A 36-year-old male presents with recurrent epistaxis and fatigue of several days’ duration. Physical examination reveals numerous ecchymoses scattered over his limbs and trunk. A CBC shows the following:
- Hgb 9.2 g/dL (normal = 13.5 – 17.5 g/dL)
- WBC 31×109/L (normal = 4.5 – 11 x 109/L)
- Platelet count 23 x 109/L (normal = 150 – 450 x 109/L)
Review of the blood smear shows numerous hypergranulated immature myeloid cells. Rare cells like the cell below are also present.
What cytogenetic abnormality is most likely present in the abnormal cells?
The answer is D, t(15;17). This is a case of acute promyelocytic leukemia (or AML-M3 in the old FAB classification). The key to the diagnosis is the cell in the image above, which is an immature myeloid cell containing innumerable Auer rods. This cell is called a faggot cell because the Auer rods resemble a bundle of sticks (or faggot). Faggot cells are specific for acute promyelocytic leukemia; they are not seen in any other hematologic malignancy.
Other clues to the diagnosis which are not entirely specific for acute promyelocytic leukemia include the anemia and thrombocytopenia (which point towards bone marrow failure), and the leukocytosis (which presumably is comprised mostly of the hypergranular myeloid cells noted on the blood smear).
Acute promyelocytic leukemia (APL) is a type of acute leukemia in which the predominant cell type is the promyelocyte. The malignant promyelocytes in APL have a distinctive appearance which is different from that of normal promyelocytes. In most cases, the malignant promyelocytes in APL contain innumerable small azurophilic granules – but in rare cases, the promyelocytes are hypogranular.
The characteristic morphologic finding in APL is the faggot cell, as shown above. When you see faggot cells, you can make the diagnosis of APL based on morphology alone, without waiting for molecular or cytogenetic studies (which will show the characteristic t(15;17) of APL – but which take some time to perform).
Making an immediate, morphologic diagnosis is critical in cases of APL, because patients with APL cannot be given routine acute myeloid leukemia chemotherapeutic agents. The granules in the malignant promyelocytes contain substances which quickly activate the coagulation system. Traditional chemotherapeutic agents cause cell lysis and release of the procoagulant substances, which puts the patient at high risk for disseminated intravascular coagulation (DIC).
Patients with APL are given a drug called all-trans retinoic acid (ATRA) that overcomes the maturation block caused by the translocation between chromosomes 15 and 17. Following ATRA therapy, the malignant promyelocytes mature into segmented neutrophils, and the risk of DIC diminishes.
The other cytogenetic translocations in this question are seen in different disorders: inv(16) is seen in some cases of acute myelomonocytic leukemia (AML-M4); t(8;21) is seen in some cases of acute myeloblastic leukemia with maturation (AML-M2); t(14;18) is seen in follicular lymphoma; and t(11;14) is seen in mantle cell lymphoma.
-Kristine Krafts, MD, is an Assistant Professor of Pathology at the University of Minnesota School of Medicine and School of Dentistry and the founder of the educational website Pathology Student.