Somatotroph Tumors and the Epigenetic Status of the GNAS Locus

Background: Forty percent of somatotroph tumors harbor recurrent activating GNAS mutations, historically called the gsp oncogene. In gsp-negative somatotroph tumors, GNAS expression itself is highly variable; those with GNAS overexpression most resemble phenotypically those carrying the gsp oncogene. GNAS is monoallelically expressed in the normal pituitary due to methylation-based imprinting. We hypothesize that changes in GNAS imprinting of gsp-negative tumors affect GNAS expression levels and tumorigenesis. Methods: We characterized the GNAS locus in two independent somatotroph tumor cohorts: one of 23 tumors previously published (PMID: 31883967) and classified by pan-genomic analysis, and a second with 82 tumors. Results: Multi-omics analysis of the first cohort identified a significant difference between gsp-negative and gsp-positive tumors in the methylation index at the known differentially methylated region (DMR) of the GNAS A/B transcript promoter, which was confirmed in the larger series of 82 tumors. GNAS allelic expression was analyzed using a polymorphic Fok1 cleavage site in 32 heterozygous gsp-negative tumors. GNAS expression was significantly reduced in the 14 tumors with relaxed GNAS imprinting and biallelic expression, compared to 18 tumors with monoallelic expression. Tumors with relaxed GNAS imprinting showed significantly lower SSTR2 and AIP expression levels. Conclusion: Altered A/B DMR methylation was found exclusively in gsp-negative somatotroph tumors. 43% of gsp-negative tumors showed GNAS imprinting relaxation, which correlated with lower GNAS, SSTR2 and AIP expression, indicating lower sensitivity to somatostatin analogues and potentially aggressive behavior.

Genetics and Mutational Landscape of Ovarian Sex Cord-Stromal Tumors

Ovarian sex cord-stromal tumors (SCST) are uncommon tumors accounting for approximately 8% of all ovarian malignancies. By far, the most common are granulosa cell tumors (GCT) which represent approximately 90% of SCST. SCST are also found in the hereditary syndromes: Peutz-Jeghers syndrome, Ollier disease and Maffucci syndrome, and DICER1 syndrome. Key genomic and genetic events contributing to their pathogenesis have been the focus of recent studies. Most of the genomic studies have been limited to GCT which have identified a number of recurring chromosomal abnormalities (monosomy and trisomy), although their contribution to pathogenesis remains unclear. Recurrent DICER1 mutations are reported in non-hereditary cases of Sertoli cell and Sertoli–Leydig cell tumors (SLCT), while recurrent somatic mutations in both the juvenile (jGCT) and adult forms of GCT (aGCT) have also been reported. Approximately 30% of jGCT contain a somatic mutation in the gsp oncogene, while a further 60% have activating mutations or duplications in the AKT gene. For aGCT, a well characterized mutation in the FOXL2 transcription factor (FOXL2 C134W) is found in the majority of tumors (primary and recurrent), arguably defining the disease. A further mutation in the human telomerase promoter appears to be an important driver for recurrent disease in aGCT. However, despite several studies involving next generation sequencing, the molecular events that determine the stage, behavior and prognosis of aGCT still remain to be determined. Further, there is a need for these studies to be expanded to other SCST in order to identify potential targets for personalized medicine.

The gsp Oncogene Disrupts Ras/ERK-Dependent Prolactin Gene Regulation in gsp Inducible Somatotroph Cell Line

The MAPK ERK1/2 cascade regulates all the critical cellular functions, and in many pathological situations, these regulatory processes are perturbed. It has been clearly established that this cascade is an integrative point in the control of the pituitary functions exerted by various extracellular signals. In particular, ERK1/2 cross talk with the cAMP pathway is determinant in the control of somatolactotroph hormonal secretion exerted via neuropeptide receptors. GH-secreting adenomas are characterized by frequent cAMP pathway alterations, such as constitutive activation of the α-subunit of the heterotrimeric Gs protein (the gsp oncogene), overexpression of Gsα, and changes in the protein kinase A regulatory subunits. However, it has not yet been established exactly how these alterations result in GH-secreting adenomas or how the ERK1/2 cascade contributes to the process of GH-secreting adenoma tumorigenesis. In this study on the conditional gsp-oncogene-expressing GH4C1 cell line, expression of the gsp oncogene, which was observed in up to 40% of GH-secreting adenomas, was found to induce sustained ERK1/2 activation, which required activation of the protein kinase A and the GTPases Ras and Rap1. All these signaling components contribute to the chronic activation of the human prolactin promoter. The data obtained here show that Ras plays a crucial role in these processes: in a physiopathological context, i.e. in the presence of the gsp oncogene, it switched from being a repressor of the cAMP/ protein kinase A ERK-sensitive prolactin gene control exerted by neuropeptides to an activator of the prolactin promoter.

Growth Hormone Response to the Hypothalamic Somatostatinergic Activity in Acromegalic Patients1

Oral glucose administration suppresses the TRH-induced TSH response by enhancing the hypothalamic somatostatinergic activity (HSA). We assessed the HSA in 13 acromegalic patients by measuring glucose-induced suppression of TRH-stimulated TSH secretion. The HSA showed wide variation with up to 64% suppression. The mean HSA of the patients (25 ± 6%) did not differ from that in normal young men (19 ± 4%) in our previous study. Six patients had normal or low HSA, and the other 7 patients had high HSA. The mean TRH-stimulated TSH levels of the patients with normal or low HSA was significantly lower than that of the patients with high HSA (5.13 ± 0.10 vs. 11.30 ± 2.80 mU/L). The HSA did not relate to sex, age, tumor size, basal GH levels, the paradoxical responses to TRH and GnRH, octreotide response, or the gsp oncogene. In the combined glucose-TRH test, glucose pretreatment completely suppressed the paradoxical increase in GH level at 30 min in 4 patients. However, it could suppress the paradoxical GH response by only 6–51% in the other 5 patients who also showed the paradoxical response in TRH test. The tumor diameter of patients with good response to the HSA was significantly larger than that of the patients with poor response (31 ± 5 vs. 14 ± 2 mm) as was the tumor grade (3.3 ± 0.3 vs. 1.7 ± 0.2). This study supports the idea that a reduction of HSA is not a primary cause of acromegaly, and the HSA seems to increase to suppress the autonomous secretion of GH from the pituitary adenomas. HSA as well as the response of tumors to HSA do not determine tumor growth.