Abstract
Glucagon cell hyperplasia and neoplasia (GCHN) is the name of an endocrine receptor disease, whose morphology was first described in 2006. Three years later, this rare disease was found to be to be caused by an inactivating mutation of the glucagon receptor (GCGR) gene. Functionally, the genetic defect mainly affects glucagon signaling in the liver with changes in the metabolism of glycogen, fatty acids and amino acids. Recent results of several studies in GCGR knockout mice suggested that elevated serum amino acid levels probably stimulate glucagon cell hyperplasia with subsequent transformation into glucagon cell neoplasia. This process leads over time to numerous small and some large pancreatic neuroendocrine tumors which are potentially malignant. Despite high glucagon serum levels, the patients develop no glucagonoma syndrome. In 2015, GCHN was identified as an autosomal recessive hereditary disorder.
Introduction
Glucagon cell hyperplasia and neoplasia (GCHN) of the endocrine pancreas is a rare disease, with only a few patients reported so far. Soon after its morphological description in 2006, its genetic background, an inactivating mutation of the glucagon receptor gene (GCGR), was found in 2009. Since this discovery coincided with an interest in inhibiting the glucagon receptor as a way to find a treatment for type 2 diabetes (Guzman et al. 2017), there were mouse models available and further developed, including GCGR knock-out mice (Gelling et al. 2003, for review see Yu 2018). According to these well-characterized mouse models, GCGR inactivation probably interrupts glucagon signaling in the liver, whereby the metabolism of glycogen, fatty acids and amino acids is disturbed. The net result of the altered function of the liver cells is hyperplastic changes of the glucagon cells of the pancreatic islets, followed by hyperglucagonemia and the development of multiple glucagon-producing neuroendocrine tumors.
History and histology
The first well-documented cases of GCHN were published in 2006 by Martin Anlauf and colleagues in an article on microadenomatosis of the endocrine pancreas in patients with and without the multiple endocrine neoplasia type 1 syndrome (Anlauf et al. 2006). Among 37 cases of multiple pancreatic endocrine neoplasms were 4 patients who had no hormonal symptoms and showed no association with mutations in the von Hippel–Lindau (VHL) or multiple endocrine neoplasia 1 (MEN1) genes. Histology and immunohistochemistry revealed that the four pancreata contained a few neuroendocrine macrotumors and many unevenly distributed microtumors all expressing almost exclusively glucagon and only occasionally a pancreatic polypeptide cell. In addition, there were normal islets besides many big islets with glucagon cell hyperplasia (Fig. 1). The tumors showed a Ki67 index below 1%.
In 2008, the first clinically well-characterized case of GCHN was described by the group of Run Yu at Cedars-Sinai Medical Center in Los Angeles (Yu et al. 2008). The patient was a 60-year-old woman with abdominal pain and sometimes mild hypoglycemia but without signs of a glucagonoma syndrome such as hyperglycemia, skin rash, stomatitis, weight loss or thromboembolism, although markedly elevated glucagon levels were detected. In the head of the pancreas, there was a 4 cm large neuroendocrine tumor associated with two microtumors. In the surrounding tissue, there was marked hyperplasia of the endocrine islets with the predominance of glucagon-expressing cells. The neuroendocrine tumor expressed no glucagon or pancreatic polypeptide. Yu and colleagues named the disorder ‘Mahvash’ disease after the patient’s first name (Yu 2018).
In 2009, Henopp et al. further characterized four patients (including the first three cases from the publication of Anlauf et al. 2006) and demonstrated in addition to multiple micro- and macro-neuroendocrine tumors with glucagon expression the presence of diffuse glucagon cell hyperplasia in the islets that increased with the size of the islets to the point that it eventually imperceptibly passed into glucagon cell microtumors (Henopp et al. 2009).
In the same year, the group of Yu (Zhou et al. 2009) sequenced the GCGR in the patient reported in 2008 and detected a homozygous point mutation (P86S, exon 4) in the extracellular domain. In vitro functional assays revealed that this mutation abrogated glucagon binding but not the localization of the receptor in the plasma membrane.
In 2015, we reported a series of six patients with GCHN (Sipos et al. 2015). Three of them harbored deleterious mutations in the GCGR gene, while the other cases revealed a wildtype GCGR gene by Sanger and pyrosequencing. The pancreatic changes of the mutation carriers were more pronounced in terms of the number and size of glucagon-positive tumors. This finding suggested that GCHN, as we called the disease (Sipos et al. 2015), seems to be caused by two different pathomechanisms, of which one is associated with a GCGR gene mutation, while the other is not and lacks so far any known genetic alteration or explanation. As to the inheritance of the GCGR mutation, we found an autosomal recessive trait in one index patient and his parents. We also detected lymph node micrometastases in one GCHN patient with wildtype GCGR, indicating that GCHN has the potential to metastasize (Sipos et al. 2015). In 2017, GCHN has been included in the WHO Classification of Tumours of Endocrine Organs in the chapter of inherited tumor syndromes (WHO 2018).
Clinical presentation and course
Patients are mostly middle-aged adults who present with non-specific symptoms such as abdominal pain, fatigue, diabetes or acute pancreatitis. Despite very high serum glucagon levels, none of the GCHN patients with GCGR mutations showed a glucagonoma syndrome, most probably owing to the receptor’s deficient signaling. The only reported patient with a glucagonoma syndrome harbored a wildtype GCGR (Otto et al. 2011, Sipos et al. 2015), a constellation that still needs explanation as to the underlying GCHN. Elevated amino acid levels were found in the only pediatric patient (Li et al. 2018) and in one adult patient (Larger et al. 2016) reported so far. The pancreatic neuroendocrine tumours (NET) can be visualized by somatostatin receptor imaging, since the glucagon-producing cells express the somatostatin receptor SSTR2 (Henopp et al. 2009).
GCHN follows a benign clinical course for most patients. However, follow-up of the patients is suggested, as the tumors have a metastatic potential. Glucagon serum level may serve as tumor marker for monitoring patients during surveillance. Some anecdotal GCHN cases with liver metastasis are known in the scientific community, but none of these cases has been published so far (Yu 2018).
Genetics and pathogenesis
GCHN in patients with mutated GCGR follows an autosomal recessive inheritance (Sipos et al. 2015). The GCGR gene is located on chromosome 17q25. The reported GCGR gene germline mutations are either homozygous GCGR sequence variants or two compound heterozygous variants leading to premature stop codons or most probably deficient protein expression (Table 1). The functional characterization of D63N, P86S, R225H, V368M point mutations revealed impaired receptor signaling function (Zhou et al. 2009, van der Velden et al. 2022).
Characteristics of GCGR mutations in GCHN cases.
Mutation | Exon | Domain | Function | Method | Patient | Reference |
---|---|---|---|---|---|---|
R8STOP | 2 | First extracellular | Loss, premature termination of translation | In silico | Female, 43 years | Sipos et al. (2015) |
IVS3-1G > T | 4 splice site | First extracellular | Loss, premature termination of translation | In silico | More family members | Tang & Yu (2016) |
D63N | 4 | First extracellular | Reduced glucagon binding, reduced G protein binding | In vitro | More family members | Yu et al. (2016), van der Velden et al. (2022) |
W83Lfs*35 | 4 | First extracellular | Loss, premature termination of translation | In silico | Male, 25 years | Sipos et al. (2015) |
P86S | 4 | First extracellular | Reduced glucagon binding, reduced G protein binding | In vitro | Female, 65 years | Zhou et al. (2009), van der Velden et al. (2022) |
R225H | 8 | Beginning of extracellular loop 3 | Reduced G protein binding | In silico, in vitro | Male, 58 years | Sipos et al. (2015), van der Velden (2022) |
IVS9-1G > A | 9, splice site | Second extracellular loop and part of the fifth transmembrane domain | Missense translation of the sequence encoded by exons 10–13, and introduction of a stop codon after amino acid 327. | In vitro | Male, 51 years | Larger et al. (2016) |
Phe320del | 11 | Fifth transmembrane domain | Probably altered G protein binding | In vitro | Female, 7 years | Li et al. (2018) |
Q327STOP | 11 | Third intracellular loop | Loss, premature termination of translation | In silico | Female, 43 years | Sipos et al. (2015) |
V368M | 12 | Sixth transmembrane domain | Reduced G protein binding | In silico, in vitro | Male, 58 years | Sipos et al. (2015), Lin et al. (2020), van der Velden (2022) |
The GCGR is expressed in humans predominantly in the liver. Its activation by glucagon which is mainly secreted in response to hypoglycemia and elevated amino acid serum levels (Janah et al. 2019, van der Velden et al. 2022) suppresses glycolysis and increases gluconeogenesis and glycogenolysis in hepatocytes. Glucagon also increases hepatic amino acid turnover and thus lowers postprandial serum amino acid levels. In addition, glucagon promotes increased beta-oxidation, decreased fatty acid synthesis and very-low-density lipoprotein (VLDL) release.
Of these glucagon effects, the disturbed amino acid metabolism plays the most important role in the development of GCHN, as several studies in GCGR knockout mice have shown (Solloway et al. 2015, Kim et al. 2019, Capozzi et al. 2022). Impaired glucagon signaling in the liver results in constantly elevated serum amino acid levels which stimulate the secretion and proliferation of glucagon cells, leading to hyperglucagonemia, glucagon cell hyperplasia and finally neuroendocrine tumors. This proliferative effect on glucagon cells is most probably mediated by amino acid transporters (e.g. slc38a5 in mice, slc38a4 in humans), which react to several amino acids, that were found to be elevated in GCHN. These amino acid transporters are coupled with downstream mechanistic targets of rapamycin signaling, which leads to glucagon cell proliferation, but not to an increase in glucagon secretion (Kim et al. 2017, 2019). Since the diagnosis of GCHN in humans was mostly made postoperatively, only few data on increased amino acid levels in GCHN patients are available. However, these data that come from the only pediatric patient and from one adult patient (Larger et al. 2016) suggest that the pathomechanisms of glucagon cell proliferation in mouse models and humans with an inactive GCGR are probably comparable.
Conclusions
GCHN is a receptor disease, in which changes in the glucagon cells follow a hyperplasia–neoplasia sequence. The molecular mechanisms underlying GCHN tumorigenesis are unique among the neuroendocrine tumors of the pancreas. The first step, the glucagon cell hyperplasia, has been well explained in recent years by the inactivating mutation of the GCGR gene that leads to disturbed glucagon signaling in the liver cells and elevated amino acid levels in the serum, with a stimulating effect on glucagon cell secretion and proliferation. The second step, the transformation of hyperplastic glucagon cells into neoplastic glucagon cells, has yet to be elucidated but might be explained by the activation of one of the driver genes in pancreatic neuroendocrine tumorigenesis. The proliferative activity and the metastatic potential of the neoplastic glucagon cells seem to be low. However, since only a small number of GCHN cases have been described and studied so far, future investigations with long-term follow-up in a larger number of GCHN cases will probably give new insights into the course and the spectrum of the disease. Of great interest in this respect is the question regarding the pathomechanism of GCHN cases with wildtype GCGR. As possible mechanisms can be hypothesized alterations in amino acid transporters, in downstream signaling or epigenetic modifications in GCGR.
Declaration of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of this review.
Funding
This work did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.
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