Hematopathology / LEUKEMIAS RESEMBLING ACUTE PROMYELOCYTIC LEUKEMIA, MICROGRANULAR VARIANT Leukemias Resembling Acute Promyelocytic Leukemia, Microgranular Variant

Acute promyelocytic leukemia (APL) should be distinguished from other subtypes of acute myeloid leukemia (AML) because of the increased risk of disseminated intravascular coagulation (DIC) and its response to arsenic compounds and retinoids. Some cases of AML seem morphologically similar to the microgranular variant of APL (French-AmericanBritish [FAB] AML-M3v) but lack the t(15;17). We evaluated 8 cases of APL-like leukemias for subtle morphologic, cytochemical, immunophenotypic, and cytogenetic differences compared with 5 cases of promyelocytic leukemia/retinoic receptor alpha (PML/RARalpha)–positive APL (FAB AML-M3v). We also evaluated both groups for the presence of DIC. No differences among the groups were noted in blast size, chromatin pattern, nuclear morphologic features, intensity of myeloperoxidase staining, or presence of Auer rods. Immunophenotypes were similar; both types of cases lacked CD34 and HLA-DR and were CD13+ and CD33+. Two cases of APL-like leukemias also were CD56+. DIC was present in 2 patients with M3v. Our study shows that there are no definitive morphologic, cytochemical, or immunophenotypic findings that can distinguish these cases from PML/RARalpha-positive APL. Distinguishing acute promyelocytic leukemia (APL) from the other subtypes of acute myeloid leukemia (AML) is important because APL responds to treatment with all-transretinoic acid1 and arsenic compounds.2 The diagnosis of the microgranular variant of APL can be difficult because the morphologic, cytochemical, and immunophenotypic features often are nonspecific. As a result, APL can be confused with other AMLs, such as acute myelomonocytic (AML-M4) or acute monocytic leukemia (AML-M5).3,4 Definitive diagnosis of the classic form of APL rests with identification of the characteristic reciprocal translocation between chromosomes 15 and 17 and/or recognition of the promyelocytic leukemia/retinoic receptor alpha (PML/RARalpha) transcript. Chemotherapy often needs to be initiated rapidly in many patients suspected of having APL. This may be accompanied by a significant risk of hemorrhage if all-transretinoic acid is not included in the induction regimen. Alltrans-retinoic acid substantially reduces the incidence of early hemorrhagic death during induction therapy for APL.5 Unfortunately, molecular and cytogenetic studies may be time consuming, requiring the use of morphologic, cytochemical, and immunophenotypic studies for initial classification. While immunofluorescent staining for the PML gene product has shown promise as a method to rapidly diagnose APL, it is not used routinely in most hematology laboratories.6 Therefore, we conducted a study to determine whether there are morphologic and cytochemical differences between PML/RARalpha-positive AML-M3v as assessed by the presence of t(15;17) and AML-M3v–like cases, which do not demonstrate t(15;17), and the usefulness of the clinical manifestations and immunophenotype in determining whether these leukemias were of promyelocytic origin. Am J Clin Pathol 2002;117:651-657 651 © American Society for Clinical Pathology Nagendra et al / LEUKEMIAS RESEMBLING ACUTE PROMYELOCYTIC LEUKEMIA, MICROGRANULAR VARIANT Materials and Methods Case Selection and Review Peripheral blood or bone marrow aspirate smears (1 peripheral blood smear and 12 bone marrow aspirates) from 5 cases of microgranular variant of APL (French-AmericanBritish type AML-M3v) demonstrating t(15;17) were compared with 8 cases of AML in which the morphologic features suggested APL; the blasts in these 8 cases had prominent nuclear indentations or folds, multiple Auer rods, or prominent myeloperoxidase activity but demonstrated no t(15;17) (APL-like leukemias). The cases were obtained for review from the Department of Pathology, University of Iowa Hospitals and Clinics (UIHC), Iowa City, and University Hospitals of Cleveland (UHC), Cleveland, OH. These cases were initially diagnosed between January 1993 and June 2000, and clinical history and follow-up, peripheral blood samples and bone marrow aspirates, and findings from cytochemical, flow cytometric, and karyotypic studies were available. Peripheral blood or bone marrow aspirate smears from the cases were prepared as unknown slides for rereview by a board-certified hematopathologist (N.R.) and a clinical laboratory scientist experienced in bone marrow pathology (G.S.). The blasts were examined for size, chromatin pattern, nuclear features, cytoplasmic features, and presence of Auer rods. Based on the morphologic features, the reviewers gave an impression (AML-M3 or AML-M3–like). The cytochemical (myeloperoxidase and alpha-naphthyl butyrate esterase), immunophenotypic, karyotypic, and molecular features (if available) also were reviewed. Flow Cytometric, Karyotypic, and Molecular Evaluation Flow cytometric data for the 13 cases were reviewed retrospectively. Analysis was performed initially on the FACSCalibur Flow Cytometer (Becton Dickinson, San Jose, CA) and/or the EPICS XL-MCL (Coulter, Hialeah, FL) on peripheral blood or bone marrow aspirate specimens. Antibody combinations selected for review included CD45, CD34, CD14, CD13, CD33, HLA-DR (Becton Dickinson), and CD56 (Coulter). For karyotype analysis, cells from unstimulated peripheral blood or bone marrow aspirate specimens were arrested at metaphase with colchicine. Chromosomes were stained by the G-banding method. The chromosome number was determined by microscopic analysis, and the cells were examined for the presence or absence of detectable structural rearrangements. Karyotypes were prepared from computerassisted images of the metaphases. Fluorescent in situ hybridization (FISH) or reverse transcriptase–polymerase chain reaction (RT-PCR) was performed in 3 of 13 cases. FISH was performed using a PML/RARalpha dual-color translocation probe (Vysis, Downers Grove, IL). RT-PCR studies were performed at ARUP Laboratories, Salt Lake City, UT. Patient RNA was isolated, reverse transcribed into complementary DNA, and subjected to PCR amplification using oligonucleotide primers specific for the PML gene on chromosome 15 and the retinoic acid receptor alpha gene on chromosome 17. PCR products were analyzed by electrophoresis and ultraviolet transillumination of ethidium bromide–stained gels.