Genetic and environmental determinants of Paget's disease of bone.
Rios Visconti, Micaela
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Genetic factors play an important role in the pathogenesis of Paget’s Disease of Bone (PDB). The most important predisposing gene is SQSTM1 which is mutated in about 10% of patients, additionally common variants at seven other loci have also been shown to predispose to PDB as well as environmental factors which are also important in the pathogenesis of PDB. Little research has been conducted on the relationship between the genetic variants that predispose to PDB and disease severity. Similarly, only limited information exists on the role that gene-environment interactions play in the pathogenesis of PDB or its severity. The aim of the present thesis was to explore these issues in participants of the Paget’s Disease Randomised Trial of Intensive versus Symptomatic Management study (PRISM) and other study cohorts. In chapter 3, I investigate the relationship between SQSTM1 mutation status, disease severity and clinical outcome in 737 patients from the PRISM study. Mutations of SQSTM1 were detected in 80/737 (10.9%) patients. Mutation carriers had an earlier age at diagnosis; a greater number of affected bones and more commonly had required orthopaedic surgery and bisphosphonate therapy than those without mutations. Quality of life was significantly reduced in carriers and during the study; fractures were more common although most of these occurred in unaffected bone. This study demonstrates that SQSTM1 mutations are strongly associated with disease severity and complications of PDB. In chapter 4, I study associations between common genetic variants identified by genome wide association (GWAS), clinical severity and extent of PDB, alone and in combination with SQSTM1 mutations. This showed that these common variants were also associated with severity and extent of PDB in PRISM, but with weaker effects than SQSTM1 mutations. The findings were replicated in a multinational study involving 1940 subjects from centres in Italy, Spain and Australia. In all cohorts the GWAS risk alleles acted in an additive manner with SQSTM1 mutations to regulate disease severity and extent. By combining information from SQSTM1 status and the new risk alleles, however, we are able to develop a genetic risk score which delineated three distinct groups with markedly differing effects on disease extent and severity. In chapter 5, I study associations between PDB, severity and extent in relation to circulating levels of IgG antibodies against various viruses including Rubella, respiratory syncytial virus, distemper, varicella zoster virus, measles and mumps. We found little evidence of an interaction between viral antibody titres and SQSTM1 in predicting disease severity with the notable exception of mumps virus where subjects with the highest levels of antibodies that were SQSTM1 positive had in increased age at diagnosis than the other genotype / viral antibody groups. Overall the studies do provide no support for the notion that patients with PDB have an abnormal antibody response to paramyxovirus or have had previous infections with these viruses more frequently than controls. This of course does not exclude the possibility that PDB patients might have a clinically occult slow virus infection which is not accompanied by an abnormality in the immune response. . This raises the possibility that genetic testing may be of value in identifying individuals at risk of developing severe disease and those at risk of complications. I also demonstrate that PBD patients have abnormalities in circulating antibodies to various viruses suggesting that the disease may be associated with disturbance in the response of the immune system to infectious agents but further investigation is required. This, perhaps, could explain the changes in the severity and prevalence of PDB that have been observed over recent years in several countries.