| dc.description.abstract | A variable proportion of neuroendocrine tumours arise in the setting of familial tumour-predisposing conditions as a result of a genetic mutation. Owing to de novo mutations or incomplete penetrance, predisposing mutations can also be identified in patients with sporadic clinical presentation. While several advances have been made in the identification of the causative genes for the more common inheritable endocrine neoplasia syndromes, in rare instances the causative mutations remain to be identified. In this thesis, I aimed at identifying and characterising novel pathogenic genetic mutations in familial and sporadic neuroendocrine tumours affecting the endocrine pancreas (familial insulinomatosis), the thyroid (familial medullary thyroid carcinoma) and the pituitary gland (X-linked acrogigantism - XLAG). In the first chapter, I describe the identification of a novel germline mutation in the islet-enriched transcription factor MAFA (c.191C>T, p.S64F) as the cause of a sexually dimorphic phenotype of familial insulinomatosis and diabetes mellitus, a condition characterised by either hyperinsulinaemic hypoglycaemia secondary to multiple pancreatic insulinomas (i.e. insulinomatosis – occurring predominantly in females) or non-insulin-dependent diabetes mellitus (occurring prevalently in males). After establishing a suitable in vitro β cell model, I showed that the mutation impairs phosphorylation at the N-terminal transactivation domain of MAFA, resulting in impaired glucose-stimulated insulin secretion as well as reduced cell proliferation and increased susceptibility of β cells to apoptosis. Studies of glucose metabolism in vivo highlighted defective early phase insulin secretion exclusively in males, while female carriers of the MAFA mutation showed hyperinsulinism, confirming the sexually biased nature of these phenotypes. In the second chapter, I have described the phenotype of two related families with autosomal dominant inheritance of medullary thyroid carcinoma (MTC), a neuroendocrine tumour arising in the thyroid gland. While the vast majority of familial MTC cases are secondary to mutations in the RET gene, no such mutations were identified in these families. Instead, by employing next-generation sequencing techniques and linkage analysis, I provided strong evidence that the disease in these families maps to a discrete region of chromosome 4. High-density array comparative genomic hybridisation eventually allowed to identify a novel microdeletion on chromosome 4p. The deletion was found to segregate with the disease in all 14 affected subjects over three different generations. Furthermore, prospective genetic screening for this deletion allowed to identify two previously unrecognised carriers who were affected with MTC and have been successfully treated with surgery. The potential mechanisms linking this deletion with MTC development have been discussed. Lastly, in the third chapter, I have characterised the clinical features of XLAG, a condition of early-onset pituitary gigantism secondary to Xq26.3 microduplications. By describing a patient with a unique microduplication, I provided definitive evidence linking the disease with the GPR101 gene, encoding an orphan G protein-coupled receptor which is highly expressed in the hypothalamus. Furthermore, while the disease occurs as a result of a germline microduplication in females, I demonstrated that affected males have somatic mosaicism, and that testing of DNA from alternative tissues should be considered to confirm the diagnosis in cases with typical phenotype and negative testing on leukocyte-derived DNA. I have described the unique histopathological features of XLAG-related pituitary tumours and have ruled out a role of GPR101 point variants in patients with acromegaly. Finally, using RNA in situ hybridisation, I have shown that Gpr101 colocalises with Ghrh in the arcuate hypothalamic nucleus in mice, implicating this GPCR in the hypothalamic regulation of growth hormone secretion. | en_US |
