Blastic plasmacytoid dendritic cell neoplasm with unusual morphology, MYC rearrangement and TET2 and DNMT3A mutations

A 71-year-old man with a history of relapsed/refractory acute myeloid leukaemia (AML) involving skin and bone marrow was referred to our institution. Bone marrow aspirate films showed medium-sized to large blast cells with slightly irregular nuclear contours, prominent nucleoli and strongly basophilic cytoplasm (top left). Cytochemistry showed the blasts to be negative for myeloperoxidase, periodic acid-Schiff and non-specific esterase. Flow cytometric immunophenotyping showed an aberrant blast population, positive for CD4, CD7, CD13 (dim), CD36 (partial), CD38, CD45 (dim), CD64 (partial/dim), CD123, HLA-DR (bright) and terminal deoxynucleotidyl transferase (TdT) (dim). All other myelomonocytic, T cell, B cell, erythroid, megakaryocytic and plasma cell markers were negative. The bone marrow trephine biopsy and clot specimens showed confluent sheets of blasts (top right) positive for CD123 (strong), TCL1, TCF4 (bottom left) and MYC (bottom right), but negative for CD56. Conventional cytogenetic analysis demonstrated: 46,XY,t(6;8)(p21;q24.3)[5]/47,idem,+18[11]/46,XY[4]. Fluorescence in situ hybridisation (FISH) analysis demonstrated MYC rearrangement in 90% of cells. Targeted next generation sequencing analysis showed DNMT3A p.R882H and TET2 p.Y1631 mutations. The diagnosis was revised to blastic plasmacytoid dendritic cell neoplasm (BPDCN). This case of BPDCN had unusual morphological and immunophenotypic features leading to an initial incorrect diagnosis of AML. Unlike typical BPDCN, the blasts had strongly basophilic cytoplasm and prominent nucleoli, in contrast to most BPDCN cases in which blasts have weakly basophilic, often tapered cytoplasm and open chromatin with inconspicuous or moderately prominent nucleoli. In addition, this case lacked CD56 expression, a rare occurrence in BPDCN, which typically is strongly positive for CD56. Immunohistochemical stains, especially CD123, TCL1 and TCF4, can be helpful for establishing the diagnosis of BPDCN in challenging cases.