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Letters to the Editor
Myeloid/lymphoid neoplasms with eosinophilia/basophilia and ETV6-ABL1 fusion: cell-of-origin and response to tyrosine kinase inhibition
ETV6-ABL1 rearrangements have been reported in a spectrum of hematologic malignancies, including B- or T-acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and myeloproliferative neoplasms (MPN).1-11 Mostly reported as single cases, ETV6-ABL1 rearranged MPN shows clinical features mimicking chronic myeloid leukemia (CML) and empirically responds to tyrosine kinase inhibitors (TKI). Therefore, these cases are commonly diagnosed as Philadelphia chromosome negative CML (Ph– CML), CML with atypi- cal ABL1 fusions, or even atypical CML. Transformation to AML, B-ALL and T-ALL has also been observed in a subset of cases.3-8 In addition, eosinophilia is a hallmark in nearly all cases, proportionally much greater than seen in CML. On the other hand, a few de novo AML and ALL cases with this fusion also present with eosinophilia,9,10 raising the possibility of a progression from an underlying chronic phase of myeloid/lymphoid neoplasm, similar to those seen in PDGFRA, PDGFRB, FGFR1 and PCM1- JAK2 rearrangements. Here we report six patients with myeloid/lymphoid proliferation and ETV6-ABL1 fusions and review the literature. Our findings support the clas- sification of such cases as myeloid/lymphoid neoplasms with eosinophilia/basophilia and ETV6-ABL1 fusion, sim- ilar to the category of myeloid/lymphoid neoplasms with eosinophilia and rearrangements of PDGFRA, PDGFRB, or FGFR1, or with PCM1-JAK2 listed in the World Health Organization classification.12
A search for cases at Memorial Sloan Kettering Cancer Center from January 2014 to December 2019 was per- formed and identified five patients with ETV6-ABL1 fusions and we also include a case from University Hospital Cleveland Medical Center (Cleveland, OH, USA). The clinicopathologic features including laborato- ry findings, pathologic evaluation, and cytogenetic and molecular results are summarized in Table 1. Two female and four male patients were included, with a median age of 49.5 years (range 23-88 years). All six patients pre- sented with myeloid proliferation and eosinophilia: four patients were diagnosed as CML, atypical CML or CML with atypical ABL1 fusion, one as essential thrombo- cythemia (ET) based on morphologic findings and peripheral blood counts, and one as myeloid/lymphoid neoplasm with eosinophilia and ETV6-ABL1 fusion. Five patients (with the exception of Patient 4) were treated with either first- or second-generation TKI (imatinib, dasatinib and nilotinib) and showed complete cytoge- netic response a few months (range 2-6 months) after initiation of treatment. Patient 1 had cytogenetic and morphologic relapse after imatinib treatment for 10 years (ABL1 mutational analysis failed) but again achieved cytogenetic remission 2 months after switching to dasatinib treatment. This patient continued to have cytogenetic remission 5 years after dasatinib. Patient 4 had a cryptic rearrangement not detected by routine karyotyping and was initially managed as ET. The patient failed multiple lines of treatment (hydroxyurea, Heat Shock Protein 90 inhibitor, ruxolitinib, anagrelide, and α-interferon), and progressed four years later to AML with marked basophilia. RNA-based sequencing studies revealed ETV6-ABL1 fusion, confirmed by fluo- rescence in situ hybridization (FISH) analysis. Combined imatinib and cytarabine treatment was initiated; howev-
er, the patient died shortly after due to comorbid compli- cations. Patient 5 responded to nilotinib treatment for 2 years then progressed to B-ALL (ABL1 mutational analy- sis failed) and obtained complete remission after HyperCVAD. Patient 6 presented as myeloid sarcoma and T-lymphoblastic lymphoma (TLL) in two separate foci of the same neck node with no increased blasts in the marrow. She was treated with dasatinib for 8 weeks. Her lymphadenopathy and eosinophilia both resolved.
All patients had peripheral eosinophilia, ranging from 1.7-44.5x109/L. Patient 1 had 11% eosinophils in periph- eral blood (PB) at relapse. Three (Patients 2-4) had eosinophilia in the marrow at the time of diagnosis. Peripheral basophilia was documented in Patients 3 and 5 at presentation, in Patient 1 at relapse (2% basophils in PB) and in Patient 4 at transformation (10% basophils in PB). Leukocytosis was widely variable, ranging from 9- 374 x109/L. Patient 3 had anemia and thrombocytopenia. Patient 4 had marked thrombocytosis but no splenomegaly. Patient 5 had anemia. Patients 2-6 had diagnostic marrow biopsy for review that showed 90- 100% cellularity and markedly increased M:E ratio. Blasts were not increased in any of the six patients at diagnosis. Megakaryocyte morphology was highly vari- able: both small and large forms in Patient 2, predomi- nantly large forms in Patient 4, increased hypolobated forms in Patients 5 and 6, unremarkable in the other two cases (Figure 1A). There was no overt dysplasia in myeloid or erythroid lineages observed in any of the cases (data not shown).
FISH analysis using ETV6 and/or ABL1 break-apart probes detected the presence of the ETV6 rearrangement in metaphase cells in four patients (Patients 1, 2, 4 and 5): ABL1 rearrangement in Patient 2, an extra normal fusion signal in Patients 1 and 6 (ABL1 gain), and normal signal pattern in both Patients 4 and 5. Patient 6 had no ETV6 FISH testing but only ABL1. FISH was not performed on the diagnostic sample from Patient 3. Next generation RNA sequencing (RNAseq NGS) with a customized 199- gene panel (Archer FusionPlex) identified the ETV6-ABL1 transcripts involving the same breakpoints with ETV6 exon 5 and ABL1 exon2 in all six cases (Figure 1D and Online Supplementary Table S1). Next generation DNA sequencing was performed using FoundationOne Heme (Foundation Medicine 406 gene panel, Patients 1, 4 and 6) and MSK-IMPACT Heme (400 gene panel, Patients 2, 3 and 5). Patients 1 and 2 were positive for ARID2 trun- cating mutations. In addition, Patient 1 had TP53 point mutation while Patient 2 had CDKN1B truncating muta- tions. Patient 5 was positive for SETD2 mutation. Patients 3, 4 and 6 were negative for additional muta- tions (Online Supplementary Table S2). While ARID2 defect has been associated with megakaryocytic dysplasia,13 in our study, one patient harboring an ARID2 mutation had no megakaryocytic atypia (Patient 1) whereas the other showed variable megakaryocyte morphology (Patient 2), suggesting that the functional significance of ARID2 mutations in such cases needs further investigation. Although CDKN1B expression level was reported to be a potential biomarker for prognostication in acute myeloid leukemia,14 the biological role of this mutation in this entity remains unclear.
To investigate the downstream signaling pathway acti- vation of ETV6-ABL1 fusion, phosphorylation levels of STAT3, STAT5 and ERK were evaluated by immunochem- istry using antibodies specific for phosphorylated proteins on the bone marrow biopsy from Patient 4. Although phospho-STAT3 was not increased, phospho-STAT5 showed a markedly increased signal, suggestive of a spe-
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haematologica | 2021; 106(2)