Definition of APL

Pancreatic Cancer Slideshow Colorectal Cancer Slideshow Top Cancer-Fighting Foods Slideshow

APL: Abbreviation for acute promyelocytic leukemia, a malignancy of the bone marrow in which there is a deficiency of mature blood cells in the myeloid line of cells and an excess of immature cells called promyelocytes. APL is due to a translocation (an exchange of chromosome material) between chromosomes 15 and 17 which is symbolized t(15;17). This translocation is not a mere marker of APL. It is the cause of APL.

APL was first recognized as a distinct disease entity in 1957. It accounts for 5-10% of cases of acute myeloid leukemia (AML). The peak incidence of APL is in young adults. APL is considered a type of AML and is classified as the M3 variant of AML in the internationally accepted French-American-British (FAB) Classification.

The signs and symptoms of APL are nonspecific and include fatigue (feeling tired), minor infections, or a tendency to bleed (hemorrhagic diathesis). There is usually pancytopenia with low levels of red blood cells (anemia), low levels of the granulocytes and monocytes (types of white blood cells that fight infections), and low levels of platelets (that are needed for blood to clot normally). Patients with APL may therefore receive transfusions.

APL is consistently associated with a disorder that resembles (but is not identical to) disseminated intravascular coagulation (DIC). There is in APL a pronounced tendency to hemorrhage (bleeding). The bleeding can manifest itself as petechiae (little bleeding spots in the skin or elsewhere), small ecchymosis (bruises), epistaxis (nose bleeds), bleeding in the mouth, hematuria (blood in the urine), bleeding from venipuncture and bone marrow sites and in girls and women who are menstruating may have menometrorrhagia (excessive irregular menstrual bleeding). The hemorrhagic diathesis (bleeding condition) may precede the diagnosis of leukemia by 2-8 weeks.

The t(15;17) translocation in APL is the result of two chromosome breaks: one in chromosome 15 and the other in chromosome 17. The break in chromosome 15 disrupts the promyelocytic leukemia (PML) gene which encodes a growth suppressing transcription factor. And the break in chromosome 17 interrupts the retinoic acid receptor alpha (RARa) gene which regulates myeloid differentiation. The translocation creates a PML/RARa fusion gene. It produces a chimeric protein that arrests the maturation of myeloid cells at the promyelocytic stage. (It reduces terminal cell differentiation.) And this leads to the increased proliferation of promyelocytes.