Acute monocytic leukemia

The pathology of AML involves abnormal proliferation and differentiation of a population of myeloid stem cells. Genetic mutations are identified in the majority of cases. A common genetic mutation identified in these cases are characterized as chromosomal translocations where information from one chromosome is exchanged to a non-homologous chromosome creating an unusual rearrangement of chromosomes. This translocation is often abbreviated as t(#of one chromosome involved, #of other chromosome involved). 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. These translocations yield the formation of chimeric proteins (RUNX1-RUNX1T1 and PML-RARA, respectively) which disrupt normal myeloid precursor development.

Secondary leukaemia, which may include AML-M5, has been associated with exposure to epipodophyllotoxins, such as etoposide.[4]

Many cases of AML-M5 are seen to have enhanced phosphorylation of the STAT3 protein due to increased induction of cytokines thus increasing cell proliferation and survival.[5] Finally, genetic mutations involved in epigenetic regulation are associated with this leukemia, as they have downstream effects on cell differentiation and proliferation. Excessive cytokine release could be a byproduct of skewed epigenetic regulation.

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. Tyrosine kinase receptor inhibitors are a prominent treatment developed to combat the over activation of cell proliferation proteins induced by AML-5. Inhibiting the STAT3 protein is another useful form of treatment.[5]

• Image at hmds.org.uk