Activation-Induced Cytidine Deaminase Acts as a Mutator in BCR-ABL1-Transformed Acute Lymphoblastic Leukemia Cells.

Acute lymphoblastic leukemia (ALL) cells are derived from B and T cell precursors and typically carry rearranged immunglobulin (Ig) or T cell receptor (TCR) variable (V) region genes devoid of somatic mutations. The Philadelphia chromsome (Ph) encoding the oncogenic BCR-ABL1 kinase defines a subset of ALL with a particularly unfavorable prognosis. Here we show that oncogenic BCR-ABL1 kinase activity induces aberrant somatic hypermutation in Ph-positive ALL cells. Under physiological conditions, somatic hypermutation is restricted to mature germinal center B cells and depends on expression of the DNA-deaminating enzyme AID. Comparing Ph-positive and Ph-negative ALL cells, AID expression was found in 24 of 28 Ph-positive but only 3 of 80 Ph-negative ALLs. As shown by RT-PCR and Western blot, expression of AID in Ph-positive ALL cells reached similar levels as in germinal center B cells.

Forced expression of BCR-ABL1 in Ph-negative ALL cells and usage of the BCR-ABL1-kinase inhibitor STI571 revealed that BCR-ABL1 kinase activity is required and sufficient to induce aberrant expression of AID in Ph-positive ALL. Consistent with aberrant AID expression in Ph-positive ALL, Ig V_H region genes were mutated in most Ph-positive but unmutated in Ph-negative cases. Of note, also non-Ig genes including BCL6 and MYC harbored somatic mutations in Ph-positive but not Ph-negative ALL cells. Likewise, Ph-positive T cell lineage ALL cells express AID and carry somatically mutated TCRβ V region genes. As demonstrated by ligation-mediated PCR, AID introduced DNA-single-strand breaks also within the tumor suppressor gene CDKN2B in Ph-positive ALL cells, which was sensitive to both inhibition of BCR-ABL1 kinase activity and silencing of AID expression by RNA interference. These findings identify AID as a BCR-ABL1-induced mutator in Ph-positive ALL cells, which may be relevant with respect to the particularly unfavorable prognosis of this leukemia subset.