Isolated monocytosis was the flag preceding abnormalities in other parameters of complete blood counts in chronic myeloid leukemia with e1a2 (minor, P190) BCR-ABL1 chimeric transcripts

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm with the molecular hallmark of BCR-ABL1 chimeric transcripts from t(9;22)(q24;q22). In more than 95% of CML, the fusion occurs in the major breakpoint cluster region (M-bcr) of BCR, most commonly producing the e14a2 or e13a2 type. CML with e1a2 BCR-ABL1 by fusion in minor bcr is rare, accounting for approximately 1% of CML. CML with e1a2 BCR-ABL1 is reportedly associated with monocytosis and poor prognosis. Here we describe a woman who underwent bone marrow (BM) study with an impression of chronic myelomonocytic leukemia based on thrombocytosis and prominent monocytosis. Genetic workup revealed, however, t(9;22) and e1a2 BCR-ABL1, which indicated CML and explained her unusual monocytosis. A review of serial complete blood counts (CBC) before the BM study demonstrated that isolated monocytosis preceded the abnormalities in other parameters of CBC. The patient is on follow-up with tyrosine kinase inhibitor (TKI, dasatinib) medication. Close monitoring is required due to the association of e1a2 BCR-ABL1 with poor response to TKI and frequent disease progression. Timely genetic workup and determination of the precise type of BCR-ABL1 transcripts are critical for the diagnosis and stratification of this rare subtype of CML with distinct genotype-phenotype correlations.