Related Experiment Video
Updated: Jan 13, 2026
Intracellular Phosphoflow Cytometry of Acute Myeloid Leukemia Patient-Derived Xenotransplants
Published on: June 6, 2025
Uncovering the PML::RARA Fusion in Cytogenetically Cryptic and FISH-Negative Acute Promyelocytic Leukemia-A Case
Busra N Delikkaya1, Jaime Eberle-Singh1, Arianna B Morton1
1Hematopathology, Department of Pathology and Genomic Medicine, Thomas Jefferson University, Philadelphia, PA 19107, USA.
Abstract:
The PML::RARA fusion resulting from t(15;17) is the genetic hallmark of acute promyelocytic leukemia (APL), typically detected by cytogenetics and/or fluorescence in situ hybridization (FISH) studies. Rarely, APL patients present with normal cytogenetics and FISH findings, complicating diagnosis and delaying life-saving therapy. We report a 23-year-old male with clinical, morphologic and immunophenotypic features consistent with APL but negative for FISH studies. Despite prompt initiation of all-trans retinoic acid (ATRA) based on clinical suspicion, the patient succumbed to intracranial hemorrhage. Quantitative reverse transcriptase PCR (qRT-PCR) confirmed a long isoform PML::RARA fusion. A review of 34 published cytogenetics- and FISH-negative cases since 1995 demonstrates that RT-PCR-based methods reliably detect cryptic fusions. While advanced genomic approaches may identify these fusions at higher resolution, their accessibility, complexity, cost, and turnaround time often limit diagnostic utility in the urgent setting of APL. Given the extreme rarity of this subset, cytogenetics and FISH remain the standard frontline tests; however, these cases underscore the critical need to incorporate molecular testing into routine workflows. Early recognition and timely therapy are essential to reducing mortality in cryptic APL, and these cases also provide insight into mechanisms of atypical leukemia biology.
Insights
Acute promyelocytic leukemia (APL) with normal cytogenetics and FISH requires molecular testing. Quantitative reverse transcriptase PCR reliably detects the PML::RARA fusion in these rare, cryptic cases.
Area of Science:
- Hematology
- Molecular Biology
- Oncology
Background:
- Acute promyelocytic leukemia (APL) is characterized by the PML::RARA fusion gene.
- Standard diagnostic methods include cytogenetics and fluorescence in situ hybridization (FISH).
- Rare cases of APL present with normal cytogenetic and FISH findings, posing diagnostic challenges.
Purpose of the Study:
- To report a case of APL with normal FISH findings.
- To highlight the utility of molecular testing in diagnosing cryptic APL.
- To review previously reported cases of FISH-negative APL.
Main Methods:
- Case report of a patient with APL features but negative FISH.
- Quantitative reverse transcriptase PCR (qRT-PCR) to detect PML::RARA fusion.
- Literature review of cytogenetics- and FISH-negative APL cases since 1995.
Main Results:
- A patient with APL morphology and immunophenotype tested negative for FISH but was positive for the PML::RARA fusion via qRT-PCR.
- RT-PCR methods effectively detect cryptic PML::RARA fusions in APL.
- Advanced genomic methods are less practical for urgent APL diagnosis.
Conclusions:
- Cytogenetics and FISH remain frontline tests for APL due to the rarity of cryptic forms.
- Molecular testing, such as RT-PCR, is crucial for diagnosing FISH-negative APL.
- Early detection and treatment of cryptic APL are vital for improving patient outcomes.
