Acute monocytic leukemia

M5 is associated with characteristic chromosomal abnormalities, often involving chromosome 11, such as t(9;11), affecting the MLL (KMTA2) locus at 11q23; however MLL translocations are also found in other leukemia subtypes. The t(8;16) translocation in AMoL is associated with hemophagocytosis.

Secondary leukaemia, which may include AML-M5, has been associated with exposure to epipodophyllotoxins, such as etoposide.[2]

In order to fulfill World Health Organization (WHO) criteria for AML-5, a patient must have greater than 20% blasts in the bone marrow, and of these, greater than 80% must be of the monocytic lineage. A further subclassification (M5a versus M5b) is made depending on whether the monocytic cells are predominantly monoblasts (>80%) (acute monoblastic leukemia) or a mixture of monoblasts and promonocytes (<80% blasts). Monoblasts can be distinguished by having a roughly circular nucleus, delicate lacy chromatin, and abundant, often basophilic cytoplasm. These cells may also have pseudopods. By contrast, promonocytes have a more convoluted nucleus, and their cytoplasm may contain metachromatic granules. Monoblasts are typically MPO-negative and promonocytes are MPO variable. Both monoblasts and promonocytes stain positive for non-specific esterase (NSE), however NSE may often be negative.

Immunophenotypically, M5-AML variably express myeloid (CD13, CD33) and monocytic (CD11b, CD11c) markers. Cells may aberrantly express B-cell marker CD20 and the NK marker CD56. Monoblasts may be positive for CD34.

AML-M5 is treated with intensive chemotherapy (such as anthracyclines) or with bone marrow transplantation.

• Image at hmds.org.uk