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An unusual case of indigestion: persistence of phagocytosed Auer rods in acute promyelocytic leukaemia

A 33-year-old woman was diagnosed with acute promyelocytic leukaemia (APL), having presented with pancytopenia and atypical promyelocytes in her peripheral blood. Bone marrow biopsy showed typical morphological features of APL with numerous heavily granulated promyelocytes containing Auer rods. At diagnosis, many histiocytes containing multiple Auer rods were also present (left, arrow), presumably from phagocytosis of leukaemic cells. Conventional cytogenetic analysis and fluorescence in situ hybridisation identified a three-way translocation with 15q22, 17q21 and 9q12 breakpoints, consistent with APL. Complex translocations, such as this, occur in approximately 5% of cases and are considered to have a similar outcome as the standard t(15;17)(q22;q21). The PML-RARA rearrangement was also demonstrated by reverse transcription polymerase chain reaction of bone marrow. Following induction with idarubicin and all-trans-retinoic acid, bone marrow biopsy at day 35 showed clearance of the abnormal promyelocytes; however, a number of phagocytic histiocytes containing Auer rods remained (right). Despite this persistent morphological abnormality, molecular and cytogenetic remission was demonstrated by the presence of a normal karyotype and absence of PML-RARA transcripts. Histiocytes containing Auer rods were not seen on any subsequent bone marrow aspirates following consolidation and maintenance therapy, and the patient remains in remission at 18 months since diagnosis. Auer rods, first described in leukaemic cells in 1906, represent crystalline forms of the enzyme myeloperoxidase. The presence of multiple Auer rods is characteristic of APL and, in the post-induction setting, their presence in blasts or promyelocytes regardless of the overall blast count is indicative of persistent disease. However, this novel appearance of numerous phagocytosed Auer rods persisting after induction therapy despite other strong evidence of remission demonstrates that Auer rods may persist in histiocytes long after the clearance of leukaemic cells and their presence does not necessarily indicate residual leukaemia.