SHORT REPORT Lacking CD56 expression in a relapsing cutaneous blastic plasmacytoid dendritic cell neoplasm after allogeneic bone marrow transplantation: FISH analysis revealed loss of 11q C Mitteldorf, †,1, * HP Bertsch, † M Baumgart, ‡ D Haase, § G Wulf, § MP Scho ¨ n, † A Rosenwald, – C Neumann, † KM Kaune † † Department of Dermatology, Venereology and Allergology, Georg August University Go ¨ ttingen, Germany, ‡ Leibniz Institute for Age Research – Fritz Lipmann Institute, Jena, Germany, § Department of Hematology and Oncology, Georg August University Go ¨ ttingen, Germany, and – Institute of Pathology, University of Wu ¨ rzburg, Wu ¨ rzburg, Germany *Correspondence: C Mitteldorf. E-mail: christina.mitteldorf@klinikum-hildesheim.de Abstract Background Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare entity characterized by a CD4+ ⁄ CD56+ ⁄ CD123+ immunophenotype and a fatal clinical course. The average survival of 12–14 months may be prolonged by allogeneic bone marrow transplantation (BMT). Objectives We report about a male patient who suffered from BPDCN with a typical histology and co-expression of CD4 ⁄ CD123 and a CD56 expression by 80% of the tumour cells. The cutaneous tumour relapse after chemotherapy and allogeneic BMT was completely negative for CD56. Methods We performed interphase fluorescence in situ hybridization (FISH) analysis of tumour tissue, asserved before and after BMT, using specific probes for chromosome 11, which encompass the CD56 gene region. Results The tumour cells revealed a partial loss of 11q as well as a monosomy of chromosome 11. Conclusion This case demonstrates for the first time that loss of CD56 expression can also occur as a secondary event after chemotherapy and BMT. In our case, DNA loss of 11q23 could be responsible for the negativity of 20% of tumour cells as observed before chemotherapy. However, the complete loss of CD56 expression in the relapsed tumour cannot be explained by the loss of 11q23 alone. Additional factors such as chemotherapy-induced mutations might also have contributed. Received: 17 June 2010; Accepted: 29 October 2010 Keywords allogenic bone marrow transplantation, blastic plasmacytoid dendritic cell neoplasm, CD4+CD56+, CD56-negative, chemotherapy, loss 11q23 Conflict of interest None declared. Funding The authors have no relevant financial interest in this article. Introduction Blastic plasmacytoid dendritic cell neoplasm (BPDCN), a rare haematological neoplasm, is presumably derived from a plasma- cytoid dendritic cell precursor. 1,2 Most of the patients show asymptomatic solitary or multiple skin lesions, frequently with contusiforme aspects. 3 In 50% of these patients, the skin is the only extramedullary manifestation at the time of diagnosis 4 with additional, mostly focal involvement of bone marrow and ⁄ or blood in the other 50%. It was suggested recently that alloge- neic bone marrow transplantation (BMT) may prolong the usu- ally short median survival time and provides a curative option in these patients. 5 1 Present address: Department of Dermatology, Venereology and Aller- gology, Klinikum Hildesheim GmbH, Weinberg 1, D-31134 Hildes- heim, Germany. ª 2010 The Authors JEADV 2010 Journal of the European Academy of Dermatology and Venereology ª 2010 European Academy of Dermatology and Venereology DOI: 10.1111/j.1468-3083.2010.03922.x JEADV