B-cell specific activation protein encoded by the PAX-5 gene is commonly expressed in merkel cell carcinoma and small cell carcinomas

Am J Surg Pathol. 2005 May;29(5):687-92. doi: 10.1097/01.pas.0000155162.33044.4f.


PAX-5 is a B cell specific transcription factor crucial for B cell ontogeny and has been detected in most of human B-cell lymphomas. In mouse, PAX-5 is also highly expressed in the central nervous system under tight temporal and spatial controls during embryogenesis. In humans, however, detection of PAX-5 in cells other than B lymphocytes has rarely been reported. We have encountered cases of Merkel cell carcinoma expressing PAX-5 during our routine evaluation of lymphoma. Because Merkel cell carcinoma is a small blue round cell tumor constantly in the differential diagnosis of lymphoma, we expanded our study in an effort to determine if PAX-5 is significantly expressed in neuroendocrine tumors. Based on our immunohistochemistry results using a monoclonal anti-PAX5 antibody with paraffin-embedded tissue sections, we report herein that PAX-5 was detected in 29 of 31 (93.5%) of Merkel cell carcinoma and 22 of 30 (73.3%) of small cell carcinoma, but in none of 17 cases of carcinoid tumor. Furthermore, the staining intensity of PAX-5 in Merkel cell carcinoma was frequently comparable with that in most B-cell lymphomas. We conclude that expression of PAX-5 is not confined to the B cell lineage and is frequently associated with neuroendocrine carcinomas.

MeSH terms

  • B-Lymphocytes / metabolism*
  • B-Lymphocytes / pathology
  • Biomarkers, Tumor / metabolism
  • Carcinoid Tumor / genetics
  • Carcinoid Tumor / metabolism
  • Carcinoid Tumor / pathology
  • Carcinoma, Merkel Cell / genetics
  • Carcinoma, Merkel Cell / metabolism*
  • Carcinoma, Merkel Cell / pathology
  • Carcinoma, Small Cell / genetics
  • Carcinoma, Small Cell / metabolism*
  • Carcinoma, Small Cell / pathology
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism*
  • Gastrointestinal Neoplasms / genetics
  • Gastrointestinal Neoplasms / metabolism
  • Gastrointestinal Neoplasms / pathology
  • Humans
  • Immunohistochemistry
  • Lung Neoplasms / genetics
  • Lung Neoplasms / metabolism*
  • Lung Neoplasms / pathology
  • PAX5 Transcription Factor
  • RNA, Messenger / metabolism
  • RNA, Neoplasm / analysis
  • Reverse Transcriptase Polymerase Chain Reaction
  • Skin Neoplasms / genetics
  • Skin Neoplasms / metabolism*
  • Skin Neoplasms / pathology
  • Transcription Factors / genetics
  • Transcription Factors / metabolism*


  • Biomarkers, Tumor
  • DNA-Binding Proteins
  • PAX5 Transcription Factor
  • PAX5 protein, human
  • RNA, Messenger
  • RNA, Neoplasm
  • Transcription Factors