Submitting Author: Johnson, Ryan Christopher, MD
Institution: Stanford University Medical Center, Department of Pathology
Additional authors:Dita Gratzinger, M.D., PhD. Stanford University Medical Center, Department of Pathology; Athena Cherry, PhD. Stanford University Medical Center, Department of Pathology
Session: AML with recurrent genetic abnormalities Part I

The patient is a 41-year-old female who presented to the ER with increased bruising for several months despite no known injuries and periodic heart palpitations. At presentation, the platelet count and hemoglobin were low, and the white count was elevated at 89 K/uL.

A peripheral blood smear examination at diagnosis revealed an elevated WBC count of 67 K/uL, while the platelet count was 18 K/uL and the hemoglobin was 9.4 g/dL. Minimal anisopoikilocytosis was present within the mature erythroids and platelets were overall normal in morphology. Leukocytes were markedly increased and left shifted with increased numbers of myelocytes and metamyelocytes; fewer numbers of promyelocytes and blasts were present. Blasts comprise 4% of circulating nucleated cells and show typical blast morphology with high nuclear to cytoplasmic ratio cells with smooth chromatin, agranular basophilic cytosplasm, and conspicuous single nucleoli. The absolute neutrophil count was 59 K/uL and the absolute lymphocyte count 4.0 K/uL. No absolute increase in eosinophils, basophils, or monocytes was noted. Many of the neutrophils showed dysplastic morphology including pelgeroid or monolobate or hypolobate nuclei accompanied by agranular cytoplasm. There was no increase in basophils.

The air-dried methanol fixed Wright-Giemsa stained bone marrow aspirate slides showed markedly cellular spicules comprised predominantly of myeloid lineage nucleated cells with markedly decreased megakaryopoiesis and erythropoiesis in the background. Minimal to no cytologic dysplasia was noted in the few megakaryocytes present, and the erythroids showed minimal to mild dyserythropoiesis. The myeloid series was comprised predominantly of myelocytes, metamyelocytes and mature neutrophils with fewer numbers of promyelocytes. Blasts comprised approximately 1% of all nucleated cells.

A bone marrow core biopsy was performed showing predominantly cortical bone and no hematopoietic space available for evaluation.

No immunohistochemistry was performed on this case. A limited flow cytometry panel was performed on the bone marrow aspirate. Flow cytometric analysis revealed approximately 2.4% of CD45 positive events to be within the typical dim CD45 low side scatter base plot. The majority of events (91.4%) of CD45 positive events were within the expected granulocyte region of the SSC/CD45 base plot. A subset (1.2% of total events) within the blast gate expressed CD34, CD33, CD38, with aberrant expression of CD56 and CD19, while lacking expression of CD10 and CD20. Monocytes showed aberrant expression of CD56.

In situ hybridization (FISH) analysis using a dual-fusion BCR/ABL probe was performed on the bone marrow aspirate and was negative. Chromosome analysis, however, revealed the presence of a balanced translocation between the long arms of chromosomes #8 and #21 (i.e., t(8;21)), which was present in 21 of 21 metaphases examined. Due to the unexpected nature of this finding, interphase FISH was performed using a dual-fusion RUNX1T1/RUNX1 probe which was identified in 88% of nucleated cells assessed.

No BCR-ABL transcripts were detected by major and minor breakpoint assessment via reverse transcriptase PCR.

The following additional tests were also negative: FLT3 internal tandem duplication, FLT3 D835 mutation, NPM1 insertion, JAK2 V617F, CEBPA, C-KIT exon 8 and 17.

The prominent features of this case include a marked absolute neutrophilia and left shifted myeloid series with dysplasia in mature neutrophils, the absence of an absolute monocytosis, eosinophilia or basophilia, and a blast count of 1% in the bone marrow and 4% in the peripheral blood. These features were initially most suggestive of a myeloproliferative neoplasm, and given the overt dysplasia within the neutrophil lineage, atypical CML. As BCR-ABL1 fusion testing via FISH was one of the first ancillary tests to be finalized and was negative, a diagnosis of atypical CML was deemed most likely.

However, interphase chromosome analysis revealed the t(8;21) translocation, which was then confirmed by FISH. The t(8;21) translocation defines a specific WHO diagnosis of acute myeloid leukemia with t(8;21), even if the overall blast count is less than 20% (oligoblastic). Indeed, as metaphase analysis showed that the translocation was present in 21 of 21 metaphases examined, this translocation was presumably also present in the non-blast myeloid proliferation as well.

While the case presentation was unusual, some typical findings of t(8;21) AML were present at diagnosis. In particular, flow cytometry revealed the small blast population to express CD19, a common finding in t(8;21) acute leukemia. Dysplastic neutrophils are also commonly seen in t(8;21) AML. A high level of suspicion needs to be maintained for oligoblastic t(8;21) AML in the setting of apparent myeloproliferative/myelodysplastic neoplasms.

This patient was treated with idarubicin/cytarabine induction and high dose cytarabine consolidation and has been in clinical remission since.

Oligoblastic acute myeloid leukemia with t(8;21)(q22;q22); RUNX1-RUNX1T1

Case 316