Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia (AML) and is treated differently than other types of AML. Optimal treatment requires rapid initiation of treatment with all-trans retinoic acid (ATRA) and supportive care measures.[1,2] The characteristic chromosomal abnormality associated with APL is t(15;17). This translocation involves a breakpoint that includes the retinoic acid receptor and leads to production of the promyelocytic leukemia (PML)-retinoic acid receptor alpha (RARA) fusion protein. Patients with a suspected diagnosis of APL can have their diagnosis confirmed by detection of the PML-RARA fusion (e.g., through fluorescence in situ hybridization [FISH], reverse transcriptase–polymerase chain reaction [RT–PCR], or conventional cytogenetics). An immunofluorescence method using an anti-PML monoclonal antibody can rapidly establish the presence of the PML-RARA fusion protein based on the characteristic distribution pattern of PML that occurs in the presence of the fusion protein.[4,5,6]
Clinically, APL is characterized by a severe coagulopathy that is often present at the time of diagnosis. Mortality during induction (particularly with cytotoxic agents used alone) due to bleeding complications is more common in this subtype than in other French-American-British classifications. A lumbar puncture at diagnosis should not be performed until evidence of coagulopathy has resolved.
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APL in children is generally similar to APL in adults, though children have a higher incidence of hyperleukocytosis (defined as white blood cell [WBC] count higher than 10 × 109 /L) and a higher incidence of the microgranular morphologic subtype.[8,9,10,11] Similar to adults, children with WBC counts less than 10 × 109 /L at diagnosis have significantly better outcome than patients with higher WBC counts.[9,10,12] The prognostic significance of WBC count is used in defining high-risk and low-risk patient populations for assigning postinduction treatment, with high-risk patients most commonly defined by WBC of 10 × 109 /L or greater.[13,14]FLT3 mutations (either internal tandem duplications or kinase domain mutations) are observed in 40% to 50% of APL cases, with the presence of FLT3 mutations correlating with higher WBC counts and the microgranular variant (M3v) subtype.[15,16,17,18,19]FLT3 mutation has been associated with an increased risk of induction death, and in some reports, an increased risk of treatment failure.[15,16,17,18,19,20,21] Data from a combined analysis of two European trials demonstrated that children younger than 4 years with APL presented with higher WBC counts, had an increased incidence of the M3v subtype, and had a higher cumulative incidence of relapse and fatal cardiac toxicity during remission than did adolescents and adults; however, overall survival (OS) was similar.[Level of evidence: 3iiA]