REVIEW
The molecular biology of thyrotroph pituitary neuroendocrine tumors
2023 Volume 70 Issue 2 Pages 135-139
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2023 Volume 70 Issue 2 Pages 135-139
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.
Pituitary neuroendocrine tumors (PitNETs), which were formerly known as pituitary adenomas and are classified in 5th Edition of the WHO Classification of Endocrine and Neuroendocrine Tumors [1], are one of the most frequent intracranial tumors. Thyrotroph PitNETs are one of the most common causes of central hyperthyroidism, with autonomous secretion of TSH from tumors in the pituitary gland. In clinical practice, thyrotroph PitNETs show syndrome of inappropriate secretion of TSH (SITSH), in which serum TSH levels are within reference limits to mildly elevated despite high serum free throxine (FT4) levels, and requires differential diagnosis from resistance to thyroid hormone (RTH). Thyrotroph PitNETs are rare diseases, with an incidence of 1.0 per million population and 0.5% to 2.8% of all PitNETs [2]. This may be due to the fact that thyrotroph cells are present in less than 5% of all pituitary cells [3]. Therefore, most previous studies about thyrotroph PitNETs have focused on a small number of cases. However, based on the increased sensitivity of serum TSH measurement and widespread recognition that thyrotroph PitNETs are the cause of SITSH, the diagnostic rate has increased up to 3-fold, and knowledge on the molecular mechanism of thyrotroph PitNET is gradually accumulating [2]. Recently, with the availability of comprehensive gene and protein analyses such as next-generation sequencing, transcriptome analysis, and proteome analysis, some progress has been made in the pathogenesis of PitNET. For example, the discovery of micro duplication of the region containing the GPR101, a G protein binding receptor, in familial gigantism [4], and the discovery of mutations in the USP8, a deubiquitinating enzyme, in corticotroph PitNET [5]. This review outlines the molecular mechanisms involved in thyrotroph PitNET.
Since thyrotroph PitNET occur in some cases of multiple endocrine neoplasia type 1 (MEN1), which is characterized by tumors of the pituitary, parathyroid, and pancreatic endocrine glands [6], and somatic mutations of the MEN1 gene have been found in other pituitary tumors [7], various genetic mutations have also been searched in sporadic thyrotroph PitNETs. The results of gene mutation analysis were summarized in Table 1.
| Gene mutation | Gene name | Gene symbol | Notes |
|---|---|---|---|
| Germline | Multiple endocrine neoplasia type 1 | MEN1 | |
| Aryl hydrocarbon receptor interacting protein | AIP | * | |
| Somatic | Astrotactin 2 | ASTN2 | ** |
| Cell wall biogenesis 43 C-terminal homolog | CWH43 | ** | |
| R3H domain containing 2 | R3HDM2 | ** | |
| Spermine oxidase | SMOX | ** | |
| Synaptotagmin-like 3 | SYTL3 | ** | |
| Zinc finger and SCAN domain containing 23 | ZSCAN23 | ** | |
| Thyroid hormone recptor beta | TRβ | *** | |
| No mutation | GNAS complex locus | GNAS | |
| G protein subunit alpha q | GNAQ | ||
| G protein subunit 11 | GNA11 | ||
| G protein subunit alpha i2 | GNAI2 | ||
| Pituitary specific factor 1 | PIT1 | ||
| Thyrotropin Releasing Hormone Receptor | TRHR | ||
| Dopamine receptor D2 | D2R |
* Only 1 of 96 AIP mutation-positive PitNETs
** This DNA varant was found in 1 of 12 cases using WES and the function is unknown
*** This mutation may be responsible for the resistance to thyroid hormone
Abbreviation GNAS: Guanine nucleotide binding protein, alpha stimulating; PitNET: Pituitary neuroendocrine tumor; WES: Whole exome sequencing
Loss of heterozygosity (LOH) of chromosome 11q13, where the MEN1 was located, was observed in 3 of 13 cases with sporadic thyrotroph PitNET, but no MEN1 mutation was found in any of the cases [8].
As for oncogenes, active mutations in the Ras gene, which are found in a variety of cancer types, were not found in thyrotroph PitNETs [2]. Somatic mutations in the GNAS encoding the α subunit of G protein, which are found in 30–40% of somatotroph PitNETs, were also absent. Furthermore, somatic mutations in the genes encoding the other G protein subunits, αq, α11, and αi2, were similarly absent [9]. PIT-1, a transcription factor that plays an important role in the development and cell differentiation of somatotroph, lactotroph, and thyrotroph pituitary cells, was examined in 14 cases of thyrotroph PitNET and no mutations were found [2].
Sporadic mutations of Thyroid hormone receptor (TR) gene have also been investigated because tumor cells of thyrotroph PitNET are resistance to the negative feedback mechanism of thyroid hormones. Mutations that replace histidine to tyrosine at codon 435 of the TRβ1 and codon 450 of the TRβ2, as well as a 135 bp deletion in exon 6 of the TRβ2, have been reported to be responsible for the resistance of tumor cells to thyroid hormone [10]. TRH receptor and dopamine D2 receptor genes were also examined as receptors involved in hormone production, but no genetic mutations were found in thyrotroph PitNET [11, 12].
We reported somatic mutation analysis using whole exome sequencing of four thyrotroph PitNET in 2017 [13]. In this report, an average frequency of 1.5 somatic mutations/tumor was observed, similar to previously reported frequencies in somatotroph and nonfunctioning pituitary tumors. Although no known gene mutations have been found to be involved in pituitary tumorigenesis, DNA variants were discovered in six genes (ASTN2, CWH43, R3DHM2, SMOX, SYTL3, and ZSCAN23) as new candidate causative gene mutations for thyrotroph PitNET. Additional searches for these candidate gene mutations were performed in total 12 cases, but no mutations were recurrent, suggesting that these gene mutations are unlikely to be the cause of tumor development.
Immunohistochemical studies on p27Kip1 expression, a cyclin-dependent kinase inhibitor, showed decreased expression in 8 of 11 thyrotroph PitNETs compared to normal pituitary tissue, suggesting that cell cycle abnormalities may be involved in tumorigenesis [14]. Pituitary-specific transcription factor PIT-1 expression was also found to be overexpressed in thyrotroph PitNETs as well as in somatotroph PitNETs [2]. Further studies are needed to determine how these differences in expression may contribute to the mechanism of tumorigenesis.
In long-standing primary hypothyroidism, loss of negative feedback from lack of circulating thyroxine and triiodothyronine leads to excessive TRH secretion from the hypothalamus [15], resulting in pituitary hyperplasia. Hormone regulatory pathways may therefore be involved in tumorigenesis of thyrotroph PitNETs. A newly discovered thyroid hormone receptor beta isoform with a shortened C-terminal (TRβ4) was reported to be abnormally expressed in thyrotroph PitNET. This abnormal expression of TRβ4 may be involved in SITSH [16]. Expression analysis of various other receptors have also been reported. In general, thyrotroph PitNETs often show no TSH response to the TRH test, but it was reported that many of the tumor cells showed the expression of TRH receptor in a study using primary cultures of thyrotroph PitNETs [2]. Expression levels of somatostatin receptor subtypes have also been examined because somatostatin analogs are effective in the treatment of thyrotroph PitNET. Almost all thyrotroph PitNETs have been reported to express one or more subtypes of somatostatin receptors (SSTRs) [17]. Furthermore, it was reported that specific gene polymorphisms and gene expression levels in SSTR subtype 2 and 5 genes were possibly associated with tumor invasiveness and therapeutic efficacy of somatostatin analogs [18, 19]. The expression analysis of SSTR subtypes in 8 thyrotroph pituitary tumors showed SSTR1 mRNA was expressed in 7 cases, SSTR3, SSTR4 and SSTR5 was expressed in 5 cases, while SSTR2 was found in all cases [20]. The mRNA expression of SSTR subtypes in PitNETs, including four thyrotroph PitNET, showed that thyrotroph PitNET expressed higher levels of SSTR2 and SSTR5 mRNA than other PitNETs. It was also reported that SSTR5 expression was stronger in a case in which tumor shrinkage was observed with somatostatin analogs [19]. In a case report of thyrotroph PitNET markedly reduced with a somatostatin analog, mRNA expression level of SSTR5 was higher compared to other SSTR subtypes [21]. Protein expressions of SSTR subtypes were examined in 9 cases of thyrotroph PitNET, and SSTR2A, SSTR2B, and SSTR5 were expressed in all tumors, SSTR1 in 8 tumors, SSTR3 in 7 tumors, and SSTR4 in only 1 tumor [22].
Thyrotroph PitNETs occur in some cases of MEN1. Because the Aryl hydrocarbon receptor interacting protein (AIP) gene is a cause of familial PitNETs, only one case developed thyrotroph PitNET in 96 AIP germline mutation-positive PitNET patients [23].
In addition, a case of RTHβ complicated with PitNET has been reported, raising the question of whether RTH predisposes to pituitary hyperplasia and adenoma development [24]. It has been reported that mutations in the TRβ impaired suppression of hypothalamic TRH secretion by thyroid hormones. Therefore, it is possible that impaired TSH suppression by thyroid hormone and persistent pituitary stimulation by TRH may be associated with the development of thyrotroph PitNET in patients with RTH [25].
The summary of comprehensive analysis were summarized in Table 2.
| Author | Year | Method | Summary | References |
|---|---|---|---|---|
| Sapkota S et al. | 2017 | WES and CNV | Six somatic DNA variants were found as candidate driver mutations. But no mutations were recurrent. Frequent chromosomal regions of copy number gains were also revealed. | 13 |
| Neou M et al. | 2020 | Pangenomic | POU1F1 lineage tumors including thyrotroph PitNETs showed chromosomal instability with copy number alterations in many chromosomes. This lineage were characterized by extensive DNA hypomethylation and transposon expression. | 27 |
| Taniguchi-Ponciano K et al. | 2020 | Transcriptome and methylome | The POU1F1 lineage tumors were characterized by upregulation of genes such as SLIT1 due to demethylation. This lineage were also characterized by alterations in pathways related to fatty acid metabolism, nitrogen metabolism, PPAR and HIPPO. | 28 |
| Taniguchi-Ponciano K et al. | 2022 | Transcriptome and proteome | POU1F1-kineage tumors showed upregulation of PIP5K1B and NEK10 and alterations in phosphatidylinositol, insulin and phospholipase D signaling pathways. | 29 |
Abreviation WES: whole exome sequencing; CNV: Copy number variation
To elucidate the mechanism of sporadic thyrotroph PitNET, we performed copy number variation analysis of 8 cases of thyrotroph PitNETs patients by genome-wide genotyping array and found that various somatic copy number alterations occurred [13]. In this study, large copy number gains extending to the entire autosomal short arm (p) or long arm (q) were found in 5 of 8 cases (62.5%), copy number gain was observed in 4 cases for 4p, 5p, 7p, and 19p, and in 3 cases for 4q, 15q, 16p, 19p, and 21q. Localized copy number gains that did not extend the entire length of the chromosome arms were found in 106 regions. In the case with the greatest number of localized copy number gains, most were found on chromosomes 1p and 2. This phenomenon is similar to the pattern of chromosomal break (chromothripsis) associated with cell carcinogenesis. Localized copy number gains common to several cases was found in 7 regions, including 1q31-1q32 with the BRINP3 gene, which has been reported to be associated with gonadotropin-producing pituitary tumors [26]. Chromosome-wide copy neutral loss of hetelozygosity (cnLOH) was also found in 4 out of 8 cases and was common in several cases on the short arms of chromosomes 1 and 8.
These gene copy number abnormalities in thyrotroph PitNETs were also reported in the pangenomic analysis of PitNETs. Thyrotroph PitNETs, together with somatotroph and lactotroph PitNETs, were classified as POU1F1/PIT1 lineage tumors, and it has been reported that tumors of this lineage, except for somatotroph PitNETs with GNAS gene mutation, showed chromosomal instability with copy number alterations in many chromosomes [27]. They also reported that tumors of this lineage are characterized by extensive DNA hypomethylation and transposon expression.
Comprehensive analysis of pituitary tumors by transcriptome and methylome analysis was also reported. This study reported that thyrotroph PitNETs, together with somatotroph and lactotroph PitNETs were characterized by upregulation of genes such as SLIT1, PRLR and SLC16A6, and this upregulation is due to demethylation of these genes [28]. They also reported that tumors of the POU1F1/PIT1 lineage were characterized by alterations in pathways related to fatty acid metabolism, nitrogen metabolism, PPAR and HIPPO.
Recently, transcriptome and proteome analysis of 42 pituitary tumors, including 4 thyrotroph PitNET, showed that thyrotroph PitNETs were classified as POU1F1/PIT1 delivered tumors along with somatotroph and lactotroph PitNETs, and CDK4, CDK7, and Cyclin-K were up-regulated, while Cyclin-JL, Cyclin-D1, CDK2, and CDKN2A were down-regulated in POU1F1/PIT1 delivered tumors. Furthermore, the kinome of POU1F1/PIT1 delivered tumors showed phosphatidylinositol, insulin and phospholipase D signaling pathways [29].
Recent comprehensive analyses have revealed comprehensive chromosomal, genetic, and epigenomic alterations in thyrotroph PitNETs, and their nature 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.
