Institutionen för molekylär medicin / Department of Molecular Medicine
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)