Chapter 104. Acute and Chronic Myeloid Leukemia (Part 3) pps

Chapter 104. Acute and Chronic
Myeloid Leukemia
(Part 3)

Immunophenotype and Relevance to the WHO Classification
The immunophenotype of human leukemia cells can be studied by
multiparameter flow cytometry after the cells are labeled with monoclonal
antibodies to cell-surface antigens. This can be important for separating AML
from acute lymphoblastic leukemia (ALL) and identifying some types of AML.
For example, AML that is minimally differentiated (immature morphology and no
lineage-specific cytochemical reactions) is diagnosed by flow-cytometric
demonstration of the myeloid-specific antigens cluster designation (CD) 13 or 33.
Similarly, acute megakaryoblastic leukemia can often be diagnosed only by
expression of the platelet-specific antigens CD41 and/or CD61. While flow
cytometry is useful, widely used, and, in some cases, essential for the diagnosis of
AML, it is only supportive in establishing the different subtypes of AML through
the WHO classification.
Clinical Features and Relevance to the WHO Classification
The WHO classification considers clinical features in subdividing AML.
For example, it identifies therapy-related AML as a separate entity and
subclassifies this group based on the specific types of prior chemotherapy
received. It also divides AML with multilineage dysplasia based upon the presence
or absence of an antecedent MDS. These clinical features contribute to the
prognosis of the specific type of AML.
Genetic Findings and Relevance to the WHO Classification
The WHO classification is the first AML classification to incorporate
genetic (chromosomal and molecular) information. Indeed, AML is first
subclassified based on the presence or absence of specific recurrent genetic
abnormalities. For example, AML FAB M3 is now designated acute
promyelocytic leukemia (APL), based on the presence of either the
t(15;17)(q22;q12) cytogenetic rearrangement or the PML/RARα product of the

translocation. Thus, the WHO classification separates APL from all other types of
AML as a first step and forces the clinician to correctly identify the entity and
tailor treatment(s) accordingly.
Chromosomal Analyses
Chromosomal analysis of the leukemic cell provides the most important
pretreatment prognostic information in AML. Two cytogenetic abnormalities have
been invariably associated with specific morphologic features: t(l5;17)(q22;q12)
with APL and inv(16)(p13q22) with AML with abnormal bone marrow
eosinophils. Many other chromosomal abnormalities have been associated
primarily with one morphologic/immunophenotypic group, including
t(8;21)(q22;q22) with slender Auer rods, expression of CD19, and abundance of
normal eosinophils, and t(9;11)(p22;q23), as well as other translocations involving
11q23, with monocytic features. Many of the recurring chromosomal
abnormalities in AML have been associated with specific clinical characteristics.
More commonly associated with younger age are t(8;21) and t(l5;17); with older
age, del(5q) and del(7q). Myeloid sarcomas (see below) are associated with t(8;21)
and disseminated intravascular coagulation (DIC) with t(15;17).
Molecular Classification
Molecular study of many recurring cytogenetic abnormalities has revealed
genes that may be involved in leukemogenesis; this information is increasingly
being incorporated into the WHO classification. For instance, the t(15;17) encodes
a chimeric protein, promyelocytic leukemia (Pml)/retinoic acid receptor α (Rarα),
which is formed by the fusion of the retinoic acid receptor α (RARα) gene from
chromosome 17 and the promyelocytic leukemia (PML) gene from chromosome
15. The RARα gene encodes a member of the nuclear hormone receptor family of
transcription factors. After binding retinoic acid, RARα can promote expression of
a variety of genes. The 15;17 translocation juxtaposes PML with RARα in a head-
to-tail configuration that is under the transcriptional control of PML. Three
different breakpoints in the PML gene lead to various fusion proteins. The Pml-
Rar α fusion protein tends to suppress gene transcription and blocks differentiation

of the cells. Pharmacologic doses of the Rar α ligand, all-trans-retinoic acid
(tretinoin), relieve the block and promote differentiation (see below). Similar
examples exist with a variety of other balanced translocations and inversions,
including the t(8;21), t(9;11), t(6;9), and inv(16).
Molecular aberrations are also being identified that are useful for
classifying risk of relapse in patients without cytogenetic abnormalities. A partial
tandem duplication (PTD) of the MLL gene is found in 5–10% of patients with
normal cytogenetics and results in short remission duration. FMS-like tyrosine
kinase 3 (Flt3) is a tyrosine kinase receptor important in the development of
myeloid and lymphoid lineages. Activating mutations of the gene FLT3 are
present in ~30% of adult AML patients due to internal tandem duplications (ITDs)
in the juxtamembrane domain or mutations of the activating loop of the kinase.
These occur more commonly in patients with normal karyotype. Continuous
activation of Flt3 and downstream target kinases, including signal transducer and
activator of transcription protein 5, Ras/mitogen-activated protein kinase, and
phosphatidylinositol 3-kinase/Akt, provides increased proliferation and
antiapoptotic signals to the myeloid progenitor cell. Presence of FLT3 ITD in
patients with normal cytogenetics predicts for short remission duration and inferior
survival. Other molecular prognostic factors in patients with normal karyotype
AML include mutations of the nucleophosmin gene (NPM1) and C/EBP α that are
associated with improved treatment outcome. In contrast, overexpression of genes
such as brain and acute leukemia, cytoplasmic (BAALC) predicts for poor
outcome. Gene expression profiles to predict outcome in normal karyotype AML
patients are under active investigation.