Acute promyelocytic leukemia (APL) is usually related to a good prognosis, however, about 20% of patients whose outcome is not favorable, early complications cause mortality in 10%, and disease relapse or drug resistance affects the other 10%. Prompt diagnosis and early initiation of treatment are critical to lowering the risk of early mortality. Although the hallmark of APL is the PML-RARA fusion, generated from t(15;17)(q24;q21), it is common to observe additional chromosomal abnormalities, with a clinical impact not yet established, associated with this neoplasm, which arouses the interest of different study groups in the literature about additional (cryptic) anomalies related to the disease. In this work, we present a FISH positive PML-RARA case, but negative in chromosome banding analysis, associated with a very poor prognosis in an adolescent with APL.

A 14-year-old girl was admitted to the pediatric hematology service of the Lagoa Federal Hospital, with a history of 30 days of edema, pain, and intense gingival bleeding, associated with fever and pancytopenia. A bone marrow (BM) sample was referred to the Laboratory of Cytogenetics of CEMO – INCA. FISH and molecular biology tests confirmed the PML-RARA fusion, helping to define the diagnosis of AML-M3, and the patient was treated under the BFM-AIOPE-2012 protocol. Treatment with ATRA was initiated, but on the fourth day, she evolved with pulmonary condensation and desaturation and chemotherapy was suspended. Computed tomography revealed multiple hemorrhage sites. Hydreia was administrated to reduce white blood cell count and treatment with ATRA and ATO was resumed. However, on the fourth day of treatment, the adolescent evolved with low saturation being then intubated, in addition to worsening of bleeding even with intense transfusional support. The patient died 20 days after the initial diagnosis.

The patient's BM sample was studied by G-banding and FISH assays. The karyotype was 46,XX, with a marker chromosome. FISH analysis revealed the PML-RARA gene fusion in 90% of the interphase nuclei analyzed, besides an extra RARA gene signal, but this abnormality was not observed in metaphase spreads. Complementary FISH with the RARA break apart probe showed a RARA gene rearrangement in 90% of the interphase nuclei analyzed, without the involvement of the secondary chromosomal abnormality. Molecular studies detected a breakpoint within PML intron 3 (bcr 3) producing a short PML-RARA isoform.

Although secondary cytogenetic abnormalities occur in about 30% of cases of APL, their prognostic importance has not been determined, and these additional cytogenetic abnormalities are often complex. We could not eliminate the secondary alteration as a cryptic abnormality, however, through FISH (in interphase nuclei), the gold standard method, we showed the PML-RARA fusion. In the past, before the advances brought by FISH methodology for detecting PML-RARA fusion, the hallmark of APL, there was controversy about the culture time needed to diagnose the t(15;17)(q24;q21). In our patient, we observed a chromosome marker that may confer a proliferative advantage for normal cells over aberrant interphasic nuclei positive for PML-RARA.

Our findings confirm that FISH, regardless of the culture time used, is the fastest and most effective method to confirm the diagnosis of this hematologic urgency. However, molecular studies in metaphases with the observed marker chromosome will be important to elucidate/eliminate the involvement of cryptic abnormalities present in the karyotype of the patient in this study. Besides, the influence of PML-RARA isoform on patient outcomes remains controversial.