Different isoforms of the B-cell mutator activation-induced cytidine deaminase are aberrantly expressed in BCR–ABL1-positive acute lymphoblastic leukemia patients.

The main reason for the unfavorable clinical outcome of BCR–ABL1-positive acute lymphoblastic leukemia (ALL) is genetic instability. However, how normal B-cell precursors acquire the genetic changes that lead to transformation has not yet been completely defined. We investigated the expression of the activation-induced cytidine deaminase (AID) and its role in clinical outcome in 61 adult BCR–ABL1-positive ALL patients. AID expression was detected in 36 patients (59%); it correlated with the BCR–ABL1 transcript levels and disappeared after treatment with tyrosine kinase inhibitors. Different AID splice variants were identified: full-length isoform; AIDΔE4a, with a 30-bp deletion of exon 4; AIDΔE4, with the exon 4 deletion; AIDins3, with the retention of intron 3; AIDΔE3-E4 isoform without deaminase activity. AID-FL predominantly showed cytoplasmic localization, as did the AID-ΔE4a and AID-ΔE3E4 variants, whereas the C-terminal-truncated AID-ΔE4 showed a slightly increased nuclear localization pattern. AID expression correlated with a higher number of copy number alterations identified in genome-wide analysis using a single-nucleotide polymorphism array. However, the expression of AID at diagnosis was not associated with a worse prognosis. In conclusion, BCR–ABL1-positive ALL cells aberrantly express different isoforms of AID that may act as mutators outside the immunoglobulin (Ig) gene loci in promoting genetic instability. [ABSTRACT FROM AUTHOR]