Pituitary-dependent hyperadrenocorticism (PDH) or Cushing's disease is a common endocrinopathy in the elderly dog caused by a pituitary adrenocorticotrophic hormone (ACTH) producing tumor (corticotroph adenoma) of unknown pathogenesis. Surgical removal of the pituitary tumor is applied as routine treatment of dogs with PDH at the Faculty of Veterinary Medicine, Utrecht University. Since 1993, more than 230 dogs have been operated. The aims of the present thesis were: (1) to analyze the long-term results and predictors for transsphenoidal hypophysectomy for the treatment of PDH in dogs; and (2) to perform molecular biological studies on the pituitary adenomas. Long-term follow-up confirmed that transsphenoidal hypophysectomy is an effective treatment for PDH in dogs in the hands of a skilled neurosurgeon. Dogs with enlarged pituitary glands have shorter survival, shorter disease-free periods and increased risk of developing permanent central diabetes insipidus. Old age and high plasma concentrations of ACTH were associated with a higher risk of PHD-related mortality. Large pituitary size, thick sphenoid bone, high preoperative urinary corticoid-to-creatinine ratio (UCCR), and high plasma alpha-melanocyte-stimulating hormone (alpha-MSH) concentration were risk factors for recurrence. Immediate post-operative (4h) measurement of the plasma concentrations of ACTH, cortisol and alpha-MSH or postoperative (6-10 wk) measurement of UCCR were predictive for the long-term outcome. UCCR in the upper reference range (5-10 x 10-6) is associated with a higher risk of recurrence than UCCR in the lower reference range (< 5 x 10-6). In the molecular biological studies, factors were investigated that promote normal differentiation of the corticotrophs during ontogenesis. It was hypothesized that a dysregulation of either the pituitary T-box transcription factor Tbx19 (Tpit), neurogenic differentiation (NeuroD1) and leukemia inhibitory factor (LIF) or LIF receptor was involved in the pathogenesis of the corticotroph adenomas. Expression and mutation analyses were performed. Tbx19 proved to be a useful marker for cells that are expressing pro-opiomelanocortin (POMC), the precursor of ACTH and alpha-MSH. No tumor-specific mutation was found in Tbx19. However, interestingly, a commonly occurring missense polymorphism was discovered in the most conserved part of the protein (the Tbox) in Bernese Mountain dogs. The function of the polymorfism is unknown. The expression of NeuroD1 was low in the neurointermediate lobe and differed among the corticotroph adenomas. There was strong co-expression of LIFR and POMC. The presence of LIFR in the intermediate lobe may serve as a non-neuronal regulatory mechanism for alpha-MSH secretion. This is highly interesting with respect to the recently recognized anti-inflammatory properties of alpha-MSH. It can be concluded that the research presented in this thesis has contributed valuable information for the prediction of surgical outcome after transsphenoidal hypophysectomy for the treatment of PDH in dogs, and provided insight in the role of pituitary ontogenic factors in pituitary physiology and oncogenesis later in life
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