APL is a distinct variant of acute myeloid leukemia (AML) and is characterized by bone marrow infiltration by promyelocytes; it is often caused by the reciprocal translocation t(15;17)(q24.1;q21.2) causing fusion of the retinoic acid receptor alpha gene (RARA on chromosome 17) with the promyelocytic leukemia gene (PML on chromosome 15), also called PML-RARA. This causes abnormal promyelocytes to predominate in the bone marrow and/or peripheral blood.
Three types exist:
- Low risk (white blood cell [WBC] count <10 000/μL and platelets >40 000/μL) with relapse-free survival of 98%
- Intermediate risk (WBC ≤10 000/μL and platelet ≤40 000/μL) with relapse-free survival of 89%
- High risk (WBC count >10 000/μL) with relapse-free survival of 70%
The incidence of APL increases in the second decade of life, plateaus in early adulthood, and then remains stable into the sixth decade, with a median age at diagnosis of 47 years. Known risk factors include exposure to chemotherapy, ionizing radiation, industrial solvents, and other toxic agents.
Peripheral blood will show either low WBC count (more often seen in the hypergranular type) or high WBC count (more often seen in the microgranular / hypogranular type) and abnormal promyelocytes, with bilobed or kidney-shaped nuclei. In the hypergranular variant, the cytoplasm is filled with granules and/or large Auer rods. The microgranular / hypogranular type is characterized by promyelocytes with either sparse granules or absent granules and is mainly identified by the bilobed-shaped nucleus.
Bone marrow biopsy is typically hypercellular, showing proliferation of abnormal promyelocytes that have abundant pink cytoplasm and convoluted nuclei.
Patients often present with symptoms due to pancytopenia, fatigue, recurrent infections, and/or bleeding. Compared with AML presentations, APL more often presents with DIC and hemorrhage.