First group WHO: AML with "recurrent cytogenetic translocations"

Although the term "de novo" is not fully appropriate (see below "secondary AML"), this category of patients is usually referred as such in the literature since MDS or chemo/radiotherapy does not usually precede them either. The most commonly identified abnormalities are reciprocal translocations: t(8;21), inv(16) or t(16;16), t(15;17), t(11;17), t(9;11), t(6;9), t(1;22) and t(8;16). Molecular studies have shown that these structural chromosome rearrangements create a fusion gene encoding a chimeric protein. Most can be detected by RT-PCR including complex and cryptic cytogenetic variants. The altered expression and/or structure of cellular gene products leads to functional activation that may contribute to the initiation or progression of leukemogenesis.
The most frequent anomalies are : t(8;21)(q22;q22) - inv/del16( p13q22)/del(16)(q22)/t(16;16)(p13q22) - t(15;17)(q22;q21) - t(11;17)(q23;q21) - 11q23 , 

A first class of secondary AML include those patients with a longstanding exposure to environmental toxins, including smoking, occupational chemicals such as benzene and related petrochemicals. The importance of detailed occupational history of all patients cannot be overstated. , 

The second category corresponds to patients who received prolonged administration of chemotherapy and/or radiotherapy for non-MDS/MPS malignancies (epithelial cancer, malignant lymphomas, myelomas, Hodgkins disease). These AML occur after a latent period of a few years. They may present with myelodysplastic features evolving rapidly to AML. Until recently these were assumed to be exclusively the result of administration of alkylating agents. These AML are frequently associated with acquired chromosomal abnormalities involving 5q, -7/del(7q) and other complex rearrangements, and more rarely with translocations. The morphological presentation and cytogenetic features of these two first types of "secondary" AML (sAML) are somewhat similar to "multilineage AML" (mAML). , 

Another situation that has been described more recently is AML developing after the administration of agents that bind to DNA-topoisomerase II. In contrast to the loss of chromosomal material after alkyliting agent exposure, balanced translocations ("de novo" type AML): 11q23, usually t(9;11), or 21q22, t(8;21) or even t(15;17) have been noted in these leukemias. This category has a morphologic presentation similar to the corresponding "de novo" AML and a much more favorable outcome with chemotherapy. , Morphologically, the diagnosis of AML is based on the cytological aspect of the blast cells and the maturation of the different cell lineages in bone marrow aspirate, in addition to quantitative parameters obtained from the peripheral blood. Blood films, although essential, are not considered sufficient for diagnosis. The major criteria required for sub-classification are based on bone marrow aspirates. This explains the care required in difficult cases, in which the bone marrow aspirate is hypocellular. In these cases, as well as those with myelofibrosis, precise diagnosis needs the additional information of histological examination of a bone marrow biopsy. When the bone marrow is hypercellular or normocellular and easy to aspirate, bone marrow biopsy is usually not essential and cytological examination of smears is sufficient. With some reservations the sub-classification criteria can also be used for the material from patients with relapsing acute leukemia.
 , MORPHOLOGICAL CATEGORIES. The categories of this fourth group reflect the previous FAB classification with eight main types of AML (from M0 to M7 AML) and one additonal category for the so-called "biphenotypic AL". AML M1 and M2 show predominantly granulocytic (neutrophil) differentiation. Very specific hypergranular cells characterize M3 AML. AML M4 and M5 both show monocytic differentiation, predominantly monocytic for M5, and mixed monocytic-granulocytic for M4. Predominantly erythroblastic and megakaryoblastic differentiation are characteristic of AML M6 and M7 AML respectively; the myeloid nature of M0 is defined only on immunological markers (myeloid and no lymphoid markers) in patients lacking morphological or cytochemical criteria for AML. Biphenotypic acute leukemias are defined for patients having both lymphoid and myeloid immunological markers.

t(8;21)(q22;q22)

DEFINITION: The translocation t(8;21)(q22;q22) is one of the most common structural aberration in acute myeloid leukemia and is found in 5-12% of AML and in one-third of karyotypically abnormal M2 cases according to the French-American-British (FAB) classification.
MORPHOLOGY AND CYTOCHEMISTRY: Among the non-random chromosomal aberrations observed in AML, t(8;21)(q22;q22) is one of the best known and usually correlates with AML M2, with well defined and specific morphological features. AML M2 FAB is the morphological type predominating in correlation with t(8;21), but some AML M1 or AML M4 cases have been also reported. Rare cases with a low bone marrow blast cell count (

Second group WHO: mAML : Multilineage AML

DEFINITION: This category is defined by the presence of multilineage dysplasia on cytological analysis. In contrast to the patients with "recurrent translocation", "multilineage AML" by definition involve all myeloid cell lineages. This category of AML occurs mainly in elderly patients and is rare in children. Translocations typical of "de novo AML" in young patients are not found in "multilineage AML". Dysplasia may be analyzed according to standard criteria (presence in >50% of each cell category). Granulocytic dysplasia (DysG) may be defined as polymorphonuclear neutrophils (PMN) with agranular or with hyposegmented nuclei (pseudo Pelger-Huet anomaly). Dysplastic features of erythroblastic precursors define Erythroid dysplasia (DysE): (megaloblastic or macroblastic aspects, karyorexis, nuclear fragments or multinuclearity). Megakaryocytic dysplasia (DysM) may be diagnosed when micromegakaryocytes, large megakaryocytes with monolobed or with multiple separated nuclei are found. A special mention has to be made of the high frequency of dysmegakaryopoiesis and the utmost importance of clearly separating abnormal megakaryocytic cells with normal ploidy and non lobed ("monolobed") nuclei from hypoploid ("micromegakaryocytes") megakaryocytes and from megakaryocytes with multiple separated nuclei.

KARYOTYPIC/MOLECULAR ANALYSIS: In this group of patients chromosomes abnormalities include gain or loss of major segments of chromosomes: -5, -7/del(7q), +8, +9, +11, del(11q), del(12p), del(17p), -18, +19, del(20q), +21 and less often specific translocations t(2;11), t(1;7)(q10;p10) and translocations involving 3q21 and 3q26.

Third group WHO: "Secondary AML"

DEFINITION The term "secondary" AML has been utilized to encompass several different situations.

Fourth group WHO: Morpholocical and Immunophenotyping classification

DEFINITION: A morphological and immunophenotypic classification remains necessary for the other situations which do not fit with the two preceding main categories, respectively: "recurrent translocations AML" (so-called "de novo") and "multilineage AML".

Basis of classification in conformity with WHO recommandations.

The classification of acute myeloid leukemia (AML) and myelodysplasic syndromes (MDS) includes clinical data (previous history, age) and biologic characteristics (morphology, cytochemistry, immunophenotype, cytogenetic and molecular biology). The separation of homogeneous classes allows us to distinguish pronostic parameters and to identify groups of patients sensitive to drugs or to specific treatment. Recurrent cytogenetic abnormalities are strong prognostic indicators in AML and MDS. Molecular studies of structural chromosomal changes have enabled the cloning of genes located at chromosomal breakpoints and have helped to characterize the proteins involved in leukemogenesis. Morphologic studies remain important because of a strong correlation with cytogenetic and molecular abnormalities.

The clinico-biological classification of acute myeloid leukemia (AML) should include morphological, cytochemical, immunophenotypic, cytogenetic and molecular characterization of the leukemia blasts. The identification of homogeneous categories would allow the development and refinement of treatment strategies.
- Recurrent cytogenetic abnormalities are important as prognostic indicators in AML. The identification of specific abnormalities is used increasingly to decide treatment. Cytogenetic findings have contributed to the understanding of morphological and clinical heterogeneity of AML. Molecular genetic analysis of recurrent translocations and inversions has led to clone genes adjacent to chromosome breakpoint and to characterize their protein products involved in the leukemogenesis process.
- Over the years, leukemia classifications have been mainly descriptive, which was open to regular criticism, revision and reassessment. During the last 20 years, classification according to morphological features of leukemia has been proposed (F.A.B. defined classification). This classification is based on cell morphology on May-Grunwald-Giemsa (MGG) staining of peripheral blood and bone marrow smears with the addition of simple cytochemical techniques

Rationale for a new classification approach
- The age-incidence of AML is subtly bimodal. Between early childhood and age 45, the annual incidence of acute myeloid leukemia (AML) remains constant at 0.8 cases/10⁵population. The incidence rises exponentially after the age of 45, exceeding 15 cases/10⁵ population by age 75. AML has been extensively characterized using cytogenetic since the mid-1970s.
- Available data have suggested an alternative classification in four main groups; a first one for patients identified with specific balanced translocations, the second group for patients with "multilineage" deregulation, a third one for "secondary" AML (after exposure to mutagenic agent or chemo/radiotherapy). Although this is a more rational model of AML classification, some patients cannot be classified into the three first groups and defined a fourth group. At least for the moment, the diagnosis of this last group of patients must rely on the classical cytologic approach (FAB) defining "morphological"-based category.

The first group is characterized by recurring chromosomal abnormalities, mainly balanced reciprocal translocations and affects children and young adults. In this group, it is assumed that there is involvement of committed precursor. This may explain the cellular involvement of a specific subset of myeloid cells for example pure granulocytic cells in t(15;17) AML, granulocytic and eosinophilic cells in t(8;21) AML, and monocytes and eosinophils in inv(16). These patients often have a high rate of complete remission with cytotoxic chemotherapy.

The second group has similar abnormalities to those which are associated with myelodysplastic syndromes, occur mainly in the elderly population and are rare in childhood. They are characterized by multilineage involvement of bone marrow cells suggesting an early commitment precursor (stem cell). Cytogenetic studies usually show complex chromosome aberrations, mainly loss of genetic material. These diseases are associated with a poor prognosis and a lesser incidence of complete remission after chemotherapy.

The third group concerns "secondary" AML (mainly after treatment for malignant diseases) usually morphologically and cytogenetically related with the second group, or more rarely with the first one, depending of the type of triggering drug used.