Growth of parathyroid glands : genetic and functional aspects

Abstract

Ca2+ is an essential ion with a variety of important functions in the body and it is crucial that the Ca2+ levels are precisely regulated. One of the hormones regulating these processes is parathyroid hormone (PTH) secreted from the parathyroid glands. Hyperparathyroidism (HPT) commonly affects postmenopausal women and is caused by a solitary benign adenoma in more than 90% of the cases. The remainder is constituted of hyperfunction in multiple glands including hyperplasia and multiple adenomas or parathyroid carcinoma. HPT is characterized by an increased proliferation of parathyroid cells as well as inadequate control of PTH secretion. The goal of this thesis was to investigate genetic alterations in parathyroid glands with different types of pathological growth and to enlighten the role of ion channels in the regulation of PTH secretion in human parathyroid cells.

The chromosome arm 1p has been indicated as a potential location for a still unknown parathyroid tumor suppressor gene(s) but despite both positional cloning efforts and candidate gene screenings the responsible gene(s) remains unknown. We investigated 42 parathyroid tumors from 38 patients with sporadic primary HPT for loss of heterozygosity (LOH) to characterize the pattern of deletions in 1p. Overall 1p LOH was detected in 43% of the tumors. Although several distinct target regions were identified, the highest frequency of LOH was noted distally in 1p32.3-36.2 comprising of a 40cM interval. A special focus was further directed on histone deacetylase 1 (HDAC1) gene as a candidate but the results do not support its role in parathyroid tumorigenesis. Taken together, our results support that deletions in 1p in sporadic adenomas are distally located and thus differ from the more proximal deletions in parathyroid carcinomas (Paper I).

Parathyroid carcinoma is an uncommon but devastating cause of primary HPT. Since inactivating germline mutations in the HRPT2 gene were recently found to be responsible for the hereditary hyperparathyroidism-jaw (HPT-JT) tumor syndrome with an increased risk of parathyroid carcinoma, we investigated the involvement of HRP72 in sporadic parathyroid carcinomas. We showed inactivating HRPT2 mutations in the majority of parathyroid carcinomas, suggesting that HRPT2 is important for the pathogenesis of parathyroid carcinoma. Furthermore, the finding that certain patients with sporadic parathyroid carcinoma can carry germline HRP72 mutations yields new considerations for the clinical management of these patients (Paper II).

Secondary HPT arises in patients with chronic renal failure and is characterized by diffuse and/or nodular hyperplasia. Chromosomal gains and losses are not commonly found in whole glands of secondary HPT but the expression of receptors important for the parathyroid growth and hormone secretion, namely CASR, CAS and VDR, is shown to be lowered. We studied the expression of CASR, CAS, VDR and PTH mRNA in 36 glands from 18 patients with secondary HPT to characterize the expression pattern in detail. We showed that the expression of all four genes was highly variable as well between different glands as within individual glands, supporting the theory that each nodule in secondary HPT is of monoclonal origin, but that the monoclonal origin of each nodule is independent (Paper III).

The role of membrane potential in the regulation of PTH secretion is not known. We investigated the relationship between extra- and intracellular Ca2+ K+channels, and membrane potential in human parathyroid cells with patch-clamp. Two Ca2+-activated K+channels were identified. At high extracellular Ca2+ the cells were hyperpolarized whereas lowering of extracellular Ca 2+ depolarized the cells. The results suggest that extracellular Ca2+ regulates membrane potential via Ca2+ - activated K+ channels and that the membrane potential may be of greater importance for the stimulus-secretion coupling than previously recognized (Paper IV).