Chromogranin A (CGA)

Characteristics

Together with secretogranins, chromogranins comprise a family of acid calcium-binding glycoproteins closely associated with the matrix of dense-core neurosecretory granules in virtually all neuroendocrine (NE) cells (Fig. 1a-b) and neurons (Fig. 1c-d). Probably, these proteins are involved in packaging and processing of neuropeptides and peptide hormones. Chromogranin A (CGA), 48-75 kDa, 439 amino acids, is the diagnostically most important of these glycoproteins, and probably the major target of the argyrophilic Grimelius staining reaction. Post-translational processing of CGA gives rise to fragments with biologic activities such as autocrine and paracrine functions.

A number of CGA fragments have been identified: vasostatins (amino acid sequences 1-17/113), chromostatin (116/124-130/143), chromacin (173/176-195), WE-14 (316-329), catestatin (344-364), mid-parastatin (347/375-384), C-terminal parastatin (411-419/424), GE-25 (367-391), and N-terminal pancreastatin (250-284/301). The functions of several of these are known. Some fragments have influence on cell growth, adhesion and activation (e.g., vascular smooth muscle cell contraction or relaxation, microglial neurotoxin secretion) as well as antimicrobial activity. e.g., chromostatin and catestatin have an inhibitory effect on catecholamine release from the adrenal medulla, secretoneurin can stimulate dopamine release.

Neoplasms

CGA is found in the large majority of epithelial NE tumours: pituitary adenomas (however,  prolactinoma, corticotropin-producing adenoma and null cell adenoma may be CGA negative while presenting CGB), parathyroid and pancreatic islet cell tumours (however, insulinoma and somatostatinoma may be CGA negative), medullary thyroid carcinoma, and carcinoid tumours of e.g., respiratory and gastrointestinal tract. As regards the latter, almost all foregut and midgut tumours are CGA positive, while hindgut carcinoids are frequently negative (while CGB positive).

CGA is detected in most cases of small cell carcinoma (particularly from the lung; (Fig. 2a), Merkel cell carcinoma, and other NE carcinomas (Fig. 2b) as well as in some cases of multiphenotypic tumours (e.g., malignant rhabdoid tumour, desmoplastic small round cell tumour). However, in the literature the proportion of these tumours reported as CGA positive is varying, e.g., 40 – 100 % in small cell lung carcinoma. This is in part depending on the Ab and the sensitivity of system combined with a variable and often low expression of CGA in these tumours.

A small number of CGA positive cells may be detected in non-neuroendocrine carcinomas as a result of a partial NE differentiation (e.g. breast, prostate). In some carcinomas (e.g., prostate, colo-rectum, stomach), the occurrence of CGA positive cells has shown to be a negative prognostic marker, while in others (e.g., breast) it seems to be of no prognostic importance.

CGA is found in the large majority of neuronal tumours: ganglioneuroblastoma, ganglioneuroma, ganglioglioma, phaeochromocytoma, paraganglioma (however, some parasympathic paraganglioma may be CGA negative). Primitive neuro­ectodermal tumours (neuroblastoma etc.) are usually  described as CGA negative, but CGA may be detected focally in sensitive systems.

CGA is not detected in glial tumours (with the exception of some oliogodendrogliomas), meningioma, choroid plexus tumours, schwannoma, malignant melanoma, or adrenocortical tumours. Carcinomas (with the exception of the above mentioned) as well as mesotheliomas and sarcomas are always CGA negative.

Application

Together with synaptophysin, CGA is the most important marker for the identification of NE and neuronal tumours and NE/neuronal differentiation in other tumours. In optimized protocols is CGA slightly more specific than synaptophysin and slightly less sensitive.

Visualization

The immunoreactivity of CGA antibodies is dependent of the neurosecretory granule density, and presence of the fragments that the antibody is directed to. Useful Abs are mAb clones LK2H10 and LK2H10+PHE5 and pAb A0430 (Dako). LK2H10 probably binds to the N-terminal part of the pancreastatin fragment of CGA, which is represented in the large majority (but not all) of neuroendocrine and neuronal tumours. In few cases a tumour negative with LK2H10 will be positive with a pAb. The staining product should be granular cytoplasmic. Heat induced epitope retrieval (HIER) is mandatory, and a high pH buffer should be preferred. Care should be taken to avoid endogenous biotin reaction giving false positive staining.

Control tissue: Appendix is appropriate: Not only the mucosal NE cells should be strongly labelled, but also the neurons of the Auerbach’s plexus, which only harbour a low content of CGA, must be demonstrated (photo).   Assessments Run 9 2003 Run 13 2005 Run 18 2006 Run 22 2008

Selected references

Hagn C, Schmid KW, Fischer-Colbrie R et al. Chromogranin A, B, and C in human adrenal medulla and endocrine tissues. Lab Invest 1986;55:405-411.

Konecki DS, Benedum UM, Gerdes HH et al. The primary structure of chromogranin A and pancreastatin. J Biol Chem  1987;262:17026-17030.

Lloyd RV, Cano M, Rosa P, et al. Distribution of chromogranin A and secretogranin I (chromogranin B) in neuroendocrine cells and tumors.  Am J Pathol 1988;130:296-304.

O’Connor DT. Chromogranin: widespread immunoreactivity in polypepetide hormone producing tissues and in serum. Regul Pept 1983;6:263-280.

Portela-Gomes GM, Grimelius L, Johansson H, Wilander E, Stridsberg M. Chromogranin A in human neuroendocrine tumors. An immunohistochemical study with region-specific antibodies. Am J Surg path 2001;25:1261-7.

Taupenot L, Harper KL, O’Connor DT:  The chromogranin-secretogranin family. N Engl J Med 2003;3:1134-1149.