Pulmonary large cell neuroendocrine carcinoma with adenocarcinoma-like features: napsin A expression and genomic alterations

Abstract

Pulmonary large cell neuroendocrine carcinoma (LCNEC) is a highly aggressive malignancy, which was recently found to comprise three major genomic subsets: small cell carcinoma-like, non-small cell carcinoma (predominantly adenocarcinoma)-like, and carcinoid-like. To further characterize adenocarcinoma-like subset, here we analyzed the expression of exocrine marker napsin A, along with TTF-1, in a large series of LCNECs (n=112), and performed detailed clinicopathologic and genomic analysis of napsin A-positive cases. For comparison, we analyzed napsin A expression in other lung neuroendocrine neoplasms (177 carcinoids, 37 small cell carcinomas) and 60 lung adenocarcinomas. We found that napsin A was expressed in 15% of LCNEC (17/112), whereas all carcinoids and small cell carcinomas were consistently negative. Napsin A reactivity in LCNEC was focal in 12/17 cases, and weak or moderate in intensity in all cases, which was significantly lower in the extent and intensity than seen in adenocarcinomas (P<0.0001). The combination of TTF-1-diffuse/napsin A-negative or focal was typical of LCNEC but was rare in adenocarcinoma, and could thus serve as a helpful diagnostic clue. The diagnosis of napsin A-positive LCNECs was confirmed by classic morphology, diffuse labeling for at least one neuroendocrine marker, most consistently synaptophysin, and the lack of distinct adenocarcinoma component. Genomic analysis of 14 napsin A-positive LCNECs revealed the presence of mutations typical of lung adenocarcinoma (KRAS and/or STK11) in 11 cases. In conclusion, LCNECs are unique among lung neuroendocrine neoplasms in that some of these tumors exhibit low-level expression of exocrine marker napsin A, and harbor genomic alterations typical of adenocarcinoma. Despite the apparent close biological relationship, designation of adeno-like LCNEC as a separate entity from adenocarcinoma is supported by their distinctive morphology, typically diffuse expression of neuroendocrine marker(s) and aggressive behavior. Further studies are warranted to assess the clinical utility and optimal method of identifying adenocarcinoma-like and other subsets of LCNEC in routine practice.

Figure 1

Example of LCNEC with focal napsin A expression (case ID 16 in Table 2). (A) H&E sections illustrate classic LCNEC morphology, including nested growth pattern with peripheral nuclear palisading, frequent rosette-like arrangements, and areas of geographic necrosis. (B) Higher-power image illustrates non-small cell cytomorphology - moderate volume of cytoplasm and evident nucleoli. Panels C and D illustrate weak and focal but convincing granular cytoplasmic napsin A labeling. Panel E illustrates diffuse labeling for synaptophysin. F. Ki67 marker confirms high proliferation rate (70%).

Figure 2

Example of LCNEC with diffuse weak to moderate napsin A expression (case ID 4 in Table 2). H&E sections (A,B) illustrate neuroendocrine morphology - nested and trabecular growth pattern with peripheral nuclear palisading, and overtly non-small cell cytology - prominent nucleoli with moderate amount of cytoplasm. Tumor has amphophilic cytoplasmic commonly seen in LCNEC. This type of morphology enters in the differential diagnosis with solid adenocarcinoma. Panels C and D illustrate napsin A labeling in the majority of tumor cells, which shows typical granular cytoplasmic reactivity with variably-sized granules. Napsin A expression is seen in the absence of entrapped pneumocytes or histiocytes, confirming the specificity of labelling. Panel E illustrates focal labeling for synaptophysin (SYN) in the same tumor areas as those labeling for napsin A. F. Ki67 confirms high proliferation rate (80%).

Figure 3

Contrast in intensity of napsin A reactivity in LCNEC (A and B) and lung adenocarcinoma (C and D). Intense (3+) labeling typical of adenocarcinomas (D) was not seen in any LCNECs (B). The figure illustrates a case on adenocarcinoma with cribriform pattern, which enters in the close differential diagnosis with LCNEC. Cribriform spaces in adenocarcinoma tend to have more undulating outlines, with occasional slit-like lumens, whereas luminal borders in LCNEC are characteristically rosette-like, with rigid/punched-out outlines (arrowheads). Spaces in rosettes also tend to be smaller, pinpoint-like, compared to more variable luminal sizes in adenocarcinoma.