The patient was diagnosed with subclinical DIC because complications from Acute Promyelocytic Leukemia (APL)
The characteristic chromosomal translocation of Acute Promyelocytic Leukemia is the break and fusion of the PML gene located on chromosome 15 and the RARA gene located on chromosome 17. This results in a t(15;17) which is detectable in more than 90% of cases. The PML gene has a physiological role in apoptotic pathways and in genomic stability. The t(15;17) breakpoint in PML can occur in three different sites; bcr1 within intron 6, bcr2 within exon 6, and bcr3 within intron 3 . The RARA receptor is active in different variations within each tissue and is important for granulopoiesis. The PML-RARA fusion transcripts impair signaling which is mediated by RARA and interact with proteins that leads to the delocalization of normal PML from its nuclear structures known as NBs. It is in this way that the PML-RARA oncoprotein negatively acts on the normal physiology of the native PML protein. APL is a subtype of AML that has distinctive morphological, biological and clinical characteristics. It is classified as AML-M3 in the French-American-British (FAB) classification system. The cure rate for APL is ~80-90% for patients who survive induction therapy with ATRA. Before ATRA, the 10-day survival rate with treatment was 9.4%. A high blast count was significantly associated with hemorrhagic events and fatality within the first 10 days. A high blast count and thrombocytopenia was associated with death within 24 hours upon admission and treatment. APL predominantly affects a wide spectrum of individuals between the ages 20 and 59 with no gender discrimination. 10-15% of all AML diagnosed in adults is APL, although it can be seen in distinct populations in a higher percentage. 28.2% of all AML diagnosed in Brazil is APL, and 20% of all AML in Venezuela is APL.
APL presents as a bleeding diathesis and coagulopathy. The more common hypergranular variant of APL presents with leukopenia while the less common microgranular variant tends to be more aggressive and presents with leukocytosis. The malignant promyelocytes have specific properties that interact with the host cells. Maligant APL cells express tumor associated procoagulants; Tissue factor (TF) and cancer procoagulant (CP). Tissue factor is an activator of coagulation and the relative expression is elevated significantly in patients with APL.
APL is characterized as a hyperfibrinolysis state. Fibrinolysis is normally activated by thrombin as the fibrin clot develops and coagulation comes to an end. Malignant promyelocytes highly express annexin-II. Annexin-II is a protein receptor that has a strong affinity to plasminogen and tissue-type plasminogen activator (tPA) which results in strong yield of plasmin which initiates fibrinolysis. Annexin-II is highly expressed in the cerebral microvascular endothelial cells explaining the high prevalence of intracerebral hemorrhage in patients with APL. Cytokine release of IL-1B and TNFa by malignant promyelocytes upregulate apoptosis and upregulate the expression of tissue factor on endothelial cells. It is also common for the cytokines to cause loss of the anti-coagulant cofactor thrombomodulin. These various factors lead to APL-associated coagulopathy commonly seen.
Patients with APL present low fibrinogen levels, low platelet count, and an elevated PT-INR, aPTT, and D-dimer. In DIC secondary to APL, fibrinogen survival is markedly decreased due to rapid consumption and the liver can’t produce the product fast enough. Sometimes more specialized tests are needed to diagnose the coagulopathy in APL. Levels of thrombin-antithrombin complex (TAT), prothrombin fragment 1 and 2, and fibrinopeptide A are all increased and all indicate coagulation activation. Decreased levels of plasminogen, and a-2-antiplasmin further support the hyperfibrinolysis state. Sometimes it is helpful to further evaluate the coagulation process and its components. Protein C and antithrombin III are synthesized in the liver and are relatively normal in APL associated coagulopathy unless the maligancy is accompanied by hepatic dysfunction.
The next installment of the mini-series will focus on the key points of what lead to the diagnosis, what I look for as a medical laboratory professional in aiding the doctor in the diagnosis, and how to treat appropriately.