder mutation on an ancestral chromosome. Because marked geographical differences exist in the prevalence of PDB, we have investigated the frequency of SQSTM1 mutations in different populations and looked for a founder effect on chromosomes bearing SQSTM1 UBA domain mutations. MATERIALS AND METHODS: We conducted mutation screening of SQSTM1 and performed haplotype analysis using the PHASE software program in 83 kindreds with familial PDB, recruited mainly through clinic referrals in the United Kingdom, Australia, and New Zealand. Similar studies were conducted in 311 individuals with PDB who did not have a family history and 375 age- and sex-matched controls from the United Kingdom. RESULTS: The proportion of patients with familial PDB who had SQSTM1 UBA domain mutations varied somewhat between referral centers from 7.1% (Sydney, Australia) to 50% (Perth, Australia), but the difference between centers was not statistically significant. Haplotype analysis in 311 British patients with PDB who did not have a family history and 375 age- and sex-matched British controls showed that two common haplotypes accounted for about 90% of alleles at the SQSTM1 locus, as defined by common single nucleotide polymorphisms (SNPs) in exon 6 (C916T, G976A) and the 3'UTR (C2503T, T2687G). These were H1 (916T-976A-2503C-2687T) and H2 (916C-976G-2503T-2687G). There was no significant difference in haplotype distribution in PDB cases and controls, but the P392L mutation was found on the H2 haplotype in 25/27 cases (93%), which is significantly more often than expected given the allele frequencies in the normal population (odds ratio, 13.2; 95% CI, 3.1-56.4; p < 0.0001). Similar findings were observed in familial PDB, where 12/13 (92%) of P392L mutations were carried on H2 (odds ratio 17.2; 95% CI, 2.2-138; p = 0.001). CONCLUSIONS: These results provide strong evidence for a founder effect of the SQSTM1 P392L mutation in PDB patients of British descent, irrespective of family history. Our results imply that these individuals share a common ancestor and that the true rate of de novo mutations may be lower than previously suspected.     

Clinical Characteristics and Evolution of Giant Cell Tumor Occurring in Paget's Disease of Bone

Patients with Paget's bone disease (PDB) have an increased risk of developing giant cell tumor (GCT). This study was performed to evaluate the clinical characteristics and evolution of GCT complicating PDB and to compare these clinical characteristics to those observed in two large PDB cohorts, the PDB Italian Registry and the United Kingdom's Multi‐Centre Randomised Controlled Trial of Symptomatic Versus Intensive Bisphosphonate Therapy for Paget's Disease (PRISM) study. A systematic literature review identified 117 cases of PDB complicated by GCT (PDB‐GCT), which involved the skeletal sites affected by PDB (110 patients) or the extraskeletal tissues adjacent to affected bones (7 patients). In contrast to what previously reported for GCT patients without GCT patients (83.2%) were white and one‐fourth of them (24.8%) had multifocal GCTs. Compared to PDB patients without GCT, PDB‐GCT patients showed a higher male/female ratio (2.1 versus 1.2) and more severe disease (age at PDB onset 52.1 ± 12.1 versus 63.3 ± 10.6 years; number of affected sites 6.1 ± 2.9 versus 2.34 ± 1.6; prevalence of polyostotic PDB 93.3% versus 60.6%). The mortality rate of PDB‐GCT patients was higher than those occurring in GCT patients without PDB (about 50% versus 0% to 5% at 5 years) or in PDB patients without GCT (log rank = 29.002). Moreover, up to 98% of PDB‐GCT cases had elevated total alkaline phosphatase levels at neoplasm diagnosis, suggestive of active PDB. Importantly, PDB‐GCT patients from Southern Italy (45.6% of all GCT patients) showed a higher prevalence of multifocal GCT (51.7%) and of positive familial history for PDB (70.8%) and GCT (65.0%). Finally, indirect evidence suggests a decline in the incidence of GCT in PDB patients. The occurrence of GCT in PDB patients is associated with severe disease and reduced life expectancy of affected patients. The increased prevalence of familial diseases in PDB‐GCT patients from Southern Italy suggests a founder effect. The observed changes over time in the incidence of GCT in PDB patients could be related to improved clinical management and/or living conditions of patients. © 2014 American Society for Bone and Mineral Research.     

Genetic Epidemiology of Paget’s Disease of Bone in Italy: sequestosome1/p62 Gene Mutational Test and Haplotype Analysis at 5q35 in a Large Representative Series of Sporadic and Familial Italian Cases of Paget’s Disease of Bone

Families affected by Paget’s disease of bone frequently harbor mutations in the SQSTM1/p62 gene. In this multicentric study we collected 345 sporadic and 12 familial PDB cases throughout Italy, identifying 12 different mutations, 5 of which are newly reported and 3, D335E, A381V, and Y383X, external to the UBA domain. Subjects with truncating mutations, E396X, showed a significantly younger age at clinical diagnosis, while the Y383X subjects had a higher average number of affected skeletal sites. All the mutants exhibited the CGTG-H2 haplotype. In two pairs and one triad of unrelated Italian PDB families from different Italian regions, we detected a common SQSTM1/p62 mutation for each P392L, M404V, and G425R group. Since the CGTG-H2 haplotype frequency was also high in normal subjects, and genetic influence due to migratory fluxes of different ethnic groups exists in the Italian population, to refine the search for a more geographically specific founder effect, we extended the haplotype analysis in these families using polymorphic microsatellite repeat markers, within and flanking the SQSTM1/p62 locus, from chromosome 5q35, other than the exon 6 and 3′UTR polymorphisms. All mutant carriers from two of the three M404V families and from the G425R families exhibited common extended chromosome 5q35 haplotypes, IT01 and IT02, respectively, which may be reflecting influences of past migrations. This may be helpful in estimating the true rate of de novo mutations. We confirm the data on the existence of both a mutational hotspot at the UBA domain of SQSTM1/p62 and a founder effect in the PDB population.     

Evaluation of the Role of the SQSTM1 Gene in Sporadic Belgian Patients with Paget’s Disease

A positional cloning effort in French Canadian families with Pagets disease of bone (PDB) resulted in the identification of a mutation in the sequestosome1 (SQSTM1) gene in a subset of both familial and sporadic PDB cases. This was confirmed in samples of mainly United Kingdom (UK) origin. In this study, we performed both mutation analysis and association studies in order to evaluate the role of this gene in a collection of isolated Belgian PDB patients. A mutation in the SQSTM1 gene was found in only 6 of 111 patients (5.4%). In all cases it involves the P392L mutation, previously shown to be common in both familial and sporadic cases. To perform association studies, we selected 8 single nucleotide polymorphisms (SNPs) and looked for linkage disequilibrium (LD) between these. Haplotype analysis indicated that typing of 3 Tag SNPs (IVS1+633A/C, IVS5–23A/G, and 976A/G) enables us to identify the most common haplotypes. Association studies for the 3 selected SNPs, based on 105 PDB cases without a SQSTM1 mutation and 159 control individuals, did not support a possible influence of natural variants in the SQSTM1 gene either on the pathogenesis of PDB or on the disease severity. In conclusion, our study confirms that the P392L mutation is a recurrent mutation causing PDB in different populations. We were not able to show an association between SQSTM1 polymorphisms and PDB in our population but this clearly needs to be extended to other populations. The presented identification of haplotype Tag SNPs will be of major help for such studies.     

Development of a molecular test of Paget's disease of bone

Depending on populations, 15 to 40% of patients have a familial form of Paget's disease of bone (PDB), which is transmitted in an autosomal-dominant mode of inheritance with incomplete penetrance. To date, only SQSTM1 gene mutations have been linked to the disease. Several single nucleotide polymorphisms (SNPs) have been associated with PDB in patient non-carriers of SQSTM1 mutations, but they have minor size effects. The current clinical practice guidelines still recommend to measure total serum alkaline phosphatase (sALP) for PDB screening. However, genetic or bone biomarkers alone may lack sensitivity to detect PDB. Thus, the objective of this study was to develop a molecular test of PDB, combining genetic and bone biomarkers, in order to detect PDB, which is frequently asymptomatic. We genotyped 35 SNPs previously associated with PDB in 305 patients, and 292 healthy controls. In addition, serum levels of 14 bone biomarkers were assayed in 51 patients and 151 healthy controls. Bivariate and multivariate logistic regression models with adjustment for age and sex were fitted to search for a combination of SNPs and/or bone biomarkers that could best detect PDB in patient non-carriers of SQSTM1 mutations. First, a combination of five genetic markers gave rise to the highest area under the ROC curve (AUC) with 95% confidence interval [95% CI] of 0.731 [0.688; 0.773], which allowed us to detect 81.5% of patients with PDB. Second, a combination of two bone biomarkers had an AUC of 0.822 [0.726; 0.918], and was present in 81.5% of patients with PDB. Then, the combination of the five genetic markers and the two bone biomarkers increased the AUC up to 0.892 [0.833; 0.951], and detected 88.5% of patients with PDB. These results suggested that an algorithm integrating first a screen for SQSTM1 gene mutations, followed by either a genetic markers combination or a combined genetic and biochemical markers test in patients non-carrier of any SQSTM1 mutation, may detect the PDB phenotype better than biomarkers already available in the clinical practice.     

Sequestosome 1 (SQSTM1) Mutations in Paget’s Disease of Bone from the United States

Paget’s disease of bone (PDB) is a localized bone disease characterized by excessive bone resorption due to overactive osteoclasts. Seven genetic loci (PDB1-PDB7) have been reported for late-onset PDB. PDB3 is the only locus where a gene, sequestosome 1 (SQSTM1), has been identified. Mutations in SQSTM1 have been associated with both sporadic and hereditary PDB in different populations. However, the SQSTM1 mutation frequency in PDB patients from a more heterogeneous population has never been reported. To investigate this, we determined the frequency of mutations in patients from the United States. Blood was collected from sporadic and hereditary PDB patients in the United States. DNA was isolated from whole blood or from serum. The SQSTM1 sequence was determined for exons and intron/exon junctions from whole blood and serum. A total of 112 (39 hereditary, 73 sporadic) samples were collected. Eight mutations were found in hereditary PDB patients, for a mutation frequency of 20.5% (95% confidence interval [CI] 10.8–35.5%) and did not differ significantly from mutation rates observed in studies in Canada, Great Britain, and The Netherlands. No mutations were found in sporadic patients, for a frequency of 0% (95% CI 0.0–5.0%), which was statistically significantly lower than the mutation rates previously observed in populations from Australia (P = 0.009), Canada (P = 0.008), Great Britain (P = 0.02), and France (P = 0.04) but not compared to rates from Belgium, The Netherlands, and Italy. Four out of five families with the P392L mutation carried it on the H2 haplotype. Mutations in SQSTM1 seem to contribute to the pathogenesis of PDB in hereditary, but not sporadic, patients in the United States.     

Epidemiogenetic study of French families with Paget's disease of bone

ObjectiveTo search for association with environmental factors and to determine SQSTM1/p62 mutations prevalence in French families with Paget's disease of bone (PDB).MethodsUnrelated patients with a confirmed diagnosis of PDB were recruited in three Rheumatology departments and informed consent obtained. First- and second-degree relatives of each index case had a physical examination, blood taken for DNA extraction and biochemical measurements, and a whole-body bone scan. Exons 7 and 8 and exon-intron boundaries of SQSTM1/p62 (p62) gene were PCR-amplified before sequencing. Haplotype carriers of the p62^P392L mutation were determined. Comparisons between PDB patients and healthy relatives were performed.ResultsWe investigated 18 families consisting of 83 individuals: 20 patients with known PDB, three relatives with newly-diagnosed PDB and 60 healthy relatives. Index cases and/or relatives with Dupuytren's disease were found in eight (44.4%) out of the 18 families. Forty-three percent of PDB patients were former or current tobacco users versus 18% of healthy relatives (P = 0.02; OR = 3.37 (1.04–11.09)). Five index cases (27.8%) were carriers of SQSTM1/p62 mutations: three p62^P392L mutations, one p62^P392L/A390X double mutation and one p62^A390X mutation. The p62^P392L mutation was carried by haplotype 2 in all four index cases.ConclusionAccurate phenotypic assessment of PDB patients’ relatives allowed for diagnosing PDB in three asymptomatic relatives. There was evidence for an aggregation of Dupuytren's disease in PDB families (not associated with SQSTM1/p62 mutation), and for an association between PDB and tobacco use. Half of PDB familial forms carried a SQSTM1/p62 mutation, p62^P392L mutation being the most frequent.     

Characterization of a Non‐UBA Domain Missense Mutation of Sequestosome 1 (SQSTM1) in Paget's Disease of Bone

Mutations affecting the ubiquitin‐associated (UBA) domain of sequestosome 1 (SQSTM1/p62) are commonly found in Paget's disease of bone (PDB) and impair SQSTM1's ability to bind ubiquitin, resulting in dysregulated NF‐κB signaling. In contrast, non‐UBA domain mutations are rarer, and little is known about how they manifest their effects. We present the first characterization at the molecular, cellular, and functional level of a non‐UBA domain missense mutation (A381V) of SQSTM1. Direct sequencing of exon 7 of the SQSTM1 gene in an Italian PDB patient detected a heterozygous C to T transversion at position 1182, resulting in an alanine to valine substitution at codon 381. Pull‐down assays showed the non‐UBA region of SQSTM1 that contains A381 is important in mediating ubiquitin‐binding affinity and that the A381V mutation exerts weak negative effects on ubiquitin binding. Structural and binding analyses of longer UBA constructs containing A381, using NMR spectroscopy and circular dichroism, showed this region of the protein to be largely unstructured and confirmed its contribution to increased ubiquitin‐binding affinity. Co‐transfections of U20S cells showed that the A381V mutant SQSTM1 co‐localized with ubiquitin with a cellular phenotype indistinguishable from wildtype. Finally, effects of the wildtype and mutant SQSTM1 on NF‐κB signaling were assessed in HEK293 cells co‐transfected with an NF‐κB luciferase reporter construct. A381V mutant SQSTM1 produced a level of activation of NF‐κB signaling greater than wildtype and similar to that of UBA domain mutants, indicating that non‐UBA and UBA domain mutations may exert their effects through a common mechanism involving dysregulated NF‐κB signaling.     

SQSTM1 and Paget’s Disease of Bone

Mutations in the Sequestosome 1 gene (SQSTM1; also known as p62) have recently been identified as the cause of 5q35-linked Pagets disease of bone (PDB). All of the mutations identified to date affect the ubiquitin-associated (UBA) domain of SQSTM1, a region of the protein that binds noncovalently to ubiquitin. In this review we consider the possible functional significance of the SQSTM1-ubiquitin interaction, and consequences of the SQSTM1 UBA domain mutations. Clarification of the in vivo roles of SQSTM1 in bone-cell function will be central to improving our understanding of the molecular pathogenesis of PDB and related conditions.