November 2013 Case 2
A 60 year old female with dizziness and shortness of breath
David Cantu, M.D.; Ryan H. Livengood, M.D.; Sharon L. Wenger, Ph.D.
A 60 year-old female with history of chronic thrombocytopenia of 11 years duration, diabetes mellitus, hypertension and coronary artery disease presented to the emergency department with dizziness and shortness of breath. No abnormalities were identified on physical examination. Laboratory testing showed a WBC count of 16.0 THOU/μL, Hgb 6.8 g/dL, Hct 19.7% and a platelet count of 30 THOU/μL. A differential count yielded 35% blasts consistent with acute leukemia; however, no Auer rods were identified on review. The patient was admitted to Ruby Memorial Hospital for additional work-up and treatment. She was stabilized with multiple transfusions.
A bone marrow biopsy showed an interstitial and diffuse blast infiltrate that had a myelomonocytic immunophenotype by flow cytometry with subset aberrant expression of CD2. Eosinophils were increased (figure 1) and eosinophilic precursors showed atypical basophilic granulation (figures 2 and 3). Fluorescent in-situ hybridization (FISH) studies appropriate for acute myeloid leukemia were performed.
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The recurrent genetic abnormality identified in the presented case was the inv16(p13;q22). The abnormality was identified by FISH (figure 4). Subsequently, karyotypic studies showed the additional abnormality of trisomy 8 (figure 5).
Inversion (16)(p13;q22) and the less common translocation t(16;16)(p13;q22) have been identified in 5–8% of de novo acute myeloid leukemia (AML). The inv(16) has also been identified in rare cases of chronic myeloid leukemia (CML) in blast crisis.
The peripheral blood of typical cases shows blasts and maturing cells of the monocyte lineage. The bone marrow trephine core biopsy is commonly hypercellular. Eosinophils may be normal in number, but are usually increased. Bone marrow aspirate smears show eosinophils at all stages of maturation. Immature eosinophilic granules are present at the promyelocyte and myelocyte stages and often absent at later stages of eosinophil maturation. The abnormal eosinophilic granules are purple-violet and larger than those typically present in normal eosinophilic precursors. Nuclear hyposegmentation is occasionally seen in the mature eosinophils, and Charcot –Leyden crystals may be present.
The naphthol-ASD-chloroacetate esterase reaction, while negative in typical eosinophils, is weakly positive in the abnormal eosinophils of AML with inv(16). Eosinophils of AML with t(8;21)(q22;q22) do not present this feature. Myeloblasts may contain Auer rods, while at least 3% of the blasts will show myeloperoxidase reactivity. The monoblasts and promonocytes show non-specific esterase reactivity in most cases. Neutrophilic maturation in the bone marrow is relatively decreased, without significant numbers of mature neutrophils.
Immunophenotyping by flow cytometry shows a heterogeneous blast phenotype. There is frequent expression of CD34 and CD117 with granulocytic differentiation (CD13, CD33, CD15, CD65 and MPO) and monocytic differentiation (CD4, CD11b, CD11c, CD14, CD36, CD64 and lysozyme). Aberrant expression of CD2 has been described but is a non-specific finding.
Cytogenetic analysis shows either inv(16) (p13.1;q22) or t(16;16)(p13;q22), the former being more common . Trisomy 8 and trisomy 22 are the most frequent additional karyotypic abnormalities. Trisomy 22 is relatively specific for inv(16)(p13.1;q22), as it is very rarely detected with other primary aberrations in AML. On the other hand, trisomy 8, as seen in the presented case, is more ubiquitous and non-specific. Trisomy 21 and del(7q) also may occur. Both the chromosomal inversion and translocation result in the fusion of the CBFB gene at 16q22 to the MYH11gene at 16p13.1. This results in the formation of the CBFB–MYH11fusion oncoprotein. The expression of CBFB–MYH11 is a central pathogenetic event without a doubt, but the expression of this fusion oncoprotein is still insufficient for leukemia development. MYH11 codes for a smooth muscle myosin heavy chain and CBFB codes for the core binding factor beta subunit. Conventional cytogenetic methods, Southern blot analysis, RT-PCR, and immunohistochemistry have been used to detect inv(16) and t(16;16). Conventional cytogenetics in addition will also identify additional chromosomal abnormalities that have been found in up to 50% of AML cases. By conventional cytogenetic analysis, the inv(16)(p13.1q22)/t(16;16)(p13.1;q22) is a subtle rearrangement that may be overlooked when metaphase preparations are not optimal. Because of this, FISH and RT-PCR methods may be necessary to document the genetic alteration in cryptic cases.
Documentation of inv(16)(p13.1q22) or t(16;16)(p13.1q22) is sufficient for the diagnosis of AML regardless of blast percentage, and cases with less than 20% bone marrow blasts should be diagnosed as AML. Clinical studies have shown that patients with AML with inv(16)(p13.1q22) or t(16;16)(p13.1q22) treated with high dose cytarabine in the consolidation phase accomplish longer complete remissions. There is decreased survival and higher risk of relapse in older patients and patients with KIT mutations (detected in 30% of cases). Trisomy 22 has been reported to have improved outcome.
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