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/Objective PAX5 is a transcription factor critical for B-cell development and its genetic alterations are common in B lymphoblastic leukemia/lymphoma (B-ALL). We report a case of PAX5 P80R mutated acute leukemia with predominantly monocytic immunophenotype followed by a genetically-related histiocytic proliferation. Methods/Case Report A 27-year-old male presented with pancytopenia, epistaxis, and blurry vision. Bone marrow exam showed 95% blasts with nuclear indentations, occasional prominent nucleoli and basophilic cytoplasm. Blasts were positive for bright CD33, HLA-DR, CD14, bright CD64, partial CD123 and partial CD19. In addition, a minute population of B lymphoblasts positive for CD19, CD20, and dim TdT was seen. The AML FISH panel showed markedly aneuploid population with all probes demonstrating 3 to 5 signals. PAX5 P80R and KRAS G13D mutations were detected by NGS. Post induction bone marrow showed no evidence of acute leukemia with normal cytogenetics and FISH results. Subsequent two bone marrow exams performed due to progressive cytopenias demonstrated a prominent histiocytic proliferation with sheets of mature appearing histiocytes with abundant cytoplasm, oval to folded nuclei and occasional hemophagocytosis. This population was positive for CD163, CD14, CD68R, CD68, cyclin D1 with variable OCT2 expression and low proliferative activity. PAX5 P80R and KRAS G13D were detected. AML FISH panel showed aneuploidy in histiocytic appearing cells and IgH gene rearrangement studies by PCR showed prominent clonal population. The patient remained pancytopenic and died of disseminated fungal infection. Results (if a Case Study enter NA) NA Conclusion PAX5 P80R mutation has been primarily reported in B-ALL and is exceedingly rare in acute myeloid leukemias. This alteration has been linked to downregulation of B-cell lineage genes affecting B-cell maturation, and not surprisingly a proportion of PAX5 P80R mutated B-ALL cases show a switch to a monocytic lineage. The reported case demonstrates diagnostic caveats including unusual features of acute leukemia at the time of initial diagnosis and subsequent genetically-related histiocytic proliferation.

PAX5 P80R mutated acute leukemia followed by genetically-related histiocytic proliferation with clonal IGH gene rearrangements