Genetic background of familial primary hyperparathyroidism

Abstract

In this thesis the aim has been to identify and characterise gene(s) involved in familial primary hyperparathyroidism (PHPT). In multiple endocrine neoplasia type 1 (MEN 1), the hyperparathyroidism-jaw tumour syndrome (HPT-JT) and familial isolated hyperparathyroidism (FIHP), in which primary hyperparathyroidism is seen in most or almost all gene carriers.

In Finland monogenic disorders are known to cluster in specific regions. MEN1 mutation screening of Finnish MEN 1 families and isolated cases identified two mutations that were traced back to their respective founder couple by genealogical studies. It is therefore worthwhile to screen for these two mutations in new Finnish MEN 1 patients (Paper I).

The diagnosis of FIHP relies on the exclusion of other familial forms of PHPT e.g. MEN 1 and HPT-JT. In an attempt to clarify the underlying cause of FIHP, the MEN1 gene and the HRPT2 gene locus were investigated by mutation screening, loss of heterozygosity (LOH) and comparative genomic hybridisation (CGH) studies. Four families were positive for MEN1 mutations. Two families have since then developed symptoms suggestive of MEN 1. Thus, genetic screening is important of FIHP families as it may predict disease and allow early detection and appropriate treatment (Paper II and III).

Solitary parathyroid adenomas have been identified in most cases responsible for the PHPT in the HPT-JT related families. In addition, an over-representation of cystic features and increased risk of parathyroid carcinoma have been noted. We characterised clinically and genetically cystic parathyroid adenomas by LOH and immunohistochemistry. In addition, mutation screening was carried out of HRPT2 gene. A significant correlation between tumours with LOH and the clinical parameters such as tumour weight and PTH was found (Paper IV). Three somatic inactivating mutations were identified in the HRTP2 gene implicating its role as a tumour suppressor gene (TSG) and the involvement in the tumourigenesis in a subset of sporadic parathyroid tumours (Paper V).

Using a positional cloning approach, we report the identification of a disease gene for the hyperparathyroidismjaw tumour syndrome (HPT-JT). A combined genotyping study followed by mutation analysis of candidate genes in the critical interval revealed thirteen different heterozygous, germline mutations in a single gene in fourteen HPT-JT kindreds. HRPT2 (1q25) is a tumour suppressor gene consisting of 17 coding exons with a predicted protein of 531 amino acids, termed parafibromin. (Paper V).

The genetic background of FIHP has been considered heterogeneous as many of the reported families have been assigned to the different loci such as the MEN1 and HRPT2. To ftirther elucidate the genetics underlying FIHP, we screened the HRPT2 gene for mutations. We identified four novel HRPT2 mutations and describe for the first time its involvement in FIHP (Paper VI).

The HRPT2 gene is evolutionarily conserved and widely expressed. To further characterize HRPT2, we raised an antibody against the encoded protein, parafibromin. By expression studies such as Western blot and immunohistochemistry, a protein of approximately 60kD was revealed in relevant tissues examined and predominantly localized to the cytosol. These findings suggest a cytoplasmatic function for this protein (Paper VII).