MINIMALLY DIFFERENTIATED ACUTE MYELOID-LEUKEMIA (AML-MO) - A DISTINCTCLINICO-BIOLOGIC ENTITY WITH POOR-PROGNOSIS

FAB proposals for the diagnosis of AML-M0 represent the formal recogni tion of a distinct entity which has been described over the past few y ears by several authors and called minimally differentiated acute myel oid leukemia. By definition, AML-M0 includes acute leukemias which do not fit morphological and cytochemical criteria for the diagnosis of A ML, and for which myeloid lineage assignment can be made by immunologi cal assay showing positivity for MPO, CD13, and CD33 and negativity fo r lymphoid markers. Involvement of an early myeloid progenitor in the leukemic process is a possible theory hypothesized to explain the exis tence of such a form. Validity of this assumption has been based on th e observation that AML-M0 frequently bears ''stem cell'' markers such as CD34, HLA-DR, Tdt, CD7, and promiscuous IgH/TCR gene rearrangements , which are thought to occur in uncommitted cells. Finally, AML-M0 ver y frequently carries cytogenetic abnormalities common to MDS or second ary AML, such as -5/5q- or -7/7q- deletions and or complex karyotype. In our experience, AML-M0 is also very often associated with the MDR p henotype, which in turn has been found strictly linked to ''stem cell' ' features, especially in MDS. These biological aspects, altogether, t ranslate into a very unfavorable prognosis, confirming even from a cli nical point of view that AML-M0 is a distinct entity. In conclusion. ' 'stem cell'' markers, MDR phenotype, complex chromosome lesions, frequ ent occurrence in elderly patients, and intrinsic chemoresistance char acterize AML-M0 and indicate the need for tailored treatments, possibl y involving the use of MDR modulators and/or differentiating agents.