Precursor Dendritic Cell Proliferation in Multiple Myeloma: A Precursor to Acute Myeloid Leukemia

<b>Background</b>: Dendritic cells (DCs) are heterogeneous antigen-presenting cells that bridge innate and adaptive immunity. Recent classifications of hematolymphoid neoplasms highlight the complex origins of DC-related neoplasms. DCs have also been associated with the progression of multiple myeloma (MM). This report presents the case of a patient with MM in whom bone marrow analysis revealed an unusual additional clonal population of immature cells, in addition to plasmacytoid DCs, that later evolved into plasmacytoid dendritic cell proliferation associated with acute myeloid leukemia (pDC-AML). <b>Methods</b>: The bone marrow of a 69-year-old man with neutropenia and thrombocytopenia was examined by morphology, immunohistochemistry, flow cytometry, cytogenetics, fluorescence in situ hybridization (FISH), and next-generation sequencing (NGS). Serial assessments were performed before and during treatment with bortezomib and dexamethasone for MM, and later with daunorubicin/cytarabine for AML. <b>Results</b>: Initial bone marrow analysis revealed coexisting clonal plasma cells with t(11;14) and a population of CD34+/CD123+/CD45RA+ cells lacking lineage markers, in addition to pDCs, suggestive of precursor DCs rather than acute undifferentiated leukemia. Cytogenetic analysis identified a small clone with isolated del(20q), which corresponded in size to the clone of undifferentiated cells and to the clone with pathogenic variants detected by NGS in the <i>BCOR</i>, <i>RUNX1</i>, and <i>SRSF2</i> genes. Myeloma therapy decreased both MM and undifferentiated cells; however, within four months, pDC-AML evolved with del(20q) and higher variant allele frequencies of the previously detected gene variants. Remission was achieved with standard AML chemotherapy. <b>Conclusions</b>: This case supports evidence that MM-associated immune dysfunction and bone marrow niche alterations may promote secondary myeloid malignancies independently of cytotoxic therapy. It demonstrates the earliest events in pDC-AML evolution. Furthermore, the immature immunophenotype raises the question of appropriate treatment, since a diagnosis of acute undifferentiated leukemia can be established.