Thymus carcinoid producing parathyroid hormone (PTH)-related protein: report of a case

What Lies behind Paraneoplastic Hypercalcemia Secondary to Well-Differentiated Neuroendocrine Neoplasms? A Systematic Review of the Literature

BACKGROUND

Neuroendocrine neoplasms (NEN) originate from neuroendocrine cells ubiquitously spread throughout the body. Hypercalcemia associated with cancer is the most common life-threatening metabolic disorder in patients with advanced stage cancer. Paraneoplastic hypercalcemia is more commonly associated with hematological malignancies, renal and breast carcinomas, and squamous cell carcinomas, but it has also been described in patients with well-differentiated NEN, where it often remains undiagnosed. Among its causes, systemic secretion of parathyroid hormone-related protein (PTHrP) and ectopic production of 1,25-dihydroxyvitamin D and parathyroid hormone (PTH) may be considered paraneoplastic causes of hypercalcemia. In order to clarify the diagnostic work up of paraneoplastic hypercalcemia in patients with NEN, we perform a systematic review, which is lacking in the literature.

METHODS

We performed a data search using MEDLINE and SCOPUS including papers from 1961 to 2021. We selected articles on paraneoplastic hypercalcemia associated with well-differentiated NEN.

RESULTS

The search led to the selection of 78 publications for a total of 114 patients. Pooled data showed that the most frequent primary tumor site associated with paraneoplastic hypercalcemia was pancreatic NEN, followed by Pheochromocytoma. In most cases, paraneoplastic hypercalcemia was caused by PTHrP production and secretion. In more than two thirds of cases, paraneoplastic hypercalcemia was present at the time of NEN diagnosis and, in metachronous cases, was related to local recurrence, distant metastasis development, or tumor progression. In most patients, a combination of therapeutic approaches was employed, and reduction of the tumor burden was essential to control the paraneoplastic syndrome.

DISCUSSION

The onset of hypercalcemia associated with cancer in patients with well-differentiated NEN represents a major clinical challenge. The complex clinical and therapeutical management of paraneoplastic hypercalcemia implies the need for a multidisciplinary approach, aimed at controlling the clinical syndrome and tumor growth.

Recent advances and conceptual changes in the classification of neuroendocrine tumors of the thymus

Neuroendocrine tumors of the thymus (TNET) are exceedingly rare neoplasms. Their histomorphology is identical to neuroendocrine tumors elsewhere in the body (in particular the lungs) and bears no similarity with thymomas and thymic carcinomas. Recent molecular findings have profoundly changed our perception of these tumors and may impact future histological classification systems.

Cushing's syndrome caused by ACTH-producing thymic typical carcinoid with local invasion and regional lymph node metastasis: a case report

Background

Ectopic ACTH-producing thymic carcinoid tumors are rare, but often behave aggressively with local invasion and distant metastasis. We herein report a case of ACTH-producing thymic typical carcinoid tumor with lymph node metastasis treated by surgery and postoperative radiation therapy.

Case presentation

A 61-year-old woman was admitted to be evaluated for hypoglycemia and hypokalemia. Laboratory data revealed elevation of serum cortisol and ACTH levels. Overnight administration of 8 mg dexamethasone did not suppress plasma ACTH. Chest CT demonstrated a tumor of 30 mm in diameter and enlargement of the lymph node at the anterior mediastinum. Ectopic ACTH syndrome was suspected and total thymectomy and lymph node dissection were performed. The histopathological examination indicated typical carcinoid tumor and mediastinal lymph node metastasis, and immunohistochemical staining was positive for ACTH. The plasma ACTH level decreased immediately after surgery. She received postoperative radiation therapy of 60 Gy.

Conclusion

Ectopic ACTH-producing thymic typical carcinoid tumors are rare, and it is important to consider this disease and perform appropriate treatment.

Neuroendocrine tumors of the thymus and mediastinum

Neuroendocrine tumors of the thymus (tNET) and mediastinum are very rare neoplasms with scarce available data. All subtypes [typical and atypical carcinoid tumors (TC and AC), large cell neuroendocrine and small cell carcinoma (SCC)] observed elsewhere in the body occur also in the mediastinum and show only few if any organ-specific morphological differences. Although all available data suggest that the broad principles that govern the biology (and hence) the classification of these tumors in general apply also to tNET, there are a few noteworthy peculiarities related e.g., to risk factors, relative frequency and also to molecular genetic features. In this review, we will briefly summarize current knowledge on tNET with a special emphasis on shared and private features in comparison e.g., with pulmonary NET, which have traditionally been regarded the next closely related NET group.

Expression of parathyroid hormone-related protein and the parathyroid hormone/parathyroid hormone-related protein receptor in rat thymic epithelial cells

Thymic epithelial cells are an important source of cytokines and other regulatory peptides which guide thymocyte proliferation and maturation. Parathyroid hormone-related protein (PTHrP), a cytokine-like peptide, has been reported to affect the proliferation of lymphocytes in vitro. The studies presented here were undertaken to test the hypotheses that PTHrP is produced locally within the thymus where it could influence thymocyte maturation and, more specifically, that thymic epithelial cells (TEC) could be the intrathymic source of PTHrP expression. To this end, immunohistochemical studies were performed to localise PTHrP and the PTH/PTHrP receptor within the adult rat thymus. Antibodies directed against 2 different PTHrP epitopes, PTHrP(1-34) and PTHrP(34-53), demonstrated prominent specific PTHrP immunoreactivity in both subcapsular and medullary TEC. In addition, faint but specific staining for PTHrP was seen in the cortex, interdigitating between cortical lymphocytes while sparing epithelial-free subcapsular areas, thus suggesting that cortical TEC could also be a source of PTHrP immunoreactivity. In contrast, PTH/PTHrP receptor immunoreactivity was only seen in medullary and occasional septal TEC; no evidence of cortical or lymphocytic PTH/PTHrP receptor immunoreactivity was detected. Immunohistochemical studies of cultured cytokeratin-positive rat TEC confirmed the results of these in situ studies as cultured TEC were immunoreactive both for PTHrP and the PTH/PTHrP receptor. Thus these results demonstrate that PTHrP is produced by the epithelial cells of the mature rat thymus. This suggests that PTHrP, a peptide with known cytokine, growth factor and neuroendocrine actions, could exert important intrathymic effects mediated by direct interactions with TEC, or indirect effects on PTH/PTHrP receptor-negative thymocytes.

Distribution and functions of parathyroid hormone-related protein in vertebrate cells

Parathyroid hormone-related protein (PTHrP) was isolated from tumors and identified as the agent of humoral hypercalcemia of malignancy (HHM) in 1987. Since then its gene structure in several mammalian and an avian species has been analyzed and its gene expression demonstrated in many adult and embryonic tissues derived from all three germ layers. The composition and structure of PTHrP peptide depends on both differential gene splicing and posttranslational processing, which result in a range of peptides of potentially diverse functions. This chapter describes the distribution of PTHrP in both normal and neoplastic adult and embryonic tissues. PTHrP is of fundamental importance to cell survival because the absence of the gene is fatal; this aspect of PTHrP function in cell physiology becomes overwhelmingly important in neoplasia. Intracrine or paracrine actions for PTHrP seem to be most likely in mammalian and avian physiology, but in fishes high circulating levels suggest classic endocrine functions as well. Much remains to be learned of the biology of this fascinating protein.