Small cell and large cell neuroendocrine carcinomas of the pancreas are genetically similar and distinct from well-differentiated pancreatic neuroendocrine tumors

Abstract

Poorly differentiated neuroendocrine carcinomas (NECs) of the pancreas are rare malignant neoplasms with a poor prognosis. The aim of this study was to determine the clinicopathologic and genetic features of poorly differentiated NECs and compare them with other types of pancreatic neoplasms. We investigated alterations of KRAS, CDKN2A/p16, TP53, SMAD4/DPC4, DAXX, ATRX, PTEN, Bcl2, and RB1 by immunohistochemistry and/or targeted exomic sequencing in surgically resected specimens of 9 small cell NECs, 10 large cell NECs, and 11 well-differentiated neuroendocrine tumors (PanNETs) of the pancreas. Abnormal immunolabeling patterns of p53 and Rb were frequent (p53, 18 of 19, 95%; Rb, 14 of 19, 74%) in both small cell and large cell NECs, whereas Smad4/Dpc4, DAXX, and ATRX labeling was intact in virtually all of these same carcinomas. Abnormal immunolabeling of p53 and Rb proteins correlated with intragenic mutations in the TP53 and RB1 genes. In contrast, DAXX and ATRX labeling was lost in 45% of PanNETs, whereas p53 and Rb immunolabeling was intact in these same cases. Overexpression of Bcl-2 protein was observed in all 9 small cell NECs (100%) and in 5 of 10 (50%) large cell NECs compared with only 2 of 11 (18%) PanNETs. Bcl-2 overexpression was significantly correlated with higher mitotic rate and Ki67 labeling index in neoplasms in which it was present. Small cell NECs are genetically similar to large cell NECs, and these genetic changes are distinct from those reported in PanNETs. The finding of Bcl-2 overexpression in poorly differentiated NECs, particularly small cell NEC, suggests that Bcl-2 antagonists/inhibitors may be a viable treatment option for these patients.

Figure 1. Histologic features of neuroendocrine neoplasms of the pancreas

(A) Small cell neuroendocrine carcinoma (small cell NEC). The neoplasm is composed of small cells with minimal cytoplasm, fusiform nuclei, inapparent nucleoli, and a high mitotic rate. Apoptotic bodies are also frequent. (B) Compared to the carcinoma shown in A, this neoplasm contains large cells with abundant cytoplasm. The nuclei have an open chromatin pattern with prominent nucleoli. Abundant mitoses are also seen. (C) Well-differentiated neuroendocrine tumor. Note the small uniform nuclei with delicate chromatin. The neoplastic cells show a trabecular growth pattern in a background of hyalinized and vascular stroma (hematoxylin and eosin stains, original magnifications x200).

Figure 2. Immunohistochemical labeling patterns for DAXX and ATRX

(A) Example of positive nuclear labeling for DAXX in a large cell neuroendocrine carcinoma. (B) Loss of nuclear labeling of DAXX in a well-differentiated neuroendocrine tumor. Endothelial cells and lymphocytes within the stroma are positive. (C) Positive nuclear labeling for ATRX in a small cell neuroendocrine carcinoma. (D) Loss of nuclear labeling of ATRX in a well-differentiated neuroendocrine tumor. Similar to that for DAXX shown in panel B, endothelial cells and lymphocytes within the stroma are positive (original magnifications x200).

Figure 3. Immunohistochemical labeling for p53, Rb1 and Bcl-2 in small cell neuroendocrine carcinomas

(A) Example of loss of nuclear labeling for p53. Scattered reactive acinar cells with positive nuclear labeling are present in the adjacent normal tissue (arrowheads). Sequencing analysis for TP53 in this patient revealed a nonsense mutation (p.E307X). (B) Example of diffusely positive nuclear labeling for p53. (C) Example of loss of nuclear labeling for retinoblastoma (Rb1) protein. Adjacent nonneoplastic cells show positive nuclear labeling (left side). (D) Diffuse cytoplasmic positivity for Bcl-2 protein. Note the reactive lymphocyte regarded as an internal positive control for this protein (arrowhead) (original magnifications x200).

Figure 4. Immunohistochemical labeling for Rb1 and p16 in a large cell neuroendocrine carcinoma

(A) Positive nuclear immunolabeling for Rb1. (B) Loss of p16 labeling in a serial section of the same large cell neuroendocrine carcinoma. Reactive acinar cells (arrowheads) serve as positive internal controls for labeling (original magnifications x200).

Figure 5. Sequencing analysis for TP53 and RB1 in small cell neuroendocrine carcinoma

This carcinoma (Case #5) contained a homozygous missense mutation (c.379T>A, p.S127T) in exon 5 of TP53, as well as a homozygous mutation of the intron 12 acceptor splice site (c.1219-2A>G) of RB1.