Lack of mutations in the RANK gene in Spanish patients with Paget disease of bone

[Paget's disease of bone: 1877-2023. Etiology, and management of a disease on epidemiologic transition]

Paget's disease of bone is characterized by the alteration, in one or several bone locations, of the equilibrium between bone formation and bone resorption. This imbalance results in a disorganized, widened bone, in many cases with increased bone density, although more fragile. A genetic predisposition for Paget's disease of bone could explain between 5% and 40% of the cases. Different environmental factors should explain the rest of the cases. Paget's disease of bone was classically considered the second most common metabolic bone disease. However, in recent decades there has been a marked decrease in both incidence and clinical severity. These changes have led to believe that the influence of some environmental factor may have diminished or even disappeared. This decrease in incidence should not be an excuse for abandoning Paget's disease of bone research, but rather it should be the reason to remain searching to try to understand better its pathogenesis.

A nonsynonymous TNFRSF11A variation increases NFκB activity and the severity of Paget's disease

Mutations in the SQSTM1 gene were identified as a common cause of Paget's disease of bone (PDB) but experimental evidence demonstrated that SQSTM1 mutation is not sufficient to induce PDB in vivo. Here, we identified two nonsynonymous single nucleotide polymorphisms (SNPs) (C421T, H141Y and T575C, V192A) in the TNFRSF11A gene, associated with PDB and with the severity of phenotype in a large population of 654 unrelated patients that were previously screened for SQSTM1 gene mutations. The largest effect was found for the T575C variant, yielding an odds ratio of 1.29 (p = 0.003), with the C allele as the risk allele. Moreover, an even more significant p-value (p = 0.0002) was observed in the subgroup of patients with SQSTM1 mutation, with an odds ratio of 1.71. Interestingly, patients with the C allele also showed an increased prevalence of polyostotic disease (68%, 53%, and 51% in patients with CC, CT, and TT genotypes, respectively; p = 0.01), as well as an increased number of affected skeletal sites (2.9, 2.5, and 2.0 in patients with CC, CT, and TT genotypes, respectively, p = 0.008). These differences increased when analyses were restricted to cases with SQSTM1 mutation. In human cell lines, cotrasfection with mutated SQSTM1 and TNFRSF11A(A192) produced a level of activation of NFκB signaling greater than cotrasfection with wild-type SQSTM1 and TNFRSF11A(V192), confirming genetics and clinical evidences. These results provide the first evidence that genetic variation within the OPG/RANK/RANKL system influences the severity of PBD in synergistic action with SQSTM1 gene mutations.

Genetic variation in the TNFRSF11A gene encoding RANK is associated with susceptibility to Paget's disease of bone

RANK (receptor activator of nuclear factor-κB), encoded by TNFRSF11A, is a key protein in osteoclastogenesis. TNFRSF11A mutations cause Paget's disease of bone (PDB)-like diseases (ie, familial expansile osteolysis, expansile skeletal hyperphosphatasia, and early-onset PDB) and an osteoclast-poor form of osteopetrosis. However, no TNFRSF11A mutations have been found in classic PDB, neither in familial nor in isolated cases. To investigate the possible relationship between TNFRSF11A polymorphisms and sporadic PDB, we conducted an association study including 32 single-nucleotide polymorphisms (SNPs) in 196 Belgian sporadic PDB patients and 212 control individuals. Thirteen SNPs and 3 multimarker tests (MMTs) turned out to have a p value of between .036 and 3.17 × 10(-4) , with the major effect coming from females. Moreover, 6 SNPs and 1 MMT withstood the Bonferroni correction (p < .002). Replication studies were performed for 2 nonsynonymous SNPs (rs35211496 and rs1805034) in a Dutch and a British cohort. Interestingly, both SNPs resulted in p values ranging from .013 to 8.38 × 10(-5) in both populations. Meta-analysis over three populations resulted in p = .002 for rs35211496 and p = 1.27 × 10(-8) for rs1805034, again mainly coming from the female subgroups. In an attempt to identify the underlying causative SNP, we performed functional studies for the coding SNPs as well as resequencing efforts of a 31-kb region harboring a risk haplotype within the Belgian females. However, neither approach resulted in significant evidence for the causality of any of the tested genetic variants. Therefore, further studies are needed to identify the real cause of the increased risk to develop PDB shown to be present within TNFRSF11A.

Pathogenesis of Paget's disease of bone

Paget's disease of bone is a common condition characterised by increased and disorganised bone turnover which can affect one or several bones throughout the skeleton. These abnormalities disrupt normal bone architecture and lead to various complications such as bone pain osteoarthritis, pathological fracture, bone deformity, deafness, and nerve compression syndromes. Genetic factors play an important role in PDB and mutations or polymorphisms have been identified in four genes that cause classical Paget's disease and related syndromes. These include TNFRSF11A, which encodes RANK, TNFRSF11B which encodes osteoprotegerin, VCP which encodes p97, and SQSTM1 which encodes p62. All of these genes play a role in the RANK-NFkappaB signalling pathway and it is likely that the mutations predispose to PDB by disrupting normal signalling, leading to osteoclast activation. Although Paget's has traditionally be considered a disease of the osteoclast there is evidence that stromal cell function and osteoblast function are also abnormal, which might account for the fact that the disease is associated with increased bone formation as well as resorption. Environmental factors also contribute to Paget's disease. Most research has focused on paramyxovirus infection as a possible environmental trigger but evidence in favour of the involvement of viruses in the disease remains conflicting. Other factors which have been implicated as possible disease triggers include mechanical loading, dietary calcium and environmental toxins. Further work will be required to identify additional genetic variants that predispose to Paget's disease and to determine how the causal mutations and predisposing polymorphisms interact with environmental factors to influence bone cell function and cause the focal bone lesions that are characteristic of the disease.

Identification of sex-specific associations between polymorphisms of the osteoprotegerin gene, TNFRSF11B, and Paget's disease of bone

We studied the role of TNFRSF11B polymorphisms on the risk to develop Paget's disease of bone in a Belgian study population. We observed no association in men, but a highly significant association was found in women, and this was confirmed in a population from the United Kingdom.

INTRODUCTION

Juvenile Paget's disease has been shown to be caused by mutations in TNFRSF11B encoding osteoprotegerin. Although mutations in this gene have never been found in patients with typical Paget's disease of bone (PDB), there are indications that polymorphisms in TNFRSF11B might contribute to the risk of developing PDB.

MATERIALS AND METHODS

We recruited a population of 131 Belgian patients with sporadic PDB and 171 Belgian controls. By means of the HapMap, we selected 17 SNPs that, in combination with four multimarker tests, contain most information on common genetic variation in TNFRSF11B. To replicate the findings observed in the Belgian study population, genotyping data of SNPs generated in a UK population were reanalyzed.

RESULTS

In our Belgian study population, associations were found for two SNPs (rs11573871, rs1485286) and for one multimarker test involving rs1032129. When subsequently analyzing men and women separately, these associations turned out to be driven by women (56 cases, 78 controls). In addition, three other tagSNPs turned out to be associated in women only. These were rs2073617 (C950T), rs6415470, and rs11573869. Reanalysis of genotyping data from a UK study population indicated that the associations found for C950T and C1181G were also exclusively driven by women (146 cases, 216 controls). Meta-analysis provided evidence for risk increasing effects of the T allele of C950T and the G allele of C1181G in the female population (p = 0.002 and 0.003, respectively). The haplotypes formed by the SNPs associated in the Belgian population were also distributed differentially between female cases and controls.

CONCLUSIONS

We showed for the first time that SNPs influencing the risk to develop PDB could be sex-specific. Further research is necessary to identify the causative variants in TNFRSF11B and to elucidate the molecular pathogenic mechanism.

Paget's disease of bone in Italy

Epidemiological studies of Paget's disease of bone (PDB) suggest a pronounced geographical variation in the prevalence of the disease and a decrease in prevalence and clinical severity over time. To analyze epidemiological and clinical features of PDB in Italy, we recently established a registry of Italian PDB cases and performed radiological, biochemical, and bone scan surveys in the towns of Siena and Turin. The overall prevalence of PDB in Italy varied between 0.7% and 2.4%. Prevalence rates increased with age and were higher in men than in women. We observed clinically confirmed familial aggregation in 15-26% of cases. Pedigree analysis indicated an autosomal dominant pattern of inheritance with variable penetrance. SQSTM1 gene analysis in two Italian studies revealed the presence of at least three different mutations accounting for both familial and sporadic cases. Interestingly, no decrease in the prevalence of PDB over time was observed, the opposite of what is described in populations of British descent. However, clinical severity of PDB cases included in the Registry in 2002-2004 seemed reduced with respect to that of PDB patients from the previous epidemiological studies, including a 1950-1956 Italian study. Of interest, a consistent association between PDB and animal-related factors and a significantly higher prevalence of the disease in rural than in urban districts were observed. These findings are in keeping with an important role of the environment in the pathogenesis of PDB, perhaps facilitating the expression of the disease in genetically susceptible subjects. Finally, there was also preliminary evidence indicating regional clustering of PDB in Italy, with a concentration of cases in rural districts of Campania and Tuscany. These districts may represent high prevalence areas of PDB in Italy, similar to what has been observed in other countries. Extrapolation estimates suggest that approximately 150,000-300,000 subjects may be affected with PDB in our country. These results confirm PDB to be the most common bone remodeling disorder in elderly people in Italy, excluding osteoporosis.

Identification and molecular characterization of a novel splice-site mutation (G1205C) in the SQSTM1 gene causing Paget's disease of bone in an extended American family

Paget's disease of bone (PDB) is a common late-onset bone disorder characterized by focal areas of abnormal bone remodeling. Positional cloning efforts resulted in the identification of seven genetic loci (PDB1-7) with putative involvement in the pathogenesis of PDB. Meanwhile, the PDB-causing gene from the PDB3 region on chromosome 5q35 has been identified as the SQSTM1 gene. All mutations identified in this gene so far are located in or close to the ubiquitin-associated (UBA) domain of the protein. In 2001, we reported genotyping results of genetic markers located in the PDB3 region in an extended American family, indicating the involvement of the PDB3 locus. Here, we report the identification of a novel mutation (G1205C) in the SQSTM1 gene in this family. The G1205C mutation is located in the splice donor site of intron 7 and reverse-transcription polymerase chain reaction experiments showed that the presence of the C allele results in the production of two abnormal mRNA transcripts. Translation of the first transcript would result in a protein that lacks amino acids 351-388, including 26 amino acids of the second PEST domain in addition to two amino acids of the UBA domain. The second mutant mRNA transcript could result in a truncated protein (390X) that lacks almost the complete UBA domain. PDB mutations that disrupt the function of the PEST domain of SQSTM1 have not been reported before, so probably the pathogenic effect of both transcripts resides in the disruption of the ubiquitin-binding properties of the protein.

[Familial expansive osteolysis otological and dental manifestations of genetic origin]

OBJECTIVES

Familial Expansive Osteolysis (FEO) ist a rare autosomal dominant bone dysplasia. The disease can show general and focal skeletal alterations, the latter having a predominantly peripheral distribution. Onset occurs after the second decade of life.

PATIENTS AND METHODS

We present the study, of 30 years, of a family consisting of 49 members covering five generations.

RESULTS

Among the 35 members studied, 18 have familial expansive osteolysis (FEO). The first clinical sign of the condition is transmission deafness at an early age. The features of the teeth has a unique and characteristic appearance. Thinning of the cortical bone leads to severe, painful, disabling deformities. Serum alkaline phosphatase, and urinary hydroxyproline and deoxipyridinoline are elevated. Calcium and parathyroid hormone are normal. Treatment with diphosphonates, calcitonin and vitamin D has been unsuccessful. We present the surgical technology and the results to short and long term of 13 interventions on 8 patients.

CONCLUSION

Progressive osteoclastic reabsorption accompanied by weak osteoblastic activity results in medullary expansion characterized by rarefaction of the bone marrow, which is replaced by fibrous tissue and fat. FEO is histologically similar to Paget disease, but the age of onset, the distribution of the bone lesions, the dental and middle ear alterations, and the clinical progression are different. These features also differentiate FEO from fibrous dysplasia, fibrocystic osteitis and imperfect osteogenesis. The gene responsible for FEO is located in the 18q21-22 chromosome region. Mutations in TNFRSF11A, the gene encoding receptor activator of nuclear factor-kappa-B (RANK), has been recently identified as the cause of FEO. A duplication of 18 base pairs in exon 1 of the TNFRSF11A gene suggests that this corresponds to the site of the anomaly and can be considered a "hot spot" for mutations.

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.