Characterization of acute promyelocytic leukemia cases lacking the classic t(15;17): Results of the European Working Party

Acute promyelocytic leukemia (APL) is typified by the t(15;17), generating the PML-RARα fusion and predicting a beneficial response to retinoids. However, a sizeable minority of APL cases lack the classic t(15;17), prompting the establishment of the European Working Party to further characterize this group. Such cases were referred to a workshop held in Monza, Italy and subjected to morphologic, cytogenetic, and molecular review, yielding 60 evaluable patients. In the majority (42 of 60), molecular analyses revealed underlying PML/RARα rearrangements due to insertions (28 of 42) or more complex mechanisms, including 3-way and simple variant translocations (14 of 42). Metaphase fluorescence in situ hybridization (FISH) demonstrated that insertions most commonly led to formation of the PML-RARα fusion gene on 15q. In 11 of 60 workshop patients, PLZF/RARα rearrangements were identified, including 2 patients lacking the t(11;17)(q23;q21). In one case with a normal karyotype, FISH analysis revealed insertion of RARα into 11q23, and PLZF-RARα was the sole fusion gene formed. Two patients were found to have t(5;17), one with a diffuse nuclear NPM staining pattern and with NPM-RARα and RARα-NPM transcripts detected. In the other with an unbalanced der(5)t(5;17)(q13;q21) and a nucleolar NPM localization pattern, an NPM/RARα rearrangement was excluded, and FISH revealed deletion of one RARα allele. In the remaining 5 workshop patients, no evidence was found for a rearrangement of RARα, indicating that in rare instances, alternative mechanisms could mediate the differentiation block that typifies this disease. This study highlights the importance of combining morphologic, cytogenetic, and molecular analyses for optimal management of APL patients and better understanding of the pathogenesis of the disease. (C) 2000 by The American Society of Hematology.