Abstract:
The subject of this thesis is the molecular genetic study of idiopathic hyperphosphatasia. The work in this thesis describes linkage analysis, mutation screening of candidate genes and the functional analysis of mutant proteins expressed in patients with a clinical diagnosis of idiopathic hyperphosphatasia.
Idiopathic hyperphosphatasia is an autosomal recessive bone disease characterized by excessive bone resorption and bone formation. Affected children are normal at birth but develop deformities of long bones, kyphosis and acetabular protrusion with increasing severity as they pass through adolescence. There is considerable variability in phenotype, with some cases diagnosed in infancy and others in later childhood. A genome-wide search of a New Zealand family affected by idiopathic hyperphosphatasia suggested linkage to a locus on the long arm of chromosome 8 (8q24). The gene TNFRSF11B encoding osteoprotegerin (OPG), which lies within 8q24, was an obvious candidate gene given the critical role of OPG in regulating osteoclast development. Mutation screening of this gene indicated an apparent disease-causing mutation in exon 3 in affected individuals of the New Zealand family. Subsequently eight families, recruited by the members of the Idiopathic Hyperphosphatasia Collaborative Group in Turkey, Germany, Argentina and the United kingdom were also screened for mutations in the TNFRSF11B gene. Recombinant wild-type and mutant OPG CDNAs were expressed in human epithelial kidney cells, and secreted OPG was collected form the conditioned medium. In vitro measurements of osteoclastic bone resorption showed that wild type OPG suppressed bone resorption, whereas the mutant forms did not, confirming them to be inactivating mutations.