Napsin A

• Cytoplasmic napsin A expression in alveolar (lung) epithelium and alveolar macrophages
• Napsin A expresion in renal tubules
• Napsin A positivity in alveolar macrophages
• Diffuse granular cytoplasmic napsin A expression in a pulmonary adenocarcinoma metastatic to the brain
• Napsin A in adenocarcinoma of the lung
• Napsin A in pulmonary adenocarcinoma. Note the granular cytoplasmic staining pattern

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Synonyms

Novel Aspartic Proteinase of the PepSIN Family

Antibodies

• TA02

Staining Pattern

• Granular cytoplasmic

Expression in Normal Tissues

• Alveolar epithelium (lung) including type 2 pneumocytes

• Alveolar macrophages
• Renal tubules

Expression in Neoplastic Entities

Benign

Tumor name	% Positive Staining	Number of Cases
Alveolar adenoma	100%	1/1
Sclerosing hemangioma	100% ("surface cells")	6/6

Intermediate/Uncertain Malignant Potential

Tumor name	% Positive Staining	Number of Cases

Malignant

Tumor name	% Positive Staining	Number of Cases
Primary adenocarcinoma of lung	83%	79/95
Primary adenocarcinoma of lung	77%	122/158
Thyroid carcinoma
Metastatic adenocarcinoma of lung origin	79%	46/58
Poorly differentiated adenocarcinoma of the lung	58%	11/19

Malignant

• Napsin A is expressed in 84.5% of primary lung adenocarcinomas but not in adenocarcinomas of other primary sites

Pertinent Negatives

This antigen is typically NOT expressed by the following entities:

• Squamous cell carcinomas of the lung
• Adenocarcinomas of the GI tract
• Breast carcinomas of any type

Practical Uses / Panels

• Napsin A has been used in a panel with TTF-1, p63 and CK5/6 in small lung biopsies to classify non-small cell carcinomas that lack morphologic differentiating features on H&E. In such cases, napsin A positivity confirms a diagnosis of adenocarcinoma.
• Napsin A can be used in a panel to indicate lung origin in a metastatic adenocarcinoma of unknown primary.

Common Pitfalls

Lung epithelium (napsin A positive) that is entrapped in a napsin A-negative tumor may cause misinterpretation.

Alveolar macrophages (napsin A positive) within a napsin A-negative tumor may cause misinterpretation. Napsin A expression diminishes with decreased differentiation of lung adenocarcinomas.

Napsin A also stains 10% of thyroid carcinomas and renal cell carcinomas.

References

Mukhopadhyay S, Katzenstein AL. Subclassification of non-small cell lung carcinomas lacking morphologic differentiation on biopsy specimens: utility of an immunohistochemical panel containing TTF-1, napsin A, p63 and CK5/6. Am J Surg Pathol 2011; 35:15-25.

Bishop JA, Sharma R, Illei PB. Napsin A and thyroid transcription factor-1 expression in carcinomas of the lung, breast, pancreas, colon, kidney, thyroid, and malignant mesothelioma. Hum Pathol. 2010 Jan;41(1):20-5.