Identification of PML/RAR alpha fusion gene transcripts that showed no t(15;17) with conventional karyotyping and fluorescent in situ hybridization

Acute promyelocytic leukemia (APL) is characterized by a reciprocal translocation, t(15; 17)(q22; q11-21), resulting in the fusion of the promyelocytic leukemia (PML) and retinoic acid receptor alpha (RAR alpha) genes. Using conventional cytogenetic methods, these translocations are normally detected in about 70-90% of patients; most negative results are due to technical problems or cryptic variants. These masked PML/RAR alpha fusions can be identified by molecular analyses, such as reverse transcriptase-polymerase chain reaction (RT-PCR) or fluorescence in situ hybridization (FISH). Approximately 5 to 10% of all APL cases reported do not show PML/RARa fusion transcripts, even with dual-colored FISH. We report three of 40 diagnosed APL cases that showed morphological, cytochemical, and immunophenotypic features of hypergranular APL, but did not show a PML/RAR alpha fusion signal or any of its variants, on FISH. All cases were identified by RT-PCR, which was further confirmed by cDNA sequencing. Conventional karyotyping showed other clonal aberrations in these cases, but failed to show t(15; 17) or any other variants or complex translocations.