Discrepancies in the Detection of PML::RARA Gene Rearrangement by Fluorescent In Situ Hybridization Using Commonly Used Dual Color Dual Fusion Probes

Background/Objectives: Acute promyelocytic leukemia (APL) is a medical emergency associated with life-threatening complications such as disseminated intravascular coagulation (DIC), necessitating prompt therapeutic intervention and rapid diagnostic confirmation. APL is characterized by a translocation of the PML gene (15q24) with the RARA gene (17q21), resulting in the PML::RARA fusion gene on the derivative chromosome 15. Atypical PML::RARA rearrangements may escape detection by standard FISH probes. This study highlights limitations of commonly used probe sets and underscores the need for alternative FISH probe sets and complementary molecular testing. Methods: Two unique APL cases with atypical PML::RARA rearrangements were identified in our laboratory. Each case was evaluated at diagnosis using two commercially available FISH probe sets from Abbott Molecular and Cytocell. Metaphase FISH was performed to characterize the atypical FISH signal pattern further, and qRT-PCR was used to confirm the presence of the PML::RARA transcript. Results: Both cases demonstrated atypical rearrangements with a single fusion signal. In the first case, the Abbott probe detected a single fusion signal, while the Cytocell probe was negative. Metaphase FISH revealed an insertion of the PML region near RARA on chromosome 17. In the second case, the Cytocell probe was positive, and the Abbott probe was negative; metaphase FISH demonstrated insertion of the RARA region near PML on chromosome 15. qRT-PCR confirmed the presence of the PML::RARA transcript in both cases. Conclusions: These findings reveal limitations in commonly used PML::RARA FISH probes and support reflex testing with alternative probes and molecular confirmation to ensure accurate diagnosis.