Processing-independent analysis in the diagnosis of gastrinomas

A novel method to detect hPG80 (human circulating progastrin) in the blood

hPG₈₀ (human circulating progastrin) is produced and released by cancer cells. We recently reported that hPG₈₀ is detected in the blood of patients with cancers from different origins, suggesting its potential utility for cancer detection. To accurately measure hPG₈₀ in the blood of patients, we developed the DxPG₈₀ test, a sandwich Enzyme-Linked Immunosorbent Assay (ELISA). This test quantifies hPG₈₀ in EDTA plasma samples. The analytical performances of the DxPG₈₀ test were evaluated using standard procedures and guidelines specific to ELISA technology. We showed high specificity for hPG₈₀ with no cross-reactivity with human glycine-extended gastrin (hG17-Gly), human carboxy-amidated gastrin (hG17-NH₂) or the CTFP (C-Terminus Flanking Peptide) and no interference with various endogenous or exogenous compounds. The test is linear between 0 and 50 pM hPG₈₀ (native or recombinant). We demonstrated a trueness of measurement, an accuracy and a variability of hPG₈₀ quantification with the DxPG₈₀ test below the 20% relative errors as recommended in the guidelines. The limit of detection of hPG₈₀ and the limit of quantification were calculated as 1 pM and 3.3 pM respectively. In conclusion, these results show the strong analytical performance of the DxPG₈₀ test to measure hPG₈₀ in blood samples.

Gastrointestinal hormone research - with a Scandinavian annotation

Gastrointestinal hormones are peptides released from neuroendocrine cells in the digestive tract. More than 30 hormone genes are currently known to be expressed in the gut, which makes it the largest hormone-producing organ in the body. Modern biology makes it feasible to conceive the hormones under five headings: The structural homology groups a majority of the hormones into nine families, each of which is assumed to originate from one ancestral gene. The individual hormone gene often has multiple phenotypes due to alternative splicing, tandem organization or differentiated posttranslational maturation of the prohormone. By a combination of these mechanisms, more than 100 different hormonally active peptides are released from the gut. Gut hormone genes are also widely expressed outside the gut, some only in extraintestinal endocrine cells and cerebral or peripheral neurons but others also in other cell types. The extraintestinal cells may release different bioactive fragments of the same prohormone due to cell-specific processing pathways. Moreover, endocrine cells, neurons, cancer cells and, for instance, spermatozoa secrete gut peptides in different ways, so the same peptide may act as a blood-borne hormone, a neurotransmitter, a local growth factor or a fertility factor. The targets of gastrointestinal hormones are specific G-protein-coupled receptors that are expressed in the cell membranes also outside the digestive tract. Thus, gut hormones not only regulate digestive functions, but also constitute regulatory systems operating in the whole organism. This overview of gut hormone biology is supplemented with an annotation on some Scandinavian contributions to gastrointestinal hormone research.

Chromogranin A in gastrinomas: Promises and pitfalls

Patients with neuroendocrine tumors are found with increasing frequency. Accordingly, knowledge about relevant tumor markers and assays for diagnosis and control has become essential. Neuroendocrine tumors release one or more granin proteins. Of these, chromogranin A (CgA) has so far become the most widely used general marker. The CgA protein is, however, extensively cleaved and otherwise modified during the biosynthetic processing. In addition, the CgA-processing in individual tumors varies considerably. But only few CgA-assays have taken the processing into account and characterized the assays with respect to precise epitope-specificity. Consequently, we do not know which fragments most CgA-assays measure. It is therefore at present difficult to compare CgA-measurements from tumor patients. Some tumors, however, release - in addition to granins - also a specific hormone that causes a clinical syndrome. This review uses gastrinomas (gastrin-producing tumors) as a starting point for discussion of CgA versus peptide hormone as tumor marker. Data available so far indicate that well-defined assays for gastrin have significantly higher diagnostic sensitivity than CgA measurements in gastrinomas. But the review suggests that CgA-quantitation using processing-independent analysis (PIA) may provide an equally high diagnostic sensitivity and in addition offer a simple possibility for estimation of the tumor-burden.

An evaluation of chromogranin A versus gastrin and progastrin in gastrinoma diagnosis and control

AIM

The value of chromogranin A (CgA) versus gastrin and progastrin in diagnosis and control of gastrinoma patients is not settled because the peptides circulate as variable mixtures. We have addressed this complexity using defined sequence-specific assays.

PATIENTS & METHODS

Six assays were applied to plasma from 40 gastrinoma patients to measure α-amidated gastrins, glycine-extended gastrins, the total progastrin product, and assays for CgA sequence (340-348) and the 'total' CgA product.

RESULTS

The gastrin/progastrin parameters did not add to the diagnosis beyond that of α-amidated gastrins, except in one patient. All gastrin parameters correlated otherwise closely. The CgA results differed. Thus, 11 patients had normal CgA concentrations. By contrast, all total CgA concentrations were elevated but correlated only moderately to gastrin.

CONCLUSION

Assays measuring α-amidated gastrins have high diagnostic value except for singular patients in whom only progastrin was elevated. By contrast, CgA measurements are not valid in diagnosis or control of gastrinomas.

The Zollinger-Ellison syndrome and mismeasurement of gastrin

BACKGROUND & AIMS

Zollinger-Ellison syndrome (ZES) is characterized by hypersecretion of gastric acid, severe peptic ulcerations in the upper small intestine, and diarrhea. It is usually diagnosed by measuring increased levels of gastrin in plasma.

METHODS

We examined the accuracy of commercial kits to measure gastrin (7 radioimmunoassays and 5 enzyme-linked immunosorbent assays), using plasma from 40 patients suspected or known to have ZES. Each sample was analyzed using the 12 kits and a reference assay that measures bioactive gastrin in plasma, irrespective of size and amino acid derivatization. Known concentrations of peptides with identical sequences to circulating gastrins were also assessed by all assays. Molecular patterns in plasma from patients with ZES were examined by chromatography and monitored by kits that measure false-low or false-high concentrations of gastrin.

RESULTS

Failure to diagnose gastrinomas has serious consequences. Four kits found false-low concentrations of gastrin in 20% to 80% of the patients. Specificity assessment showed that the antibodies used in these kits bound only gastrin-17. Three kits found false-high concentrations of gastrin, because the reagents had increased reactions to sulfated gastrins or to unspecific factors in plasma. Thus, only 5 of 12 kits tested accurately measure plasma concentrations of gastrin.

CONCLUSIONS

Seven of 12 tested commercial kits inaccurately measure plasma concentrations of gastrin; these assays used antibodies with inappropriate specificity that were insufficiently validated. Misdiagnosis of gastrinoma based on lack of specificity of assays for gastrin results in ineffective or inappropriate therapy for patients with ZES.

Juvenile polyposis of the stomach--a novel cause of hypergastrinemia

BACKGROUND

A 38-year-old female presented with a 3-year history of postprandial abdominal pain, refractory nausea, vomiting and hematemesis. She appeared malnourished and her symptoms were refractory to previous treatment with acid-suppressive drugs, prokinetics and antiemetics. Her medical history was significant for a diagnosis of juvenile polyposis syndrome at the age of 14 resulting in a transverse colectomy, and a diagnosis of Crohn's disease in her residual colon at the age of 35 resulting in a total colectomy.

INVESTIGATIONS

Physical examination, blood analysis, esophagogastroduodenoscopy with biopsy, abdominal endoscopic ultrasound, abdominal CT scan, MRI, 24 h urine analysis, MIBG scintigraphy, ocreotide scintigraphy, fluorodeoxyglucose-PET scan and genetic testing for defined polyposis syndromes (SMAD4, BMPR1A).

DIAGNOSIS

Juvenile polyposis syndrome with outlet obstruction of the stomach and excessive hypergastrinemia.

MANAGEMENT

Continuous acid-suppressive therapy, prokinetic therapy and total parenteral nutrition. Repetitive endoscopic polypectomy (also known as debulking) was performed twice and was followed by gastrectomy with duodenoesophageal anastomosis.

Cell-specific precursor processing

The singular gene for a peptide hormone is expressed not only in a specific endocrine cell type but also in other endocrine cells as well as in entirely different cells such as neurons, adipocytes, myocytes, immune cells, and cells of the sex-glands. The cellular expression pattern for each gene varies with development, time and species. Endocrine regulation is, however, based on the release of a given hormone from an endocrine cell to the general circulation from whose cappilaries the hormone reaches the specific target cell elsewhere in the body. The widespread expression of hormone genes in different cells and tissues therefore requires control of biogenesis and secretion in order to avoid interference with the function of a specific hormonal peptide from a particular endocrine cell. Several mechanisms are involved in such control, one of them being cell-specific processing of prohormones. The following pages present four examples of such cell-specific processing and the implications of the phenomenon for the use of peptide hormones as markers of diseases. Notably, sick cells - not least the neoplastic cells - often process prohormones in a manner different from that of the normal endocrine cells.

The art of measuring gastrin in plasma: a dwindling diagnostic discipline?

The gastrointestinal hormone gastrin is measured in plasma in physiological, pathophysiological and diagnostic investigations. In the diagnosis of hypergastrinaemic diseases such as gastrinomas and gastric achlorhydria, measurement of gastrin concentrations in circulation is crucial. Gastrin circulates, however, not as a single peptide but as a mixture of peptides of different lengths and amino acid derivatizations. Moreover, in hypergastrinaemia the peptide pattern changes. Consequently, diagnostic gastrin measurements require immunoassays that recognize the pathological plasma patterns, which are characterized by a predominance of the large peptides (gastrin-34 and gastrin-71) and less, if any, of the shorter main form of gastrin in normal tissue, gastrin-17. Alternatively, and in specific cases, "processing-independent assays" (PIA) for progastrin may be considered, since hypersecreting gastrin cells also release substantial amounts of biosynthetic precursors and processing intermediates. Recently, gastrin kits that do not take the pathological plasma patterns into account have been marketed and may miss the diagnosis. Therefore, proper diagnosis of gastrinomas and other hypergastrinaemic diseases requires insight into cellular gastrin synthesis and peripheral metabolism, and also into the design of useful immunoassays. This review discusses the art of measuring gastrin in plasma with adequate diagnostic specificity.

Processing-independent quantitation of chromogranin a in plasma from patients with neuroendocrine tumors and small-cell lung carcinomas

BACKGROUND

Most neuroendocrine tumors express chromogranin A (CgA). The posttranslational processing of neuroendocrine proteins such as CgA is often specific for the individual tumor. To cope with this variability and improve tumor diagnosis, we developed a processing-independent analysis (PIA) method to measure the total CgA product.

METHODS

For PIA, samples underwent trypsin treatment followed by measurement of CgA by the "CgA(340-->)" assay, in which the antiserum binds an epitope starting at amino acid 340 of CgA and including amino acid residues located in the C-terminal direction. The diagnostic accuracy of the CgA PIA and 3 sequence-specific assays for CgA were evaluated on plasma samples from patients with neuroendocrine tumors and small-cell lung carcinomas. Furthermore, we investigated whether the CgA plasma concentrations correlated with the tumor burden.

RESULTS

Size-exclusion chromatography of plasma showed that CgA immunoreactivity mainly consisted of high-molecular-weight forms, indicating that neuroendocrine tumors may secrete large amounts of poorly processed CgA. Accordingly, trypsination of plasma from 54 patients with neuroendocrine tumors or small-cell lung carcinomas increased the CgA(340-->) immunoreactivity up to 500-fold. Both the CgA(340-->) assay and the PIA measured significantly higher plasma concentrations in patients with very extensive disease than in patients with less widespread disease. The diagnostic sensitivity was 0.91 when using the CgA(340-->) assay and 0.82 using the CgA PIA.

CONCLUSION

The CgA(340-->) assay and CgA PIA are both useful for diagnosis of neuroendocrine tumors and small-cell lung carcinomas and both assays correlate with tumor burden.

Serum gastrin in Zollinger-Ellison syndrome: I. Prospective study of fasting serum gastrin in 309 patients from the National Institutes of Health and comparison with 2229 cases from the literature

The assessment of fasting serum gastrin (FSG) is essential for the diagnosis and management of patients with the Zollinger-Ellison syndrome (ZES). Although many studies have analyzed FSG levels in patients with gastrinoma, limited information has resulted from these studies because of their small size, different methodologies, and lack of correlations of FSG levels with clinical, laboratory, or tumor features in ZES patients. To address this issue, we report the results of a prospective National Institutes of Health (NIH) study of 309 patients with ZES and compare our results with those of 2229 ZES patients in 513 small series and case reports in the literature. In the NIH and literature ZES patients, normal FSG values were uncommon (0.3%-3%), as were very high FSG levels >100-fold normal (4.9%-9%). Two-thirds of gastrinoma patients had FSG values <10-fold normal that overlap with gastrin levels seen in more common conditions, like Helicobacter pylori infection or antral G-cell hyperplasia/hyperfunction. In these patients, FSG levels are not diagnostic of ZES, and gastrin provocative tests are needed to establish the diagnosis. Most clinical variables (multiple endocrine neoplasia type 1 status, presence or absence of the most common symptoms, prior medical treatment) are not correlated with FSG levels, while a good correlation of FSG values was found with other clinical features (prior gastric surgery, diarrhea, duration from onset to diagnosis). Increasing basal acid output, but not maximal acid output correlated closely with increasing FSG. Numerous tumoral features correlated with the magnitude of FSG in our study, including tumor location (pancreatic > duodenal), primary size (larger > smaller) and extent (liver metastases > local disease). In conclusion, this detailed analysis of FSG in a large number of patients with ZES allowed us to identify important clinical guidelines that should contribute to improved diagnosis and management of patients with ZES.