FAB classification M5.
May develop after exposure to epipodophyllotoxin and anthracycline chemotherapy drugs.
Associated with chromosomal translocations including t(8;16)(p11;p13) and MLL locus translocations as well as others.
Must have greater than 20% blasts and 80% of these must be monocytic.
Promonocytes are considered comparable to blasts.
M5a refers to the presence of greater than 80% monoblasts or M5b a mixture of monoblasts and promonocytes with fewer than 80% monoblasts.
Monoblasts have delicate lacy chromatin, circular nucleus and abundant cytoplasm with basophilic coloring and psuedopod formation.
Often associated with hyperleucocytosis, coagulation abnormalities and extramedullary involvement.
Monoblasts are typically myeloperoxidase negative and promonocytes are variable with this stain.
Both monoblasts and promonocytes in acute monoblastic leukemia are non-specific esterase positive, although often the test is negative.
Immunophenotypically there is variable expression of myeloid CD13, CD33 and monocytic CD11b, CD11c markers.
Monoblasts may be positive for CD34.
Cells may ab2242antly express B cell marker CD20 and the NK CD56.
M5 leukemia associated with chromosomal abnormalities involving 11q23 or t(9;11) affecting the MLL locus.
MLL translocation can be found in other AML subtypes.
MLL is a negative prognostic finding in M5 AML compared with other genetic alterations involving MLL such as t(9;11) the t(8;16) translocation in MLL is associated with hematophagocytosis.