G protein mutations in pituitary tumors: a study on Turkish patients

GH/PRL-secreting pituitary macroadenoma associated with GNAS p.Gln227Leu mutation: pediatric case report and review

GH-secreting pituitary adenomas (GHomas) are rare in the pediatric population. Guanine nucleotide-binding protein, alpha stimulating (GNAS) somatic mutations are often found in patients with GHoma. Here, we report an 8-year-old girl with GH-secreting pituitary adenoma successfully treated by operative tumor resection and postoperative treatment with octreotide long-acting release (LAR). Tumor DNA sequence analysis revealed a somatic heterozygous c.680A>T (p.Gln227Leu) mutation in GNAS. We reviewed 1,084 cases of GHomas, 409 (37.7%) of which harbored GNAS mutations. In pediatrics cases, aged 15 years or younger, 11 harbored a GNAS mutation, and GNAS p.Arg201Cys was identified in five cases. No other cases of codon 227 mutation were detected. These cases suggest that, in pediatric patients, the clinical features of GHoma may differ from those observed in adults. This is possibly related to octreotide or dopamine agonist resistance. Of six patients with surgical resistance, only one was reactive when treated with octreotide. Our case shows that octreotide LAR is an effective choice for treating GNAS-induced GHoma. This is the first report detailing the effectiveness of octreotide LAR in a GNAS codon 227 mutation-induced GHoma in a pediatric case. Examination of the relationship between genetic variation and clinical features in pediatric patients will enable us to assess the long-term effects of surgical and medical treatment of GHomas.

The curious case of Gαs gain-of-function in neoplasia

BACKGROUND

Mutations activating the α subunit of heterotrimeric Gs protein are associated with a number of highly specific pathological molecular phenotypes. One of the best characterized is the McCune Albright syndrome. The disease presents with an increased incidence of neoplasias in specific tissues.

MAIN BODY

A similar repertoire of neoplasms can develop whether mutations occur spontaneously in somatic tissues during fetal development or after birth. Glands are the most "permissive" tissues, recently found to include the entire gastrointestinal tract. High frequency of activating Gαs mutations is associated with precise diagnoses (e.g., IPMN, Pyloric gland adenoma, pituitary toxic adenoma). Typically, most neoplastic lesions, from thyroid to pancreas, remain well differentiated but may be a precursor to aggressive cancer.

CONCLUSIONS

Here we propose the possibility that gain-of-function mutations of Gαs interfere with signals in the microenvironment of permissive tissues and lead to a transversal neoplastic phenotype.

Cyclic 3',5'-adenosine monophosphate (cAMP) signaling in the anterior pituitary gland in health and disease

The cyclic 3',5'-adenosine monophosphate (cAMP) was the first among the so-called "second messengers" to be described. It is conserved in most organisms and functions as a signal transducer by mediating the intracellular effects of multiple hormones and neurotransmitters. In this review, we first delineate how different members of the cAMP pathway ensure its correct compartmentalization and activity, mediate the terminal intracellular effects, and allow the crosstalk with other signaling pathways. We then focus on the pituitary gland, where cAMP exerts a crucial function by controlling the responsiveness of the cells to hypothalamic hormones, neurotransmitters and peripheral factors. We discuss the most relevant physiological functions mediated by cAMP in the different pituitary cell types, and summarize the defects affecting this pathway that have been reported in the literature. We finally discuss how a deregulated cAMP pathway is involved in the pathogenesis of pituitary disorders and how it affects the response to therapy.

Mutation analysis of inhibitory guanine nucleotide binding protein alpha (GNAI) loci in young and familial pituitary adenomas

Pituitary adenomas are neoplasms of the anterior pituitary lobe and account for 15-20% of all intracranial tumors. Although most pituitary tumors are benign they can cause severe symptoms related to tumor size as well as hypopituitarism and/or hypersecretion of one or more pituitary hormones. Most pituitary adenomas are sporadic, but it has been estimated that 5% of patients have a familial background. Germline mutations of the tumor suppressor gene aryl hydrocarbon receptor-interacting protein (AIP) predispose to hereditary pituitary neoplasia. Recently, it has been demonstrated that AIP mutations predispose to pituitary tumorigenesis through defective inhibitory GTP binding protein (Gαi) signaling. This finding prompted us to examine whether germline loss-of-function mutations in inhibitory guanine nucleotide (GTP) binding protein alpha (GNAI) loci are involved in genetic predisposition of pituitary tumors. To our knowledge, this is the first time GNAI genes are sequenced in order to examine the occurrence of inactivating germline mutations. Thus far, only somatic gain-of-function hot-spot mutations have been studied in these loci. Here, we have analyzed the coding regions of GNAI1, GNAI2, and GNAI3 in a set of young sporadic somatotropinoma patients (n = 32; mean age of diagnosis 32 years) and familial index cases (n = 14), thus in patients with a disease phenotype similar to that observed in AIP mutation carriers. In addition, expression of Gαi proteins was studied in human growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH)-secreting and non-functional pituitary tumors. No pathogenic germline mutations affecting the Gαi proteins were detected. The result suggests that loss-of-function mutations of GNAI loci are rare or nonexistent in familial pituitary adenomas.

Hope for GWAS: relevant risk genes uncovered from GWAS statistical noise

Hundreds of genetic variants have been associated to common diseases through genome-wide association studies (GWAS), yet there are limits to current approaches in detecting true small effect risk variants against a background of false positive findings. Here we addressed the missing heritability problem, aiming to test whether there are indeed risk variants within GWAS statistical noise and to develop a systematic strategy to retrieve these hidden variants. Employing an integrative approach, which combines protein-protein interactions with association data from GWAS for 6 common diseases, we found that associated-genes at less stringent significance levels (p < 0.1) with any of these diseases are functionally connected beyond noise expectation. This functional coherence was used to identify disease-relevant subnetworks, which were shown to be enriched in known genes, outperforming the selection of top GWAS genes. As a proof of principle, we applied this approach to breast cancer, supporting well-known breast cancer genes, while pinpointing novel susceptibility genes for experimental validation. This study reinforces the idea that GWAS are under-analyzed and that missing heritability is rather hidden. It extends the use of protein networks to reveal this missing heritability, thus leveraging the large investment in GWAS that produced so far little tangible gain.

Impact of gsp oncogene on the mRNA content for somatostatin and dopamine receptors in human somatotropinomas

INTRODUCTION

It has been reported in some series that gsp+ somatotropinomas are more sensitive to somatostatin analogues (SA) and dopamine's actions which may be related to their somatostatin receptor (SSTR) and dopamine receptor (DR) profile. No previous studies have been undertaken to evaluate the SSTR and DR profile related with the gsp status in somatotropinomas.

OBJECTIVES

To determine if (1) gsp status is correlated with response to octreotide LAR (LAR) and tumor expression patterns of SSTR1-5 and DR1-5 and (2) cAMP level can directly modulate SSTR and DR mRNA levels.

METHODS

Response to SA was evaluated by GH and IGF-I percent reduction after 3 and 6 months of treatment with LAR. Conventional PCR and sequencing were used to identify gsp+ tumors. Quantitative real-time PCR was used to determine SSTR and DR tumor expression. Primary pituitary cell cultures of primates were used to study whether SSTR and DR expression is regulated by forskolin.

RESULTS

The response to LAR did not significantly differ between patients with gsp+ and gsp- tumors; however, gsp+ tumors expressed higher levels of SSTR1, SSTR2, DR2 and a lower level of SSTR3. Forskolin increased SSTR1, SSTR2, DR1 and DR2 expression in cell cultures.

CONCLUSION

Elevated SSTR1, SSTR2, and DR2 tumor expression may help improve responsiveness to SA and DA therapy; however, this study may not have been appropriately powered to observe significant effects in the clinical response. Elevated cAMP levels could be directly responsible for the upregulation in SSTR1, SSTR2 and DR2 mRNA levels observed in gsp+ patients.

Prevalence of gsp oncogene in somatotropinomas and clinically non-functioning pituitary adenomas: our experience

The purpose of the present study is to evaluate the prevalence of the gsp oncogene in Brazilian patients harboring somatotropinomas and non-functioning pituitary adenomas (NFPA). Patients and methods Deoxyribonucleic acid was extracted from 54 somatotropinomas and 14 NFPA. Exons 8 and 9 (including codons 201 and 227, respectively) of the GNAS gene were amplified by polymerase chain reaction (PCR). The PCR products were then purified and sequenced using the same primers. Results The gsp oncogene was found in nine tumors (eight somatotropinomas). The prevalence among somatotropinomas was 15% and among NFPA was 7%. The mutation was found in codon 201 in eight tumors and in codon 227 in one tumor (a somatotropinoma). No differences were found in age, sex, GH, and IGF-I levels or tumor volume at diagnosis between gsp+ and gsp- patients. Conclusion We found a lower than expected prevalence of gsp mutations in somatotropinomas and a similar prevalence in NFPA compared to previous studies from other countries.

Non-functioning pituitary adenomas infrequently harbor G-protein gene mutations

BACKGROUND

Mutations of the genes encoding the alpha subunit of the stimulatory G protein (Gs) and of the inhibiting Gi2 protein (GNAS1 and GNAI2 genes, respectively) have been described in various endocrine neoplasias, including pituitary tumors.

AIM

To search for mutations of GNAS1 and GNAI2 in a continuous series of non-functioning pituitary adenoma (NFPA) patients neurosurgically treated.

SUBJECTS AND METHODS

The surgical samples of 22 patients who have been defined and characterized on a clinical, biochemical, histological, and immunohistochemical point of view have been processed for investigating the presence of the above mutations by PCR amplification of the hot spots exons 8 and 9 of GNAS1, and exons 5 and 6 of GNAI2, followed by direct sequencing. Moreover, the promoter region of GNAI2, in order to assess the prevalence of single nucleotide polymorphisms (SNP), was investigated in the same series.

RESULTS

A CGT>TGT mutation at codon 201 of GNAS1 gene in a single case of NFPA was found, but no mutation of GNAI2A was demonstrated.

CONCLUSIONS

This finding suggests and confirms that G-protein mutations are rare and not crucial in NFPA development. Additionally, we found a silent SNP at codon 318 in the promoter of the Gi2alpha gene in one out of the 22 NFPA.

Oncogenesis and mutagenesis of pituitary tumors

Although pituitary tumors may be present in up to 10% of the population, the pathophysiology of these lesions is not well characterized. Pituitary tumors are composed of monoclonal cell populations with disrupted control of replication pathways. The oncogenes and tumor suppressor genes that are common in other malignancies (i.e. jun, fos, myc, and p53) are rarely involved in the development of these tumors. However, oncogenes, such as gsp, can be present in up to 40% of hormonally active adenomas. The process of pituitary oncogenesis further appears to involve oncogenes such as cyclin E, cyclin D1, and the pituitary tumor transforming gene (PTTG). Finally, the cAMP signaling cascade plays a significant role in generation of both benign and malignant pituitary tumors. In this review, the biology of pituitary adenomas is explored with a special emphasis on potential targets for the development of targeted therapeutics.

Does the prevalence of gsp mutations in GH-secreting pituitary adenomas differ geographically or racially? Prevalence of gsp mutations in Japanese patients revisited

OBJECTIVE

The prevalence of gsp mutations in GH-secreting pituitary adenomas was thought to differ geographically or racially, given its exceptionally lower incidence among Japanese patients (4.4-9.3%) compared to other regions (30-50%). However, this notion is now being challenged after a recent paper reported a 53.3% incidence among Japanese with acromegaly. We have since re-evaluated the prevalence of gsp mutations on a larger scale.

PATIENTS

One hundred Japanese acromegaly patients with surgically confirmed GH-secreting pituitary adenomas were enrolled.

METHODS

mRNAs from primary cultured adenomas were used for reverse transcriptase-polymerase chain reaction and direct sequencing of the Gsalpha subunit. Patient data were reviewed from medical charts.

RESULTS

There were 53 gsp mutations (53%), consisting of 42 Arg201Cys, one Arg201His, one Arg201Ser, 8 Gln227Leu, and one Gln227Arg mutation. Age at operation, sex ratio, basal serum GH and IGF-I levels were no different with or without the mutations. In contrast, patients responded differently to most dynamic tests with statistical significance: serum GH levels in gsp-positive patients had blunted response to GHRH, were well suppressed by bromocriptine, and had higher rates of paradoxical response to TRH. Notably, paradoxical response to LHRH was observed exclusively in gsp-negative patients. Octreotide suppressed GH levels strongly regardless of gsp status. These clinical characteristics are similar to those of Caucasian patients.

CONCLUSION

We conclude that the prevalence of gsp mutations in Japanese acromegaly patients is comparable to those of other reports from various regions. Therefore, Japanese patients do not stand as an example for geographical or racial difference in the prevalence of gsp mutations in GH-secreting pituitary adenomas.

Bases moleculares dos adenomas hipofisários com ênfase nos somatotropinomas

Esta revisão descreve as bases moleculares dos adenomas hipofisários com ênfase nos tumores secretores de GH (somatotropinomas). São discutidos os papéis de genes de supressão tumoral (como RB1, MEN-1) e de oncogenes (como gsp, PTTG) na iniciação e progressão destes tumores. A caracterização destes marcadores moleculares pode ajudar na compreensão do comportamento tumoral, auxiliando a conduta terapêutica. Entretanto, apesar dos recentes avanços, ainda não é totalmente conhecida a seqüência de alterações genéticas envolvidas na patogênese destes adenomas.