The gene fusions BCR‐ABL1, TCF3‐PBX1, and ETV6‐RUNX1 are recurrent in B‐cell acute lymphoblastic leukemia (B‐ALL) and are found with low frequency in coexistence with CRLF2 (cytokine receptor‐like ...factor 2) rearrangements and overexpression. There is limited information regarding the CRLF2 abnormalities and dominant‐negative IKZF1 isoforms associated with surrogate markers of Jak2, ABL, and Ras signaling pathways. To assess this, we evaluated 24 Mexican children with B‐ALL positive for recurrent gene fusions at diagnosis. We found CRLF2 rearrangements and/or overexpression, dominant‐negative IKZF1 isoforms, and surrogate phosphorylated markers of signaling pathways coexisting with recurrent gene fusions. All the BCR‐ABL1 patients expressed CRLF2 and were positive for pCrkl (ABL); most of them were also positive for pStat5 (Jak2/Stat5) and negative for pErk (Ras). TCF3‐PBX1 patients with CRLF2 abnormalities were positive for pStat5, most of them were also positive for pCrkl, and two patients were also positive for pErk. One patient with ETV6‐RUNX1 and intracellular CRLF2 protein expressed pCrkl. In some cases, the activated signaling pathways were reverted in vitro by specific inhibitors. We further analyzed a TCF3‐PBX1 patient at relapse, identifying a clone with the recurrent gene fusion, P2RY8‐CRLF2, rearrangement, and phosphorylation of the three surrogate markers that we studied. These results agree with the previous reports regarding resistance to treatment observed in patients with recurrent gene fusions and coexisting CRLF2 gene abnormalities. A marker phosphorylation signature was identified in BCR‐ABL1 and TCF3‐PBX1 patients. To obtain useful information for the assessment of treatment in B‐ALL patients with recurrent gene fusions, we suggest that they should be evaluated at diagnosis for CRLF2 gene abnormalities and dominant‐negative IKZF1 isoforms, in addition to the analyses of activation and inhibition of signaling pathways.
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