Aberrant expression of thyroid hormone receptor beta isoform may cause inappropriate secretion of TSH in a TSH-secreting pituitary adenoma

The molecular biology of thyrotroph pituitary neuroendocrine tumors

Pituitary neuroendocrine tumors (PitNETs), which were formerly known as pituitary adenomas, are classified in 5th Edition of the WHO Classification of Endocrine and Neuroendocrine Tumors. Since thyrotroph PitNETs are rare PitNETs, most previous studies about former thyroid stimulating hormone (TSH)-secreting pituitary adenoma have focused on a small number of cases. However, the diagnostic rate of thyrotroph PitNET has increased because of increased sensitivity of serum TSH measurement and widespread recognition that thyrotroph PitNETs are the cause of syndrome of inappropriate secretion of TSH (SITSH). Therefore, knowledge on the molecular mechanism of thyrotroph PitNET is gradually accumulating. Recently, comprehensive chromosomal, genetic, and epigenomic alterations in thyrotroph PitNET have been revealed with the availability of comprehensive gene and protein analyses, and the nature of thyrotroph PitNET is gradually being elucidated. However, further analysis is needed to determine whether the causes of these changes are directly responsible for the development of tumors.

Transcriptomic Profiles of Normal Pituitary Cells and Pituitary Neuroendocrine Tumor Cells

The pituitary gland is one of the most cellularly diverse regions of the brain. Recent advancements in transcriptomic biology, such as single-cell RNA sequencing, bring an unprecedented glimpse into the molecular composition of the pituitary, both in its normal physiological state and in disease. Deciphering the normal pituitary transcriptomic signatures provides a better insight into the ontological origin and development of five types of endocrine cells, a process involving complex cascades of transcription factors that are still being established. In parallel with these observations about normal pituitary development, recent transcriptomic findings on pituitary neuroendocrine tumors (PitNETs) demonstrate both preservations and changes in transcription factor expression patterns compared to those seen during gland development. Furthermore, recent studies also identify differentially expressed genes that drive various tumor behaviors, including hormone hypersecretion and tumor aggression. Understanding the comprehensive multiomic profiles of PitNETs is essential in developing molecular profile-based therapies for PitNETs not curable with current treatment modalities and could eventually help align PitNETs with the breakthroughs being made in applying precision medicine to other tumors.

Recent advancements in the molecular biology of pituitary adenomas

INTRODUCTION

Pituitary adenomas are a common and diverse group of intracranial tumors arising from the anterior pituitary that are usually slow-growing and benign, but still pose a significant healthcare burden to patients. Additionally, they are increasing in both incidence and prevalence, leading to a need for better understanding of molecular changes in the development of these tumors.

AREAS COVERED

A PubMed literature search was conducted using the terms 'pituitary adenoma' in combination with keywords related to secretory subtype: lactotroph, somatotroph, corticotroph, gonadotroph and null cell, in addition to their transcription factor expression: PIT1, TPIT, and SF-1. Articles resulting from this search were analyzed, as well as relevant articles cited as their references. In this review, we highlight recent advances in the genetic and epigenetic characterization of individual pituitary adenoma subtypes and the effect it may have on guiding future clinical treatment of these tumors.

EXPERT OPINION

Understanding the molecular biology of pituitary adenomas is a fundamental step toward advancing the treatment of these tumors. Yet crucial knowledge gaps exist in our understanding of the underlying molecular biology of pituitary adenomas which can potentially be addressed by turning to differentially activated molecular pathways in tumor relative to normal gland.

Progress in the Pathogenesis, Diagnosis, and Treatment of TSH-Secreting Pituitary Neuroendocrine Tumor

TSH-secreting pituitary neuroendocrine tumor (PitNET) is one of the causes of central hyperthyroidism. The incidence of TSH PitNET is far lower than that of other PitNETs. The clinical manifestations of TSH PitNETs mainly include thyrotoxicosis or thyroid goiter, secretion disorders of other anterior pituitary hormones, and mass effect on the pituitary gland and its surrounding tissues. The application of high-sensitivity TSH detection methods contributes to the early diagnosis and timely treatment of TSH PitNETs. Improvements in magnetic resonance imaging (MRI) have advanced the noninvasive visualization of smaller PitNETs. Treatments for TSH PitNETs include surgery, drugs, and radiotherapy. This review focuses on the progress in pathogenesis, diagnosis, and treatment of TSH PitNETs to provide more information for the clinician.

Insights into pituitary tumorigenesis: from Sanger sequencing to next-generation sequencing and beyond

Introduction: This review explores insights provided by next-generation sequencing (NGS) of pituitary tumors and the clinical implications.Areas covered: Although syndromic forms account for just 5% of pituitary tumours, past Sanger sequencing studies pragmatically focused on them. These studies identified mutations in MEN1, CDKN1B, PRKAR1A, GNAS and SDHx causing Multiple Endocrine Neoplasia-1 (MEN1), MEN4, Carney Complex-1, McCune Albright Syndrome and 3P association syndromes, respectively. Furthermore, linkage analysis of single-nucleotide polymorphisms identified AIP mutations in 20% with familial isolated pituitary adenomas (FIPA). NGS has enabled further investigation of sporadic tumours. Thus, mutations of USP8 and CABLES1 were identified in corticotrophinomas, BRAF in papillary craniopharyngiomas and CTNNB1 in adamantinomatous craniopharyngiomas. NGS also revealed that pituitary tumours occur in the DICER1 syndrome, due to DICER1 mutations, and CDH23 mutations occur in FIPA. These discoveries revealed novel therapeutic targets and studies are underway of BRAF inhibitors for papillary craniopharyngiomas, and EGFR and USP8 inhibitors for corticotrophinomas.Expert opinion: It has become apparent that single-nucleotide variants and small insertion/deletion DNA mutations cannot explain all pituitary tumorigenesis. Integrated and improved analyses including whole-genome sequencing, copy number, and structural variation analyses, RNA sequencing and epigenomic analyses, with improved genomic technologies, are likely to further define the genomic landscape.

Thyrotropin-secreting pituitary adenomas: a systematic review and meta-analysis of postoperative outcomes and management

PURPOSE

TSH-secreting pituitary adenomas are rare pituitary tumors. An efficient treatment is essential to limit the mortality and morbidity in untreated patients. The aim of this study is to summarize the evidence about the postoperative outcomes and management of this rare pathology.

METHODS

A systematic search and meta-analysis of surgical series was performed.

RESULTS

Our analysis included 23 articles (536 patients). No sex difference was observed and mean age at diagnosis was 45 years. Hyperthyroidism was reportedly clinical in 67% and biochemical in 90% of patients. Co-secretion of other pituitary hormones was present in 42% of cases. Macroadenomas were found in 79% of patients, showing in 44% and 30% of cases respectively extrasellar extension and cavernous sinus invasion. The pooled rate of postoperative biochemical remission was 69.7% and a gross total resection (GTR) was observed in 54% of patients. The extent of resection was significantly increased in microadenomas (p < 0.001) and cavernous sinus invasion was predictive of lower GTR rate (p < 0.001). A biochemical remission was achieved in 66% of patients after adjuvant radiation therapy and in 76% after adjuvant medical treatment. The combination of both allowed remission in 67% of cases. At final follow-up the overall biochemical remission rate was significantly improved (85.8%) when compared to the postoperative biochemical remission (p < 0.001).

CONCLUSION

When compared to the early postoperative period, at last follow-up biochemical remission was significantly greater (p < 0.001). GTR was achieved in half of patients; the size of tumor and cavernous sinus invasion determined the extent of resection.

A case of TSH-secreting pituitary adenoma with cyclic fluctuations in serum TSH levels

A 29-year-old man was referred to our department due to adrenal insufficiency with the inappropriate secretion of TSH (SITSH). Magnetic resonance imaging revealed a pituitary tumor. A weak TSH response in the TRH test, elevated sex hormone binding globulin (SHBG) levels, and the absence of a family medical history of SITSH or TRβ gene mutations supported the diagnosis of TSH-secreting pituitary adenoma (TSHoma). However, complete TSH suppression and a blunted cholesterol response in the T3 suppression test as well as normal glycoprotein α-subunit (α-GSU) levels were not compatible with TSHoma. Since TSH, FT3, and FT4 spontaneously returned to normal ranges after admission, he was discharged. One month after his discharge, thyrotoxicosis with elevated serum TSH levels relapsed. After admission, his serum TSH levels returned to within the normal range. After his discharge from the second admission, his serum TSH levels fluctuated in accordance with serum FT3 and FT4 levels and symptoms, such as palpitations. Ten months after his discharge, he was admitted to our department again due to adrenal insufficiency and thyrotoxicosis with elevated serum TSH levels, suggesting cyclic SITSH. Although resistance to thyroid hormone (RTH) was not completely excluded, the pituitary tumor was removed by transsphenoidal surgery (TSS). A pathological diagnosis confirmed TSHoma. We herein report a case of TSHoma in which serum TSH, FT3, and FT4 levels fluctuated periodically. To the best of our knowledge, this is the first case report of "cyclic TSHoma", which needs to be considered when making a differential diagnosis of SITSH.

Whole-Exome Sequencing Study of Thyrotropin-Secreting Pituitary Adenomas

CONTEXT

Thyrotropin (TSH)-secreting pituitary adenomas (TSHomas) are a rare cause of hyperthyroidism, and the genetic aberrations responsible remain unknown.

OBJECTIVE

To identify somatic genetic abnormalities in TSHomas.

DESIGN AND SETTING

A single-nucleotide polymorphism (SNP) array analysis was performed on 8 TSHomas. Four tumors with no allelic losses or limited loss of heterozygosity were selected, and whole-exome sequencing was performed, including their corresponding blood samples. Somatic variants were confirmed by Sanger sequencing. A set of 8 tumors was also assessed to validate candidate genes.

PATIENTS

Twelve patients with sporadic TSHomas were examined.

RESULTS

The overall performance of whole-exome sequencing was good, with an average coverage of each base in the targeted region of 97.6%. Six DNA variants were confirmed as candidate driver mutations, with an average of 1.5 somatic mutations per tumor. No mutations were recurrent. Two of these mutations were found in genes with an established role in malignant tumorigenesis (SMOX and SYTL3), and 4 had unknown roles (ZSCAN23, ASTN2, R3HDM2, and CWH43). Similarly, an SNP array analysis revealed frequent chromosomal regions of copy number gains, including recurrent gains at loci harboring 4 of these 6 genes.

CONCLUSIONS

Several candidate somatic mutations and changes in copy numbers for TSHomas were identified. The results showed no recurrence of mutations in the tumors studied but a low number of mutations, thereby highlighting their benign nature. Further studies on a larger cohort of TSHomas, along with the use of epigenetic and transcriptomic approaches, may reveal the underlying genetic lesions.

Thyrotropin-secreting pituitary adenomas: epidemiology, diagnosis, and management

Inappropriate secretion of TSH was first described in 1960 in a patient with evidence of hyperthyroidism and expanded sella on imaging. It was later found that a type of pituitary adenoma that secretes TSH (thyrotropinoma) was the underlying cause. The objective of the present review article is to summarize data on the epidemiology, pathogenesis, diagnosis, and management of thyrotropinomas. The prevalence of thyrotropinomas is lower than that of other pituitary adenomas. Early diagnosis is now possible thanks to the availability of magnetic resonance imaging and sensitive laboratory assays. As a corollary, many patients now present earlier in the course of their disease and have smaller tumors at the time of diagnosis. Treatment also has evolved over time. Transsphenoidal surgery is still considered definitive therapy. Meanwhile, radiation therapy, including radiosurgery, is effective in achieving tumor control in the majority of patients. In the past, radiation therapy was used as second line treatment in patients with residual or recurrent tumor after surgery. However, the availability of somatostatin analogs, which can lead to normalization of thyroid function as well as shrink these tumors, has led to an increase in the role of medical therapy in patients who are not in remission after pituitary surgery. In addition, dopamine agonists have shown some efficacy in the management of these tumors. Better understanding of the molecular pathogenesis of thyrotropinomas may lead to rationally designed therapies for patients with thyrotropinomas.

Molecular characterization of human thyroid hormone receptor β isoform 4

Thyroid hormone exerts a pleiotropic effect on development, differentiation, and metabolism through thyroid hormone receptor (TR). A novel thyroid hormone receptor β isoform (TRβ4) was cloned using PCR from a human pituitary cDNA library as a template. We report here the characterization of TRβ4 from a molecular basis. Temporal expression of TRβ4 during the fetal period is abundant in the brain and kidney, comparable with the adult pattern. Western blot analysis revealed that TRs are ubiquitination labile proteins, while TRβ1 is potentially stable. TRβ1, peroxisome proliferator-activated receptors (PPAR), and vitamin D receptor (VDR), which belong to class II transcription factors that function via the formation of heterodimeric complexes with retinoid X receptor (RXR), were suppressed by TRβ4 in a dose-dependent manner. Thus, TRβ4 exhibits ligand-independent transcriptional silencing, possibly as a substitute for dimerized RXR. In this study, TRβ1 and TRβ4 transcripts were detected in several cell lines. Quantitative RT-PCR assay showed that the expression of TRβ4 in human embryonic carcinoma cells of the testis was suppressed by sex hormone in a reciprocal manner to TRβ1. In contrast, TRβ4 was expressed under a high dose of triiodothyronine (T3) in a reciprocal manner to TRβ1. Finally, in transiently transfected NIH-3T3 cells, green fluorescence protein (GFP)-tagged TRβ4 was mostly nuclear in both the absence and the presence of T3. By mutating defined regions of both TRβs, we found that both TRβ1 and TRβ4 had altered nuclear/cytoplasmic distribution as compared with wild-type, and different to T3 and the nuclear receptor corepressor (NCoR). Thus, site-specific DNA binding is not essential for maintaining TRβs within the nucleus.

Evaluation and management of thyrotropin-secreting pituitary adenomas

Thyrotropin (TSH)-secreting pituitary adenomas are uncommon pituitary neoplasms. They have been increasingly recognized with the advent of ultrasensitive TSH assays and advanced pituitary imaging. Clinically, patients may often present with signs and symptoms of hyperthyroidism. Some patients may present with tumor-related local mass effect, anterior hypopituitarism or syndromes related to co-secretion of other pituitary hormones. Their diagnosis and differential diagnosis from other causes of hyperthyroidism and ‘inappropriate TSH secretion’ often presents a clinical challenge. While trans-sphenoidal pituitary adenomectomy remains the therapeutic modality of choice, medical treatment with somatostatin receptor agonists or dopamine agonists is often needed as adjunctive therapy or perhaps as a potential alternative to surgery. Radiation therapy to the sella may be advised as a third-line therapy.

2013 European thyroid association guidelines for the diagnosis and treatment of thyrotropin-secreting pituitary tumors

Hyperthyroidism is mainly due to autoimmune thyroid disorders or toxic goiter, and very rarely to the presence of thyrotropin (TSH)-secreting pituitary adenomas (TSHomas). These tumors are characterized by high levels of circulating free thyroid hormones (FT4 and FT3) in the presence of nonsuppressed serum TSH concentrations. Failure to correctly diagnose TSHomas may result in inappropriate thyroid ablation, which results in a significant increase of pituitary tumor mass. The diagnosis is mainly achieved by measuring TSH after T3 suppression and TRH stimulation tests. These dynamic tests, together with pituitary imaging and genetic testing are useful in distinguishing TSHomas from the syndromes of resistance to thyroid hormone action. The treatment of choice is surgery. In cases of surgical failure, somatostatin analogs have been found to be effective in normalizing TSH secretion in more than 90% of patients.

Balance between somatostatin and D2 receptor expression drives TSH-secreting adenoma response to somatostatin analogues and dopastatins

CONTEXT

First-line therapy for thyrotropin-secreting pituitary adenomas (TSHomas) is neurosurgery, while medical treatment rests mainly on somatostatin analogues. Clinically available sst(2) -preferring analogues, octreotide and lanreotide, induce normalization of hormone levels in approximately 90% of patients and tumour shrinkage in 45%.

OBJECTIVE

We evaluated somatostatin 1, 2, 3 and 5 and dopamine D2 receptor expression in tumour samples from three TSHomas, and the relationships between receptor expression, in vitro antiproliferative response and clinical data, including octreotide test and three months of therapy with octreotide long-acting repeatable (LAR). TSHoma cell proliferation was tested in vitro using octreotide, cabergoline and two chimeric compounds, BIM-23A760 and BIM-23A387.

RESULTS

All patients showed significant TSH lowering to acute octreotide test, but a hormonal response to long-term treatment was observed in only two patients, showing a high sst(5) /sst(2) ratio. Patient 2, characterized by high expression of sst(2) and sst(1) and a relative lower expression of sst(5) , experienced tachyphylaxis after prolonged octreotide treatment. In vitro, the somatostatin/dopamine receptor agonist BIM-23A760 caused the highest antiproliferative effect among those tested. Combined treatment with octreotide and cabergoline displayed an additive effect of magnitude comparable to that of the other chimeric compound (BIM-23A387). Octreotide resistance was confirmed in cells isolated from the nonresponder patient, although it could be overcome by treatment with the chimeric compounds.

CONCLUSIONS

  A high sst(5) /sst(2) ratio might be predictive of a positive outcome to long-term treatment with somatostatin analogues in TSHomas. Moreover, combined somatostatin and D(2) receptor targeting might be considered as a potential tool to improve the response rate in octreotide-resistant tumours.

Pituitary resistance to thyroid hormones: pathophysiology and therapeutic options

Thyroid hormone secretion suppresses the expression of thyroid stimulating hormone (TSH), both of which are strictly controlled by a negative feedback loop between the hypothalamus-pituitary and thyroid. Pituitary resistance to thyroid hormone (PRTH) is defined as resistance to the action of thyroid hormone that is more severe in the pituitary than at the peripheral tissue level. Although the molecular basis of PRTH is not well understood, the clinical issue mainly involves imbalance between the hypothalamus-pituitary and peripheral thyroid hormone responsivity, which may induce peripheral thyrotoxic phenomena. Here, we review the pathogenesis and molecular aspects of PRTH, present a single case with inappropriate TSH secretion suffering from thyrotoxicosis treated with PTU, and discuss the possible choice of therapeutic options to correct the imbalance of thyroid hormone responsivity in both the hypothalamus-pituitary and peripheral tissues.