Genetic Loci for Paget's Disease of Bone

Paget's disease of the bone is a skeletal disorder of unknown cause. This disease is characterised by excessive and abnormal bone remodelling brought about by increased bone resorption followed by disorganised bone formation. Increased bone turnover results in a disorganised mosaic of woven and lamellar bone at affected skeletal sites. This produces bone that is expanded in size, less compact, more vascular, and more susceptible to deformity or fracture than normal bone. Symptoms of Paget's disease may include bone pain, bone deformity, excessive warmth over bone from hypervascularity, secondary arthritis, and a variety of neurologic complications caused in most instances by compression of the neural tissues adjacent to pagetic bone. Genetic factors play a role in the pathogenesis of Paget's disease but the molecular basis remains largely unknown. The identification of the molecular basis of Paget's disease is fundamental for an understanding of the cause of the disease, for identifying subjects at risk at a preclinical stage, and for the development of more effective preventive and therapeutic strategies for the management of the condition. With this in mind, the aim of this project is to identify genetic loci, in a large pedigree, that may harbour genes responsible for Paget's disease of bone. A large Australian family with evidence of Paget's disease was recruited for these studies (Chapter 3). This pedigree has characterised over 250 individuals, with 49 informative individuals affected with Paget's disease of bone, 31 of whom are available for genotypic analysis. The pattern of disease in these individuals is polystotic, with sites of involvement including the spine, pelvis, skull and femur. Although the affected individuals have a severe early-onset form of the disease, the clinical features of the pedigree suggest that the affected family members have Paget's disease and not familial expansile osteolysis (a disease with some similarities to Paget's disease), as our patients have extensive skull and axial skeletal involvement. The disease is inherited as an autosomal dominant trait in the pedigree with high penetrance by the sixth decade. Due to the large size of this family and multiple affected members, this pedigree is a unique resource for the detection of the susceptibility gene in Paget's disease. The first susceptibility loci for Paget's disease of bone have been mapped by other investigators to chromosome 6p21 (PDB1) and 18q21.1-q22 (PDB2) in different pedigrees. Linkage analysis of the Australian pedigree in these studies was performed with markers at PDB1: these data showed significant exclusion of linkage, with LOD scores < - 2 in this region (Chapter 4). Linkage analysis of microsatellite markers from the PDB2 region excluded linkage with this region also, with a 30 cM exclusion region (LOD score < -2.0) centred on D18S42 (Chapter 4). This locus on chromosome 18q21.1-q22 contains a serine protease (serpin) cluster with similarities to chromosome 6p21. Linkage analysis of this region also failed to provide evidence of linkage to this locus (Chapter 4). These data are consistent with genetic heterogeneity of Paget's disease of bone. A gene essential for osteoclast formation encoding receptor activator of nuclear factor-kB (RANK), TNFRSF11A, has been previously mapped to the PDB2 region. Mutations in the TNFRSF11A gene have been identified segregating in pedigrees with Familial Expansile Osteolysis and early onset familial Paget's disease, however, linkage studies and mutation screening have excluded the involvement of RANK in the majority of Paget's disease patients. For the Australian pedigree, mutation screening at the TNFRSF11A locus revealed no mutations segregating with affected individuals with Paget's disease (Chapter 4). Based on these findings, our hypothesis is that a novel susceptibility gene relevant to the pathogenesis of Paget's disease of bone lies elsewhere in the genome in the affected members of this pedigree; this gene should be identifiable using a microsatellite genome-wide scan followed by positional cloning. A genome-wide scan of the Australian pedigree was carried out, followed by fine mapping and multipoint analysis in regions of interest (Chapter 5). The peak 2-point LOD scores from the genome-wide scan were LOD = 2.75 at D7S507 and LOD = 1.76 at D18S70. Two additional regions were also considered for fine mapping: chromosome 19p11-q13.1 with a LOD of 1.58 and chromosome 5q35-qter with a LOD of 1.57. Multipoint and haplotype analysis of markers flanking D7S507 did not support linkage to this region (Chapter 5). Similarly, fine mapping of chromosome 19p11-q13.1 failed to support linkage to this region (Chapter 5). Linkage analysis with additional markers in the region on chromosome 5q35-qter revealed a peak multipoint LOD score of 6.77 (Chapter 5). A distinct haplotype was shown to segregate with all members of the family, except the offspring of III-5 and III-6. Haplotype analysis of markers flanking D18S70 demonstrated a haplotype segregating with Paget's disease in a large sub-pedigree (descendants of III-3 and III-4) (Chapter 5). This sub-pedigree had a significantly lower age at diagnosis than the rest of the pedigree (51.2 + 8.5 vs. 64.2 + 9.7 years, p = 0.0012). Linkage analysis of this sub-pedigree demonstrated a peak two-point LOD score of 4.23 at marker D18S1390 (q = 0.00), and a peak multipoint LOD score of 4.71, at marker D18S70. An implication of these data is that 18q23 harbours a novel modifier gene for reducing the age of onset of Paget's disease of bone. A number of candidate Paget's genes have previously been identified on chromosome 18q23, including the nuclear factor of activated T cells (NFATc1), membrane-associated guanylated kinase (MAGUK) and a zinc finger protein. Candidate gene sequencing of these genes in these studies has failed to identify mutations segregating with affected family members in the sub-pedigree linked to chromosome 18q23 (Chapter 6). More recently, a mutation in the gene encoding the ubiquitin-binding protein sequestosome 1 (SQSTM/p62) has been shown to segregate with affected members of Paget's disease families of French-Canadian origin. In this study, a single base pair deletion (1215delC) was identified as segregating with the majority of affected members in the pedigree (Chapter 6). This deletion introduces a stop codon at amino acid position 392 which potentially results in early termination of the protein and loss of the ubiquitin binding domain. The three affected members of the family that do not share the affected haplotype do not carry a mutation in the coding region of SQSTM/p62. Screening of affected members from 10 further Paget's disease families identified the previously reported P392L mutation in 2 (20%) families. No SQSTM1/p62 coding mutations have been found in the remaining 8 families or in 113 aged matched controls. In conclusion, this project has identified genetic loci and mutations that segregate with individuals affected with Paget's disease. Further investigation of the functional significance of the genetic changes at these loci is expected to lead to a better understanding of the molecular basis of this disease.