Plasma cells with Auer-rod-like inclusions in a patient with MGUS and acute myeloid leukaemia with NPM1 mutation

Abstract

A 76-year-old man with asymptomatic monoclonal gammopathy of undetermined significance (MGUS) type IgM kappa, presented with fever, joint pain, leucocytosis and monocytosis. On admission to hospital the blood count showed anaemia (haemoglobin concentration 102 g/L), leucocytosis with monoblasts (leucocytes 86.5 × 10⁹ /L; monoblasts 48.5 × 10⁹ /L) and thrombocytopenia (platelets 111 × 10⁹ /L). A non-progressive paraprotein level of 3.5 g/L (IgM kappa) was confirmed.

Bone marrow aspirate showed hypercellularity with 85% blasts, and acute monocytic leukaemia was diagnosed. The other diagnostic procedures, including trephine biopsy, immunophenotyping, genetic studies and next-generation sequencing, allowed classification according to the WHO criteria as acute myeloid leukaemia (AML) with NPM1-Type A mutation, FLT3-ITD negative, with additional DNMT3A and PTPN11 mutations, but no chromosomal anomalies. The patient was treated with azacitidine 75 mg/m² on days 1–5 and days 8–9, and with venetoclax 400 mg on days 1–14, achieving a complete remission after one cycle of chemotherapy. Bone marrow examination after two cycles of therapy showed a significant reduction of blasts (<2%).

Interestingly, plasma cells containing numerous intracellular Auer-rod-like, needle-shaped cytoplasmatic inclusions were observed (upper, plasma cells with and without intracytoplasmic inclusions, May–Grünwald–Giemsa stain, ×100 objective, total magnification ×1000). Retrospectively, the presence of rare plasma cells with the same cytoplasmatic inclusions were detected in the first bone marrow slide prepared at diagnosis of AML before chemotherapy (bottom, hypercellular bone marrow showing blasts and a single plasma cell with cytoplasmatic inclusions, May–Grünwald–Giemsa stain, ×60 objective, total magnification ×600). No Auer rods were detected in the myeloid blasts in either bone marrow smears.

Cytoplasmatic inclusions that are sometimes indistinguishable from Auer rods have been described in B-cell, myeloid or plasma cell malignancies. This case of MGUS and AML with NPM1 mutation where ‘bundled’ Auer-rod-like structures were present in abnormal plasma cells of MGUS but not in blast cells emphasises the surprises that morphology can sometimes present.

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